Electron Loss from 1,3,3-Trimethyl-2-azabicyclo[2.2.2]octane Derivatives. Is σ Coupling Thermodynamically Important?

The ease of one-electron oxidation was compared for σ coupled and non-σ-coupled γ-oxopiperidine nitroxides and their saturated analogues (CO→ CH2). For the non-σ-coupled pair, 1,3,3-trimethyl-2-oxyl-2-azabicyclo-[2.2.2]octan-5-one (9) has IPV = 7.29 eV, E°′ = 0.73 V and its CO → CH2 analogue 11 IPV = 6.88, E°′ = 0.52 V. The σ-coupled pair 9-oxyl-9-azabicyclo[3.3.1]nonan-3-one (12) has IPV = 7.78 eV, E°′ = 0.83 V and its CO → CH2 analogue 13 IPV = 7.39 eV, E°′ = 0.58 V. Replacement of CO by CH2 in the non-σ-coupled example has as large an effect of IPV and only a 13% smaller effect on E°′ than in the σ-coupled example, making it appear that a σ-coupled transmission of positive charge from the nitrogen to the carbonyl in the radical ccation is unimportant.