DISSOCIATION MECHANISMS OF ENERGY-SELECTED CHLOROBUTANE IONS - EXPERIMENT AND THEORY

The breakdown diagrams and photoionization efficiency curves of isomer ic chlorobutane ions were obtained by energy-selected molecular ions w ith the photoelectron-photoion coincidence (PEPICO) technique. The los s of HCl from 1- and 2-chlorobutane molecular ions was found to be fas t, and accurate dissociation onsets were determined from the crossover energy in the breakdown diagrams and appropriately corrected for the precursor thermal energy. In spite of the low activation energy, the i sochlorobutane molecular ion was found to dissociate slowly near the d issociation limit, the time-of-flight distributions having been analyz ed in terms of two exponential decay rates. The dissociation reaction involved a large kinetic energy release that pointed to a 1,2-eliminat ion leading to a stable C4H+. ion. Ab initio calculations indicated th at the reaction path involved an H-atom transfer through a barrier, wh ich is a favorable case for a tunneling model to explain the slow deco mposition rate. The kinetic energy release of chlorine radical loss fr om tert-chlorobutane was determined as a function of the ion internal energy. At low energies the kinetic energy release was nearly statisti cal but the dissociation from the excited electronic states resulted i n a large and non-statistical kinetic energy release.