Rotation of Methyl Radicals in Molecular Solids

Electron spin resonance (ESR) measurements were carried out to study the rotation of methyl radicals (CH3) in solid carbon monoxide, carbon dioxide, and nitrogen matrices. The radicals were produced by dissociating methane by plasma bursts generated by a focused 193 nm ArF excimer laser radiation during the gas condensation on the substrate. The ESR spectra exhibit anisotropic features that persist over the temperature range examined, and in most cases this indicates a restriction of rotation about the C-2 symmetry axis. A nonrotating CH3 was also observed in a CO2 matrix. The intensity ratio between the symmetric (A) and antisymmetric (E) nuclear spin states was recorded as a function of temperature for each molecular matrix. The rotational energy levels are modified from their gas phase structure with increasing crystal field strength. An anomalous situation was observed where the A/E ratio extended below the high temperature limit of 1/2.