Studies of Sterically Controlled Solvent Induced NMR Chemical Shifts

Abstract

Recently Homer and Percival have postulated that intermolecular van der Waals dispersion forces can be characterized by three mechanisms. The first arises via the mean square reaction field due to the transient dipole of a particular solute molecule that is considered situated in a cavity surrounded by solvent molecules; this was characterized by an extended Onsager approach. The second stems from the extra cavity mean square reaction field of the near neighbour solvent molecules. The third originates from square electric fields due to a newly characterized effect in which peripheral solute atoms are "buffeted" by the peripheral atoms of adjacent solvent molecules. Extensive justification for the use of these terms in total has been provided elsewhere for simple isotropic molecular systems. The present work shows that the overall treatment is similarly successful when applied to the proton chemical shifts of more complex molecules, and it is suggested and demonstrated that the buffeting shifts may prove useful in elucidating features of molecular structure. More importantly, the present work provides strong experimental evidence for the separate existence of the three individual contributions to dispersion forces. In the course of this work, a novel method of measuring volume magnetic susceptibility is proposed and evaluated. Additionally, the theoretical and practical implications of errors that may arise when measuring chemical shifts with Fourier Transform NMR Spectrometers that are field/frequency locked to signals arising from materials in various physical environments are evaluated