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

Abstract

The same e.s.r. spectrum is observed when either cis- or trans-stilbene is photolysed in a solution of mercury(II) trifluoroacetate in trifluoroacetic acid. This spectrum is ascribed to the near-planar trans radical cation in which rotation about the Ph–C bonds is fast on the e.s.r. time scale, with a(2H olefinic) and a(2H para) 4.53, a(4H ortho) 2.78,a(4H meta) 0.715 G. In contrast, the trans radical anion shows restriction of rotation about the Ph–C bonds. The spectra of the radical cations derived from trans-4,4′-dimethyl- and trans-4,4′-di-t-butyl-stilbene are compatible with this interpretation. These stilbene radical cations do not cyclise to phenanthrenes: solutions of phenanthrene and dimethylphenanthrene under the same conditions show the different spectra of the corresponding phenanthrene radical cations. A satisfactory simulation of the spectrum of the former compound can be obtained with the hyperfine coupling constants a[H(1,8)]2.94, a[H(2,7)] 1.34, a[H(3,6)] 2.94, a[H(4,5)] 1.34, a[H(9,10)] 4.91 G.