N.M.R. Studies of the Formation of Molecular Complexes in the Liquid Phase

Abstract

This thesis concerns the investigation by nuclear magnetic resonance spectroscopy of complexes formed transiently between polar aliphatic molecules (solutes) and aromatic solvents. The complexes are studied by the use of 1H chemical shifts and spin-lattice relaxation times. The purpose of the work is to confirm the validity of procedures which have been developed previously to obtain more meaningful parameters than hitherto describing the formation of such complexes, and to investigate the mechanism of their formation. The methods proposed for the evaluation of equilibrium quotients and the screening induced in the solute proton in the complex relative to that in the free state rely on measurements of its 1H shift variation with the composition of mixtures of the solute, the aromatic and an inert solvent. Chemical shift data for the interaction between chloroform and [2H6] benzene occurring in several inert solvents have been measured to a higher accuracy than previously achieved, are analysed using the procedures referred to, and confirm the validity of these. Some observations are made concerning the effect of inert solvents and references used in these investigations. The mechanism of formation of transient complexes is studied by analysing the variations in the magnitude of equilibrium quotients with the composition of three-component mixtures. The interactions occurring between several solutes separately with benzene, in cyclohexane, are investigated. A model for complex formation is proposed, which is quantitatively explained in terms of molecular ordering in the liquid medium. Measurements of spin-lattice relaxation times in [2H] chloroform-benzene-cyclohexane mixtures of varying composition are analysed, and are shown to substantiate the model for complex formation deduced from the chemical shift analysis. The conventional procedures used for the deoxygenation of samples used for spin-lattice relaxation time measurements are shown to be unsatisfactory, and a versatile chemical alternative is described.