Electron Spin Resonance Study of the Pairing Theorem for Alternant Hydrocarbons : 13C Splittings in the Anthracene Positive and Negative Ions

Abstract

The pairing theorem for alternant hydrocarbons predicts that the pi-electron spin densities at corresponding positions of the negative and positive ions of a hydrocarbon should be the same, but significant differences have been found previously for the proton hyperfine splittings in electron-spin resonance spectra. In order to examine the validity of the theorem more thoroughly, we have measured the 13C splittings in the ESR spectra of the anthracene negative and positive ions. These have been obtained by studying the satellites arising from 13C nuclei present in natural abundance and also by investigating anthracene labeled with 50% 13C in the 9-position. All of the four possible splittings were determined for the negative ion and assigned to the appropriate positions in the radical, and three of the splittings were obtained for the positive ion. The assignments of the splittings, and also their signs, were established by linewidth studies, counter-ion effects, and calculation. The negative ions were produced by reduction with lithium, sodium, and potassium, and also electrolytically. An apparatus for the electrolytic generation of radicals in vacuo is described. The positive ions were formed in concentrated sulfuric acid. The 13C splittings in the positive and negative ions of anthracene were found to be almost identical. This result is most readily interpreted as implying that the pairing theorem is valid to a high degree of approximation for the spin densities and for those of the sigma—pi parameters (Q's) which govern the 13C splittings.