ELECTRON SPIN RESONANCE STUDIES OF STERIC EFFECTS IN TETRAKIS(ALKYLTHIO)ETHYLENE CATION RADICALS

The temperature dependence of the esr spectra of cation radicals of the type (RS)2C+=C(SR)2,I+, has been investigated (R = methyl, ethyl, isopropyl, and phenyl). The radicals were produced by oxidation of the parent ethylenes with aluminum chloride in methylene chloride. The esr room-temperature spectrum of the methyl derivative, Ia+, was attributed to interaction with 12 equivalent protons: |aH| = 2.70 G. The spectrum observed at –90° was analyzed in terms of interaction with two sets of six equivalent protons: |aH| = 0.93G, |aH| = 4.22G. At intermediate temperatures, the spectrum exhibited an alternating line-width effect. These results are in accord with a temperature-dependent intramolecular process which can be formally described as an interconversion of two identical conformers of Ia+ (see eq A). The line-width variation was analyzed using the theory of Freed and Fraenkel to obtain k, the rate constant for the interconversion process, Ea = (8.6 ± 0.6) kcal mole−1. A similar temperature dependence of the methylene proton coupling constant in the ethyl derivative, Ib+, was observed and similarly interpreted. In addition, restricted rotation about two of the ethyl carbon-sulfur bonds is postulated to account for the low-temperature spectrum of Ib+. It is suggested that the alkyl proton coupling constants in this type radical can be related to the sulfur spin density, ρs, by the following expression: aH = QH〈cos2 θ〉. The 13C coupling constant was observed in the esr spectrum of Ia+ enriched in 13C at the ethylenic position: at +30°, ac = +3.72 G; at -90°, ac = +3.87 G. © 1969, American Chemical Society. All rights reserved.