CHEMICAL SHIFTS IN THE NUCLEAR MAGNETIC RESONANCE SPECTRA OF MOLECULES CONTAINING POLAR GROUPS

Abstract

It is shown that the electric field at a particular nucleus arising from polar groups in other parts of a molecule can lead to chemical shifts proportional to the first power of the field strength. The change Δσ in the proton screening constant of an X—H bond when it is subjected to an electric field E is approximately given by[Formula: see text]where Ez is the component of E in the bond direction. A field along the X—H bond draws the electrons in the enriched region between the nuclei away from the proton, thereby causing its resonance to occur at lower magnetic field strengths, while a field in the H—X direction leads to resonance at higher fields. The electric field produced by the polarization of neighboring solvent molecules may also be important and lead to a solvent shift related to the dielectric constant of the solvent. The direction of the internal field, and that due to the solvent, can easily be estimated when the molecule is rigid, so that the model can be useful in assigning spectra. It gives a simple and reasonably accurate account of the observed spectra of substituted benzenes and of some solvent effects on the proton resonances of cis- and trans-1,2-dichloroethene.