ANOMALOUS MASS EFFECTS IN ISOTOPIC EXCHANGE EQUILIBRIA

We have carried out a systematic analytical and computational study of the crossover behavior in the isotopic exchange equilibria between dihydrogen and the hydrogen halides. The reactions have been studied for the isotopomers H, D, T, and H-4. The crossover temperatures are related to the H-H and H-X force constants and the masses of the exchanging and nonexchanging hydrogen atoms. When the mass of the hydrogen atom in HX increases, the crossover temperature increases. When the mass of the nonexchanging hydrogen atom in dihydrogen increases, the crossover temperature decreases. Both of these effects are shown to be enthalpy controlled. The change in the crossover temperatures of the reactions H-2 + 2*HX with m*H is shown to be entropy controlled. Graphical methods are presented which relate the range of force constants in HX molecules to the existence of a crossover in an isotopic exchange with dihydrogen. It is shown that not all isotopomers give the crossover in a given exchange reaction. Exchange reactions which show the crossover for H-D and H-T exchange do not necessarily show the crossover for T-D exchange or mass independent isotope fractionation. Conversely it is shown that some reactions show T-D crossover or mass independent isotope fractionation, without H-D and H-T crossovers. Such reactions have large mass independent isotope fractionation separation factors. The results obtained are illustrative of the type of behavior found in equilibrium and kinetic systems when isotopic substitution in one of the molecular species leads to a large change in the fraction of the total vibrational kinetic energy associated with the exchanging isotopomer.