Photofragmentation of mono- and dichloroethylenes: Translational energy measurements of recoiling Cl and HCl fragments

Abstract

Translational energy (Et ) spectra of Cl and HCl fragments from vinylchloride, trans-dichloroethylene, cis-dichloroethylene, and 1,1′-dichloroethylene have been measured for the π*←π excitation at 193 nm. Et distribution and angular dependence of the Cl fragment indicate that the two-center dissociation occurs in a time faster than a rotation period and the recoiling organic radical (the counter fragment) is highly vibrationally excited. In dichloroethylenes, the presence of a second channel producing Cl atoms has been confirmed and attributed to the dissociation from the lower (n,σ*) state through a (π,σ*) state. The Et distribution of HCl fragments is nonstatistical and found to converge to null population at an energy less than half of the total available energy. This convergence point coincides with the value of the local available energy for the elimination reaction (activation energy —ΔH0reaction ). The yield of HCl molecules relative to Cl atoms is estimated to be ∼1.1 for vinylchloride. High efficiency of the HCl elimination is attributed to a rapid internal conversion from the (π,π*) state to the lowest (π,σ*) state from which the pathway to the transition state for HCl elimination is opened in ground electronic manifolds.

Investigating the C−Cl Antibonding Character in the ππ* Excited State of Vinyl, Allyl, and Propargyl Chloride: Emission Spectra and ab Initio Calculations

“…These results indicate that the excited state reached in vinyl chloride is nearly exclusively ππ* CdC in the Franck-Condon region, in agreement with the calculations of Umemoto and co-workers. 1 Other low lying states exhibit transitions from π CdC and n Cl molecular orbitals to σ* C-Cl and Rydberg states but do not mix with the nominally ππ* state in the Franck-Condon region or carry significant oscillator strength. As suggested by the emission spectra, allyl chloride and propargyl chloride show increased contributions of σ* C-Cl character to the excited state reached at 199 nm.…”

“…In the photodissociation of vinyl chloride in its first ultraviolet absorption band, the branching between C-Cl fission and HCl elimination products has been investigated by a number of groups, 1,4,5 and there is good agreement on a C-Cl/HCl ratio of approximately 1:1. The observed C-Cl bond fission evidences two distinct kinetic energy distributions, both of which are peaked well away from zero kinetic energy, indicating significant exit channel barriers to reverse reaction for each of the two different C-Cl bond fission channels.…”

“…Ab initio calculations on vinyl chloride have further indicated the importance of ππ* CdC/CtC , nσ* CsCl , and n Cl 3s Rydberg configurations in this excitation band. 1 It is also likely that there are nearby triplet states which may also mix slightly with these configurations through spin-orbit coupling.…”

“…Umemoto and co-workers speculate that, upon photoexcitation, twisting of the CdC bond leads to mixing of the ππ* and πσ* configurations. 1 Internal conversion to the πσ* state may serve as an intermediate state which facilitates eventual internal conversion to the ground state.…”

“…In particular, vinyl chloride (H 2 CdCHCl) has been extensively studied through a variety of experimental techniques which probe its photodissociation products. [1][2][3][4][5] Photodissociation of vinyl chloride in its first ultraviolet absorption band gives four different bond fission channels, two involving C-Cl fission and two involving HCl elimination. Recent crossed laser-molecular beam studies have also focused on the related systems allyl chloride 6,7 (H 2 CdCHsCH 2 Cl) and propargyl chloride 8,9 (HCtCsCH 2 -Cl).…”