Kinetic Energy Release in Unimolecular Ionic Reactions. Thermochemical Aspects

Abstract

The kinetic energy (T) released in the unimolecular reactions of metastable gaseous ions has been compared with literature values obtained by several methods for the same reactions occurring in the ion source. The internal energy difference between the long-lived and the short-lived reactant ions has a relatively small effect upon the kinetic energy release when rearrangement and elimination reactions are studied, large kinetic energy releases being observed by both methods. Simple cleavages, which are typically accompanied by far smaller energy releases, show large relative variations when the time-scale of the reaction is changed with the values measured in the field-free region being much lower (e.g., 0.002 eV vs 0.18 eV in the case of H loss from propane). These and other results provide strong evidence that metastable reactions occur from ions whose average internal energies are only slightly in excess of the activation energy for the process in question. The consequences of this result for studies on energy partitioning by mass spectrometry and for improving thermochemical data on ions are stressed and illustrated. Franklin has emphasized the importance of T values, extrapolated to ions of threshold internal energy, as a means of correcting for the excess energy of the activated complex and so improving the quality of thermochemical measurements obtained by mass spectrometry. Since the metastable technique is directly applicable to ions of approximately threshold energy, no extrapolation is necessary, and a rapid and simple method of obtaining improved thermochemical data is therefore available. It is further suggested that these results should also provide a basis for correcting measured appearance potentials for the excess energy term arising from the presence of a reverse activation energy.