Molecular geometry and the vapor pressures of the isotopic ethylenes : chemical and structural isomers

The theory of isotope effects on vapor pressures for systems with large quantum effects, as manifested by the cross over phenomenon and deviation from the rule of the mean is illustrated by the large body of experimental data on the isotopic ethylenes which has recently become available. An effect of molecular geometry has been found, viz, a difference in vapor pressures of cis-, trans-, and gem-dideuteroethylenes. This has been shown to be due to the hindered rotation about the C = C axis in the liquid. A further consequence of the hindered rotation has been established, namely a perturbation of the out of plane molecular vibrations. This is a new effect for which a one to one correspondence with the molecular symmetry has been established. Quantitative calculations show that this rotation-vibration coupling further enhances the effect of the hindered rotation.A theoretical calculation of the vapor pressure differences of the isotopic ethylenes from molecular data is made through statistical mechanical theory and the calculation of the energy states of the isotopic ethylenes. The agreement with the experimental data, which encompasses a significant number of different parameters, is quantitative