Quasi-elastic (QENS) and inelastic neutron scattering (INS) on hexamethylbenzene

摘要

The Quasi-elastic Neutron scattering (QENS) spectra of polycrystalline hexamethylbenzene (HMB) were measured for temperatures from 10K to room temperature (phase III and phase II) for momentum transfer 1.9 angstrom(-1). The Inelastic Neutron scattering (INS) and QENS spectra for momentum transfer 0.5-2.9 angstrom(-1) were measured at T = 20, 100 and 130 K for energy transfer up to 200 meV. The low-resolution diffraction patterns, used as the phase indicator, were also obtained.In the phase III (below 117 K), we see practically no quasi-elastic broadening. In phase 11, the broadening changes with the temperature are in good agreement with the Arrhenius law. The estimated activation barrier to reorientation is 6 kJ/mol. The fitted mean time between instantaneous 120 degrees jumps of CH3 groups changes from 10(-11) at T = 130 K to 2 x 10(-13)s at room temperature.On the basis of EISF versus momentum transfer dependency it is hardly possible to decide what is the geometry of the reorientation. Both reorientation of the CH3 groups around the three-fold symmetry axis and reorientation of the whole molecule around the six-fold symmetry axis of the benzene ring could describe our results, the former being more probable.The measured INS spectra are compared with the quantum chemical ab initio calculations performed for an isolated HMB molecule. (c) 2005 Elsevier B.V. All rights reserved.