Unstable intermediates. Part 110.—Hyperconjugation by groups containing heavy atoms: electron spin resonance studies of carbon radicals having β-SnR3, –PR2, –PR+3, –AsR2, –AsR+3 and –AsO3H groups1

Hyperfine coupling to β-atoms such as Sn, P or As in carbon π-radicals is found to be remarkably large, corresponding to as much as 6 % delocalization into the valence s orbital on the heavy atom. The radicals all have a preferred conformation in which the metal atom is constrained above the radical plane, so that overlap between the carbon 2p orbital nominally containing the unpaired electron, and the carbon-metal σ-bond orbital is a maximum. This magnetic interaction is shown to be about four times larger than that for similarly placed –CR3 or –NR+3 groups.These observations are linked with those of others on rate enhancements by CH2X groups in electrophilic aromatic substitution. They are also significant with respect to the stereochemical course of some olefin additions.