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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry