Temperature dependence of the isotopic liquid-vapor fractionation factor for nitric oxide

The 15N/14N isotope fractionation factor between NO liquid and vapor has been determined in the temperature range 110–173°K. The results are in good agreement with the isotopic vapor pressure data of Clusius et al., which cover the range 110–121°K. The isotope fractionation data follow the T−2 law up to 135°K. A refinement of the T−2 law analysis of the isotopic vapor pressure data given by Bigeleisen in 1960 has been carried out by the harmonic oscillator cell model to include T−1 and T0 terms. It is shown that the T−2 terms from the dimerization of the monomer in the liquid and the hindered translation and rotations of the dimer dominate the isotope fractionation factor. The falloff from the T−2 law for the liquid-vapor fractionation factor in NO is steeper than in argon. This is reconciled with the dissociation of the dimer in the liquid and the association of the monomer in the vapor at high temperatures.