Vapor Pressure of the Isotopic Ethylenes. IV. Liquid Ethylene-d3 and -d 4

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Abstract
The purification of ethylene gas samples (200–400 cc NTP) to a chemical purity of 99.9997% is described. The vapor pressures of liquid ethylene-d3 and ethylene-d4 have been measured and compared with that of C2H4 in the temperature range 115°–180°K. The reduced partition function ratios of liquid C2HD3/C2H4 and C2D4/C2H4 compared to the ideal-gas reference state are derived from the vapor-pressure measurements. The experimental reduced partition function ratios of the liquid are in satisfactory agreement with cell model calculations of Stern, Van Hook, and Wolfsberg. The latter calculations have been extended to include interaction force constants between the internal and external modes as well as the external modes among themselves. The experimental partition function ratios show significant curvature in a plot of T lnfe / fgvs 1 / T in contrast to the almost linear behavior inherent in the harmonic oscillator model. Analysis of the curvature shows that the mean square intermolecular force in liquid ethylene decreases by a factor of 2 in the temperature range from 104°–180°K. Correlations of the liquid partition function ratio with the liquid density are consistent with a short-range intermolecular force.