Optical and spectroscopic study of the phase transition at 116 K in hexamethylbenzene

Abstract

The phase transition of hexamethylbenzene (HMB) at 116 K has been studied by optical and spectroscopic techniques. In Phase III HMB appears to crystallize in a dimetric group system as shown by typical conoscopic patterns. The morphology of the (001) pinacoid supports the hypothesis of an hexagonal or trigonal system.

Spectrophotometric measurements of the optical retardation show that pinacoids in Phase III are optically isotropic. The phase change shows the typical behaviour of a non-isothermal transition. Raman spectra in the region of lattice vibrations suggest that in Phase III a more compact structure occurs with equivalent librational modes in the plane of the benzene rings. Both the isotropy of the u.v. absorption in the region of the electronic transition ¹B_2uâ†�¹A_1g, observed in Phase III in the plane of the benzene ring, and the disappearance of the 0–0 band in (001), indicate that in the colder phase the potential of the crystal field has some elements of symmetry common to those of the free molecule.