β-proton hyperfine splittings in the vinyl radical

Abstract

The valence-bond method is used to evaluate the β-proton hyperfine splittings in vinyl-type radicals (HC=CH2) as a function of the H-C=C bond angle. Since only spin polarization is included, use is made of semiempirical exchange integrals to compensate for the neglect of charge transfer and the presence of other approximations in the present treatment. It is demonstrated theoretically that both β-proton splitting constants should be positive and that the coupling with the proton trans to the sigma orbital occupied by the unpaired electron should be larger than the cis coupling. If the measured values of 68 and 34 Oe found in the vinyl radical are assigned accordingly (no experimental sign determination or assignment of cis versus trans has been made), good agreement with the calculated results is obtained for a bond angle in the reasonable range of 130°-150°.