Diffusion coefficient of ionic solvation shell molecules

It is shown that, for a tightly bound ion-solvation shell complex, the mean square displacement for solvation molecules is characterized by a long lasting transitory. This initial portion is related to the rotational relaxation of the complex and can reach up to several hundred picoseconds for a representative example such as the Mg2+ ion in water. As the diffusion coefficient is usually fitted using much shorter time spans, unnoticed overestimations are possible. It is argued that, instead of computing the aforementioned diffusion coefficient from the mean square displacement, it should be defined taking as a basic guideline the ratio between the rotational relaxation time of the complex and the lifetime within the first solvation shell

On the performance of molecular polarization methods. II. Water and carbon tetrachloride close to a cation

Our initial study on the performance of molecular polarization methods close to a positive point charge [M. Masia, M. Probst, and R. Rey, J. Chem. Phys. 121, 7362 (2004)] is extended to the case in which a molecule interacts with a real cation. Two different methods (point dipoles and shell model) are applied to both the ion and the molecule. The results are tested against high-level ab initio calculations for a molecule (water or carbon tetrachloride) close to Li+, Na+, Mg2+, and Ca2+. The monitored observable is in all cases the dimer electric dipole as a function of the ion-molecule distance for selected molecular orientations. The moderate disagreement previously obtained for point charges at intermediate distances, and attributed to the linearity of current polarization methods (as opposed to the nonlinear effects evident in ab initio calculations), is confirmed for real cations as well. More importantly, it is found that at short separations the phenomenological polarization methods studied here substantially overestimate the dipole moment induced if the ion is described quantum chemically as well, in contrast to the dipole moment induced by a point-charge ion, for which they show a better degree of accord with ab initio results. Such behavior can be understood in terms of a decrease of atomic polarizabilities due to the repulsion between electronic charge distributions at contact separations. It is shown that a reparametrization of the Thole method for damping of the electric field, used in conjunction with any polarization scheme, allows to satisfactorily reproduce the dimer dipole at short distances. In contrast with the original approach (developed for intramolecular interactions), the present reparametrization is ion and method dependent, and corresponding parameters are given for each case

On the coupling between molecular diffusion and solvation shell exchange

The connection between diffusion and solvent exchanges between first and second solvation shells is studied by means of molecular dynamics simulations and analytic calculations, with detailed illustrations for water exchange for the Li+ and Na+ ions, and for liquid argon. First, two methods are proposed which allow, by means of simulation, to extract the quantitative speed-up in diffusion induced by the exchange events. Second, it is shown by simple kinematic considerations that the instantaneous velocity of the solute conditions to a considerable extent the character of the exchanges. Analytic formulas are derived which quantitatively estimate this effect, and which are of general applicability to molecular diffusion in any thermal fluid. Despite the simplicity of the kinematic considerations, they are shown to well describe many aspects of solvent exchange/diffusion coupling features for nontrivial systems

Computational study of γ-butyrolactone and Li+/γ-butyrolactone in gas and liquid phases

A comprehensive study of structural and dynamical properties of γ-butyrolactone (GBL) and the extent to which they are affected in the vicinity of a lithium ion, both in gas and liquid phases, is reported. The isolated GBL molecule is found to be nonplanar, with a barrier of ≈9 kJ/mol to ring inversion. As expected, the lithium ion coordinates the carbonyl oxygen with an almost collinear configuration relative to the carbon−oxygen bond but with a slight tilting toward the lactone oxygen. This configuration holds for clusters of up to four molecules and in the liquid phase as well (where a tetrahedral first solvation shell is found). A high level ab initio vibrational analysis, with a new assignment of bands has been performed, which shows substantial red and blue shifts upon lithium solvation, which decrease in a nontrivial way upon increasing the cluster size. To study the solvent effect of the vibrational spectrum, an accurate intramolecular force field has been developed, based on the concept of relaxed potential energy profiles. The inclusion of stretch and bend anharmonicity is shown to be essential in order to explain, not only the absolute value, but the sign of the shifts, particularly for the carbonyl stretching which is substantially downshifted. The shifts obtained for the rest of the bands, together with the diffusion coefficients for bulk GBL and for lithium, are in fair agreement with experimental results