Ab initio analysis of the effects of aqueous solvation on the resonance Raman intensities of N-methylacetamide

The resonance Raman spectrum of the simple peptide N-methylacetamide (NMA) is very different in the vapor phase than when dissolved in aqueous solution. The spectrum for an acetonitrile solution is intermediate. The major difference is that the amide I mode, primarily involving C=O stretching, is very strong in the vapor-phase spectrum but very weak or absent in the aqueous solution spectrum. Since resonance Raman scattering reflects the geometry change associated with electronic excitation, this suggests that there is a large effect of solvation on the geometry of the excited electronic state. The present work describes the development of quantitative ab initio quantum chemical procedures for calculating resonance Raman spectra with application to NMA both isolated and in solution. It is shown that a simple cluster model involving hydrogen-bonding of water molecules to NMA provides an adequate description of the effect of aqueous solvation on resonance Raman spectra of NMA. The major effect appears to be due to a change in the geometry of the resonant ππ* excited electronic state. However, there is also a significant change in the ground-state geometry and the form of the normal modes upon hydrogen-bonding. To fully describe this ground-state effect it is necessary (and sufficient) to treat the solvent beyond the NMA(H₂O)₃ cluster using a self-consistent reaction field.