Proton magnetic shielding of a pair of interacting methane molecules

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Abstract
A decrease in the proton magnetic shielding constant σ of methane gas is observed when the gas density is increased, even after allowing for bulk magnetic polarization. The decrease is linear with density. Thus σ = σ0 + σ1/tV m where σ0 is the shielding constant at zero density and V m is the molar volume. At room temperature, experiment gives σ1 = -8 × 10-6 cm3 mole-1. The density dependence is caused by the van der Waals forces of which the London dispersion interaction predominates. An attempt is made to calculate σ1 by considering the interaction which alters the shielding constant as being made up of separate interactions involving each of the atoms in the perturbing molecule. In this way account is taken of the ‘site effect’ due to the finite size of the interacting molecules. The calculation gives σ1 = -6 × 10-6 cm3 mole-1. It is suggested that the difference between the observed and calculated σ1 is probably caused by treating the shielding change due to dispersion forces as being R -6 dependent and ignoring higher-order terms.

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