Electron spin resonance studies of hyperconjugation in aromatic ions

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Abstract
We have found strong evidence that the main interaction between methyl groups and the ring system in the positive ions of aromatic hydrocarbons involves hyperconjugation rather than an inductive effect. The methyl proton hyperfine splittings have been compared in the e.s.r. spectra of both positive and negative ions of 9-methylanthracene and 9,10-dimethylanthracene. The splittings are about twice as large in the positive ions, which is expected if hyperconjugation takes place, but cannot be explained by an inductive model. The ring proton splittings have also been measured, and show small variations which can be explained satisfactorily by either theory.