Reaction fields, van der Waals forces and related nuclear magnetic resonance chemical shifts

Abstract

Following suggestions in the literature, intermolecular dispersion forces are characterized by two mechanisms. The first is due to the reaction field of the solvent and is characterized by a modified Onsager approach that accounts for both long- and short-range interactions. The second is due to a localized non-continuum ‘buffeting’ action of peripheral solvent atoms on peripheral solute atoms and is sterically controlled by the latter. It is shown that there is no necessity to introduce a ‘site factor’ into the reaction-field part of the treatment and that this has only been necessary previously because of the neglect of the buffeting interaction. The latter produces a nuclear screening σ_BI=–BK(2β_T–ξ_T)²r^–6 which for a particular solute is governed by the solvent parameter, K, and β_T and ξ_T, which are measures of the steric accessibility of a solute resonant nucleus to encounters by the solvent. The equation for σ_BI is tested exhaustively, and in so doing values of K are deduced that are in relative agreement with theoretical estimates; additionally, acceptable values of the screening coefficient B for proton and fluorine shifts and of solvent anisotropy screenings, σ_a, are deduced. Moreover, it is shown that β_T and ξ_T can be calculated accurately on a hard-atom contact basis which justifies the implicit neglect of repulsion forces. The reaction field and buffeting theories are used to calculate van der Waals a values that are most satisfactorily consistent with those deduced from the appropriate equation of state. The overall consistency of the approach suggests that β_T and ξ_T can be deduced reliably from experimental data, and therefore may afford a new method of elucidating molecular structure.