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 temperatur e range 4.2-40 K. This was achieved by X-ray radiolysis of Ar matrix contai ning 0.2 mol % CH4. The high-resolution EPR spectra of CD3 and CHD2 radical s were also obtained under similar conditions using CD4 and CH2D2 instead o f CH4, respectively. At the lower temperatures, the EPR line shapes of thes e radicals are dominated by hyperfine (hf) patterns with anomalous intensit y, 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" l ines of the corresponding isotropic beta-proton methyl rotor [Sornes, A. R. ; Benetis, N. P. Chem. Phys. 1998, 226, 151](1) are absent from their regul ar positions. The deuteron rotor is giving a peculiar spectrum at the lowes t experimental temperature, i.e., an extremely strong central singlet super imposed on a much weaker fast motional spectrum. The quantum effects are at tributed to spin-rotation coupling through the anisotropic part of the hf i nteraction and exchange symmetry of at least two identical fermions or boso ns of the studied radicals. The experimental findings are consistent with a three-dimensional, free quantum-rotor motional model.