You can find below the publication list of all members of the theoretical chemistry group at ENS. For the list of each individual member, please consult their personal webpage from the Members page.
1994 |
Solvent Barriers in Unimolecular Ionizations. 2. Electron Transfer Perspective for Alkyl Iodide Ionizations Article de journal J R Mathis; J T Hynes Journal of Physical Chemistry, 98 (21), p. 5460-5470, 1994, (cited By 8). @article{Mathis19945460, title = {Solvent Barriers in Unimolecular Ionizations. 2. Electron Transfer Perspective for Alkyl Iodide Ionizations}, author = {J R Mathis and J T Hynes}, doi = {10.1021/j100072a011}, year = {1994}, date = {1994-01-01}, journal = {Journal of Physical Chemistry}, volume = {98}, number = {21}, pages = {5460-5470}, abstract = {The analysis of the alkyl iodide ionizations RI $rightarrow$ R+ + I- begun in I (preceding paper in this issue) is continued by focusing on a nuclear (r) coordinate-dependent electron transfer (ET) perspective. Considerations presented within provide an understanding of the factors that determine, e.g., whether or not charge transfer plus bond breakage occurs in a stepwise or concerted fashion. Two different ET rate constants are investigated: a conventional r-dependent ET rate k1ET-generalized to these SN1 ionizations - and a new ET rate k2ET. k2ET accounts for the r-dependence of the solvent barrier location and thus incorporates into the rate analysis dividing surface curvature or, equivalently, the strong transition state electronic structure variation along the nuclear coordinate. This added component - which is not present in the standard ET rate - is essential in providing a reasonable estimate of the ionization rate while still attempting to retain an ET perspective. k1ET can be more than an order of magnitude smaller than k2ET and does not provide a reasonable estimate of the ionization rate. In the absence of the electronic structure variation effects, k2ET reduces to k1ET. Comparison of k2ET with the variationally optimized result kv from I shows that they are comparable, but with k2ET being slightly larger. Trajectory calculations, carried out to test the no-recrossing assumption for the k2ET dividing surface, show that there is some recrossing. This recrossing correction brings k2ET into accord with kv, so that kv is the best estimate of the actual rate. The results are applicable to other reaction classes, including the SRN1 reaction mechanism and inner sphere electron transfer reactions. textcopyright 1994 American Chemical Society.}, note = {cited By 8}, keywords = {}, pubstate = {published}, tppubtype = {article} } The analysis of the alkyl iodide ionizations RI $rightarrow$ R+ + I- begun in I (preceding paper in this issue) is continued by focusing on a nuclear (r) coordinate-dependent electron transfer (ET) perspective. Considerations presented within provide an understanding of the factors that determine, e.g., whether or not charge transfer plus bond breakage occurs in a stepwise or concerted fashion. Two different ET rate constants are investigated: a conventional r-dependent ET rate k1ET-generalized to these SN1 ionizations - and a new ET rate k2ET. k2ET accounts for the r-dependence of the solvent barrier location and thus incorporates into the rate analysis dividing surface curvature or, equivalently, the strong transition state electronic structure variation along the nuclear coordinate. This added component - which is not present in the standard ET rate - is essential in providing a reasonable estimate of the ionization rate while still attempting to retain an ET perspective. k1ET can be more than an order of magnitude smaller than k2ET and does not provide a reasonable estimate of the ionization rate. In the absence of the electronic structure variation effects, k2ET reduces to k1ET. Comparison of k2ET with the variationally optimized result kv from I shows that they are comparable, but with k2ET being slightly larger. Trajectory calculations, carried out to test the no-recrossing assumption for the k2ET dividing surface, show that there is some recrossing. This recrossing correction brings k2ET into accord with kv, so that kv is the best estimate of the actual rate. The results are applicable to other reaction classes, including the SRN1 reaction mechanism and inner sphere electron transfer reactions. textcopyright 1994 American Chemical Society. |
Dynamics of Twisted Intramolecular Charge Transfer Complexes in Polar Solvents Article de journal T Fonseca; H J Kim; J T Hynes Journal of Molecular Liquids, 60 (1-3), p. 161-200, 1994, (cited By 59). @article{Fonseca1994161, title = {Dynamics of Twisted Intramolecular Charge Transfer Complexes in Polar Solvents}, author = {T Fonseca and H J Kim and J T Hynes}, doi = {10.1016/0167-7322(94)00744-6}, year = {1994}, date = {1994-01-01}, journal = {Journal of Molecular Liquids}, volume = {60}, number = {1-3}, pages = {161-200}, abstract = {The excited-state electron transfer and time-dependent fluorescence (TDF) for twisted intramolecular charge transfer (TICT) molecules in polar solvents are studied theoretically. This reaction class involves critical solvent stabilization of the charge-separated state with a large dipole moment compared to the less polar, locally-excited state; it is also accompanied by a significant solute geometry distortion compared to vacuum, i.e., from a planar to a twisted configuration. A theoretical framework for TICT dynamics is constructed and illustrated throughout for a model dimethylaminobenzonitrile (DMABN) solute. By employing a model two valence-bond state description for the solute in a dielectric continuum solvent for illustration, we obtain a two-dimensional free energy surface in terms of the solute torsional angle texttheta and a solvent coordinate s that gauges the nonequilibrium solvent orientational polarization. The TICT rate constant and TDF are analyzed via the minimum free energy solution-phase reaction path (SRP), which is found to be strongly curved on the reactive free energy surface. For slow aprotic solvents, the SRP is mainly along the texttheta coordinate near the transition state. There is little motion in the solvent coordinate s; the solvent lags the solute twisting motion and there is nonadiabatic nonequilibrium solvation. Near the reactant and product states, however, the SRP is almost completely along s with little solute torsional motion. This indicates that the solvent orientational polarization fluctuation is important in initiating the TICT reaction. By contrast, for faster solvents (on the same surface), the SRP becomes nearly parallel to s at the transition state, while it is almost along texttheta near the reactant and product states. Thus the reactive mode relevant for electron transfer at the transition state changes markedly with the solvent time scale. The consequences of the SRP curvature on the TICT rate constant and its contrasts with traditional activated electron transfer are also discussed. The SRP analysis also shows that the dynamics relevant for TDF vary during the course of the reaction. In particular, for DMABN in acetonitrile solvent, about the first 70% of the dynamic Stokes shift upon photoexcitation occurs mainly via the solute torsional dynamics with a minor s participation, followed by the remaining relaxation along s with a minimal texttheta motion near the perpendicular geometry. Thus the TDF dynamics probed via DMABN involve the solute twisting dynamics as well as the solvation dynamics; the former dominates at the early and middle stages of TDF until the latter eventually takes over towards the end. As the solvent becomes faster, the solvent motions begin to participate in the TDF dynamics progressively earlier; as a result, the TDF probes the concomitant motions of both the solute and solvent. This picture provides a theoretical explanation for the recent experimental findings that the dynamic Stokes shift measured with DMABN is much faster than that with coumarin dyes (with no torsional degrees of freedom) for the same solvents. Finally, it is found that the variations of the activation free energy with solvent polarity and overall reaction free energetics correlate well with Hammond postulate behavior and experimentally observed trends for activated TICT reactions. textcopyright 1994.}, note = {cited By 59}, keywords = {}, pubstate = {published}, tppubtype = {article} } The excited-state electron transfer and time-dependent fluorescence (TDF) for twisted intramolecular charge transfer (TICT) molecules in polar solvents are studied theoretically. This reaction class involves critical solvent stabilization of the charge-separated state with a large dipole moment compared to the less polar, locally-excited state; it is also accompanied by a significant solute geometry distortion compared to vacuum, i.e., from a planar to a twisted configuration. A theoretical framework for TICT dynamics is constructed and illustrated throughout for a model dimethylaminobenzonitrile (DMABN) solute. By employing a model two valence-bond state description for the solute in a dielectric continuum solvent for illustration, we obtain a two-dimensional free energy surface in terms of the solute torsional angle texttheta and a solvent coordinate s that gauges the nonequilibrium solvent orientational polarization. The TICT rate constant and TDF are analyzed via the minimum free energy solution-phase reaction path (SRP), which is found to be strongly curved on the reactive free energy surface. For slow aprotic solvents, the SRP is mainly along the texttheta coordinate near the transition state. There is little motion in the solvent coordinate s; the solvent lags the solute twisting motion and there is nonadiabatic nonequilibrium solvation. Near the reactant and product states, however, the SRP is almost completely along s with little solute torsional motion. This indicates that the solvent orientational polarization fluctuation is important in initiating the TICT reaction. By contrast, for faster solvents (on the same surface), the SRP becomes nearly parallel to s at the transition state, while it is almost along texttheta near the reactant and product states. Thus the reactive mode relevant for electron transfer at the transition state changes markedly with the solvent time scale. The consequences of the SRP curvature on the TICT rate constant and its contrasts with traditional activated electron transfer are also discussed. The SRP analysis also shows that the dynamics relevant for TDF vary during the course of the reaction. In particular, for DMABN in acetonitrile solvent, about the first 70% of the dynamic Stokes shift upon photoexcitation occurs mainly via the solute torsional dynamics with a minor s participation, followed by the remaining relaxation along s with a minimal texttheta motion near the perpendicular geometry. Thus the TDF dynamics probed via DMABN involve the solute twisting dynamics as well as the solvation dynamics; the former dominates at the early and middle stages of TDF until the latter eventually takes over towards the end. As the solvent becomes faster, the solvent motions begin to participate in the TDF dynamics progressively earlier; as a result, the TDF probes the concomitant motions of both the solute and solvent. This picture provides a theoretical explanation for the recent experimental findings that the dynamic Stokes shift measured with DMABN is much faster than that with coumarin dyes (with no torsional degrees of freedom) for the same solvents. Finally, it is found that the variations of the activation free energy with solvent polarity and overall reaction free energetics correlate well with Hammond postulate behavior and experimentally observed trends for activated TICT reactions. textcopyright 1994. |
On the Activation Free Energy of the Cl- + CH3Cl SN2 Reaction in Solution Article de journal J R Mathis; R Bianco; J T Hynes Journal of Molecular Liquids, 61 (1-3), p. 81-101, 1994, (cited By 39). @article{Mathis199481, title = {On the Activation Free Energy of the Cl- + CH3Cl SN2 Reaction in Solution}, author = {J R Mathis and R Bianco and J T Hynes}, doi = {10.1016/0167-7322(94)00754-3}, year = {1994}, date = {1994-01-01}, journal = {Journal of Molecular Liquids}, volume = {61}, number = {1-3}, pages = {81-101}, abstract = {The activation free energetics of the identity SN2 reaction Cl- + CH3Cl $rightarrow$ ClCH3 + Cl- in solution are examined theoretically. Two diabatic valence bond states, $psi$1[Cl-1/CH3Cl] and $psi$2[ClCH3/Cl-], are employed within the framework of the Kim-Hynes theory of solvation [H. J. Kim and J. T. Hynes, J. Chem. Phys. 96, 5088 (1992)] to calculate the free energies of the reactant and transition states in five solvents of different polarity. An electronically adiabatic vacuum potential energy surface for the reaction is presented, in terms of the energies associated with the two diabatic states $psi$1 and $psi$2 and the electronic coupling between them. Emphasis is placed on exposing the differences between two limiting solvent electronic polarization response regimes: the Born-Oppenheimer (BO) limit, where the solvent electronic polarization time scale is much smaller than that of the solute electronic motion, and the self-consistent (SC) limit, where the solute electronic motion time scale is much smaller than that of the solvent electronic polarization. It is found that the activation free energies calculated within the two limiting regimes can differ by as much as 7.7 kcal/mol, which is extremely significant, given the exponential sensitivity of the reaction rate constant. For the solvents studied, the BO activation free energies are always lower than the experimental estimates, whereas the SC results are invariably higher. The activation free energies calculated by the full Kim-Hynes theory, which properly treats the solvent electronic polarization, gives a result in between the BO and SC limits. textcopyright 1994.}, note = {cited By 39}, keywords = {}, pubstate = {published}, tppubtype = {article} } The activation free energetics of the identity SN2 reaction Cl- + CH3Cl $rightarrow$ ClCH3 + Cl- in solution are examined theoretically. Two diabatic valence bond states, $psi$1[Cl-1/CH3Cl] and $psi$2[ClCH3/Cl-], are employed within the framework of the Kim-Hynes theory of solvation [H. J. Kim and J. T. Hynes, J. Chem. Phys. 96, 5088 (1992)] to calculate the free energies of the reactant and transition states in five solvents of different polarity. An electronically adiabatic vacuum potential energy surface for the reaction is presented, in terms of the energies associated with the two diabatic states $psi$1 and $psi$2 and the electronic coupling between them. Emphasis is placed on exposing the differences between two limiting solvent electronic polarization response regimes: the Born-Oppenheimer (BO) limit, where the solvent electronic polarization time scale is much smaller than that of the solute electronic motion, and the self-consistent (SC) limit, where the solute electronic motion time scale is much smaller than that of the solvent electronic polarization. It is found that the activation free energies calculated within the two limiting regimes can differ by as much as 7.7 kcal/mol, which is extremely significant, given the exponential sensitivity of the reaction rate constant. For the solvents studied, the BO activation free energies are always lower than the experimental estimates, whereas the SC results are invariably higher. The activation free energies calculated by the full Kim-Hynes theory, which properly treats the solvent electronic polarization, gives a result in between the BO and SC limits. textcopyright 1994. |
Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. 4. Quantum Theory in a Multidiabatic State Formulation Article de journal R Bianco; J J I Timoneda; J T Hynes Journal of Physical Chemistrytextregistered, 98 (47), p. 12103-12107, 1994, (cited By 19). @article{Bianco199412103, title = {Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. 4. Quantum Theory in a Multidiabatic State Formulation}, author = {R Bianco and J J I Timoneda and J T Hynes}, doi = {10.1021/j100098a001}, year = {1994}, date = {1994-01-01}, journal = {Journal of Physical Chemistrytextregistered}, volume = {98}, number = {47}, pages = {12103-12107}, abstract = {A theory of the solute electronic structure for chemical reaction systems in solution [Kim H. J., Hynes, J. T. J. Chem. Phys. 1992, 96, 5088], in terms of a solute description via two chemically relevant valence bond (VB) states and a dielectric continuum solvent model, is generalized to account for more than two solute VB states. This extension is required for a number of important solution reaction systems. The electronic polarization of the solvent is quantized via a coherent states treatment, and its orientational polarization spans both nonequilibrium and equilibrium solvation regimes. The nonlinear Schr\"{o}dinger equation and system free energy are obtained in a solvent coordinates framework. textcopyright 1994 American Chemical Society.}, note = {cited By 19}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theory of the solute electronic structure for chemical reaction systems in solution [Kim H. J., Hynes, J. T. J. Chem. Phys. 1992, 96, 5088], in terms of a solute description via two chemically relevant valence bond (VB) states and a dielectric continuum solvent model, is generalized to account for more than two solute VB states. This extension is required for a number of important solution reaction systems. The electronic polarization of the solvent is quantized via a coherent states treatment, and its orientational polarization spans both nonequilibrium and equilibrium solvation regimes. The nonlinear Schrödinger equation and system free energy are obtained in a solvent coordinates framework. textcopyright 1994 American Chemical Society. |
1993 |
Well and Barrier Dynamics and Electron Transfer Rates. A Molecular Dynamics Study Article de journal B B Smith; A Staib; J T Hynes Chemical Physics, 176 (2-3), p. 521-537, 1993, (cited By 66). @article{Smith1993521, title = {Well and Barrier Dynamics and Electron Transfer Rates. A Molecular Dynamics Study}, author = {B B Smith and A Staib and J T Hynes}, doi = {10.1016/0301-0104(93)80259-C}, year = {1993}, date = {1993-01-01}, journal = {Chemical Physics}, volume = {176}, number = {2-3}, pages = {521-537}, abstract = {Molecular dynamics simulations are carried out for model electronically adiabatic electron transfer (ET) reactions. The reactants are immersed in various model dipolar aprotic solvents, ranging from slightly overdamped to strongly overdamped. In all cases, the solvent barrier recrossings which give the solvent dynamical influence on the ET rate are determined by motion in the barrier top region, and not in the solvent wells as some current notions would have it. The MD reaction transmission coefficients are reproduced by Grote-Hynes theory to within the simulation error bars, while other analytic descriptions such as Zusman theory (in its range of applicability) and Kramers theory are less satisfactory, and fail significantly in the overdamped solvent regime. The reasons for these results as well as their implications for the interpretation of adiabatic ET rates are briefly discussed. textcopyright 1993.}, note = {cited By 66}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecular dynamics simulations are carried out for model electronically adiabatic electron transfer (ET) reactions. The reactants are immersed in various model dipolar aprotic solvents, ranging from slightly overdamped to strongly overdamped. In all cases, the solvent barrier recrossings which give the solvent dynamical influence on the ET rate are determined by motion in the barrier top region, and not in the solvent wells as some current notions would have it. The MD reaction transmission coefficients are reproduced by Grote-Hynes theory to within the simulation error bars, while other analytic descriptions such as Zusman theory (in its range of applicability) and Kramers theory are less satisfactory, and fail significantly in the overdamped solvent regime. The reasons for these results as well as their implications for the interpretation of adiabatic ET rates are briefly discussed. textcopyright 1993. |
Solute Electronic Structure and Solvation in Chemical Reactions in Solution Article de journal J T Hynes; H J Kim; J R Mathis; J J i Timoneda Journal of Molecular Liquids, 57 (C), p. 53-73, 1993, (cited By 13). @article{Hynes199353, title = {Solute Electronic Structure and Solvation in Chemical Reactions in Solution}, author = {J T Hynes and H J Kim and J R Mathis and J J {i Timoneda}}, doi = {10.1016/0167-7322(93)80047-Y}, year = {1993}, date = {1993-01-01}, journal = {Journal of Molecular Liquids}, volume = {57}, number = {C}, pages = {53-73}, abstract = {A review is given of a recent theoretical approach to the issue of the title. Compared to past efforts, novel aspects of the theory include incorporation of nonequilibrium solvation, attention to time scales of the solvent electronic polarization and construction of reaction paths and free energetics. The theory is illustrated with examples of electron and proton transfer and SN1 ionization. textcopyright 1993.}, note = {cited By 13}, keywords = {}, pubstate = {published}, tppubtype = {article} } A review is given of a recent theoretical approach to the issue of the title. Compared to past efforts, novel aspects of the theory include incorporation of nonequilibrium solvation, attention to time scales of the solvent electronic polarization and construction of reaction paths and free energetics. The theory is illustrated with examples of electron and proton transfer and SN1 ionization. textcopyright 1993. |
Vibrational Relaxation Times for a Model Hydrogen-Bonded Complex in a Polar Solvent Article de journal M Bruehl; J T Hynes Chemical Physics, 175 (1), p. 205-221, 1993, (cited By 50). @article{Bruehl1993205, title = {Vibrational Relaxation Times for a Model Hydrogen-Bonded Complex in a Polar Solvent}, author = {M Bruehl and J T Hynes}, doi = {10.1016/0301-0104(93)80238-5}, year = {1993}, date = {1993-01-01}, journal = {Chemical Physics}, volume = {175}, number = {1}, pages = {205-221}, abstract = {Molecular dynamics computer simulations are carried out to estimate, via a Landau-Teller formula, the vibrational relaxation times for the proton stretch, proton bend and symmetric stretch in a model hydrogen-bonded complex solute in a model polar aprotic solvent. The estimated times are all small when compared to those of nonpolar reference systems. Analysis indicates that Coulomb solute-solvent forces play a central role in causing these short times, but that their precise role is vibrational mode-specific. Interactions of the solute with first solvent shell molecules provide the dominant source of the relaxation forces. textcopyright 1993.}, note = {cited By 50}, keywords = {}, pubstate = {published}, tppubtype = {article} } Molecular dynamics computer simulations are carried out to estimate, via a Landau-Teller formula, the vibrational relaxation times for the proton stretch, proton bend and symmetric stretch in a model hydrogen-bonded complex solute in a model polar aprotic solvent. The estimated times are all small when compared to those of nonpolar reference systems. Analysis indicates that Coulomb solute-solvent forces play a central role in causing these short times, but that their precise role is vibrational mode-specific. Interactions of the solute with first solvent shell molecules provide the dominant source of the relaxation forces. textcopyright 1993. |
A Theoretical Model for SnI Ionic Dissociations in Solution. 3. Analysis of Tert-Butyl Halides Article de journal J R Mathis; H J Kim; J T Hynes Journal of the American Chemical Society, 115 (18), p. 8248-8262, 1993, (cited By 56). @article{Mathis19938248, title = {A Theoretical Model for SnI Ionic Dissociations in Solution. 3. Analysis of Tert-Butyl Halides}, author = {J R Mathis and H J Kim and J T Hynes}, doi = {10.1021/ja00071a038}, year = {1993}, date = {1993-01-01}, journal = {Journal of the American Chemical Society}, volume = {115}, number = {18}, pages = {8248-8262}, abstract = {The SN1 activation free energies and transition-state structure for the series tert-butyl chloride, -bromide, and -iodide in several solvents over a wide polarity range are examined theoretically. The analysis is effected by using a two-state valence bond representation for the solute electronic structure, in combination with a two-dimensional free energy formalism in terms of the alkyl halide nuclear separation coordinate and a solvent coordinate. The calculated tert-butyl halide activation free energies are in good agreement with experiment. In a fixed solvent, a decreasing activation free energy trend is found (Cl > Br > I) as well as a decreasing transition-state ionic character. Both fixed solvent trends, and others, are shown to arise from a decreasing electronic coupling variation between the covalent and ionic solute valence bond states, in fundamental contrast with earlier interpretations. A Br\onsted plot for the series in a fixed solvent shows a slope (greater than unity) which is consistent with the Hammond postulate. As solvent polarity increases, the calculated activation free energies decrease for the series, yet the trend of decreasing transition state solvent stabilization with increasing solvent polarity is exhibited by all three tert-butyl halides, confirming and extending earlier results [Kim, H. J.;Hynes, J. T.J. Am. Chem.Soc. 1992, 114, 10508, 10528]. This trend is at variance with the conventional Hughes-Ingold interpretation for increasing SN1 reaction rates with increasing solvent polarity but is consistent with the Hammond postulate. An approximately linear correlation is established for the transition-state ionic character and a suitably defined measure of the tert-butyl halide bond extension at the transition state. In addition, it is also demonstrated how a Br\onsted plot for each halide in solvents of different polarity, whose slope is shown analytically to be proportional to the square of the transition state ionic character, could be used as an experimental test of these unconventional predictions. textcopyright 1993, American Chemical Society. All rights reserved.}, note = {cited By 56}, keywords = {}, pubstate = {published}, tppubtype = {article} } The SN1 activation free energies and transition-state structure for the series tert-butyl chloride, -bromide, and -iodide in several solvents over a wide polarity range are examined theoretically. The analysis is effected by using a two-state valence bond representation for the solute electronic structure, in combination with a two-dimensional free energy formalism in terms of the alkyl halide nuclear separation coordinate and a solvent coordinate. The calculated tert-butyl halide activation free energies are in good agreement with experiment. In a fixed solvent, a decreasing activation free energy trend is found (Cl > Br > I) as well as a decreasing transition-state ionic character. Both fixed solvent trends, and others, are shown to arise from a decreasing electronic coupling variation between the covalent and ionic solute valence bond states, in fundamental contrast with earlier interpretations. A Brønsted plot for the series in a fixed solvent shows a slope (greater than unity) which is consistent with the Hammond postulate. As solvent polarity increases, the calculated activation free energies decrease for the series, yet the trend of decreasing transition state solvent stabilization with increasing solvent polarity is exhibited by all three tert-butyl halides, confirming and extending earlier results [Kim, H. J.;Hynes, J. T.J. Am. Chem.Soc. 1992, 114, 10508, 10528]. This trend is at variance with the conventional Hughes-Ingold interpretation for increasing SN1 reaction rates with increasing solvent polarity but is consistent with the Hammond postulate. An approximately linear correlation is established for the transition-state ionic character and a suitably defined measure of the tert-butyl halide bond extension at the transition state. In addition, it is also demonstrated how a Brønsted plot for each halide in solvents of different polarity, whose slope is shown analytically to be proportional to the square of the transition state ionic character, could be used as an experimental test of these unconventional predictions. textcopyright 1993, American Chemical Society. All rights reserved. |
Dynamical Theory of Proton Tunneling Transfer Rates in Solution: General Formulation Article de journal D Borgis; J T Hynes Chemical Physics, 170 (3), p. 315-346, 1993, (cited By 214). @article{Borgis1993315, title = {Dynamical Theory of Proton Tunneling Transfer Rates in Solution: General Formulation}, author = {D Borgis and J T Hynes}, doi = {10.1016/0301-0104(93)85117-Q}, year = {1993}, date = {1993-01-01}, journal = {Chemical Physics}, volume = {170}, number = {3}, pages = {315-346}, abstract = {A general dynamical theory is presented for the rate constant of weak coupling, nonadiabatic proton-tunneling reactions in solution. The theory incorporates the critical role of the solvent and the vibration of the separation of the heavy particles between which the proton transfers, including their dynamics. The formulation which allows the computation of the quantum rate constant k via classical molecular dynamics simulation techniques is presented, as are a number of approximate analytic results for k in a variety of different important regimes. The frequent appearance of (nearly) classical Arrhenius behavior for k - even though the intrinsic reactive event is quantum proton tunneling - is discussed, together with the solvent and vibrational contributions to the apparent activation energy. In certain weak solvation limits, however, non-Arrhenius behavior for k is found and is related to vibrational Franck-Condon features in the reaction. textcopyright 1993.}, note = {cited By 214}, keywords = {}, pubstate = {published}, tppubtype = {article} } A general dynamical theory is presented for the rate constant of weak coupling, nonadiabatic proton-tunneling reactions in solution. The theory incorporates the critical role of the solvent and the vibration of the separation of the heavy particles between which the proton transfers, including their dynamics. The formulation which allows the computation of the quantum rate constant k via classical molecular dynamics simulation techniques is presented, as are a number of approximate analytic results for k in a variety of different important regimes. The frequent appearance of (nearly) classical Arrhenius behavior for k - even though the intrinsic reactive event is quantum proton tunneling - is discussed, together with the solvent and vibrational contributions to the apparent activation energy. In certain weak solvation limits, however, non-Arrhenius behavior for k is found and is related to vibrational Franck-Condon features in the reaction. textcopyright 1993. |
Vibrational Predissociation in Hydrogen-Bonded OH...O Complexes via OH Stretch-OO Stretch Energy Transfer Article de journal A Staib; J T Hynes Chemical Physics Letters, 204 (1-2), p. 197-205, 1993, (cited By 97). @article{Staib1993197, title = {Vibrational Predissociation in Hydrogen-Bonded OH...O Complexes via OH Stretch-OO Stretch Energy Transfer}, author = {A Staib and J T Hynes}, doi = {10.1016/0009-2614(93)85627-Z}, year = {1993}, date = {1993-01-01}, journal = {Chemical Physics Letters}, volume = {204}, number = {1-2}, pages = {197-205}, abstract = {A theory for vibrational predissociation of hydrogen-bonden OH...O complexes via OH stretch-OO stretch energy transfer is constructed, motivated by a recent experiment on vibrational predissociation of hydrogen-bonded ethanol dimer in solution. The theory employs a vibrationally adiabatic perspective, exploiting the frequency separation between the OH and OO stretches. The nonadiabatic coupling responsible for the hydrogen bond rupture subsequent to OH stretch excitation is found analytically. A lifetime for ethanol dimer of $approx$ 10 ps is estimated, which is within a factor of two of the experimental result. General trends are predicted and discussed for other OH...O complexes. textcopyright 1993.}, note = {cited By 97}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theory for vibrational predissociation of hydrogen-bonden OH...O complexes via OH stretch-OO stretch energy transfer is constructed, motivated by a recent experiment on vibrational predissociation of hydrogen-bonded ethanol dimer in solution. The theory employs a vibrationally adiabatic perspective, exploiting the frequency separation between the OH and OO stretches. The nonadiabatic coupling responsible for the hydrogen bond rupture subsequent to OH stretch excitation is found analytically. A lifetime for ethanol dimer of $approx$ 10 ps is estimated, which is within a factor of two of the experimental result. General trends are predicted and discussed for other OH...O complexes. textcopyright 1993. |
Electronic Friction and Electron Transfer Rates at Metallic Electrodes Article de journal B B Smith; J T Hynes The Journal of Chemical Physics, 99 (9), p. 6517-6530, 1993, (cited By 55). @article{Smith19936517, title = {Electronic Friction and Electron Transfer Rates at Metallic Electrodes}, author = {B B Smith and J T Hynes}, doi = {10.1063/1.465843}, year = {1993}, date = {1993-01-01}, journal = {The Journal of Chemical Physics}, volume = {99}, number = {9}, pages = {6517-6530}, abstract = {A theory is presented for the rate constant k for electron transfer between a metal electrode and a redox couple solute in solution, in or near the electronically adiabatic regime. The departure of k from its electronically adiabatic transition state theory limit k TST is described via Grote-Hynes theory, and includes two sources of friction. The electronic friction arises from excitation of electron hole pairs in the metal, i.e., electronic nonadiabaticity effects. The solvent friction arises from solvent dynamical effects. Both features can result in significant reduction of k below k TST , and their interplay can lead to interesting nonmonotonic variations with reaction overpotential. textcopyright 1993 American Institute of Physics.}, note = {cited By 55}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theory is presented for the rate constant k for electron transfer between a metal electrode and a redox couple solute in solution, in or near the electronically adiabatic regime. The departure of k from its electronically adiabatic transition state theory limit k TST is described via Grote-Hynes theory, and includes two sources of friction. The electronic friction arises from excitation of electron hole pairs in the metal, i.e., electronic nonadiabaticity effects. The solvent friction arises from solvent dynamical effects. Both features can result in significant reduction of k below k TST , and their interplay can lead to interesting nonmonotonic variations with reaction overpotential. textcopyright 1993 American Institute of Physics. |
A Simple Basis Set Approach to Solute Electronic Structure and Free Energy in Solution Article de journal H J Kim; R Bianco; B J Gertner; J T Hynes Journal of Physical Chemistry, 97 (9), p. 1723-1728, 1993, (cited By 42). @article{Kim19931723, title = {A Simple Basis Set Approach to Solute Electronic Structure and Free Energy in Solution}, author = {H J Kim and R Bianco and B J Gertner and J T Hynes}, doi = {10.1021/j100111a001}, year = {1993}, date = {1993-01-01}, journal = {Journal of Physical Chemistry}, volume = {97}, number = {9}, pages = {1723-1728}, abstract = {A truncated basis set approach is developed to describe the electronic character of reacting solutes in polar and polarizable solvents and is illustrated by application to the ionization of tert-butyl chloride and to lithium hydride near its equilibrium configuration. This simple approach is shown to be numerically quite accurate, when compared to the available results of a nonlinear Schr\"{o}dinger equation method. Further, it can be analytically analyzed to shed light on the general limitations of commonly employed limiting approximation methods and the ingredients necessary to remedy those shortcomings. textcopyright 1993 American Chemical Society.}, note = {cited By 42}, keywords = {}, pubstate = {published}, tppubtype = {article} } A truncated basis set approach is developed to describe the electronic character of reacting solutes in polar and polarizable solvents and is illustrated by application to the ionization of tert-butyl chloride and to lithium hydride near its equilibrium configuration. This simple approach is shown to be numerically quite accurate, when compared to the available results of a nonlinear Schrödinger equation method. Further, it can be analytically analyzed to shed light on the general limitations of commonly employed limiting approximation methods and the ingredients necessary to remedy those shortcomings. textcopyright 1993 American Chemical Society. |
1992 |
A Theoretical Model for S N 1 Ionic Dissociation in Solution. 2. Nonequilibrium Solvation Reaction Path and Reaction Rate Article de journal H J Kim; J T Hynes Journal of the American Chemical Society, 114 (26), p. 10528-10537, 1992, (cited By 68). @article{Kim199210528, title = {A Theoretical Model for S N 1 Ionic Dissociation in Solution. 2. Nonequilibrium Solvation Reaction Path and Reaction Rate}, author = {H J Kim and J T Hynes}, doi = {10.1021/ja00052a056}, year = {1992}, date = {1992-01-01}, journal = {Journal of the American Chemical Society}, volume = {114}, number = {26}, pages = {10528-10537}, abstract = {The theoretical formulation developed in the preceding article [Kim, H. J.; Hynes, J. T. J. Am. Chem. Soc., preceding paper in this issue] is applied to determine the reaction path and rate constant for the S N 1 ionization process in solution RX $\longrightarrow$ R + + X - , illustrated for t-BuCl. It is found that the intrinsic solution reaction path (SRP), which is the analogue of the familiar minimum energy path of gas-phase reaction studies, differs considerably from the conventionally assumed equilibrium solvation path (ESP). In particular, the SRP near the transition state lies mainly along the RX separation coordinate r. There is little motion in the solvent coordinate s; the solvent lags the solute nuclei motion and there is nonadiabatic nonequilibrium solvation. Near the reactant configuration RX, however, the critical motion initiating the reaction is that of the solvent, i.e., the solvent orientational polarization. The contrasts with activated electron transfer are also pointed out. The connection of the two-dimensional (r, s) free energy surface to the potential of mean force is made, particularly in connection with the ionization activation free energy, as is the connection to the conventional transition-state theory (TST) rate constant k TST , which assumes equilibrium solvation. The deviation of the actual rate constant k from its TST approximation (the transmission coefficient n = k/k TST ) due to nonequilibrium solvation is examined, via both linear and nonlinear variational transition state theory. Despite the pronounced anharmonicity of the (r, s) free energy surface arising from the electronic mixing of the covalent and ionic valence bond states, a simple harmonic nonadiabatic solvation analysis is found to be suitable. This analysis predicts progressively larger and more significant departures from equilibrium solvation TST with increasing solvent polarity. textcopyright 1992, American Chemical Society. All rights reserved.}, note = {cited By 68}, keywords = {}, pubstate = {published}, tppubtype = {article} } The theoretical formulation developed in the preceding article [Kim, H. J.; Hynes, J. T. J. Am. Chem. Soc., preceding paper in this issue] is applied to determine the reaction path and rate constant for the S N 1 ionization process in solution RX $łongrightarrow$ R + + X - , illustrated for t-BuCl. It is found that the intrinsic solution reaction path (SRP), which is the analogue of the familiar minimum energy path of gas-phase reaction studies, differs considerably from the conventionally assumed equilibrium solvation path (ESP). In particular, the SRP near the transition state lies mainly along the RX separation coordinate r. There is little motion in the solvent coordinate s; the solvent lags the solute nuclei motion and there is nonadiabatic nonequilibrium solvation. Near the reactant configuration RX, however, the critical motion initiating the reaction is that of the solvent, i.e., the solvent orientational polarization. The contrasts with activated electron transfer are also pointed out. The connection of the two-dimensional (r, s) free energy surface to the potential of mean force is made, particularly in connection with the ionization activation free energy, as is the connection to the conventional transition-state theory (TST) rate constant k TST , which assumes equilibrium solvation. The deviation of the actual rate constant k from its TST approximation (the transmission coefficient n = k/k TST ) due to nonequilibrium solvation is examined, via both linear and nonlinear variational transition state theory. Despite the pronounced anharmonicity of the (r, s) free energy surface arising from the electronic mixing of the covalent and ionic valence bond states, a simple harmonic nonadiabatic solvation analysis is found to be suitable. This analysis predicts progressively larger and more significant departures from equilibrium solvation TST with increasing solvent polarity. textcopyright 1992, American Chemical Society. All rights reserved. |
A Theoretical Model for SN1 Ionic Dissociation in Solution. 1. Activation Free Energetics and Transition-State Structure Article de journal H J Kim; J T Hynes Journal of the American Chemical Society, 114 (26), p. 10508-10528, 1992, (cited By 127). @article{Kim199210508, title = {A Theoretical Model for SN1 Ionic Dissociation in Solution. 1. Activation Free Energetics and Transition-State Structure}, author = {H J Kim and J T Hynes}, doi = {10.1021/ja00052a055}, year = {1992}, date = {1992-01-01}, journal = {Journal of the American Chemical Society}, volume = {114}, number = {26}, pages = {10508-10528}, abstract = {The rate-determining ionic dissociation RX$rightarrow$ R+ + X- for SN1 reactions in a polar solvent is examined theoretically. These unimolecular dissociations involve critical and extensive solvent stabilization of the ionic state compared to the covalent state to induce an electronic curve crossing to allow the heterolysis. Our approach is via a nonlinear Schrddinger equation theory recently developed to account for the mutual influence of solute electronic structure and solvent polarization\textemdashboth equilibrium and nonequilibrium. The theory deals quantitatively with the competition between the electronic coupling between covalent and ionic valence bond states\textemdashwhich tends to mix these states\textemdashand solvation which tends to localize the system in one of them. Novel aspects of the theory as applied to SN1 ionizations include removal of the restriction of the pioneering Ogg-Polanyi solvent-equilibrated diabatic curve intersection approach\textemdashwhich predicts an invariant 50% ionic character of the transition state, connection to (and contrasts with) an electron-transfer perspective for the reaction, and exposure of the role of the solvent electronic polarization in stabilizing the delocalized transition state. By adopting a simple model for tert-butyl chloride in a dielectric continuum model for solvents of differing polarity, we implement the theory to obtain a two-dimensional free energy surface in terms of the R-X separation r and a solvent coordinate s, which gauges the nonequilibrium solvent orientational polarization. Along s, a single stable well potential results, in contrast to activated electron-transfer reactions\textemdashthis clarifies the issue of electron transfer versus electron shift in the reaction. One measure of this contrast is the predicted lack of applicability of Marcus activation-reaction free energy relations for this reaction class. Activation free energy barriers for heterolysis are calculated, compared with experimental results, and the trends are examined and explained. The change in the ionization transition-state structure with solvent polarity is also analyzed. The ionic character of the transition state is found to decrease with increasing solvent polarity; the transition state becomes less ionic for more polar solvents, in direct contrast to prevalent notions. The activation free energy $Delta$Gtextdaggerdblitself decreases with increasing solvent polarity; this trend is in accord with experiment and standard conceptions. However, the dominant source of this trend of $Delta$Gtextdaggerdbl with solvent polarity\textemdashthe variation of the electronic coupling between the covalent and ionic states\textemdashcontrasts fundamentally with that conventionally envisioned via, e.g., a Hughes-Ingold perspective. In addition, our analytic relationship connecting the activation free energy and the solvent polarity differs markedly from that often used to determine the transition-state ionic character. Solvent polarity dependence of a Bronsted coefficient is suggested as an experimental probe of the new perspective. Finally, locally stable ion pair products in weakly polar and nonpolar solvents are found and discussed. textcopyright 1992, American Chemical Society. All rights reserved.}, note = {cited By 127}, keywords = {}, pubstate = {published}, tppubtype = {article} } The rate-determining ionic dissociation RX$rightarrow$ R+ + X- for SN1 reactions in a polar solvent is examined theoretically. These unimolecular dissociations involve critical and extensive solvent stabilization of the ionic state compared to the covalent state to induce an electronic curve crossing to allow the heterolysis. Our approach is via a nonlinear Schrddinger equation theory recently developed to account for the mutual influence of solute electronic structure and solvent polarization—both equilibrium and nonequilibrium. The theory deals quantitatively with the competition between the electronic coupling between covalent and ionic valence bond states—which tends to mix these states—and solvation which tends to localize the system in one of them. Novel aspects of the theory as applied to SN1 ionizations include removal of the restriction of the pioneering Ogg-Polanyi solvent-equilibrated diabatic curve intersection approach—which predicts an invariant 50% ionic character of the transition state, connection to (and contrasts with) an electron-transfer perspective for the reaction, and exposure of the role of the solvent electronic polarization in stabilizing the delocalized transition state. By adopting a simple model for tert-butyl chloride in a dielectric continuum model for solvents of differing polarity, we implement the theory to obtain a two-dimensional free energy surface in terms of the R-X separation r and a solvent coordinate s, which gauges the nonequilibrium solvent orientational polarization. Along s, a single stable well potential results, in contrast to activated electron-transfer reactions—this clarifies the issue of electron transfer versus electron shift in the reaction. One measure of this contrast is the predicted lack of applicability of Marcus activation-reaction free energy relations for this reaction class. Activation free energy barriers for heterolysis are calculated, compared with experimental results, and the trends are examined and explained. The change in the ionization transition-state structure with solvent polarity is also analyzed. The ionic character of the transition state is found to decrease with increasing solvent polarity; the transition state becomes less ionic for more polar solvents, in direct contrast to prevalent notions. The activation free energy $Delta$Gtextdaggerdblitself decreases with increasing solvent polarity; this trend is in accord with experiment and standard conceptions. However, the dominant source of this trend of $Delta$Gtextdaggerdbl with solvent polarity—the variation of the electronic coupling between the covalent and ionic states—contrasts fundamentally with that conventionally envisioned via, e.g., a Hughes-Ingold perspective. In addition, our analytic relationship connecting the activation free energy and the solvent polarity differs markedly from that often used to determine the transition-state ionic character. Solvent polarity dependence of a Bronsted coefficient is suggested as an experimental probe of the new perspective. Finally, locally stable ion pair products in weakly polar and nonpolar solvents are found and discussed. textcopyright 1992, American Chemical Society. All rights reserved. |
Dielectric Friction and Solvation Dynamics: A Molecular Dynamics Study Article de journal M Bruehl; J T Hynes Journal of Physical Chemistry, 96 (10), p. 4068-4074, 1992, (cited By 40). @article{Bruehl19924068, title = {Dielectric Friction and Solvation Dynamics: A Molecular Dynamics Study}, author = {M Bruehl and J T Hynes}, doi = {10.1021/j100189a028}, year = {1992}, date = {1992-01-01}, journal = {Journal of Physical Chemistry}, volume = {96}, number = {10}, pages = {4068-4074}, abstract = {The van der Zwan-Hynes relation connecting the solvation time and the dielectric friction for a solute in a polar solvent is tested via molecular dynamics computer simulation. For partially and fully ionic diatomic solute pairs in a model polar aprotic solvent, for which there is considerable dielectric friction, the relation is found to be satisfied to within a factor of 2 and to correctly follow the trends observed for different solute pairs. Deficiencies of the relation are also discussed. Pronounced solute rotational caging associated with strong electrostatic solute-solvent interactions is also observed. textcopyright 1992 American Chemical Society.}, note = {cited By 40}, keywords = {}, pubstate = {published}, tppubtype = {article} } The van der Zwan-Hynes relation connecting the solvation time and the dielectric friction for a solute in a polar solvent is tested via molecular dynamics computer simulation. For partially and fully ionic diatomic solute pairs in a model polar aprotic solvent, for which there is considerable dielectric friction, the relation is found to be satisfied to within a factor of 2 and to correctly follow the trends observed for different solute pairs. Deficiencies of the relation are also discussed. Pronounced solute rotational caging associated with strong electrostatic solute-solvent interactions is also observed. textcopyright 1992 American Chemical Society. |
Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. III. Quantum Theory Article de journal H J Kim; J T Hynes The Journal of Chemical Physics, 96 (7), p. 5088-5110, 1992, (cited By 159). @article{Kim19925088, title = {Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. III. Quantum Theory}, author = {H J Kim and J T Hynes}, doi = {10.1063/1.462752}, year = {1992}, date = {1992-01-01}, journal = {The Journal of Chemical Physics}, volume = {96}, number = {7}, pages = {5088-5110}, abstract = {The electronic structure of a solute in a polar and polarizable solvent depends on the nonequilibrium (or equilibrium) state of the solvent. Here we present a theory for this phenomenon, at the level of a dielectric continuum description of the solvent, characterized by an orientational polarization Por and an electronic polarization Pel. The entire range of electronic coupling between solute electronic states is considered. The present theory supersedes, in important respects, our earlier work [H. J. Kim and J. T. Hynes, J. Phys. Chem. 94, 2737 (1990); J. Chem. Phys. 93, 5194 (1990); 93, 5211 (1990)] by including a full quantization of Pel; this is a feature recently shown in a model study for weak electronic coupling [J. N. Gehlen, D. Chandler, H. J. Kim, and J. T. Hynes, J. Phys. Chem. (to be published)] to be necessary for, e.g., a correct description of electron transfer activation free energies and transition states. The quantization of Pel is effected via a coherent state formulation, coupled with a multiconfiguration self-consistent representation of the solute-Pel wave function. Nonequilibrium free energies and solute electronic structure are found and depend explicitly on Ae comparative time scales of a transferring electron in the solute and of Pel. The corresponding equilibrium relations are also found. For activated electron transfer, the current theory recovers (and generalizes) the conventional expression for the activation free energy, in both the electronically nonadiabatic and adiabatic regimes, for small electronic coupling. For larger electronic coupling-but still within the regime of activated electron transfer-the theory predicts an increased activation free energy compared to conventional results, associated with the (partial) solvation by the electronic polarization of delocalized solute electronic structure of the transition state. This trend is the same as that previously reported by us, but is smaller in magnitude due to the finite time scale of the transferring electron. The same is true for even stronger electronic coupling, characteristic of delocalized complexes, SN1, SN2, and proton transfers. Our previous predictions of novel spectroscopic aspects for delocalized complexes are confirmed. textcopyright 1992 American Institute of Physics.}, note = {cited By 159}, keywords = {}, pubstate = {published}, tppubtype = {article} } The electronic structure of a solute in a polar and polarizable solvent depends on the nonequilibrium (or equilibrium) state of the solvent. Here we present a theory for this phenomenon, at the level of a dielectric continuum description of the solvent, characterized by an orientational polarization Por and an electronic polarization Pel. The entire range of electronic coupling between solute electronic states is considered. The present theory supersedes, in important respects, our earlier work [H. J. Kim and J. T. Hynes, J. Phys. Chem. 94, 2737 (1990); J. Chem. Phys. 93, 5194 (1990); 93, 5211 (1990)] by including a full quantization of Pel; this is a feature recently shown in a model study for weak electronic coupling [J. N. Gehlen, D. Chandler, H. J. Kim, and J. T. Hynes, J. Phys. Chem. (to be published)] to be necessary for, e.g., a correct description of electron transfer activation free energies and transition states. The quantization of Pel is effected via a coherent state formulation, coupled with a multiconfiguration self-consistent representation of the solute-Pel wave function. Nonequilibrium free energies and solute electronic structure are found and depend explicitly on Ae comparative time scales of a transferring electron in the solute and of Pel. The corresponding equilibrium relations are also found. For activated electron transfer, the current theory recovers (and generalizes) the conventional expression for the activation free energy, in both the electronically nonadiabatic and adiabatic regimes, for small electronic coupling. For larger electronic coupling-but still within the regime of activated electron transfer-the theory predicts an increased activation free energy compared to conventional results, associated with the (partial) solvation by the electronic polarization of delocalized solute electronic structure of the transition state. This trend is the same as that previously reported by us, but is smaller in magnitude due to the finite time scale of the transferring electron. The same is true for even stronger electronic coupling, characteristic of delocalized complexes, SN1, SN2, and proton transfers. Our previous predictions of novel spectroscopic aspects for delocalized complexes are confirmed. textcopyright 1992 American Institute of Physics. |
Solvation Free Energies and Solvent Force Constants Article de journal T Fonseca; B M Ladanyi; J T Hynes Journal of Physical Chemistry, 96 (10), p. 4085-4093, 1992, (cited By 58). @article{Fonseca19924085, title = {Solvation Free Energies and Solvent Force Constants}, author = {T Fonseca and B M Ladanyi and J T Hynes}, doi = {10.1021/j100189a032}, year = {1992}, date = {1992-01-01}, journal = {Journal of Physical Chemistry}, volume = {96}, number = {10}, pages = {4085-4093}, abstract = {A theoretical formulation for the solvent force constant kq, which gauges electrical potential fluctuations for an ion in solution and whose charge dependence is a measure of nonlinear aspects of solvation, is presented in terms of the solute charge (q) variation of the solvation free energy. This formulation allows the calculation of kq via integral equation theories. This is illustrated by a series of calculations for ionic solutes in model dipolar-quadrupolar solvents via the reference hypernetted chain (RHNC) integral equation approach. It is found that the q variation of kq can be comprehended in terms of the cooperative (or competing) contributions of the solvent dipole and quadrupole to the acceleration of the solvation free energy. By contrast, traditional notions of dielectric saturation prove to be of much less direct relevance, due in part to the importance of competing electrostriction effects. The formalism is also applied to available simulation and integral equation solvation free energy studies of aqueous ionic solvation to infer the behavior of kq. The extensions for the formalism to more complex solutes (e.g., ion pairs), to higher order fluctuations (e.g., electric field), and to the solvent frequency and effective mass are briefly indicated. textcopyright 1992 American Chemical Society.}, note = {cited By 58}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theoretical formulation for the solvent force constant kq, which gauges electrical potential fluctuations for an ion in solution and whose charge dependence is a measure of nonlinear aspects of solvation, is presented in terms of the solute charge (q) variation of the solvation free energy. This formulation allows the calculation of kq via integral equation theories. This is illustrated by a series of calculations for ionic solutes in model dipolar-quadrupolar solvents via the reference hypernetted chain (RHNC) integral equation approach. It is found that the q variation of kq can be comprehended in terms of the cooperative (or competing) contributions of the solvent dipole and quadrupole to the acceleration of the solvation free energy. By contrast, traditional notions of dielectric saturation prove to be of much less direct relevance, due in part to the importance of competing electrostriction effects. The formalism is also applied to available simulation and integral equation solvation free energy studies of aqueous ionic solvation to infer the behavior of kq. The extensions for the formalism to more complex solutes (e.g., ion pairs), to higher order fluctuations (e.g., electric field), and to the solvent frequency and effective mass are briefly indicated. textcopyright 1992 American Chemical Society. |
Vibrational Relaxation of a Dipolar Molecule in Water Article de journal R M Whitnell; K R Wilson; J T Hynes The Journal of Chemical Physics, 96 (7), p. 5354-5369, 1992, (cited By 192). @article{Whitnell19925354, title = {Vibrational Relaxation of a Dipolar Molecule in Water}, author = {R M Whitnell and K R Wilson and J T Hynes}, doi = {10.1063/1.462720}, year = {1992}, date = {1992-01-01}, journal = {The Journal of Chemical Physics}, volume = {96}, number = {7}, pages = {5354-5369}, abstract = {The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau-Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH 3 Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH 3 Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH 3 Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH 3 Cl molecule and that most of these important molecules are at positions perpendicular to the CH 3 Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray. textcopyright 1992 American Institute of Physics.}, note = {cited By 192}, keywords = {}, pubstate = {published}, tppubtype = {article} } The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau-Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH 3 Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH 3 Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH 3 Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH 3 Cl molecule and that most of these important molecules are at positions perpendicular to the CH 3 Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray. textcopyright 1992 American Institute of Physics. |
Free Energies of Electron Transfer Article de journal J N Gehlen; D Chandler; H J Kim; J T Hynes Journal of Physical Chemistry, 96 (4), p. 1748-1753, 1992, (cited By 122). @article{Gehlen19921748, title = {Free Energies of Electron Transfer}, author = {J N Gehlen and D Chandler and H J Kim and J T Hynes}, doi = {10.1021/j100183a047}, year = {1992}, date = {1992-01-01}, journal = {Journal of Physical Chemistry}, volume = {96}, number = {4}, pages = {1748-1753}, abstract = {A simple class of Hamiltonian models of electron-transfer systems is analyzed. We focus on the activation free energy governing the charge transfer, and especially the role of fast solvent electronic polarization. The analysis of the free energy is carried out exactly and also by a self-consistent mean field approximation that has been used in this context. Comparison with the exact analysis in the limit that the solvent electronic polarization is much faster than the time scale of the transferring electron indicates that the predictions of the mean field approximation are generally erroneous. The correct activation free energy is in agreement with traditional estimates. textcopyright 1992 American Chemical Society.}, note = {cited By 122}, keywords = {}, pubstate = {published}, tppubtype = {article} } A simple class of Hamiltonian models of electron-transfer systems is analyzed. We focus on the activation free energy governing the charge transfer, and especially the role of fast solvent electronic polarization. The analysis of the free energy is carried out exactly and also by a self-consistent mean field approximation that has been used in this context. Comparison with the exact analysis in the limit that the solvent electronic polarization is much faster than the time scale of the transferring electron indicates that the predictions of the mean field approximation are generally erroneous. The correct activation free energy is in agreement with traditional estimates. textcopyright 1992 American Chemical Society. |
1991 |
Molecular Solvent Vibrational Effects on the Friction for Barrier Crossing Reactions Article de journal A G Zawadzki; J T Hynes Journal of Molecular Liquids, 48 (2-4), p. 183-196, 1991, (cited By 0). @article{Zawadzki1991183, title = {Molecular Solvent Vibrational Effects on the Friction for Barrier Crossing Reactions}, author = {A G Zawadzki and J T Hynes}, doi = {10.1016/0167-7322(91)80009-S}, year = {1991}, date = {1991-01-01}, journal = {Journal of Molecular Liquids}, volume = {48}, number = {2-4}, pages = {183-196}, abstract = {According to the Grote-Hynes Theory, the reduction of the rate constant of a barrier crossing reaction in solution from its Transition State Theory value is governed by the time-dependent friction exerted by the solvent on the reaction system. The influence of an internal vibrational degree of freedom for a molecular solvent on this friction, and thus on the rate constant, is examined via a collisional model, and the consequences are assessed. textcopyright 1991.}, note = {cited By 0}, keywords = {}, pubstate = {published}, tppubtype = {article} } According to the Grote-Hynes Theory, the reduction of the rate constant of a barrier crossing reaction in solution from its Transition State Theory value is governed by the time-dependent friction exerted by the solvent on the reaction system. The influence of an internal vibrational degree of freedom for a molecular solvent on this friction, and thus on the rate constant, is examined via a collisional model, and the consequences are assessed. textcopyright 1991. |
Chemical Reaction Rates and Solvation Dynamics in Electrolyte Solutions: Ion Atmosphere Friction Article de journal G van der Zwan; J T Hynes Chemical Physics, 152 (1-2), p. 169-183, 1991, (cited By 74). @article{vanderZwan1991169, title = {Chemical Reaction Rates and Solvation Dynamics in Electrolyte Solutions: Ion Atmosphere Friction}, author = {G {van der Zwan} and J T Hynes}, doi = {10.1016/0301-0104(91)80043-H}, year = {1991}, date = {1991-01-01}, journal = {Chemical Physics}, volume = {152}, number = {1-2}, pages = {169-183}, abstract = {We consider the influence of ion atmosphere dynamics on a model dipolar isomerization in an electrolyte solution, as well as on the time dependent fluorescence (TDF) ofa dipolar solute. The impact of the resulting ion atmosphere friction on reducing the reaction rate below the equilibrium solration transition state theory value can sometimes be pronounced. Connections of this friction to the TDF dynamics are indicated for solvents both of low and high polarity. textcopyright 1991.}, note = {cited By 74}, keywords = {}, pubstate = {published}, tppubtype = {article} } We consider the influence of ion atmosphere dynamics on a model dipolar isomerization in an electrolyte solution, as well as on the time dependent fluorescence (TDF) ofa dipolar solute. The impact of the resulting ion atmosphere friction on reducing the reaction rate below the equilibrium solration transition state theory value can sometimes be pronounced. Connections of this friction to the TDF dynamics are indicated for solvents both of low and high polarity. textcopyright 1991. |
Molecular-Dynamics Simulation for a Model Nonadiabatic Proton Transfer Reaction in Solution Article de journal D Borgis; J T Hynes The Journal of Chemical Physics, 94 (5), p. 3619-3628, 1991, (cited By 293). @article{Borgis19913619, title = {Molecular-Dynamics Simulation for a Model Nonadiabatic Proton Transfer Reaction in Solution}, author = {D Borgis and J T Hynes}, doi = {10.1063/1.459733}, year = {1991}, date = {1991-01-01}, journal = {The Journal of Chemical Physics}, volume = {94}, number = {5}, pages = {3619-3628}, abstract = {It is shown how a dynamical theory for proton transfer rates in solution can be implemented in a molecular-dynamics simulation for a model reaction system. The reaction is in the nonadiabatic limit, in which the transfer occurs via quantum tunneling of the proton. The importance of the coupling of the proton to the solvent and to an intramolecular vibration is illustrated, and the simulation results are successfully compared with analytic rate-constant expressions in several limiting regimes. textcopyright 1991 American Institute of Physics.}, note = {cited By 293}, keywords = {}, pubstate = {published}, tppubtype = {article} } It is shown how a dynamical theory for proton transfer rates in solution can be implemented in a molecular-dynamics simulation for a model reaction system. The reaction is in the nonadiabatic limit, in which the transfer occurs via quantum tunneling of the proton. The importance of the coupling of the proton to the solvent and to an intramolecular vibration is illustrated, and the simulation results are successfully compared with analytic rate-constant expressions in several limiting regimes. textcopyright 1991 American Institute of Physics. |
Direct and Indirect Solvent Coupling Vibrational Dephasing Mechanisms in Hydrogen-Bonded Molecules Article de journal S J Klippenstein; J T Hynes Journal of Physical Chemistry, 95 (12), p. 4651-4659, 1991, (cited By 15). @article{Klippenstein19914651, title = {Direct and Indirect Solvent Coupling Vibrational Dephasing Mechanisms in Hydrogen-Bonded Molecules}, author = {S J Klippenstein and J T Hynes}, doi = {10.1021/j100165a013}, year = {1991}, date = {1991-01-01}, journal = {Journal of Physical Chemistry}, volume = {95}, number = {12}, pages = {4651-4659}, abstract = {Infrared absorption spectral bandshapes are examined theoretically for the OH stretching vibration in hydrogen-bonded complexes in solution. Two distinct dephasing mechanisms are considered: the indirect mechanism in which the OH vibration is coupled to one or more internal vibrations in the complex, which are in turn coupled to the solvent, and the direct mechanism in which the OH vibration is directly coupled to the solvent. Attention is focused on intramolecularly H-bonded complexes for which extensive spectral data are available for a range of solvent polarity. It is concluded for the complexes considered that the direct mechanism is dominant. In particular, good fits to the experimental data are obtained via this mechanism for spectral width features in a given solvent and as a function of solvent polarity. Some distinctions between intra- and intermolecularly H-bonded complexes are given, and some suggestions for further work are made. textcopyright 1991 American Chemical Society.}, note = {cited By 15}, keywords = {}, pubstate = {published}, tppubtype = {article} } Infrared absorption spectral bandshapes are examined theoretically for the OH stretching vibration in hydrogen-bonded complexes in solution. Two distinct dephasing mechanisms are considered: the indirect mechanism in which the OH vibration is coupled to one or more internal vibrations in the complex, which are in turn coupled to the solvent, and the direct mechanism in which the OH vibration is directly coupled to the solvent. Attention is focused on intramolecularly H-bonded complexes for which extensive spectral data are available for a range of solvent polarity. It is concluded for the complexes considered that the direct mechanism is dominant. In particular, good fits to the experimental data are obtained via this mechanism for spectral width features in a given solvent and as a function of solvent polarity. Some distinctions between intra- and intermolecularly H-bonded complexes are given, and some suggestions for further work are made. textcopyright 1991 American Chemical Society. |
Solvation Dynamics for an Ion Pair in a Polar Solvent: Time-Dependent Fluorescence and Photochemical Charge Transfer Article de journal E A Carter; J T Hynes The Journal of Chemical Physics, 94 (9), p. 5961-5979, 1991, (cited By 364). @article{Carter19915961, title = {Solvation Dynamics for an Ion Pair in a Polar Solvent: Time-Dependent Fluorescence and Photochemical Charge Transfer}, author = {E A Carter and J T Hynes}, doi = {10.1063/1.460431}, year = {1991}, date = {1991-01-01}, journal = {The Journal of Chemical Physics}, volume = {94}, number = {9}, pages = {5961-5979}, abstract = {The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time-dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution. textcopyright 1991 American Institute of Physics.}, note = {cited By 364}, keywords = {}, pubstate = {published}, tppubtype = {article} } The results of a molecular dynamics (MD) computer simulation are presented for the solvation dynamics of an ion pair instanteously produced from a neutral pair, in a model polar aprotic solvent. These time-dependent fluorescence dynamics are analyzed theoretically to examine the validity of several linear response theory approaches, as well as of various theoretical descriptions (e.g., Langevin equation) for the solvent dynamics per se. It is found that these dynamics are dominated for short times by a simple inertial Gaussian behavior, a feature which is absent in many current theoretical treatments, and which is related to the approximate validity of linear response theory. Nonlinear aspects, such as an overall spectral narrowing, but a transient initial spectral broadening, are also discussed. A model photochemical charge transfer process is also briefly considered to elucidate aspects of the connection between solvation dynamics and chemical kinetic population evolution. textcopyright 1991 American Institute of Physics. |
Nonequilibrium Free Energy Surfaces for Hydrogen-Bonded Proton-Transfer Complexes in Solution Article de journal J Juanós I Timoneda; J T Hynes Journal of Physical Chemistry, 95 (25), p. 10431-10442, 1991, (cited By 108). @article{Juan\'{o}sITimoneda199110431, title = {Nonequilibrium Free Energy Surfaces for Hydrogen-Bonded Proton-Transfer Complexes in Solution}, author = {J Juan\'{o}s I Timoneda and J T Hynes}, doi = {10.1021/j100178a034}, year = {1991}, date = {1991-01-01}, journal = {Journal of Physical Chemistry}, volume = {95}, number = {25}, pages = {10431-10442}, abstract = {A theory is presented for the electronic structure and multidimensional free energy surfaces for hydrogen-bonded complexes AH$cdots$B capable of proton transfer to form an ion pair A-$cdots$HB+ in solution. Two diabatic states, neutral and ionic, are electronically coupled to each other and electrostatically coupled to the surrounding solvent and are treated via a nonlinear Schr\"{o}dinger equation approach. The theory includes nonequilibrium solvation of the complex, a feature important for proton-transfer dynamics and spectroscopic phenomena. Representative calculations for a model OH$cdots$N complex are presented. The importance of the solvent polarization for the electronic structure of the complex is illustrated by comparison with the results obtained by solvation of the in vacuo electronic structure. textcopyright 1991 American Chemical Society.}, note = {cited By 108}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theory is presented for the electronic structure and multidimensional free energy surfaces for hydrogen-bonded complexes AH$cdots$B capable of proton transfer to form an ion pair A-$cdots$HB+ in solution. Two diabatic states, neutral and ionic, are electronically coupled to each other and electrostatically coupled to the surrounding solvent and are treated via a nonlinear Schrödinger equation approach. The theory includes nonequilibrium solvation of the complex, a feature important for proton-transfer dynamics and spectroscopic phenomena. Representative calculations for a model OH$cdots$N complex are presented. The importance of the solvent polarization for the electronic structure of the complex is illustrated by comparison with the results obtained by solvation of the in vacuo electronic structure. textcopyright 1991 American Chemical Society. |
Molecular Dynamics of a Model S N 1 Reaction in Water Article de journal W P Keirstead; K R Wilson; J T Hynes The Journal of Chemical Physics, 95 (7), p. 5256-5267, 1991, (cited By 106). @article{Keirstead19915256, title = {Molecular Dynamics of a Model S N 1 Reaction in Water}, author = {W P Keirstead and K R Wilson and J T Hynes}, doi = {10.1063/1.461697}, year = {1991}, date = {1991-01-01}, journal = {The Journal of Chemical Physics}, volume = {95}, number = {7}, pages = {5256-5267}, abstract = {Results are presented from a computer simulation of the dynamics of a model S N 1 reaction in water, very loosely based on the reaction t-BuCl $rightarrow$ t-Bu + + Cl - . Two diabatic electronic states are considered, covalent and ionic, which cross in the presence of the polar solvent. The curve crossing is treated in the electronically adiabatic limit, which gives rise to coupled reagent and solvent dynamics involving a mixed covalent/ionic adiabatic potential surface. The reaction dynamics are analyzed in terms of a simple solute reaction coordinate defined to be the t-Bu to Cl separation distance. By employing constraint dynamics techniques, the potential of mean force is determined as a function of this reaction coordinate. The time evolution of the reaction is followed in terms of the full molecular dynamics of all reagent and solvent atoms. Beginning with the largely covalently bound reactant t-BuCl, the following was observed: (i) how energy flows out of the water solvent bath into a solvent-reactant fluctuation driving the system to the top of the barrier, (ii) how barrier recrossings lower the reaction rate below the transition state theory prediction, and (iii) how the products slide down the barrier toward separated t-Bu + and Cl - , dissipating their excess energy back into the solvent. The calculated transmission coefficient measuring the departure of the rate constant from its transition state theory value is 0.53 $pm$ 0.04. This is found to agree with the Grote-Hynes theory prediction, and also with its nonadiabatic solvation, frozen solvent limit, to within the estimated error bars. By contrast, Kramers' theory incorrectly predicts a much smaller value. textcopyright 1991 American Institute of Physics.}, note = {cited By 106}, keywords = {}, pubstate = {published}, tppubtype = {article} } Results are presented from a computer simulation of the dynamics of a model S N 1 reaction in water, very loosely based on the reaction t-BuCl $rightarrow$ t-Bu + + Cl - . Two diabatic electronic states are considered, covalent and ionic, which cross in the presence of the polar solvent. The curve crossing is treated in the electronically adiabatic limit, which gives rise to coupled reagent and solvent dynamics involving a mixed covalent/ionic adiabatic potential surface. The reaction dynamics are analyzed in terms of a simple solute reaction coordinate defined to be the t-Bu to Cl separation distance. By employing constraint dynamics techniques, the potential of mean force is determined as a function of this reaction coordinate. The time evolution of the reaction is followed in terms of the full molecular dynamics of all reagent and solvent atoms. Beginning with the largely covalently bound reactant t-BuCl, the following was observed: (i) how energy flows out of the water solvent bath into a solvent-reactant fluctuation driving the system to the top of the barrier, (ii) how barrier recrossings lower the reaction rate below the transition state theory prediction, and (iii) how the products slide down the barrier toward separated t-Bu + and Cl - , dissipating their excess energy back into the solvent. The calculated transmission coefficient measuring the departure of the rate constant from its transition state theory value is 0.53 $pm$ 0.04. This is found to agree with the Grote-Hynes theory prediction, and also with its nonadiabatic solvation, frozen solvent limit, to within the estimated error bars. By contrast, Kramers' theory incorrectly predicts a much smaller value. textcopyright 1991 American Institute of Physics. |
Activation to the Transition State: Reactant and Solvent Energy Flow for a Model S N 2 Reaction in Water Article de journal B J Gertner; R M Whitnell; K R Wilson; J T Hynes Journal of the American Chemical Society, 113 (1), p. 74-87, 1991, (cited By 141). @article{Gertner199174, title = {Activation to the Transition State: Reactant and Solvent Energy Flow for a Model S N 2 Reaction in Water}, author = {B J Gertner and R M Whitnell and K R Wilson and J T Hynes}, doi = {10.1021/ja00001a014}, year = {1991}, date = {1991-01-01}, journal = {Journal of the American Chemical Society}, volume = {113}, number = {1}, pages = {74-87}, abstract = {We have performed molecular dynamics calculations on a model C1 - + CH 3 C1 S N 2 reaction in water in order to elucidate how the reactants obtain sufficient energy from the solvent to climb the potential energy barrier to reaction. This system, consisting of ionic and dipolar reagents in a polar solvent, is representative of a large class of chemical reactions with strong Coulombic reagent-solvent coupling. We find that the change in internal energy of the reactants during the barrier-climbing process involves three distinct epochs: (i) vibrational activation of the methyl chloride in the initial C1 - CH 3 C1 ion-dipole complex, (ii) gradual increase in kinetic and potential energies of the reactants, and (iii) fast dumping of reactant kinetic energy into reactant potential energy resulting in the reactants reaching the top of the potential energy barrier, with the symmetric structure Cl $delta-$ CH 3 $delta$+ Cl $delta-$ . The energy that the reagents gain during this process comes primarily from the potential energy of the water solvent. We also show that many water molecules are involved in this energy transfer, but almost as much energy is removed from the reactants by these water molecules as is deposited in them over the course of the barrier climbing. The critical change in the charge distribution as the reactants climb the barrier occurs over a very short time, and we present evidence that the total energy of the water solvent molecules remains essentially constant, consistent with the frozen solvent nonadiabatic solvation model used previously to understand the deviations from the transition-state rate for this system (Bergsma et al. J. Chem. Phys. 1987, 86, 1356; Gertner et al. J. Chem. Phys. 1987, 86, 1377; 1989, 90, 3537). We also find that the water solvent undergoes a substantial, though not complete, reorganization well before the change in the charge distribution of the reactants. This reorganization is crucial, although not sufficient, for the success of the barrier climbing. Many of these results for this strongly coupled system contrast starkly with those found by Benjamin et al. (J. Am. Chem. Soc. 1990, 112, 524) for a neutral symmetric atom exchange reaction in a rare gas solvent (Bergsma et al. Chem. Phys. Lett. 1986, 123, 394; J. Chem. Phys. 1986, 85, 5625) where the forces between solvent and reagents are short range and the coupling is much weaker. Thus, there is a rich variety of energy flow phenomena and solvent dynamics that must be considered in order to understand the detailed molecular dynamics of how chemical reactions take place in solution and how these dynamics arise from the particular system's reagent, solvent, and solvent-reagent forces. textcopyright 1991, American Chemical Society. All rights reserved.}, note = {cited By 141}, keywords = {}, pubstate = {published}, tppubtype = {article} } We have performed molecular dynamics calculations on a model C1 - + CH 3 C1 S N 2 reaction in water in order to elucidate how the reactants obtain sufficient energy from the solvent to climb the potential energy barrier to reaction. This system, consisting of ionic and dipolar reagents in a polar solvent, is representative of a large class of chemical reactions with strong Coulombic reagent-solvent coupling. We find that the change in internal energy of the reactants during the barrier-climbing process involves three distinct epochs: (i) vibrational activation of the methyl chloride in the initial C1 - CH 3 C1 ion-dipole complex, (ii) gradual increase in kinetic and potential energies of the reactants, and (iii) fast dumping of reactant kinetic energy into reactant potential energy resulting in the reactants reaching the top of the potential energy barrier, with the symmetric structure Cl $delta-$ CH 3 $delta$+ Cl $delta-$ . The energy that the reagents gain during this process comes primarily from the potential energy of the water solvent. We also show that many water molecules are involved in this energy transfer, but almost as much energy is removed from the reactants by these water molecules as is deposited in them over the course of the barrier climbing. The critical change in the charge distribution as the reactants climb the barrier occurs over a very short time, and we present evidence that the total energy of the water solvent molecules remains essentially constant, consistent with the frozen solvent nonadiabatic solvation model used previously to understand the deviations from the transition-state rate for this system (Bergsma et al. J. Chem. Phys. 1987, 86, 1356; Gertner et al. J. Chem. Phys. 1987, 86, 1377; 1989, 90, 3537). We also find that the water solvent undergoes a substantial, though not complete, reorganization well before the change in the charge distribution of the reactants. This reorganization is crucial, although not sufficient, for the success of the barrier climbing. Many of these results for this strongly coupled system contrast starkly with those found by Benjamin et al. (J. Am. Chem. Soc. 1990, 112, 524) for a neutral symmetric atom exchange reaction in a rare gas solvent (Bergsma et al. Chem. Phys. Lett. 1986, 123, 394; J. Chem. Phys. 1986, 85, 5625) where the forces between solvent and reagents are short range and the coupling is much weaker. Thus, there is a rich variety of energy flow phenomena and solvent dynamics that must be considered in order to understand the detailed molecular dynamics of how chemical reactions take place in solution and how these dynamics arise from the particular system's reagent, solvent, and solvent-reagent forces. textcopyright 1991, American Chemical Society. All rights reserved. |
1990 |
Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. I. Formulation Article de journal H J Kim; J T Hynes The Journal of Chemical Physics, 93 (7), p. 5194-5210, 1990, (cited By 161). @article{Kim19905194, title = {Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. I. Formulation}, author = {H J Kim and J T Hynes}, doi = {10.1063/1.459665}, year = {1990}, date = {1990-01-01}, journal = {The Journal of Chemical Physics}, volume = {93}, number = {7}, pages = {5194-5210}, abstract = {A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free-energy expression for the quantum solute-solvent system, a time-independent nonlinear Schr\"{o}dinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peeq, to the solute electronic wave function $psi$ and the solvent orientational polarization Por. When P or is arbitrary, there is nonequilibrium solvation. When P or. is equilibrated to Peeq and $psi$, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor-acceptor solute system in a two state basis set description. Solution of the nonlinear Schr\"{o}dinger equation in the presence of arbitrary Por. yields nonequilibrium solvation stationary states (NSS) for the solute-solvent system, including the solvent-dependent solute electronic structure, and the associated free energies. When Por = P oreq, the corresponding equilibrium solvation states (ESS) and their characteristics are obtained. The NSS are classified into three distinct regimes, according to the relative strengths of the electronic coupling, which tends to delocalize the solute electronic distribution, and the solvent polarization, which tends to localize it. The ESS stability characteristics are also important in this classification. Two of the regimes correspond to activated electron transfer processes, and differ according to whether there is or is not a continuous free-energy path leading from localized reactants to localized products. The third regime, in which the electronic coupling dominates the solvent polarization, corresponds to stable delocalized states between which spectroscopic transitions are of interest. Finally, the inclusion of electronic exchange in the theory leads to the necessity of more than one solvent coordinate in order to describe the free-energy surface for the solute-solvent system. textcopyright 1990 American Institute of Physics.}, note = {cited By 161}, keywords = {}, pubstate = {published}, tppubtype = {article} } A theoretical formulation is developed to describe the electronic structure of an immersed solute, electrostatically coupled to a polar and polarizable solvent. The solvent is characterized, in the dielectric continuum approximation, by electronic and orientational polarizations. Starting from a general free-energy expression for the quantum solute-solvent system, a time-independent nonlinear Schrödinger equation is derived. The nonlinearity arises from the assumed equilibration of the solvent electronic polarization Peeq, to the solute electronic wave function $psi$ and the solvent orientational polarization Por. When P or is arbitrary, there is nonequilibrium solvation. When P or. is equilibrated to Peeq and $psi$, equilibrium solvation obtains. The theory is illustrated for a model symmetric electron donor-acceptor solute system in a two state basis set description. Solution of the nonlinear Schrödinger equation in the presence of arbitrary Por. yields nonequilibrium solvation stationary states (NSS) for the solute-solvent system, including the solvent-dependent solute electronic structure, and the associated free energies. When Por = P oreq, the corresponding equilibrium solvation states (ESS) and their characteristics are obtained. The NSS are classified into three distinct regimes, according to the relative strengths of the electronic coupling, which tends to delocalize the solute electronic distribution, and the solvent polarization, which tends to localize it. The ESS stability characteristics are also important in this classification. Two of the regimes correspond to activated electron transfer processes, and differ according to whether there is or is not a continuous free-energy path leading from localized reactants to localized products. The third regime, in which the electronic coupling dominates the solvent polarization, corresponds to stable delocalized states between which spectroscopic transitions are of interest. Finally, the inclusion of electronic exchange in the theory leads to the necessity of more than one solvent coordinate in order to describe the free-energy surface for the solute-solvent system. textcopyright 1990 American Institute of Physics. |
Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. II. Strong Coupling Limit Article de journal H J Kim; J T Hynes The Journal of Chemical Physics, 93 (7), p. 5211-5223, 1990, (cited By 74). @article{Kim19905211, title = {Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure. II. Strong Coupling Limit}, author = {H J Kim and J T Hynes}, doi = {10.1063/1.459666}, year = {1990}, date = {1990-01-01}, journal = {The Journal of Chemical Physics}, volume = {93}, number = {7}, pages = {5211-5223}, abstract = {The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 ( 1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron-donor- acceptor solute system D -A$rightleftharpoons$DA - in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck-Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed. textcopyright 1990 American Institute of Physics.}, note = {cited By 74}, keywords = {}, pubstate = {published}, tppubtype = {article} } The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 ( 1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron-donor- acceptor solute system D -A$rightleftharpoons$DA - in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck-Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed. textcopyright 1990 American Institute of Physics. |
Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure Article de journal H J Kim; J T Hynes International Journal of Quantum Chemistry, 38 (24 S), p. 821-833, 1990, (cited By 22). @article{Kim1990821, title = {Equilibrium and Nonequilibrium Solvation and Solute Electronic Structure}, author = {H J Kim and J T Hynes}, doi = {10.1002/qua.560382480}, year = {1990}, date = {1990-01-01}, journal = {International Journal of Quantum Chemistry}, volume = {38}, number = {24 S}, pages = {821-833}, abstract = {When a molecular solute is immersed in a polar and polarizable solvent, the electronic wave function of the solute system is altered compared to its vacuum value; the solute electronic structure is thus solvent-dependent. Further, the wave function will be altered depending upon whether the polarization of the solvent is or is not in equilibrium with the solute charge distribution. More precisely, while the solvent electronic polarization should be in equilibrium with the solute electronic wave function, the much more sluggish solvent orientational polarization need not be. We call this last situation ``non-equilibrium solvation.'' We outline a nonlinear Schr\"{o}dinger equation approach to these issues. The nonlinearity arises from the self-consistent aspect that the solute electronic Hamiltonian depends on the solvent electronic polarization which is induced by the solute charge distribution. We illustrate the predictions of the theory for electron transfer reactions, ionic dissociations, and solvation dynamics in polar solvents. Special features of interest include activation barriers that differ markedly from standard predictions, and novel solvent dynamical features. Copyright textcopyright 1990 John Wiley & Sons, Inc.}, note = {cited By 22}, keywords = {}, pubstate = {published}, tppubtype = {article} } When a molecular solute is immersed in a polar and polarizable solvent, the electronic wave function of the solute system is altered compared to its vacuum value; the solute electronic structure is thus solvent-dependent. Further, the wave function will be altered depending upon whether the polarization of the solvent is or is not in equilibrium with the solute charge distribution. More precisely, while the solvent electronic polarization should be in equilibrium with the solute electronic wave function, the much more sluggish solvent orientational polarization need not be. We call this last situation ``non-equilibrium solvation.'' We outline a nonlinear Schrödinger equation approach to these issues. The nonlinearity arises from the self-consistent aspect that the solute electronic Hamiltonian depends on the solvent electronic polarization which is induced by the solute charge distribution. We illustrate the predictions of the theory for electron transfer reactions, ionic dissociations, and solvation dynamics in polar solvents. Special features of interest include activation barriers that differ markedly from standard predictions, and novel solvent dynamical features. Copyright textcopyright 1990 John Wiley & Sons, Inc. |