How large a 3 JHCCH coupling can be? Prediction of giant vicinal couplings in quinonoid systems.

Abstract

Second-order polarization propagator approximation (SOPPA) and density functional theory (DFT) B3LYP computations revealed the presence of giant 3 JHCCH couplings, up to more than 60 Hz, in conjugated quinonoid systems, very far above the typical limit of ~15 Hz. Strong hyperconjugative interactions of the C-H σ orbitals with the quinonoid 8-e- π system, which allows effective propagation of the spin polarization, seem to be responsible for those extraordinarily large couplings. Computation on several model systems showed the additive nature of the contributions from the available coupling paths to the total coupling.