Refinements on solvation continuum models: Hydrogen-bond effects on the OH stretch in liquid water and methanol

We present a study on the IR vibrational shifts of the OH stretch mode for water and methanol when passing from gas to liquid phase. Both a purely continuum solvation model (here the polarizable continuum model) and its semicontinuum extensions, obtained explicitly considering small clusters of H-bonded molecules of liquid, have been tested. The definition of such clusters as the new quantum mechanical system to be coupled to a continuum description of the remainder liquid has allowed us to take into account both the "local permanent specific" interaction due to the H-bond and the long-range effects present in all liquids. The analysis mainly focused on harmonic approximations and has been extended to include mechanical anharmonicities in order to have a more reliable comparison with the experiments. Finally, careful attention has been paid to the analysis of the "intrinsic" parameters of continuum models (i.e., the shape and size of the molecular cavity containing the QM system) and to their effects on the vibrations.