UV RAMAN DETERMINATION OF THE PI-PI-ASTERISK EXCITED-STATE GEOMETRY OF N-METHYLACETAMIDE - VIBRATIONAL ENHANCEMENT PATTERN

We have examined the near-IR, visible, and UV Raman spectra of N-methy lacetamide (NMA) isotopomers and characterized their Raman excitation profiles. We use the normal mode eigenvectors for these isotopomers to calculate a set of excited state bond displacements which are able to model the observed spectra of all of the isotopomers in water. The pi pi excited state geometry for NMA in water differs from that of the ground state in that the CN bond elongates, while the CC and the N-CH3 bonds contract by smaller amounts. The CO bond elongates by about 33% of that of the CN bond. Our results indicate that the alterations in the resonance Raman enhancement for NMA in non-hydrogen-bonding solven ts involve both ground state normal mode composition changes and chang es in the excited state geometry relative to the ground state. The CO bond becomes more elongated in the pi pi excited state in the non-hyd rogen-bonded solvents. The changes in the resonance Raman intensity in gas phase NMA result from an even larger NMA pi pi excited state geo metry expansion along the CO bond relative to the ground state. The C- CH3 symmetric bending mode is enhanced, not due to its CH bending moti on but because of the contribution of other internal coordinates.