Kinetic energy release in metastable ion transitions. A study by ion kinetic energy spectrometry and rrkm theory

An approach to energy partitioning using experimental data in conjunction with theoretical calculations is presented, with the McLafferty and related rearrangements occurring in the molecular ions of ketones, esters, amides and acid chlorides representing the specific reactions examined. The kinetic energy release associated with statistical partitioning of the nonfixed energy of the activated complex was calculated from unimolecular reaction theory and compared with experimental data. Calculations of the energy dependence of the rate constant and the average statistically released kinetic energy were made for 24 activated complexes corresponding to the two basic structures, cyclic and linear. The frequencies used in the calculations were all modeled upon the 2‐pentanone case with sufficient variation in the frequencies used to allow for any reasonable type of perturbation of the linear and cyclic structures. To further test that the complexes chosen were reasonable, A‐factors were determined. From a comparison of the calculated and experimental results, the contributions of the nonfixed energy of the activated complex and of the reverse activation energy to the energy release were separated. This appears to be applicable, not only to the 2‐pentanone case, but across the series of reactions studied. It is also shown that, despite the fact that these reactions release relatively small kinetic energies, the fraction due to the reverse activation energies may be large. The consequences of these observations for ion enthalpy determinations are emphasized.