Kinetic shift in chlorobenzene ion fragmentation and the heat of formation of the phenyl ion

The fragmentation of chlorobenzene ion has been studied by photoelectron–photoion coincidence techniques. By varying the residence time it is possible to obtain breakdown curves as a function of residence time. The parent–daughter transition region shifts to lower energies as the residence time is increased (kinetic shift). The shift is of the order of 0.4 eV in going from 0.7 to 8.9 μs. A systematic analysis of the breakdown curves and residence time effects has been carried out using quasiequilibrium theory. The experimental results and analysis lead to ΔHf00 (phenyl ion) =275±1 kcal/mol (1151±4 kJ/mol. The systematic analysis shows that this experiment leads to a quite accurate rate-energy curve in the range of 104–106 s−1. The sensitivity of the QET model has been studied, and the limitations to the determination of activated complex parameters is critically discussed. The parameters obtained in this work are rather similar to those of an analogous neutral process, i.e., thermal decomposition of bromobenzene.