Structure and vibrational dynamics of the strongly hydrogen-bonded model peptide: N-methyl acetamide

Density functional theory-based methods have been used to calculate the vibrations, in the harmonic approximation, of n-methyl acetamide in the solid state. Good agreement is obtained with previously published inelastic neutron scattering spectra. The starting point for the calculation is the crystal structure, which has to be measured at the same temperature as the vibrational spectra. Unit cell and atomic coordinates have been obtained using powder neutron diffraction on the methyl-deuterated material at 2 K. The controversial assignment of the $\mathrm{N–H}$ stretch mode at ∼1600 ${\mathrm{cm}}^{\text{−1}},$ made in the original analysis of the vibrational spectra, is not supported by the calculations presented here. Neither is evidence found for the proposed double-well potential for the proton in the hydrogen bond.