Dynamics of Crystalline N-Methylacetamide:  Temperature Dependence of Infrared and Inelastic Neutron Scattering Spectra

Abstract

N-methylacetamide may be viewed as a simple model for polypeptides. Unusual intensities have previously been observed in its amide vibrational spectra and were attributed to contradictory causes, namely, Fermi resonance, vibrational polarons, and dynamic proton transfer along the H bond. We report the first inelastic neutron scattering study of three specifically deuterated derivatives at different temperatures along with new infrared spectra. The analysis of the librations in the INS spectra indicates a degeneracy between the N-CH 3 methyl torsional mode and another degree of freedom assigned to one component of the skeletal torsion. No direct evidence for a methyl-methyl coupling is observed. However γ(CO) appears strongly coupled to the motions of the neighboring C-CH 3 group. A new absorption band ("X") is observed at approximately 1590 cm -1 in the IR spectrum of the amide-1/amide-2 region. This calls into question the previous assignment of a band at ∼1600 cm -1 as the overtone of γ(NH). In addition, all observed peaks in the amide-2 region shift to higher frequency on cooling so that none of them could be assigned to the ν(NH) mode, as previously proposed. Previous theoretical explanations of the anomalous amide spectra of N-methylacetamide based on either Fermi resonance or the proton-transfer model are therefore in disagreement with our new experimental data.

Polymorphism of Paracetamol: A New Understanding of Molecular Flexibility through Local Methyl Dynamics

“…Interestingly, in both forms, the temperature evolution of the intensity ( I ) of these modes follows an exponential decay of the form I ∝ 1 − e − W / k normalB T This corroborates well with our zero-point periodic calculations and the experimental IINS data from FDS for form I that demonstrate that these bands are assigned to methyl librational modes mixed with collective vibrations (which are dependent on thermal energy). Similar librations were evidenced in N -methylacetamide in the early studies of paracetamol and acetanilide and in phenacetin . For form II, however, our computational results show that the methyl librational modes are mixed with phenyl ring rotations.…”

“…In the inset of Figure 2a,b, the modes located at approximately 95 and 100 meV for the monoclinic form and at 105 and 120 meV for the orthorhombic form can be assigned to the out-ofplane bending of the N−H amide group. 40 These bands also contain O−H and N−H bending modes, highlighting differences in the strength and symmetry of the hydrogen bonding of the donor −NH and −OH groups. The most striking finding in this spectral region is that the −OH bend modes occur at a much higher energy in form II (+8 meV).…”

Spectral response of crystalline acetanilide andN-methylacetamide: Vibrational self-trapping in hydrogen-bonded crystals

“…This splitting has been observed before and is of unclear origin. [30][31][32] In ACN the NH main peak at 3295 cm −1 is observed both in the parallel and perpendicular measurements, however, with a relative shift of 11 cm −1 [ Fig. 2(a)].…”

Dynamics of the intermolecular hydrogen bonds in the polymorphs of paracetamol in relation to crystal packing and conformational transitions: a variable-temperature polarized Raman spectroscopy study

“…The structure and vibrational spectra of N-methylacetamide (CH 3 -(CQO)-NH-CH 3 ) (NMA), a single amide containing methyl groups at both extremities, have been the subject of numerous investigations. [3][4][5][6][7][8] A complex study by incoherent inelastic and quasi-elastic neutron scattering, IR-spectroscopy, DSC, and neutron diffraction has revealed several unusual phenomena in this system, including dynamic transitions at about 230 K and 100 K, and has raised the question of a possible proton transfer. The authors of ref.…”