Structural Effects in the Vapor Pressures of Isotopic Molecules. O18 and N15 Substitution in N2O

The relative vapor pressure ratios of the isotopic N₂O molecules have been determined by distillation in a column under total reflux. It is shown that the vapor pressures are in the sequence $${\mathrm{N}}^{14}{\mathrm{N}}^{14}{\mathrm{O}}^{16}>{\mathrm{N}}^{14}{\mathrm{N}}^{15}{\mathrm{O}}^{16}>{\mathrm{N}}^{15}{\mathrm{N}}^{14}{\mathrm{O}}^{16}>{\mathrm{N}}^{14}{\mathrm{N}}^{14}{\mathrm{O}}^{18}.$$ The absolute vapor pressure ratios have been determined from the kinetic behavior of the column and the decrease in separation by production from the column. Theoretical calculations of the relative vapor pressure ratios can be made from the atomic masses and the random structure of liquid N₂O. The calculated relative isotope effects are in good agreement with experiment. The absolute value of ln $P_{{\mathrm{N}}^{14}{\mathrm{N}}^{14}{\mathrm{O}}^{16}}{\mathrm{/P}}_{{\mathrm{N}}^{14}{\mathrm{N}}^{15}{\mathrm{O}}^{16}}$ is calculated from the specific heat of N₂O to be 8.2×10^—4 at 184°K, in good agreement with the experimental value ${\text{7.8±1.4×10}}^{\text{−4}}$ . The difference in vapor pressures of the isotopic isomers ${\mathrm{N}}^{14}{\mathrm{N}}^{15}{\mathrm{O}}^{16}$ and ${\mathrm{N}}^{15}{\mathrm{N}}^{14}{\mathrm{O}}^{16}$ is attributed to hindered rotation of the molecule in the liquid.