Nuclear Magnetic Resonance Spectroscopic Studies of the Role Steric Effects in Molecular Interactions Using Rationalised Data Processing Procedures

Abstract

The n.m.r. technique has been used extensively to study the molecular interactions between aliphatic molecules (solutes) and aromatic solvent molecules. Several problems have become apparent regarding the evaluation of data determined from such studies, namely that the equilibrium quotient (K) and the full aromatic solvent induced shifts (4,) are found to depend on the concentration scale and inert diluent used, regardless of the data processing method. In this thesis, therefore, an attempt has been made to devise a thermodynamically valid data processing technique which overcomes these shortcomings. By the application of a correction for the difference between the molar volume of the aromatic and the inert solvent to the Benesi-Hildebrand data processing method, rationalised results have been shown to be obtained using both the mole fraction and molal scales, The use of the molarity scale has also been validated. The new data processing method was used to determine the equilibrium quotients and excess shielding values at different temperatures pertaining to the interactions of chloroform (solute) with benzene and alkylated benzenes. Values of ΔG°, ΔH° and ΔS° were subsequently calculated. From a consideration of these data it would appear that the approach of the solute to the aromatic is sterically hindered by substituents of greater bulk than three methyl groups. Such substituents in fact tend to exclude the solute. Finally, as a result of a study of a reference independent shift technique, a new method of determining magnetic susceptibilities to an accuracy of better than 1% was devised.