No evidence for proton transfer along the N-H center dot center dot centerdot O hydrogen bond in N-methylacetamide: Neutron single crystal structureat 250 and 276 K

The crystal structure of N-methylacetamide (C3H7NO), M-r = 73.095, has been determined from single-crystal neutron diffraction data at two temperature s, 250 and 276 K, above and below the previously reported phase transition located at 274 K in this work. Crystal data: 250 K [276 K]: space group Pn2 la [Pn2(1)m], a = 9.671(2) [4.878(1)] Angstrom, b = 6.613(6) [6.567(1)] Ang strom, c = 7.218(1) [7.332(2)] Angstrom; V = 465.7(6) [234.9(5)] Angstrom ( 3); D-n = 1.043 [1.034] g.cm(-3), R(F-2) = 0.168 [0.134], wR(F-2) = 0.062 [ 0.051], S = 1.18 [1.11]. This new investigation of the structure of N-methy lacetamide was undertaken in order to assess a recent suggestion based on i nelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anom alies. While we found no evidence for proton transfer along the N-H...O hyd rogen bond (d(NH) = 1.025(17) Angstrom, and d(H...O) 1.856(14) Angstrom for at T = 250 K) at either temperature evidence for some molecular disorder i s present in accord with our previous C-13 NMR studies. In addition, we fin d short intramolecular contacts between the amide hydrogen atom and those o n both neighboring methyls, which may well affect the vibrational propertie s of the respective molecular groups.