Kinetics of unimolecular ionic reactions kinetic energy release in metastable ion fragmentations

Abstract The kinetic energy release for metastable ion fragmentations has been measured as a function of ion life-time by varying the ion accelerating voltage. These measurements have been used, assuming statistical partitioning of the non-fixed energy of the activated complex, to estimate the energy dependence of the unimolecular rate constant. By taking data at accelerating voltages as low as several hundred electron volts and by using two instruments of different geometries, ion life-times could be varied by a factor of 10. A decrease of less than 10% from an initial value of 75 meV (measured from the width of the peak at half maximum) was observed as the ion life-time was increased from 4 to 45 μs for H loss from the benzene molecular ion. No detectable variation in kinetic energy release from an initial value of 27 meV was observed between 4 and 37 μs for HCN loss from the benzonitrile molecular ion. Experiments in which the distribution of kinetic energies was determined confirmed these results. The energy dependence of the rate constant, estimated on the basis of statistical theory, was significantly greater than suggested by previous ion abundance studies for both the benzene and benzonitrile reactions. This discrepancy suggests that statistical theory may not apply to these reactions. This conclusion was supported by comparison of the experimental results with RRKM calculations which showed that for the theory to be applicable, the benzonitrile reaction cannot be occurring from the ground state. Reaction from a electronic state some 2 eV above the ground state explains the average value of the kinetic energy release but not its variation with ion life-time. It is concluded that this reaction cannot be described in conventional terms.