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

Abstract

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 N14N14O16>N14N15O16>N15N14O16>N14N14O18.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 PN14N14O16/PN14N15O16 is calculated from the specific heat of N2O to be 8.2×10—4 at 184°K, in good agreement with the experimental value 7.8±1.4×10−4. The difference in vapor pressures of the isotopic isomers N14N15O16 and N15N14O16 is attributed to hindered rotation of the molecule in the liquid.