Intermolecular interactions in nuclear magnetic resonance. X. A site-specific continuum model for the gas-to-liquid shifts of nonpolar solutes. Applications to proton medium shifts and the determination of cavity radii

Abstract

Based on the Onsager reaction field theory of the dielectric polarization of a continuum, a model has been developed to calculate the Van der Waals contribution σw to the gas-to-liquid medium shifts of nonpolar solutes. The addition of a site factor correction, to account for the off-center location of the measured nucleus in the solute molecule, provides a marked improvement. The agreement between experimental and calculated σw's is good (±0.05 ppm), but a separate scale factor is required for each solute. This nonuniversality is shown to be related to the Onsager approximation for the solute cavity radius; a3 = 3V1/4πNA. It is shown that Van der Waals shifts can be used for the determination of the Onsager cavity radii; for a number of molecules these cavity radii are given. It is claimed that this method is as precise, but much easier to apply, than the Böttcher–Onsager optical method.No evidence could be found for the existence of a [Formula: see text] contribution to the medium shifts due to permanent electric dipoles of solvent molecules. Combination of experimental medium shifts in anisotropic solvents with calculated σw contributions allows the evaluation of the neighbor anisotropy contribution σa. The σa effect is found to be solute dependent; there is a V1−1/6 proportionality related to the molar volume V1 of the solute and in addition there are site effects with site factors both greater and less than unity, depending on molecular shape. The solvent dependence of σa includes not only the magnetic susceptibility anisotropy Δχ2 and the molar volume (V2−1/3) but also a term corresponding to the geometric anisotropy of the solvent molecule.The solvent CS2 is found to behave as an isotropic molecule.