No evidence for proton transfer along the n-h⋯o hydrogen bond in n-methylacetamide: neutron single crystal structure at 250 and 276 K

The crystal structure of N-methylacetamide (C ₃H ₇NO), M _r = 73.095, has been determined from single-crystal neutron diffraction data at two temperatures, 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 ₁a [Pn2 ₁m], a = 9.671(2) [4.878(1)] Å, b = 6.613(6) [6.567(1)] Å, c = 7.218(1) [7.332(2)] Å; V = 465.7(6) [234.9(5)] Å 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-methylacetamide was undertaken in order to assess a recent suggestion based on inelastic neutron scattering spectroscopy that transfer of the amide proton along the peptide hydrogen bond may be responsible for the vibrational anomalies. While we found no evidence for proton transfer along the N-H⋯O hydrogen bond (d(NH) = 1.025(17) Å, and d(U-O) = 1.856(14) Å for at T = 250 K) at either temperature evidence for some molecular disorder is present in accord with our previous ¹³C NMR studies. In addition, we find short intramolecular contacts between the amide hydrogen atom and those on both neighboring methyls, which may well affect the vibrational properties of the respective molecular groups.