Molecular motions and lambda phase transition: Raman and FAR-1R studies of neat and isotopic mixed hexamethylbenzene crystal

Raman and far-IR studies of hexamethylbenzene (HMB) low temperature crystalline phases (neat and isotopic mixed) are presented. The Raman phonon spectrum changes drastically during the X-phase transition. Site splittings on two intramolecular modes of HMB-h(18) (but on only one corresponding HMB-d(18) mode) arc observed in the room temperature phase (phase II) spectrum. These splittings disappear in the lower temperature phase (phase III). The methyl torsional bands are identified and a significant shift is observed for them during the phase transition. Also, while the lambda-phase transition takes place at 113 degrees K (+/- 2 degrees K) for HMB-h(18) crystal, the transition temperature is 133 degrees K (+/- 2 degrees K) for HMB-d(18). Our results suggest that (for phonon interactions) the symmetry in phase III is close to D-3d and reduces to C-i in phase II. Furthermore, the results support the mechanism of phase transition which involves a tilting of the methyl groups out of the benzene ring.