Molecular Geometry and the Vapor Pressure of Isotopic Molecules. The Equivalent Isomers cis-, gem-, and trans-Dideuteroethylenes

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Abstract
The vapor pressures of trans-, cis-, and gem-dideuteroethylenes have been intercompared directly by mercury manometry over the temperature range 120°—180°K. The trans isomer has been compared with ordinary ethylene over the same temperature range. The vapor pressures of cis- and gem-dideuteroethylene are equal to within a few hundredths of a percent. The trans compound has a greater vapor pressure than either the cis or gem compounds (about 0.1% at 130°K and 0.05% at 180°K). At these same temperatures the trans-dideuteroethylene has a vapor pressure 1.2% and 1.6%, respectively, greater than ordinary ethylene. The difference between the dideuteroethylenes is correlated with differences in molecular geometry. It is shown that the trans is the most volatile of the dideutero isomers, because it has the smallest moment of inertia about the C=C axis. The vapor pressure data are then direct evidence for hindered rotation in the liquid.