The electron spin resonance spectrum of the biphenyl radical cation

Abstract

The e.s.r. spectrum of the biphenyl radical cation, Ph₂^+· has been observed from the photolysis of benzene or biphenyl and mercury(II) trifluoroacetate in trifluoroacetic acid. The hyperfine coupling constants of Ph₂^+· and Ph₂^–· are compared with the predictions of various modifications of the Hückel-McConnell approximation, and it is concluded that this provides no evidence that these species are anything but planar. UHF/MINDO/3 calculations have been carried out on Ph₂, Ph₂^+·, and Ph₂^–·, and lead to dihedral angles of 6.5 and 6.8° for the radical cation and anion, respectively, with negligible energy differences between the planar and fully optimised structures.