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

Journal Article · Mon Sep 15 04:00:00 EDT 1986 · J. Chem. Phys.; (United States)

DOI:https://doi.org/10.1063/1.450988· OSTI ID:5109564

Jones, L H; Ekberg, S A; Swanson, B I

High resolution infrared absorption spectra have been recorded for the ..nu../sub 4/ vibrational mode of CH/sub 4/ trapped in rare gas solids. In argon, krypton, and xenon hosts two dominant sites are observed which we call site R-italic and site A-italic,B-italic. For both sites we believe that the CH/sub 4/ molecule replaces one host atom. The fine structure of site R-italic at a temperature of 5 K is that predicted for a tetrahedral molecule rotating in an octahedral field; singlet peaks for P-italic(1), Q-italic(1), R-italic(0), and a doublet for the R-italic(1) transitions. The barrier for this rotation is not clearly defined. Site A-italic,B-italic shows site symmetry splitting and orientational ordering suggestive of C-italic/sub 3//sub v-italic/ symmetry. Rotational structure is also associated with site A-italic,B-italic; it is not clear whether the hindered rotation for this site takes place about only the C-italic/sub 3/ axis or about axes perpendicular to C-italic/sub 3/ as well. From annealing studies and the effects of deposition temperature we conclude that site R-italic is for CH/sub 4/ molecules trapped in the normal cubic-close-packed environment while site A-italic,B-italic is for CH/sub 4/ trapped in hexagonal-close-packed pockets induced by stacking faults formed during deposition. Temporal changes of peak intensities while the matrices stand at 5 K are indicative of a slow relaxation of nuclear spin from a triplet state (J-italic = 1) to a quintet state (J-italic = 0). For site A-italic,B-italic the triplet to quintet spin relaxation is considerably faster than for site R-italic; for example, for a matrix of CH/sub 4/ in krypton (1/10 000) at 5 K the spin relaxation half-lives are 87 minutes and 26 minutes for sites R-italic and A-italic,B-italic, respectively.

• Research Organization: Los Alamos National Laboratory, Los Alamos, New Mexico 87545
• OSTI ID: 5109564
• Journal Information: J. Chem. Phys.; (United States), Journal Name: J. Chem. Phys.; (United States) Vol. 85:6; ISSN JCPSA
• Country of Publication: United States
• Language: English

Related Subjects

640302* -- Atomic
Molecular & Chemical Physics-- Atomic & Molecular Properties & Theory
74 ATOMIC AND MOLECULAR PHYSICS
ALKANES
ANNEALING
ARGON
CONFORMATIONAL CHANGES
ELEMENTS
FLUIDS
GASES
HEAT TREATMENTS
HYDROCARBONS
INFRARED SPECTRA
KRYPTON
MATRIX ISOLATION
METHANE
MOTION
NONMETALS
ORGANIC COMPOUNDS
RARE GASES
RESOLUTION
ROTATION
SPECTRA
TRAPPING
ULTRALOW TEMPERATURE
XENON