The electron spin resonance spectrum of the biphenyl radical cation

ESR observation of redox processes on benzene-loaded NaPdY, NaPtY and NaCeY zeolites

“…This is not untypical for radical cations of benzene dimers which have been observed by ESR and ENDOR spectroscopy on different catalytic materials such as silica gel 12,13 and HY molecular sieve 13 after -and/or x-ray irradiation, with slightly smaller hyperfine splitting constants of 2.1, 12 2.38 13 and 2.29 G. 13 Corio and Shih also observed the dimer radical cation on oxygen-pretreated H Zeolon. It should be noted that the spectra are also compatible with an assignment to the biphenyl radical cation (Ph 2 C ) which was observed in trifluoroacetic acid, with coupling constants of 3.15 G for the ortho, 0.51 G for the meta and 6.3 G for the para protons, 15 and on H-ZSM-5 with similar values, although with a considerable anisotropy and unresolved meta couplings. 16 The intensity ratios of the inner lines of Ph 2 C would be almost indistinguishable from those of the cation of the benzene dimer.…”

“…16 The intensity ratios of the inner lines of Ph 2 C would be almost indistinguishable from those of the cation of the benzene dimer. Although Ph 2 C had been generated also from benzene by photolysis in trifluoroacetic acid in presence of mercury(II) acetate, 15 it is difficult to imagine that it is formed under the conditions of the present experiments.…”

Electron spin resonance spectra of radical cations derived from stilbenes and phenanthrenes

“…Phenanthrenes.-The benzene radical cation reacts with benzene to give the biphenyl radical cation. 20 Lewis found that no significant amount of phenanthrene was formed when stilbene was irradiated in the presence of dicyanoanthracene in the absence or presence of oxygen,I6 but Bendig et al showed that irradiation in the presence of electron acceptors such as tetracyanoethylene, and of oxygen, did lead to the formation of phenanthrene. 26 To exclude the possibility that some of the spectra which we have ascribed to stilbenes were in fact derived from phenanthrenes, we have therefore briefly investigated the spectra of the phenanthrene (V) and 3,6-dimethylphenanthrene (VI) radical cations.…”

Unpaired Electrons as Probes of Catalytic Systems

“…Hence we should consider the McConnell Q-value this implies 124 , which amounts to 3 × 12 = 36 G, or 2.3 × 12 = 27.6 G. Now the latter value is quite acceptable for α-protons in a π-electron radical, whereas the former is much too high, and hence does not support that the species could be a simple benzene dimer radical cation. The biphenyl radical cation has been measured in a liquid acidic solution 125 with couplings to the two para protons of 6.3 G and to the four equivalent ortho protons of 3.15 G. [It is assumed that the meta proton coupling of 0.51 G, resolved in the solution spectrum would not be resolved in the solid state and is accommodated within the linewidths]. Interestingly, there is little difference between this simulation and that assuming 12 equivalent protons, for the benzene dimer radical cation, although we have excluded this species on the basis of the unrealistically large Q-value.…”