Nuclear Magnetic Resonance Spectroscopic Studies of Molecular Interactions with Special Reference to their Stereo-Specificity

Abstract

Molecular interactions between polar aliphatic molecules (solute) and aromatic solvent molecules have been studied widely by nuclear magnetic resonance spectroscopy. Most of the studies have been carried out using solutes containing a single or group of equivalent protons and because of the paucity of information they provide, many problems connected with the interactions, specially their stereo-specificity, remain unexplained. In an attempt to elucidate these problems solutes with several non-equivalent protons have been investigated during the work described herein. The complexes studied are those formed between vinylsolutes and benzene or non-polar alkyl substituted benzenes. The specific intention in this work was to determine the time-average structure of the complexes, the thermo-— dynamic parameters pertaining to their formation, the nature of the interactions involved and the effect of substituents in the solute and solvent molecules. The equilibrium quotients for the formation of each of the complexes and the additional shielding in the fully complexed state for the different non-equivalent protons in the solute have been determined from the dependence of the observed solvent—induced shift on the mole fraction of the aromatic solvent. Values of ΔGº, ΔHº and ΔSº were determined from the equilibrium quotient values obtained at different temperatures. The data accumulated are strongly indicative of the formation of weakly bound complexes. Significantly the thermodynamic parameters are similar for the different non-equivalent protons in the same solute and suggest the formation of only one type, 1:1, of complex. Structures are deduced for these complexes in which the solute molecule tends to adopt a preferred time-average orientation relative to the solvent. The structures depend on the nature of the substituents in the interacting molecules and indicate the highly stereospecific nature of the interactions. It is proposed that, both dipole—induced dipole interactions and steric factors govern the complex formation to a great extent.