Ultraviolet photodissociation of vinyl chloride.

摘要

In this research, we have performed a systematic study of the UV photodissociation of vinyl chloride. This study is very important for our understanding how a system behaves when multiple reaction pathways are available and for assessing the impact of gas phase photochemistry of chloroethylenes on the troposphare.; We first determined the "chemical branching" in a deuterium labeled study. It was found that photo-induced elimination of HCl from vinyl chloride occurs 75% of the time by an {dollar}alpha ,alpha{dollar} mechanism and 25% by {dollar}alpha ,beta{dollar} mechanism. Surprisingly, the rotational state distributions of HCl and DCl are the same for both pathways. Qualitatively different HCl and DCl rotational distribution functions were observed for v{dollar}sp{lcub}primeprime{rcub}{dollar} = 0 and v{dollar}sp{lcub}primeprime{rcub}>0{dollar}.; In an attempt to identify the microscopic mechanism for HCl elimination, we used velocity aligned Doppler spectroscopy to measure the kinetic energy distribution functions of different rovibrational states of HCl produced in the 193 nm photodissociation of vinyl chloride. The average kinetic energy was found to vary inversely with the vibrational energy of HCl. To reconcile all of our observations we propose a mechanism in which three-center HCl elimination followed by vinylidene-acetylene isomerization occurs in a concerted, non-synchronous fashion. It is further proposed that isotopic scrambling in vinyl chloride occurs by hydrogen atom migration, producing the stable {dollar}rm CHsb3CCl{dollar} intermediate. The dichotomy between the rotational state populations for HCl(v{dollar}sp{lcub}primeprime{rcub}{dollar} = 0) and HCl(v{dollar}sp{lcub}primeprime{rcub}>0{dollar}) is explained by a vibrationally adiabatic mechanism.; Magic angle Doppler spectroscopy was used to measure the kinetic energy distribution functions of Cl({dollar}rmsp2Psb{lcub}3/2{rcub}{dollar}) and Cl({dollar}rmsp2Psb{lcub}1/2{rcub}{dollar}) produced in the 193 nm photodissociation of vinyl chloride. Bimodal energy distributions were observed for both spin-orbit states, indicating competitive reactions on two potential energy surfaces. The Cl({dollar}rmsp2Psb{lcub}1/2{rcub}{dollar}) product was found to have more kinetic energy than Cl({dollar}sp2Psb{lcub}3/2{rcub}{dollar}). Adiabatic correlation, with some scrambling in the asymptotic region, is proposed to explain the spin-orbit branching ratios.; Finally, we studied the photodissociation of 2-chloro-1,1-difluoroethylene at 193 nm. We observed a Boltzmann-like rotational state distribution for HCl(v{dollar}sp{lcub}primeprime{rcub}{dollar} = 0 and 1). These results are consistent with our understanding of the VCl reaction.