Vibrational Spectra of Liquid and Crystalline CF4

The Raman and infrared spectra of CF₄ have been recorded in its liquid and two crystalline phases. In the liquid, rotational wings along side some of the fundamentals and the appearance of some forbidden infrared bands reveal the high degree of molecular disorder characteristic of this phase. Very little change occurs in the spectra at the liquid–solid transition; the spectra of Phase I thus appear to be those of a plastic crystal. Marked changes are observed upon cooling to Phase II. The splitting of the fundamentals in the Phase II spectra of CF₄ is best interpreted in terms of a $S_4$ tetragonal structure, with the molecules lying on sites of $S_4$ symmetry.