Theoretical Studies of Solvent Effects on Nuclear Spin—Spin Coupling Constants. II A Cubic Closest Packed Cluster Model

Abstract

A theoretical study of the solvent dependence of nuclear spin—spin coupling constants is presented in terms of a model which assumes a cubic closest packed cluster in which six solvent molecules are arranged around the solute molecule. This model is described theoretically by means of a group function modification of self-consistent perturbation theory. Calculations for individual molecular groups were performed by the semiempirical INDO (intermediate neglect of differential overlap) approximation of self-consistent-field molecular orbital theory. Intermolecular integrals associated with solute and solvent molecules were evaluated theoretically. This model provides some improvement over the previous theoretical study in this series, which was based on the reaction field model. The improvement can be attributed to the inclusion of electrostatic effects arising from the real charge distributions of the solvent molecules, especially those which are perpendicular to the solute dipole moment axis. A most important result is the good agreement found for the solvent dependence of the geminal H–H coupling constant in formaldehyde, as the reaction field results for this case were of the wrong sign. The calculated solvent dependence of the coupling constants of 1, 1-difluoroethylene in solvents of almost identical dielectric constant exhibits a greater range of values than either the experimental values or the reaction field results. On this basis, it is concluded that the cluster model is too specific and that improvement in the theoretical description of this phenomenon will incorporate rotational effects associated with the solvent molecules.