Electron Spin Resonance Spectra of Monomeric and Dimeric Cation Radicals

Abstract

Dimer cation radicals of anthracene, naphthalene, 2,3,6,7-tetramethylnaphthalene, and pyrene have been obtained using the technique of Lewis and Singer in which oxidation is carried out with antimony pentachloride in methylene chloride at low temperature. The monomer cation radicals of anthracene, 1,4,5,8-tetramethylnaphthalene, and pyrene were also produced using this technique. In the spectra of the dimers, lines were observed corresponding to the same number of groups of equivalent protons as in the monomer, but each group contained twice the number of protons. The larger of the ring-proton splittings in the dimers were slightly less than one-half the corresponding monomer cation splittings, and were larger than one-half the corresponding monomer anion splittings, but this was not always true of the smallest splittings for a particular compound. The values of the dimer splittings are for the most part in good agreement with the charge-effect treatment of Colpa and Bolton. The methyl-proton splittings in the monomer and dimer cations have been analyzed, and an earlier treatment by Bolton, Carrington, and McLachlan has been corrected. Levy's result for the inclusion of contributions from unpaired electrons on the ring and methyl-group carbon atoms proves useful for the positive ions but is unimportant for the anions. Detailed analysis of the methyl-proton splittings indicates, however, that sufficient discrepancies remain to cause uncertainties about this entire treatment. The structure of the dimers cannot be rigorously determined, but a sandwich in which the two halves are parallel, with one above the other, seems reasonable. The dimer cations appear to be formed more readily than the monomer cations from hydrocarbons that are difficult to oxidize, while the monomer is more easily formed than the dimer for the compounds with smaller oxidation potentials. Steric hindrance may account for the inability to produce the dimer of 1,4,5,8-tetramethylnaphthalene. The theoretical treatment of the stability of the dimers is not yet in a satisfactory state, and it is not fully understood why anion dimers have not been generated. The g values of the dimers may prove useful for obtaining structural information, but again the theory is not sufficiently complete.