Medium Effects in Proton Magnetic Resonance. I. Gases

Abstract

The position of the proton resonance signal of a molecule in a gas is found to depend upon the gas pressure. Four distinct phenomena are considered to contribute to the displacement which the signal undergoes as the density is changed: (a) bulk susceptibility, (b) van der Waals interactions, (c) electric fields, and (d) neighbor-molecule magnetic anisotropy. Theoretical expressions are presented for each of these effects. Experiments were performed on CH4, C2H6, and HCl as pure gases and as mixtures with a variety of foreign gases. For HCl at 30°C, the chemical shift changes by —0.41 ppm as the pressure is raised to 55 atm. Good correlation between theory and experiments is obtained in all cases. Parameters describing the influence of an electric field on the proton screening constant are deduced for C–H bonds and for HCl. The bulk susceptibility contribution is large, van der Waals and neighbor-molecule anisotropy effects are small, and the electric fields from permanent dipoles and quadrupoles and from induced dipoles are all important in some instances.