Infrared Spectra and Structures of the Crystalline Phases of CH4 and CD4

Infrared spectra of CH4 and CD4, in the region of the fundamentals ν3 and ν4, have been obtained in all of their crystalline phases at temperatures ranging from 5° to 40°K. In addition, the spectra of dilute solutions of CH4 and CD4 in one another have been studied through the same temperature range. The spectra of the dilute solutions consist of single sharp lines for both fundamentals, demonstrating that the barriers to molecular rotation are high in all the phases. The fine structure observed in the low-temperature phases of the pure crystals is inconsistent with any of the structural models heretofore proposed but a model for phase II is suggested which is consistent with the spectra and which shows very efficient packing. Phase III of CD4 is probably complex and of low symmetry. Phase I is probably disordered.