Potential energy profiles for unimolecular reactions of organic ions

Abstract

Simple organic ions, generated in a mass spectrometer, are investigated when undergoing unimolecular dissociation some 10 8 vibrations after generation and excitation. Under these conditions, the slow step in the pathway to dissociation products normally occurs with only a few kilojoules per mole of excess energy in the transition state. Moreover, the kinetic energy released in the dissociation step can be measured. By the use of heats of formation of reactants, intermediates and products, and deuterium labelling, potential energy profiles for reaction can be constructed. It is shown that a general cause of the release of a range of kinetic energies in a unimolecular reaction is a rate-determining isomerization, followed by a more energetically facile dissociation with excess energy in the transition state for dissociation. The approach is exemplified by the construction of a potential energy profile for the unimolecular reactions of ionized acetic acid and its enol form