Solvent effects on the vibrational frequencies of the phenolate anion, the para-cresolate anion, and their radicals.

The effects of aqueous solvation on the structure and vibrational frequencies of phenol, para-cresol, and their respective radicals are calculated at the B3LYP/6-31+G(d,p) level of theory using the conductor-like polarizable continuum model (C-PCM) alone and in combination with an explicit water molecule H-bonded to the phenolic oxygen. Calculated vibrational frequencies are compared to experimental frequencies obtained in aqueous buffer at high pH. For all models, the C-PCM provides the best overall agreement between theory and experiment at a modest computational effort, as demonstrated by the lowest mean absolute deviations in the computed frequencies. In addition, the C-PCM provides anion Wilson mode 7a 18O isotope shifts in excellent agreement with experiment and improves agreement between the computed and observed radical Wilson mode 7a 2H isotope shift. On the basis of a quantitative comparison of the anion and radical normal modes by vibrational projection analysis and total energy decomposition, ...