Knowledge Bank

We have obtained the infrared and Raman spectra of $CF4$ in its two crystalline phases. No splitting of the bands is observed in the high-temperature phase whereas all the fundamentals, except the symmetric stretch, yield doublets in the low-temperature modification. This is consistent with either $D2d$ or $S4$ site symmetry in the latter, and indicates that the structure is different from that of Phase II of $CH4$. Results on $SiF4$ infer that the structure is probably similar to that of Phase II of $CF4$. This is at variance with the structure obtained from X-ray diffraction studies.