Molecular complexes. Part 10.—Theory for complex formation and nuclear magnetic resonance evidence for short range molecular order in liquids

Abstract

For reactions of the type A + B ⇌ AB occurring in the presence of an inert solvent, S, it is shown that the aromatic induced n.m.r. chemical shift, Δ_c, varies with the composition of the mixtures studied in a similar fashion to the equilibrium quotient, K_x. This is explained in terms of a model based on short range order in liquids, which allows aromatic molecules, B, to be preferentially attracted to the solute, A, in the form of solvation shells; experimental and theoretical justification of this is given. The cage model used to describe the formation of bimolecular complexes requires that a polar solute (A) exchanges rapidly between a limiting number of aromatic molecules in the solvation shells, which in turn exchange with molecules in the surrounding medium. The model is used to explain the variation in observed shifts caused by complex formation.