Inertial rotation and matrix interaction effects on the EPR spectra of methyl radicals isolated in 'inert' cryogenic matrices

The CW-EPR lineshapes of methyl and small methyl-like radicals trapped in noble gas matrices at liquid He temperatures are substantially different from the expected classical EPR spectra. At low temperatures they show small or negligible anisotropy in studies using different experimental techniques and have a temperature dependence that differs from systems whose motional dynamics is diffusion controlled. At liquid He temperatures, before the Boltzmann statistics take over in the classical high temperature realm, the spectral intensities are dominated by quantum statistics. These properties, which were obtained experimentally at temperatures about 5 K and lower, and up to about 20 K, can be attributed to quantum effects of inertial rotary motion and its coupling to the nuclear spin of the radical. Methyl-like radicals have nuclear-exchange symmetry and contain the lightest possible isotopes, protons, and deuterons. In the ideal case of absent radical-matrix interaction, the methyl rotation about the central heavier carbon atom guaranties minimal moments of inertia. However, the theoretical interpretation of the above effects and other related quantum effects, as well as recognition of the important physics which lead to them, is not a simple matter. The literature accumulated on the subject over the years is successful but contains several unresolved questions. Recently obtained spectra of methyl radicals in Kr, N2 and CO matrices, which are less inert than the smaller noble gas Ar, were shown to exhibit greater, but certainly slight, overall anisotropic spectral features while in earlier experimental studies the anisotropy was practically absent. Even gases of smaller radii such as Ne and H2 at liquid He temperatures show interesting differences as hosts of methyl radicals compared to Ar. Investigation of other possible causes of this difference, not excluding the experimentally controlled ones related to the sample preparation and the MW power saturation of the CW-EPR measurement, were conducted in this work. (topical review)