INELASTIC NEUTRON-SCATTERING STUDY OF THE PROTON DYNAMICS IN N-METHYLACETAMIDE AT 20-K

Inelastic neutron-scattering (INS) spectra of four isotopic derivatives of N-methylacetamide (CH3CONHCH3, CD3CONHCH3, CH3CONHCD3 and CD3CONHCD3) at 20 K are presented from 30 to 4000 cm-1. The band frequencies are compared with those observed in the infrared and Raman at low temperature. The quantitative simulation of the INS intensities, in the harmonic force field approximation, shows that the proton dynamics for the (N)H proton are totally different from those proposed previously. The valence-bond approach is not consistent with observation and the proton dynamics are independent of the molecular frame. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode is at almost-equal-to 1575 cm-1. This is a dramatic change compared to all former assignments at almost-equal-to 3250 cm-1 based on the infrared and Raman data. These unforeseen proton dynamics are associated with the weakening of the N-H bond due to the ionic character of the hydrogen bond (N(delta-)...H+...O(delta'-)) and proton transfer. The infrared and Raman spectra are reconsidered and a new assignment scheme is proposed for the amide bands in terms of dynamical proton exchange between the amidic (...OCNH...) and imidolic (...HOCN...) forms in infinite chains of hydrogen-bonded molecules.