Proton Spin Coupling by Pi Electrons

Abstract

A theoretical formulation is developed for the pi-electron contribution, AHH′(π), to the spin coupling between pairs of protons in hydrocarbon molecules. By means of the correspondence between the σ—π interaction in unsaturated molecules and related free radicals, the proton spin coupling is expressed in terms of hyperfine constants and triplet state energies. With known values for these quantities, AHH′(π) is found to be in agreement with experimental measurements available for molecules in which the pi electrons are expected to dominate the coupling. Of particular interest are the large couplings (1.3–8 cps) calculated for certain systems with protons separated by three or four carbon atoms. Also absolute signs are predicted by the theory, with AHH′(π) equal to —6.7 cps for allene and +7.8 cps for butatriene.