Structure of selenium-centered radicals. An electron spin resonance study of R2SeHal trapped in single crystals

X-irradiated single crystals of Ph3SeBr, Ph2SeBr2, and Ph2SeCl2 have been studied by ESR spectroscopy. In each crystal a radical species exhibiting anisotropic hyperfine interaction with the halogen nuclei has been trapped. The anti g tensors, the magnetic hyperfine tensors to the halogen nuclei, and the 81Br quadrupolar interaction tensors have been determined. The 77Se magnetic hyperfine tensor has also been obtained for the radical species trapped in Ph2SeBr2 and Ph2SeCl2. These tensors lead to the identification of Ph2SeHal. The spin distribution is discussed; the unpaired electron lies probably in a Se-Hal sigma* orbital which is oriented perpendicularly to the CSeC plane. The spin distribution seems to be very sensitive to the nature of the host matrix. 2 figures, 2 tables