Solvent effects on vibrational modes: ab initio calculation, scaling and solvent functions with applications to the carbonyl stretch of dialkyl ketones

We report calculations on harmonic vibrational frequencies for the carbonyl stretch of a series of ketones of general formula R-CO-Me in various solvents. The frequencies are calculated at the B3LYP/6-31G(d) level by using recent versions of the Polarizable Continuum Model (PCM). A comparison with experimental data is carried out with the help of different strategies for the scaling of computed values in solution. It is found that a solvent-specific scaling of the standard PCM frequencies is able to reproduce the fine details in the experimental solvent shifts. A parallel comparison of experimental, non-scaled and scaled frequencies with respect to solvent functions of various nature permits to identify the cases in which specific solute-solvent interactions play a role. For such cases more refined versions of PCM, using a quantum mechanical description of the solute accompanied by a small set of solvent molecules, may be used, if there is the need of refining the standard description.