Photolysis of bromoethylene at 184.9nm

Abstract

A vacuum ultraviolet photolysis of vinyl bromide at 184.9 nm (154. 6kcal/mol) has been studied over the pressure range of 2-50 Torr at room temperature using the low pressure mercury are resonance lamp. The result demonstrates the two decomposition mode, i.e., one for molecular elimination of HBr to form acetylene, and the other for C-Br bond cleavage to form vinyl radical. The major products of the reaction were $C_2H_2,\;\;C_2H_4,\;\; 1,3-C_4H_6,\;\;C_2H_5Br$, and $CH_3CHBr_2$. The branching ratio betwen the molecular elimination and the radical process were 0.56 to 0.44. The $H_2S$ was added as a radical capture to obtain information about the primary process. The presure effect by adding inert gas, i.e., $SF_6$ and He, was also observed in the presence and absence of $H_2S$. The ratio of quantum yield, $\Phi(C_2H_2)/\Phi(C_2H_4)$, was decreased as the pressure increased in the absence of $H_2S$, but that was constant as the pressure increased in the presence of $H_2S$. This observation may be attributed by the fact that "hot" vinyl radical, produced from C-Br bond cleavage, decompose to acetylene and hydrogen atom. This process has been quenched by addition of $H_2S$ or inert gas.