Molecular motions and [lambda] phase transition: Raman and far-ir studies of neat and isotopic mixed hexamethylbenzene crystal

Abstract

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 [lambda]-phase transition. Site splittings on two intramolecular modes of HMB-h18 (but on only one corresponding HMB-d18 mode) are 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[deg]K (+/- 2[deg]K) for HMB-h18 crystal, the transition temperature is 133[deg]K (+/- 2[deg]K) for HMB-d18. Our results suggest that (for phonon interactions) the symmetry in phase Ill is close to D3d and reduces to Ci 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.