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

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 Vh 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 Vh 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-Juelich. 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 deg. ±5 deg. 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