Abstract
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 such that the reaction is under thermodynamic control. 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; 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).