Molecular Dynamics of Monomeric Water Dissolved in Very Hydrophobic Solvents:  the Current State of the Art of Vibrational Spectroscopy Analyzed from Analytical Model and MD Simulations

The aim of the present paper is to evaluate the influence of the solute−solvent interactions on the infrared spectra of water diluted in liquid CCl 4 and in supercritical xenon, considered as the standard ‘inert' solvent. This investigation is based upon FTIR spectra analyzed at the light of both analytical treatments and molecular dynamics simulations. For water in supercritical xenon, the rotational relaxation processes mainly determine the shape of the IR profiles associated with the ν 1 and ν 3 stretching modes. The water molecule rotates almost “freely” due to the isotropic character of the van der Waals interactions applied on the solute. Both the J-model for asymmetric molecular rotor and the molecular dynamics simulations properly account for the band shapes associated with the ν 3 and ν 1 vibrational modes of water. Thus, the rotational dynamics of water is primarily governed by “collisional” interactions with the neighboring solvent molecules. For water dissolved in liquid CCl 4, a structural analysis based upon the simulated radial distribution functions provides evidence for the existence of a short-ranged C···H−O arrangement between the solute and its neighboring solvent molecules. It is also found that the reorientational dynamics of water are more perturbed than those in SC xenon fluid, due to the weakly anisotropic character of the water-CCl 4 interactions. In particular, the reorientational motions of the z symmetry axis of water appear to be more specifically affected. We emphasize that a correct treatment of the rotational dynamics of water in liquid CCl 4 is provided only by simulation methods that, in contrast to the analytical J model, include the details of the intermolecular solute−solvent potential. Although the transition dipole moment of the ν 3 mode of water is only weakly affected by the interactions, the oscillator strength of the ν 1 internal mode is found to be enhanced compared to its gas-phase value, a result related to the increase of the transition dipole moment due to the water−solvent interactions. Finally, we argue that the spectral properties can be interpreted without invoking a specific H-bond contribution in the intermolecular potential.