Characterization of transient radicals in the reduction product of the –PCCP– system: EPR and theoretical studies

The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–PCCP–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent 31P nuclei. Additional couplings with two equivalent 13C are observed with Ar–P13C13CP–Ar. The 31P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initio SCI) were performed on the various isomers of the two radical anions: [H–PCCP–H]˙− and [H–PCH–CHP–H]˙−. Although the optimized geometries of these two species are clearly different, neither of them leads to 13C/31P hyperfine tensors in conflict with the experimental results. The absence of any 1H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–PCCP–Ar]˙− more probable.