VIBRATIONAL ASSIGNMENTS OF TRANS-N-METHYLACETAMIDE AND SOME OF ITS DEUTERATED ISOTOPOMERS FROM BAND DECOMPOSITION OF IR, VISIBLE, AND RESONANCE RAMAN-SPECTRA

We have measured the IR absorption, Raman, and resonance Raman spectra of aqueous and neat N-methylacetamide (NMA) and its ND and CCD3 isotopic derivatives and subjected them to a detailed line shape analysis. The amide I and II bands of NMA in H2O are composed of at least two broad bands with different intensities and depolarization ratios. The two subbands of amide I are interpreted as arising from a vibrational mixing between the amide I vibration and the bending motion of water molecules hydrogen bonded to the peptide group. The amide II subbands mast likely arise from two, nearly isoenergetic, NMA conformers, which differ in the orientation of the N-methyl hydrogens. In neat NMA conformational heterogeneity and a noncoincidence between the isotropic and anisotropic parts of the Raman tenser cause an even more complex structure of amide I. The amide III mode, even at visible excitation, and the band arising from the C-methyl symmetric bending motion exhibit depolarization ratios close to 0.33, which indicates that their intensities are entirely determined by the 190 nm pi --> pi* transition. The C-methyl symmetric end is enhanced mainly by a CC stretch contribution. In contrast amide I and II show a dispersion of their depolarization ratios owing to additional contributions to their off-resonance spectra from electronic transitions at shorter wavelengths. The spectrum of the ND isotopic derivative exhibits an amide II' doubler at 1500 cm(-1), which results from a Fermi resonance interaction between the amide II' mode and a combination of amide IV' (632 cm(-1)) and a skeletal deformation mode (873 cm(-1)).