High-resolution EPR spectra of CH 3 , 13 CH 3 , and even CH 2 D 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 % CH 4 . The high-resolution EPR spectra of CD 3 and CHD 2 radicals were also obtained under similar conditions using CD 4 and CH 2 D 2 instead of CH 4 , 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 R-proton-and the R-deuteron-rotor spectra. In contrast to the -proton methyls > • C-CH 3 , the hf coupling is anisotropic and no rotation-hindering barrier is present here. The "E" lines of the corresponding isotropic -proton methyl rotor 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.

Single crystals of 2-aminoethyl hydrogen sulfate, H3N+CH2CH20SOs-, were X-irradiated at 295 K and studied using EPR, ENDOR, and FSE techniques. Among several radicals trapped at this temperature, two carboncentered radicals were identified. These are identical in structure but are formed in each of the two molecules constituting the asymmetric unit of the crystal, their common structure being H3N+CH2CHOSO3-. The aand j3-proton hyperfine coupling tensors of these two radicals show that one of them has a planar center, while the other exhibits a nonplanar site for the lone electron density. This is probably due to differences in the electrostatic interactions between each of the two radical centers and the lone pairs centered on their neighboring oxygen atoms. A revised model for obtaining the lone electron orbital spin density from the a-tensor in a nonplanar radical by considering a three-center approximation of the lone electron orbital is proposed. UHF-INDO calculations are carried out for a number of hypothetical radical configurations. The a-proton tensor is calculated for these geometries, and the three-center model is discussed in light of the results from the MO calculations.

The temperature-dependent EPR line shapes from the methyl rotor of the X irradiation-induced CH 3 C ˙(COOH) radical in powder MMA (methyl malonic acid) and the X irradiation-induced radical CD 3 C ˙(COOH) 2 in methylspecifically deuterated powder MMA are studied experimentally for the temperature range 4.8 K (5 K) to K (65 K). The hydrogenated system is simulated using a quantum inertial dynamical model with a hindering potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling frequency is negligible for the deuterated rotor, resulting in a stopped rotor low-temperature spectrum, while being sufficiently large for the hydrogenated system for this to exhibit tunneling. From the low-temperature deuterated analogue spectrum, the potential twist angle is estimated to δ ) (50°( 2°(+n‚60°, n ∈ Z). The siteexchange activation energy of the deuterium rotor is observed to be 387 K, substantially lower than the hindering potential depth of 618 K. The hydrogenated system exchange rotation rate assumes a linear behavior with 754 K activation energy in the classical region of temperatures above 50 K.