Resonance Raman examination of the two lowest amide ππ* excited states

The UV Raman spectra of N-methylacetamide (NMA) were measured in H2O and D2O between the visible spectral region and the UV region down to 184 nm. The Raman excitation profiles indicate that all of the amide bands are resonance enhanced by the first dipole-allowed π→π* transition at 190 nm. The Raman excitation profile data indicate the presence 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. 190 nm occurs to an excited ππ* state where the major geometry change, relative to the ground state, involves a dominating C—N bond elongation, with smaller C—C and N—C bond contractions, and a more modest C=O bond elongation. The C—N ground-state double bond character may decrease to the point where free rotation may occur. This may explain the previously observed monophotonic photoisomerization of trans-NMA to cis-NMA. The second allowed transition at ca. 165 nm occurs to an excited state whose major geometry change involves C=O bond elongation.