Electron Spin Resonance Studies of Substituent Effects. 5. Combined Effects of Orbital Symmetry and Substituents on the Methylene Hyperfine Splitting in 1,2-Indansemidiones

摘要

Orbital symmetry can have a profound effect on methylene hyperfine splitting in free radicals where the methylene bridges two pi atoms. The anomalously large methylene hyperfine splitting in cyclohexadienyl radical (1) was explained by Whiffen as resulting from the like sign of the molecular orbital coefficients in the semibc-cupied molecular orbital (SOMO) in the parent pentadienyl radical. Whiffen showed that the dependence of the methylene splitting for a methylene group bridging two pi atoms is of the form (c_1+ c_2)~2, where c_1 and c_2 are Huckel coefficients in the SOMO, rather than the naively expected c_1~2 + c_2~2. The first expression gives far greater splitting constants than the second expression, provided the coefficients are of like sign. Whiffen also predicted an anomalously low methylene hyperfine splitting for cyclobutenyl radical (2) because the pertinent molecular orbital coefficients in the SOMO in the parent allyl radical are of unlike sign. When the ESR spectrum of cyclobutenyl radical was obtained later, Whiffen's prediction was verified.