Unstable intermediates. Part 117.—Electron spin resonance studies of dimeric (σ*) radicals of type R₃X—XR₃with a three electron X—X bond, when X is phosphorus or arsenic

Abstract

Seven hyperfine components separated by about 450 G were observed in the e.s.r. spectrum of γ-irradiated triethylarsine at 77 K. This is indicative of two equivalent interacting arsenic atoms, and an analysis of the hyperfine tensor components suggests that the species is the dimer cation (R3As—AsR3)+ with a σ2, σ*1 structure as in VK centres. Similar species are formed in γ-irradiated trialkyl phosphines, but are not formed from phosphonium or arsonium salts, nor from the phosphines or arsines in methanol glasses. Irradiation of Me2PS—PSMe2 gave a similar di-phosphorus radical.The magnitudes of the derived orbital populations for phosphorus or arsenic suggest that, although some flattening of the local pyramidal character of the R3M groups occurs, this is far less than that observed for the monomeric R3M+ radicals.