EPR study of methyl radical in van-der-Waals solids

High-resolution EPR spectra of CH3 radicals trapped in solid N2 matrix were obtained in a temperature range 7–28 K. The analysis of the spectrum suggests that, in the whole temperature range, the radical performs both fast rotational motion around the C3-symmetry axis and libration motion about the C2-symmetry axes. A comparison study of the methyl libration motion in gas matrices of linear molecules, N2, CO, N2O and CO2, was carried out using a model of the infinite potential well with flat bottom. As a result, averaged angular deviations of the radical orientation and dimensions of the potential wells were estimated. We assessed contributions of both the classical motion and the quantum-mechanical correction to the radical reorientation. A possibility of utilizing the trapped CH3 radical as a spin probe to study order–disorder structural phase transition at the microscopic level was discussed. An empirical formula obtained earlier for the matrix shift of the isotropic hyperfine coupling (hfc) constant of CH3 in matrices of spherical particles was shown to hold true for matrices of linear molecules as well. The experimental results and their treatment made it possible to report an estimate of the hfc tensor anisotropy for the free methyl radical. This information is of special interest to the theory of atoms and molecules.