Gas-Phase Studies of NMR Shielding and Indirect Spin-Spin Coupling in ¹³C-Enriched Ethane and Ethylene

¹³C and ¹H NMR spectra were observed as the function of density in 1,2-¹³C-enriched ethane and ethylene for the pure gaseous compounds and their binary mixtures with xenon and carbon dioxide gases as the solvents. All the chemical shifts and indirect spin-spin couplings were linearly dependent on the solvent density. The appropriate NMR parameters (σ and ⁿJ) in isolated ¹³C₂H₆ and ¹³C₂H₄ molecules and the coefficients responsible for the binary molecular interactions were determined and compared with previous similar measurements and selected calculated shielding data. The newly obtained ¹³C shielding values in the isolated ethane and ethylene molecules suggest visible secondary isotope effects due to the additional carbon-13 atom. All the investigated shielding parameters depend on intermolecular interactions, and the dependence of ¹³C shielding is much more marked. In contrast, the indirect spin-spin couplings in ¹³C₂H₆ and ¹³C₂H₄ molecules are almost independent of solvent molecules. Their ⁿJ values determined in liquids over sixty years ago are generally consistent with the same ⁿJ parameters in isolated ¹³C₂H₆ and ¹³C₂H₄ molecules.