Radical Reactions in an X-Irradiated Phosphaalkene: A single-crystal ESR study

Single crystals of 1-[2,4,6-tri(tert-butyl)phenyl]-2-phenylphosphaethene ( PPPE ) and of ²D- and ¹³C-enriched PPPE were studied by ESR after X-ray irradiation. Two phosphorus-centered radicals were trapped in the crystals. The first one was characterized by its ³¹P, ¹H-, and ¹³C-hyperfine tensors, the second one exhibited coupling with ³¹P only. Comparison of these parameters with those predicted by ab initio calculations on some phosphinyl species indicates that these two radicals probably result, on the one hand, from an addition of a H-atom to the C-atom of the P?C bond and, on the other, from a cyclization of the parent molecule. The proposed mechanisms are consistent with the mutual orientations of the hyperfine eigenvectors and bond directions in the undamaged molecule. A C-centered radical which results from an addition of a H-atom to the P-atom of the phosphaethylene bond is also detected.