Vibrational spectra and dynamics of conformation and hydrogen bonding of N-methylacetamide. II. Dynamics of the NH...O Hydrogen bond and NH (ND) stretching band structure

The infrared and Raman spectra of crystalline CH₃CONHCH₃, CD₃CONHCH₃, CH₃CONHCD₃, CD₃CONHCD₃ and their N deuterated derivatives have been investigated at various temperatures in the 3400-2000 cm^1̄ range. A new interpretation of the NH (ND) stretching subbands is given and is based on the assumption of a strong coupling between NH stretching and deformation modes (γ NH and τ CN). Suitable potential functions are derived from experimental data and they allow to calculate the infrared and Raman spectra in the stretching region. The calculated spectra as well as the calculated isotopic and temperature shifts are in good agreement with the observed ones. The potential surface of the bending modes in the excited state has only one rather flat central minimum corresponding to a planar geometry of the amide group which can be contrasted to the four-minimum potential surface of the fundamental state.