Electron Spin Resonance of Radical Cations Produced by the Oxidation of Aromatic Hydrocarbons with SbCl5

Abstract

Electron spin resonance (ESR) studies have been made of radical cations prepared by the oxidation of 24 aromatic hydrocarbons with SbCl5 in CH2Cl2 solvent. The transfer of reagents, reactions, and ESR measurements were carried out at temperatures below −78°C in the absence of air. The stability of the radicals in solution, and the resolution of ESR hyperfine structure, depended critically on temperature and on the concentrations of hydrocarbon and SbCl5. The coupling constants for the ESR spectra of the radical cations of perylene, anthracene, 9,10-dimethylanthracene, and naphthacene were essentially identical to those for the respective ions in H2SO4. The ESR spectra for the radical cations of pyrene, naphthalene, dibenzo-(a,c)triphenylene,Δ9,9′-bifluorene, and tetraphenylethylene have also been reduced to coupling constants. The positive radical ion resulting from the oxidation of naphthalene is a symmetrical dimer with the unpaired electron divided equally between the two molecules. The generality of McConnell's relationship aH=Qρ was examined by comparing experimental coupling constants with Hückel spin densities for available radical anion and cation data. A least-squares fitting procedure yielded Q=28.6 for anions and Q=35.7 for cations. Quantitative comparisons were made between experimental coupling constants and those computed from the Colpa—Bolton and Giacometti—Nordio—Pavan theories. Both theories were found to account appropriately for the differences in magnitude between the coupling constants for aromatic negative and positive ions.