High-resolution EPR and quantum effects on CH3, CH2D, CHD2, and CD3 radicals under argon matrix isolation conditions

High-resolution EPR spectra of CH3, (CH3)-C-13, and even CH2D radicals with their natural abundances have been observed in Ar matrix in the temperature range 4.2-40 K. This was achieved by X-ray radiolysis of Ar matrix containing 0.2 mol % CH4. The high-resolution EPR spectra of CD3 and CHD2 radicals were also obtained under similar conditions using CD4 and CH2D2 instead of CH4, respectively. At the lower temperatures, the EPR line shapes of these radicals are dominated by hyperfine (hf) patterns with anomalous intensity, attributed to quantum effects. The application of the Pauli principle in combination with D-3 point-group symmetry results in interesting exclusion of EPR transitions for both the alpha-proton- and the alpha-deuteron-rotor spectra. In contrast to the beta-proton methyls >C-.-CH3, the hf coupling is anisotropic and no rotation-hindering barrier is present here. The "E" lines of the corresponding isotropic beta-proton methyl rotor [Sornes, A. R.; Benetis, N. P. Chem. Phys. 1998, 226, 151](1) are absent from their regular positions. The deuteron rotor is giving a peculiar spectrum at the lowest experimental temperature, i.e., an extremely strong central singlet superimposed on a much weaker fast motional spectrum. The quantum effects are attributed to spin-rotation coupling through the anisotropic part of the hf interaction and exchange symmetry of at least two identical fermions or bosons of the studied radicals. The experimental findings are consistent with a three-dimensional, free quantum-rotor motional model.