Abstract
The radical , labelled with 13C in the position shown, has been prepared by X-irradiation of single crystals of . The 13C hyperfine coupling tensor of this radical confirms previous conclusions from a study of the corresponding unlabelled species that the electron is essentially localized on the carbon atom. At room temperature the methylene group undergoes rapid reorientation about the P-C bond while comparison of the 13C tensors at 300 K and 77 K suggests that the barrier to rotation has a strong two-fold component but contains intermediate minima.