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 aqueo us 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 wea k or absent in the,aqueous solution spectrum. Since resonance Raman sc attering reflects the geometry change associated with electronic excit ation, this suggests that here is a large effect of solvation on the g eometry of the excited electronic state. The present work describes th e development of quantitative ab initio quantum chemical procedures fo r calculating resonance Raman spectra with application to NMA both iso lated and in solution. It is shown that a simple cluster model involvi ng hydrogen-bonding of water molecules to NMA provides an adequate des cription of the effect of aqueous solvation on resonance Raman spectra of NMA. The major effect appears to be due to a change in the geometr y of the resonant pi pi excited electronic state. However, there is a lso 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 bey ond the NMA(H2O)(3) cluster using a self-consistent reaction field.