Molecular Geometry and the Vapor Pressure of Isotopic Molecules. C2H3D and C12H2=C13H2

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Abstract
The isotopic vapor pressure ratio between monodeuteroethylene and ordinary ethylene has been measured in the temperature range 120°—180°K by differential manometry. The inverse isotope effect with C212H3D is less than one-half the effect observed with the dideuteroethylenes. This deviation from the rule of the geometric mean gives further support to the structural theory of isotopic vapor pressure effects and confirms the importance of quantum corrections of the order of (ℏ/kT)4 and higher. Vapor—liquid equilibrium studies of C212H4 and C12H2=C13H2 were made in a packed column under total reflux using C212H3D as an internal monitor to calibrate the column. Whereas C212H3D goes to the top of the column, C12H2=C13H2 concentrates in the boiler. Absolute values of the carbon-13 effect were calculated from the measured relative effects and the manometric data for C212H3D, and the vapor pressure ratios were checked by analysis of the kinetic behavior of the column. A test of the data was made in the framework of the applicable theory. The temperature dependencies of the isotope effects are predicted from the theory within the experimental error. The relative deuterium and carbon-13 isotope effects as well as the differences between the dideuteroethylene isomers are discussed in terms of the structure of the ethylene molecule. It is shown that there is a large normal A/T2 dependence due to hindered rotation in the liquid. The zero-point energy change on condensation leads to a term of the form B/T and accounts for the higher vapor pressures of the deutero-compounds. The zero point energy changes obtained from the vapor pressure data are in quantitative agreement with theoretical calculations. A new type of effect is found, namely a perturbation of the internal vibrations by the hindered rotation. This perturbation is different for the different molecules and a strict correspondence is established with the molecular symmetry.