The E.S.R. spectra of triarylmethyl radicals

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Abstract
High resolution E.S.R. spectra of a number of triarylmethyl radicals have been measured and interpreted. The observed meta and para-proton splitting constants conform to spin densities calculated according to McLachlan's formula. Ortho-protons show a systematic deviation which is ascribed to the influence of multiply excited states on the spin density distribution. Substituents have a very small influence on the spin density distribution, and this is explained with perturbation theory. In mono-para-substituted radicals the splitting constants of substituted and unsubstituted rings are identical whereas mono-meta-chlorine substituted triphenylmethyl probably has a slightly smaller angle of twist in the substituted than the unsubstituted ring. This is ascribed to the influence of the substituent on the moment of inertia of the aryl group. Finally the splitting constants of methyl protons are discussed in terms of hyperconjugation. It is shown that the fact that meta-methylproton splitting constants are relatively smaller than those of para-methylprotons must be ascribed to the influence of configuration interaction on the the π-electron spin density distribution.