Solvent Effects on Geminal H–H Couplings: A New Method for Determining Signs of Coupling Constants

Solvent-dependent variations of geminal coupling constants have been observed in styrene oxide, styrene sulfide, 1,4-diphenylazetidinone, and 4-methyl−1,3-dioxolane. These data, in conjunction with previously reported observations, suggest that Jgem always becomes more negative in the absolute sense in solvents having higher dielectric constants. The reaction field of the solvent seems to be the primary causative factor. The magnitude of the effect is shown to depend on the orientation of the dipole moment of the solute molecule with respect to the H–C–H plane. Qualitative theoretical considerations support the existence of a direct correlation between the observed changes and the absolute sign of Jgem and suggest that the magnitude of the change in Jgem also depends on the relative magnitude of the hyperconjugative contribution to Jgem.