INTERMOLECULAR AND INTRAMOLECULAR EFFECTS ON THE H-1 AND C-13 SHIELDING IN SOME GASEOUS HYDROCARBONS AT VARIOUS TEMPERATURES - EXPERIMENTAL RESULTS

The proton and C-13 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 C-13 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 d-sigma-0/dT, the temperature dependence of the shielding extrapolated to zero density, are also presented for both proton and C-13 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 C-13 shielding relative to the C-13 shielding in the other hydrocarbons are in error.