Unlike other chemical shifts in gaseous systems which have been found to have strictly linear dependence on density, we have found the ¹²⁹Xe chemical shift in pure xenon gas to have a quadratic and cubic dependence in addition to the dominant linear dependence on density. This implies the importance of three or more body interactions in xenon. In mixtures of xenon with another gas (Ar, Kr, CO₂, HCl, CH₄, CH₃F, CH₂F₂, CHF₃, CF₄), the dependence of the ¹²⁹Xe chemical shift on the density of the other gas is found to be linear within experimental error, and varying from 2300–11 700 ppm/mol cc⁻¹. These shifts are orders of magnitude greater than the reported H and F shifts in gases. Analysis of the results show that the density dependence cannot adequately be reproduced by the contributions, ${\Sigma }_1{\mathrm{\hspace{0.17em}=\hspace{0.17em}\Sigma }}_b{\mathrm{\hspace{0.17em}-\hspace{0.17em}B〈\epsilon }}^2{\mathrm{〉\hspace{0.17em}-\hspace{0.17em}B〈F}}^2〉$ , which had been adequate for H and F shifts. The general formulation for calculation of the $A$ and $B$ parameters, the coefficients of the linear and quadratic electric field terms in the theory of chemical shift in gases, is developed. An approximate calculation of $B$ for atoms is given, and the repulsive and anisotropic contributions are estimated. The sensitivity of the chemical shift to the form of the intermolecular potential is suggested in the case of Xe and the fluoromethanes where the results are consistent with a noncentral field potential but not with a central field potential like the Lennard‐Jones.

We have measured the 15N nuclear magnetic resonance frequencies in 15N-labeled molecules (NNO, NNO, NH3, N2, and HCN) in gas phase samples and also in CH3NO2 as neat liquid. By using the previously determined temperature dependence of samples of the these gases at various densities, we are able to reduce the measured frequencies to the zero-density limit at 300 K, and obtain shielding differences between rovibrationally averaged isolated molecules at this shielding measurements from molecular beam studies to provide an 15N absolute shielding scale based on 15NH3.

Studies of density dependence of ¹²⁹Xe chemical shift in xenon gas at room temperature have shown that while the chemical shielding does have quadratic and cubic dependence on density over densities up to 250 amagat, σ(ρ, T) = σ₀ + σ₁(T)ρ + σ₂(T)ρ² + σ₃(T)ρ³, the curve is essentially linear up to about 100 amagat. We have now obtained σ₁(T), the linear density coefficient of chemical shielding, for pure xenon over the temperature range 240–440 °K. The experimental values of σ₁(T) can be fitted by a fourth degree polynomial: σ₁(τ) = 0.536 − 0.135 × 10⁻²τ + 0.132 × 10⁻⁴ τ² − 0.598 × 10⁻⁷τ³ + 0.663 × 10⁻¹⁰τ⁴ (ppm/amagat), where τ = T − 300 °K. Comparison is made with σ₁(T) for other nuclei and with σ₁(T) predicted by various theoretical models.