Infrared Laser Photochemistry of Trans-1, 2-Dichloroethylene. Evidence For A CI Atom Chain Reaction

The TEA-CO2 laser induced reaction of trans-l,2-dichloroethylene (TDCE) was investigated at 925 cm-1. The laser radiation was focused to yield beam waist fluences of approximately 125 J cm-2. The major reaction product was cis-1, 2-dichloroethylene (CDCE), with a few per cent of chloroacetylene, and minor amounts of acetylene, dichloroacetylene, chloroethylene and an unidentified C4 compound also being formed. The reaction of pure TDCE was studied as a function of number of laser pulses and total pressure (0.02 to 5 torr). Some experiments were also done with added ethane (20%) and propane (2%). Evidence was obtained that the formation of the cis isomer occurs via two mechanisms, (1) a unimolecular isomerization, and (2) a CI atom chain reaction. The results are consistent with laser induced decomposition of TDCE occurring through the three lowest energy channels: unimolecular structural isomerization (57.4 kcal/mol); molecular HCI elimination (69 kcal/mol); and C-CI bond scission (89 kcal/mol).