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 2 D‐ and 13 C‐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 31 P, 1 H‐, and 13 C‐hyperfine tensors, the second one exhibited coupling with 31 P 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 PC 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.