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.

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