The Whiffen effect of symmetry-enhanced and symmetry-forbidden hyperconjugation in spin-paired molecules

Abstract

In the EPR spectroscopy of organic radicals, the Whiffen effect describes the way in which the hyperconjugative β-proton coupling of an H–C σ-bond in a cyclic radical can be enhanced or forbidden according to the symmetry of the π-orbital with which it hyperconjugates. This principle of the symmetry-control of hyperconjugation has been largely ignored in the context of spin-paired molecules. It is examined here as it applies to H–C and M–C (M = metal) hyperconjugation in spin-paired carbocations and neutral molecules, where the effect shows up principally in relative isomer stabilities, in bond lengths and angles, and in one-bond NMR coupling constants.

Systems in which the effect has important consequences include the Wheland intermediates of electrophilic aromatic substitution, the loose complexes formed between metal cations and arenes, and η¹-cyclopentadienylmetal compounds.