RADICAL REACTIONS IN A SINGLE-CRYSTAL OF PHOSPHAALKENE - EPR AND AB-INITIO CALCULATIONS OF PHOSPHONIUMYL RADICAL CATIONS

Two radiogenic radicals trapped in a single crystal of [2,4,6-tri-tert -butylphenyl]-2-phenylphosphaethene have been studied by EPR and have been identified, from their P-31 hyperfine tensors, as being phosphoni umyl radical cations. The spectra modifications caused by C-13 or D-2 enrichment of the phosphaalkene moiety show that these species result from an intramolecular cyclization which can lead to two possible conf ormations of the radical. The experimental P-31, C-13 and H-1 hyperfin e tensors are compared with those predicted by ab initio calculations on model phosphoniumyl radical cations. These calculations show that t hese interactions are very sensitive to the geometry of the radical an d that their measurement can yield precise structural information.