Density Dependence of 129Xe Chemical Shifts in Mixtures of Xenon and Other Gases

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.