Neutron and Raman scattering studies of the methyl dynamics in solid toluene and nitromethane

Abstract

Solid toluene has been studied in its α phase at 5 K by high resolution Inelastic Neutron Scattering (I.N.S.). The tunnel splitting of the methyl torsion group state has been found to be 0.196 cm-1 (24.3 μeV) for C6D5CH3 and 0.0088 cm-1 (1.1 μeV) for C6D5CD3. Simple cosine (V3 , V6) or square well potentials are able to reproduce this splitting but the calculated first torsional transitions (∼ 56 cm -1) are then slightly higher than the value of 46.8 cm-1 deduced from the analysis of the Raman and I.N.S. spectra obtained for several isotopic derivatives in a large temperature range. The possibility of coupling of the first torsional excited states with some phonons is invoked to explain these observations. A similar Raman and I.N.S. study of solid nitromethane allows the —CH3 torsion to be situated at about 53 cm-1 and leads to similar conclusions