Electron addition to trimetyl phosphite induced by ionising radiation: an electron spin resonance study

Abstract

A range of solutions containing trimethyl phosphite have been exposed to 60 CO γ-rays at 77K and studied by electron spin resonance (ESR) spectroscopy in the hope of obtaining unambiguous evidence for the formation of [·P(OMe) 3 ] − radical anions, and of establishing their preferred structure. It is suggested that one of two phosphorus centered π-type radicals detected in this study is the parent radical anion, having a planar T structure, the other being ·P(OMe) 2 formed by dissociative electron capture. Using an aprotic glassy medium (methyltetrahydrofuran), the only significant electron adduct had A ‖ = 267 G with a small A ⊥ value. This is assigned to the primary radical anion having a planar T structure, with SOMO that is large 3p on phosphorus, normal to the radical plane. For methanolic solutions the same radical anion was formed, but extensive protonation also occurred giving a phosphoranyl-type radical with large 1 H and 31 P hyperfine splittings. Another phosphorus centre with an even larger hyperfine splitting is thought to be ·P(OMe) 4 , formed by addition of methanol to the radical cation or by attack of MeO· radicals on P(OMe) 3 . In concentrated methanol solutions, the cation dimer, (MeO) 3 P∸P(OMe) + 3 was clearly detected by ESR spectroscopy. In CD 3 OD glasses, the 2 H adduct gave well defined triplets, thus confirming the above assignment. Methyl radical signals were also clearly defined. A second π-type phosphorus centred radical is thought to be ·P(OMe) 2 .