Intermolecular and intramolecular effects on the 1H and 13C shielding in some gaseous hydrocarbons at various temperatures—experimental results

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Abstract
The proton and 13C shielding constants in CH4, C2H6, C3H8, and some other gaseous hydrocarbons have been studied as functions of density at temperatures in the range 180–370 K. The linear coefficients describing the density dependence of the shielding, after correcting for bulk susceptibility, increase substantially as the temperature is reduced, indicating stronger intermolecular interactions. (Some of the required magnetic susceptibilities were determined in this work by an NMR method). The 13C measurements for CH4 are close to those of an earlier study; the results for the other gases are new. The linear coefficient is substantially greater for the carbon shielding of the methyl group in propane than for the methylene group at any temperature, but there is virtually no distinction between the linear coefficients for the proton shielding in this gas. Values for do0/dT, the temperature dependence of the shielding extrapolated to zero density, are also presented for both proton and 13C shielding in the hydrocarbons. They are positive and negative in different instances. It is shown from this and earlier gas‐phase studies that standard literature values of the methane 13C shielding relative to the 13 shielding in the other hydrocarbons are in error.