Effect of a Polar Solvent on the 1JCH Spin-coupling Constants in Chlorine-substituted Ethanes

The directly-bonded 13C–H spin-coupling constants (1JCH) for two chlorine-substituted ethanes, CHCl2CHCl2 and CHCl2CH2Cl, have been measured in various solvents with a wide range of dielectric constants and/or as a function of the temperature. Also, these two effects were estimated by the finite perturbation theory based on the CNDO/2 approximation with the solvaton theory. Both experimentally and theoretically, the dielectric constant of a solvent affects the 1JCH more than the temperature. It is shown that the solvent effect on the 1JCH spin couplings in CHCl2CHCl2 and CHCl2CH2Cl arises mainly from electronic changes in the solute molecules caused by intermolecular solute-solvent interaction, more than from the change in the population of the rotational isomers.