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.

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The electron spin resonance of polycrystalline graphite is shown to be due to charge carriers, as for single crystals. For lampblack-base graphite, the variation in g value with temperature agrees approximately with that for single crystal graphite if it is assumed that the spins effectively average out the anisotropic interactions by rapid intercrystallite diffusion. The magnitude and temperature dependence of the spin susceptibility are found to be in excellent agreement with McClure's theoretical predictions for the charge carriers in perfect graphite. Complications of measuring these materials are discussed, including effects of skin depth and microwave heating and the dependence upon crystallite size and sample purity. It is found that two types of impurities can affect the resonance; those which become ionized and shift the Fermi level, and large un-ionized atoms which do not shift the Fermi level but act as efficient scattering centers for shortening the spin—lattice relaxation time.

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