Neutron inelastic scattering of the methyl torsional mode in hydrogenated and deuterated 4-methylpyridine crystals at 5 K

Abstract

Inelastic neutron scattering from the 4-methylpyridine crystal (C 6H 7N) and its perdeuterated analogue (C 6D 7N) were recorded at 5 K in the 0–9.5 cm −1 range. The main intensities appear at 4.16 and 0.8 cm −1 for the hydrogenated and deuterated compounds, respectively. They are ascribed to the methyl torsional mode and the isotopic ratio (4.16/0.8 ≈5) indicates a tunnel splitting consistent with a periodic potential. In addition, there is a weak band at 3.82 cm −1 for C 6H 7N which is ascribed to the τ (CH 3) mode, although the analogous band could not be identified in the deuterated derivative. The methyl torsion dynamics is thus likely to be more complicated than suggested by a simple periodic potential.