UMR 8640 : Physico-Chimie Théorique

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(January 2017)

Versatile Electrification of Two-dimensional Nanomaterials in Water

Nature Communications 10,1656 (2019)

 

The recent emergence of nanofluidics has highlighted the exceptional properties of graphene and its boron-nitride counterpart as confining nanomaterials for water and ion transport. Surprisingly, ionic transport experiments have unveiled a consequent electrification of the water/carbon surfaces, with a contrasting response for its water/boron-nitride homologue. In this paper, we report free energy calculations based on ab initio molecular dynamics simulations of hydroxide OH ions in water near graphene and hexagonal boron nitride (h-BN) layers. Our results disclose that both surfaces get charged through hydroxide adsorption, but two strongly different mechanisms are evidenced. The hydroxide species shows weak physisorption on the graphene surface while it exhibits also strong chemisorption on the h-BN surface. Interestingly OH is shown to keep very fast lateral dynamics and interfacial mobility within the physisorbed layer on graphene. Taking into account the large ionic surface conductivity, an analytic transport model allows to reproduce quantitatively the experimental data.

 

 

A molecular density functional theory approach to electron transfer reactions

Chem. Sci.2019, Advance Article 

 

Beyond the dielectric continuum description initiated by Marcus theory, the standard theoretical approach to study electron transfer (ET) reactions in solution or at interfaces is to use classical force field or ab initio molecular dynamics simulations. We present here an alternative method based on liquid-state theory, namely molecular density functional theory, which is numerically much more efficient than simulations while still retaining the molecular nature of the solvent. We begin by reformulating molecular ET theory in a density functional language and show how to compute the various observables characterizing ET reactions from an ensemble of density functional minimizations. In particular, we define within that formulation the relevant order parameter of the reaction, the so-called vertical energy gap, and determine the Marcus free energy curves of both reactant and product states along that coordinate. Important thermodynamic quantities such as the reaction free energy and the reorganization free energies follow. We assess the validity of the method by studying the model Cl0 / Cl+ and Cl0 / Cl- ET reactions in bulk water for which molecular dynamics results are available. The anionic case is found to violate the standard Marcus theory. Finally, we take advantage of the computational efficiency of the method to study the influence of a solid–solvent interface on the ET, by investigating the evolution of the reorganization free energy of the Cl0 / Cl+ reaction when the atom approaches an atomistically resolved wall.

 

Spin in a Closed-Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction

Angew. Chem. Int. Ed., 2019, 58, 821

 

Inert metal surfaces present more chances of hosting organic intact radicals than other substrates, but large amounts of delocalized electronic states favor charge transfer and thus spin quenching. Lowering the molecule–substrate interaction is a usual strategy to stabilize radicals on surfaces. In some works, thin insulating layers were introduced to provide a controllable degree of electronic decoupling. Recently, retinoid molecules adsorbed on gold have been manipulated with a scanning tunneling microscope (STM) to exhibit a localized spin, but calculations failed to find a radical derivative of the molecule on the surface. Now the formation of a neutral radical spatially localized in a tilted and lifted cyclic end of the molecule is presented. An allene moiety provokes a perpendicular tilt of the cyclic end relative to the rest of the conjugated chain, thus localizing the spin of the dehydrogenated allene in its lifted subpart. DFT calculations and STM manipulations give support to the proposed mechanism.

Chiral Crystal Packing Induces Enhancement of Vibrational Circular Dichroism

Angew. Chem. Int. Ed. 2018, 57, 13344 –13348

 

We demonstrate that molecular vibrations with originally low or zero intensity in a vibrational circular dichroism (VCD) spectrum attain chirality in molecular crystals by coordinated motion of the atoms. Ab initio molecular dynamics simulations of anharmonic solid-state VCD spectra of l-alanine crystals reveal how coherent vibrational modes exploit the space groupQs chirality, leading to nonlocal, enhanced VCD features, most significantly in the carbonyl region of the spectrum. The VCD-enhanced signal is ascribed to a helical arrangement of the oscillators in the crystal layers. No structural irregularities need to be considered to explain the amplification, but a crucial point lies in the polarization of charge, which requires an accurate description of the electronic structure. Delivering a quantitative atomic conception of supramolecular chirality induction, our ab initio scheme is applicable well beyond molecular crystals, for example, to address VCD in proteins and related compounds.

New avenues for the large-scale harvesting of blue energy

Nature Reviews Chemistry 1, Article number: 0091 (2017)

 

Salinity gradients have been identified as promising clean, renewable and non intermittent sources of energy — so-called blue energy. However, the low efficiency of current harvesting technologies is a major limitation for large-scale viability and is mostly due to the low performances of the membrane processes currently in use. Advances in materials fabrication with dedicated chemical properties can resolve this bottleneck and lead to a new class of membranes for blue-energy conversion. In this Perspective, we briefly present current technologies for the conversion of blue energy, describe their performances and note their limitations. We then discuss new avenues for the development of a new class of membranes, combining considerations in nanoscale fluid dynamics and surface chemistry. Finally, we discuss how new functionalities originating from the exotic behaviour of fluids in the nanoscale regime can further boost energy conversion, making osmotic energy a tangible, clean alternative.