A single-crystal ESR and quantum chemical study of electron-capture trialkylphosphine sulfide and selenide radical anions with a three-electron bond

Abstract

A low-temperature ESR study of electron-capture phosphoranyl radicals in X-irradiated single crystals of tri- alkylphosphine sulfides and selenides (R3PX: X = S, Se; R = CH3, C2HS, C&l,) is presented. The principal values and direction cosines of the g tensors and ,'P and Se hyperfine coupling tensors are determined and correlated with the X-ray structure analysis of the parent compounds. All studied compounds reveal the formation of a R3PX- radical anion, with a three-electron PLX bond in which the unpaired electron is nearly equally distributed over phosphorus and the substituent X. It is shown that the site symmetry of the radicals corresponds to the crystallographic point symmetry of the precursor molecules, resulting in pure trigonal (C3) radicals for (C2HS),PX- and C, symmetry for (CH,),PX- and (C6H11)3PX-. Ab initio quantum chemical calculations (4-31G* SCF and CASSCF) on the model H$S- predict that the three-electron bond is unstable and dissociates into PH, and SO-. The calculations do not reproduce the experimental couplings and the observed spin density distribution, but tend to localize the unpaired electron entirely on sulfur. The origin of this deficiency appears to be predominantly due to a poor description of the negative charge of the nonbonding electron pairs of the sulfur atom. Via the introduction of a positive charge in the vicinity of the sulfur atom, the theoretical spin density distribution can be brought in accordance with the experimental data. The use of quantum chemical calculations for the simulation of trapped radicals in the solid state is discussed.