The electron spin resonance spectrum of the biphenyl radical cation
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.