Methyl group tunnelling rotation frequency in γ-irradiated methyl malonic acid crystals

At low temperatures in molecular crystals, hindered methyl group torsional oscillator-nuclear spin states exhibit an energy splitting which is simply related to the rate of tunnelling rotation of the group through +or-120 degrees about the threefold axis. In free radicals in radiation damaged crystals where there exists a large hyperfine interaction between the methyl group protons and the unpaired electron spin, the tunnelling splitting may be measured from the electron nuclear double resonance (ENDOR) transition frequencies. A study at 4.2K of the free radical CH 3 C(COOH) 2 formed by heat treatment of irradiated methyl malonic acid crystals yields a value of the splitting equivalent in frequency units to 160+or-30 MHz. It is found that those ENDOR transitions whose frequencies depend most upon the tunnelling splitting are anomalously broad. This implies that the splitting has a wide though strongly peaked distribution. Some methyl groups are found to be in states in which the tunnelling splitting is inverted, providing experimental support for theoretical models which have invoked low lying states of this kind to account for the averaging out of the tunnelling splitting at those higher temperatures where classical models for methyl group rotation apply.