Methyl group tunnelling frequency measurement by electron nuclear double resonance in a powder

The frequency of tunnelling rotation of methyl groups in free radicals in gamma -irradiated single crystals has been previously obtained from small second-order shifts of the frequencies of electron nuclear double resonance lines. This technique is here extended to powders. Though the powder average gives rise to a line whose overall width greatly exceeds the shift to be measured, the lineshape exhibits a well defined edge on account of the axial symmetry of the hyperfine coupling tensor and this permits the shift to be measured with only marginally less accuracy that in a single crystal. Measurements are described on irradiated powdered samples of magnesium lactate for which the tunnelling frequency (the energy splitting of the ground molecular state divided by H) is found to be 1900+or-300 MHz.