The ESR spectrum and structure of the dimer radical cation of dimethyl selenide (Me2Se-SeMe+2) in a γ-irradiated single crystal

The dimer radical cation of dimethyl selenide has been identified in a γ-irradiated single crystal of the parent compound, its main (^evenSe) ESR spectrum showing hyperfine coupling to twelve equivalent hydrogens. Also, the relative intensity of the naturally abundant⁷⁷Se satellite spectrum is in agreement with a hyperfine interaction involving two anisotropically equivalent selenium atoms. The principal values and direction cosines of the⁷⁷Se hyperfine andg tensors have been determined and are shown to be satisfactorily interpreted in terms of a centrosymmetric structure, the unpaired electron occupying an antibonding (σ^*) orbital built almost entirely from the p_σ orbitals of the two selenium atoms. Analysis of theg tensor components indicates that the CSeC planes are perpendicular to the direction of the Se-Se bond. It is also shown that the isotropic¹H splitting andg factor obtained previously from solution ESR studies of the dimethyl sulphide dimer cation are consistent with the structure advanced here for the selenium dimer. The results seem to rule out the possibility of any significant d orbital participation in the bonding.