The elimination of internal and external references in nuclear magnetic resonance determinations of fast equilibria with particular reference to electron-donor–acceptor (EDA) complex formation

Abstract

N.m.r. determinations of the association of electron donors (D) and electron acceptors (A), in solutions where [D]≫[A], generally involve the use of a reference, either internal or external. Where two or more n.m.r. probes are available in the A molecule, the difference(s) of the shifts of these probes can be used to calculate the association constants K₁ and K₂ for the formation of DA and D₂A, respectively. These differences are independent of any reference. The method therefore provides a check on the reliability of references in determinations in which they are used. Specifically, we have applied this method to experimental data which had been obtained orginally to provide shifts relative to an internal reference. The general agreement between the two sets of values of K₁ and of K₂, also of the corresponding shift differences ΔΔ₀(1) and ΔΔ₀(2) provides a new and independent justification for using an internal reference in the original experiments.