Phase equilibrium isotope effects in molecular solids and liquids

The vapor pressures of three isotopic samples of N2O enriched in 15N and 18O were compared with normal N2O over the temperature range 143-2190K. Over the temperature range studied in the solid and in the liquid in the neighborhood of the triple point, the vapor pressures are in the sequence 14N216O greater than 14N15N16O greater than 15N14N16O greater than 14N218O. The crossover phenomena have been observed for 15N14N16O/14N218O at 2070 K and for 14N15N16O/14N216O at 2150. The experimental data confirm hindered translation in both the solid and the liquid as well as the potential energy coupling of the translation and rotation found in CO2. Shifts in the internal stretching and bending modes on condensation lead to significant differences in the vapor pressures of the isomers 15N14N16O and 14N15N16O. The latter has an abnormally large change in vapor pressure on melting compared with the other isotopic N2O molecules studied. This is associated with the coupling of the internal bend cooridnate with the translation in the solid, which vanishes in the liquid. The isotope effect in the high temperature liquid is explained in terms of an anharmonicity of the external motions in addition to the temperature dependence of the stretching force constant. Model calculations of the logarithm of the reduced partition function ratios of the solid and liquid, ln(fc/fg), are made in an anharmonic version of the Stern-Van Hook-Wolfsberg cell model. This model provides a good representation of the experimentally determined ln(fc/fg) for solid and liquid N2O. (U.S.)