Hindered rotation and site structure of methane trapped in rare gas solids

High resolution infrared absorption spectra have been recorded for the ν4 vibrational mode of CD4 trapped in solid argon and krypton. As with CH4, evidence for two dominant sites was found: one with a cubic close‐packed (CCP) environment and the other with a hexagonal close‐packed (HCP) environment. The fine structure for the CCP site resembles that for a tetrahedral molecule rotating in an octahedral field: singlet peaks for P(1), Q(1), and R(0), and a doublet for R(1). The fine structure for the HCP site is more complex and is not assigned to specific rotational transitions. By high temperature annealing it was possible to eliminate the HCP site completely with increase in concentration of the more stable CCP site. The slow relaxation of nuclear spin found for CH4, from an F state (J=1) to an A state (J=0), was found to be two orders of magnitude faster for CD4.