An ab initio study of solvent shifts in vibrational spectra

Using analytical second derivatives of the generalized conductorlike screening model (GCOSMO) we calculate vibrational frequency shifts for several molecules (acetone, methylamine, formic acid, acetic acid, and trans-NMA) solvated in water. In these calculations, results from dielectric continuum approach with and without several explicit water molecules are compared with traditional supermolecule approach. The simple GCOSMO model, where all solvent molecules are treated as a continuum medium, reproduces quite accurately solvent shifts in solutes having moderate hydrogen bondings with water, such as acetone and methylamine. To represent strong solvent effects in formic acid and acetic acid, one should add at least one explicit water molecule in GCOSMO calculations. Solvent effects on solute structure correlate well with frequency shifts. Geometry optimizations and frequency calculations in the GCOSMO-supermolecule approach require only 10%-20% more computational effort than similar calculations in the gas phase. Therefore, this method provides a promising and effective tool for studying reactivity, structural, and spectroscopic properties of realistic solutes. (C) 1996 American Institute of Physics.