Isotopic fractionation by isotope-exchange reactions as a function of temperature

A simple model is used to predict the fractionation of isotopes as a function of temperature via ion-molecule reactions. It is developed for reactions proceeding via a single intermediate where the isotopic mixing is completed within the lifetime of the intermediate. The fractionation efficiency is K/(K+1) where K is the equilibrium constant of the isotope-exchange reaction. The model is applied to deuterium and 18O fractionation via the prototypical reactions OH- + H2O and H3O+ + H2O; and satisfactory agreement is found with the data available for 300 K. the model is also applied to 15N fractionation via N2H+ + N2 and comparison is made with the data available for 80 K. The model predicts simply the fractionation efficiencies for such isotope-exchange reactions at the temperatures of interstellar clouds (10 - 100 K)