Isotope Effects on Internal Frequencies in the Condensed Phase Resulting from Interactions with the Hindered Translations and Rotations. The Vapor Pressures of the Isotopic Ethylenes

Interaction of the hindered translations and rotations with the internal vibrations in the condensed phase leads to isotope-dependent shifts of the internal frequencies. This isotope dependence is a necessary consequence of the fact that the coordinates representing the translations and rotations of the molecule as a whole are, in general, isotope dependent. This external—internal interaction is investigated and the XH2 system is employed as a simple example to demonstrate the nature of the interaction. It is shown that the experimentally determined vapor pressures of the isotopic ethylenes may be rationalized when the external—internal interaction is taken into account. A force field based on a simple cell model for liquid ethylene is obtained which yields good agreement with the experimentally determined vapor-pressure isotope effects. The force field, while not unique, fits (and is in fact partially based on) other available data on liquid ethylene.