Energy Transfer by Collisions in Cis- and Trans-Dichloroethylene Vapors

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Abstract
Some recent measurements by Sette of ultrasonic absorption in liquid halogenated hydrocarbons have indicated strong structural differences upon which, it was hoped, further light would be given by a study of some of these compounds in the vapor state. Ultrasonic velocity and absorption in cis‐ and trans‐dichloroethylene have been measured at 34.7°C, at frequencies of 0.425, 1.985, and 2.982 Mc sec−1 and at pressures from 12 to 250 mm Hg, giving a range of f/p of 1.27 to 125 Mc/sec atmos. The results show that energy equilibrium in the molecule is more quickly attained in the trans‐ than in the cis‐form. Without consideration of departure from the ideal gas law, the results may be explained by the assumption of two regions of f/p in which relaxation occurs, one relaxation frequency of 3.5 Mc/sec atmos being common to both, there being also one of 30 for the cis and 90 for the trans. The molecular absorption per wavelength computed from dispersion data are in good qualitative agreement with those measured. The values of Cv/R for both forms fall from 7.5 at one Mc/sec atmos to about 3.5 at 100 Mc/sec atmos. The results indicate the pre‐eminent importance of short‐range forces in determining the energy transfer by collisions.