Vapor pressures of the isotopic ethylenes. V. Solid and liquid ethylene-d1, ethylene-d2 (c i s, t r a n s, and g e m), ethylene-d3, and ethylene-d4

Abstract

A simple chromatographic method is described for the purification of gases. The method can process of the order of 0.5 liter/day and reduce impurities to the ppm level with greater than 90% recovery of feed. Samples of all the deuterated ethylenes have been purified by this procedure over activated alumina columns and their vapor pressures measured over the temperature range 94–180 °K. They are in excellent agreement with the previous measurements on the liquid from the Brookhaven Isotope Chemistry Laboratory. In contrast to the liquid, the deuterated ethylenes show the normal vapor pressure isotope effect in the solid. A new vapor pressure curve for solid C2H4 is derived from the measurements log10P (mm Hg) =9.6921−1011.5/T (94<T<104 °K). The triple point temperatures of the deuteroethylenes have been found to increase with deuteration. A difference is found in the triple point temperatures of the equivalent isomers trans-, cis-, and gem-dideuteroethylene. The results are interpreted in terms of an anharmonic version of the BSVHW model of the solid and liquid applied to the statistical mechanical theory of condensed phase isotope effects. It is found that the force constants associated with the librational motions of the molecule are 50% larger in the solid than the liquid at the triple point. After correction of the temperature dependence of the isotopic vapor pressure data for translation and rotation through a corresponding states method, it is found that agreement with the large body of experimental data can only be obtained by introduction of a plausible temperature dependence to the C–H stretching, CH2 in-plane wagging, and the CH2 torsional modes in the liquid. A force field refined in this manner is in agreement with all the experimental isotopic vapor pressure data within 10−4 in ln(P′/P) over the entire temperature range.