Assignment of the three methyl tunneling lines in a trichloromesitylene single crystal

Abstract From quantum chemistry calculations, for an isolated molecule of trichloromesitylene (TCM), the three methyl groups (Me) are equivalent with one C–H bond in the ring plane. The potential V h hindering their rotation is lower than 10 meV. However, in the solid state at 4 K, inelastic neutron scattering (INS) found three different tunneling excitation energies at 4.3, 9.5 and 13.1 μeV. This implies three different V h about 5 times larger than in the gas. In the triclinic lattice of TCM, all the molecules are parallel and each species of Me has a specific orientation. In order to assign each tunnelling excitation to a specific Me we have used the backscattering spectrometer at FZ-Julich. Rotating a single crystal around an axis approximately perpendicular to the molecular planes, we have observed at 4 K three maxima of INS. These maxima are separated by 60°±5° as expected. This experiment has demonstrated the possibility to assign a specific excitation to each methyl group in TCM. Perturbations of the spectra due to the structural phase transition around 160 K and crystal shape are discussed.