The mechanism of the unimolecular dissociation of trichloroethylene CHCl=CCl2 in the ground electronic state

Abstract

The unimolecular dissociation of trichloroethylene in its electronic ground state has been investigated using an infrared multiphoton dissociation combined with photofragmentation translational spectroscopy to measure product translational energies. The main reaction channel was found to be HCl elimination on the basis of observed product time-of-flight (TOF) spectra. A center-of-mass translational energy distribution for this channel provides direct evidence for competition between two channels, three- and four-centered HCl eliminations. Cl elimination was found to be a minor but significant channel from observed Cl+ and C2HCl+TOF spectra. The branching ratios were determined as 0.28, 0.55, and 0.17 for the three- and four-centered HCl eliminations and the Cl elimination, respectively. The three-centered channel exhibits a ‘‘statistical’’ translational energy distribution which is typical for a reaction with no potential energy barrier in the reverse reaction, that is to say, no exit barrier reaction. In contrast, the four-centered channel exhibits a ‘‘nonstatistical’’ translational energy distribution having a peak at around 2 kcal/mol in energy, indicating that a significant exit barrier exists in the channel. The fraction of potential energy converted to translational energy was estimated to be around 10%. Ab initio calculations at the QCISD(T)/6-311+G**//MP2(FC)/6-31G* level were employed to confirm the reaction mechanism. The agreement in the energetics is quite good.