Raman spectra of cyclohexane crystal I and crystal II

Abstract

The Raman spectra of the crystals I and II of cyclohexane have been studied. The crystal I, stable between the freezing point and 186°K, has been found to be most easily super-cooled under rapid cooling to 77°K when the sample has contained a small amount of impurity. The crystal II, stable below 186°K, has been obtained under rapid cooling of pure cyclohexane or through annealing of the supercooled crystal I over a wide range of temperature. The Raman spectrum of crystal I has exhibited splittings only for two bands 2890 and 2920 cm −1. The weak bands 1057 and 1421 cm −1 which appear only in the crystal have been assigned to the fundamentals of a 2 g species. The occurrence of a 2 g fundamentals has led to the space group of T h 2 for the structure of crystal I. A remarkable effect of temperature on the band widths of the degenerate fundamentals in crystal I has suggested the existence of considerable freedom for molecular rotation in the crystal lattice at high temperatures. The Raman spectrum of crystal II at 77°K is quite different from that of crystal I at the same temperature. It has been found that in crystal II each fundamental of e g species has split into two components, while all the fundamentals of a 1 g and a 2 g species have appeared as single bands. Possible assignments for the bands 2890 and 2920 cm −1 have been suggested on the basis of this rule.