Resonance Raman spectroscopy of N-methylacetamide: Overtones and combinations of the C-N stretch (amide II′) and effect of solvation on the C=O stretch (amide I) intensity

Resonance Raman spectra of N-methylacetamide (NMA) and its isotopic derivatives are reported for the vapor phase, acetonitrile, and aqueous solution with excitation wavelengths of 218, 200, and 192 nm. The 192-nm vapor-phase spectra of NMA and that of N-deuterated NMA (D-NMA), as well as that of an aqueous solution of D-NMA obtained with 200-nm excitation, exhibit identifiable overtones and combinations with considerable intensity. This intensity distribution is discussed in terms of the change of the molecular geometry along the C-N stretching coordinate upon electronic excitation. The resonance Raman spectra of NMA in the gas phase and in acetonitrile solution have considerable intensity in the amide I (C=O) stretching vibration. This mode is absent or very weak in the spectra obtained with aqueous solutions. Spectra obtained in mixtures of acetonitrile and water show that the amide I vibration decreases roughly linearly in intensity with water content. Various hypotheses concerning this dramatic effect of solvation on the resonance Raman spectrum of NMA are presented and discussed.