Isotope effects upon hydrogen atom loss from molecular ions

Abstract The isotope effect (TH/TD) upon the kinetic energy release and the isotope effect (kH/kD) upon ion abundance for unimolecular H· loss from molecular ions has been determined for several compounds. It is suggested that the isotope effect upon abundance might provide a convenient method of estimating the relative life‐time of ions which fragment in the metastable region for different instruments or different experimental conditions. The value of kH/kD varies from <2 to >1000 for different molecular ions and this variation is apparently largely due to the rate of increase of the reaction rate with internal energy in the threshold region. The magnitude of the isotope effect is thus related to the entropy of activation. The isotope effect upon energy release was found to be slightly less than unity in almost every case studied; this included both reactions in which the reverse activation energy is very small and those in which it is appreciable.