RESONANCE RAMAN EXAMINATION OF THE 2 LOWEST AMIDE PI-PI-ASTERISK EXCITED-STATES

The UV Raman spectra of N-methylacetamide (NMA) were measured in H2O a nd D2O between the visible spectral region and the UV region down to 1 84 nm. The Raman excitation profiles indicate that all of the amide ba nds are resonance enhanced by the first dipole-allowed pi-->pi transi tion at 190 nm. The Raman excitation profile data indicate the presenc e of destructive interference from the second allowed transition at ca . 165 nm. For example, the amide I and amide I' bands disappear in the 184 nm UV Raman spectra and the relative intensities of the amide II and III bands change. The first allowed electronic transition at ca. 1 90 nm occurs to an excited pi pi state where the major geometry chang e, relative to the ground state, involves a dominating C-N bond elonga tion, with smaller C-C and N-C bond contractions, and a more modest C= O bond elongation. The C-N groundstate double bond character may decre ase to the point where free rotation may occur. This may explain the p reviously observed monophotonic photoisomerization of trans-NMA to eis -NMA. The second allowed transition at ca. 165 nm occurs to an excited state whose major geometry change involves C=O bond elongation. (C) 1 998 John Wiley & Sons, Ltd.