Molecular complexes. Part 7.—Some comments on the processing of data obtained from nuclear magnetic resonance studies

Abstract

The processing of data obtained by n.m.r. studies of 1 : 1 molecular complexes formed in reactions of the type A + B ⇌ A…B, occurring in the presence of an inert solvent S, is examined in detail. The thermodynamics of the reaction are considered, with particular reference to the Benesi-Hildebrand equation and the iterative procedures which are normally used to obtain the equilibrium quotient and chemical shift differences between the “free” and fully complexed A (Δ_c). It is shown that by the normal use of the processing procedures, Δ_c is expected to be dependent on the concentration scale adopted for its evaluation, and that whichever scale is used the corresponding quotient is expected to depend on the nature of S. It is demonstrated that this anomalous behaviour may be obviated by more critical use of the Benesi-Hildebrand method, and the modifications to it and the conditions for its proper use are explained. The arguments are substantiated by investigations of the chloroform-benzene reaction in a variety of inert materials, and of several reactions of chloroform with aromatic compounds in cyclohexane.