Reactions of cis- and trans-2-Butenes Photosensitized by Hg(3P1) and Cd(3P1)

A comparative study has been made of the reactions of cis- and trans-2-butenes photosensitized by mercury(3P1) at room temperature and by cadmium(3P1) at temperatures ranging from 275 to 350°C. In both cases, the main reaction was cis-trans isomerization. The initial quantum yield of isomerization was the same for the two butenes. The rate of decomposition was about 10−2 and 10−3 times less than that of isomerization in the Hg- and Cd-photosensitizations, respectively. The ratio of trans- to cis-2-butene after prolonged irradiation was unity and did not vary with pressure in the Cd-photosensitization, while in the Hg-photosensitization it increased with decreasing initial pressure. This is probably due to the difference in the rates of decomposition of trans- and cis-2-butenes. It was found that the pressure dependence of Hg-photosensitized cis-trans isomerization could not be explained in terms of the simple mechanism proposed before. A possible mechanism is discussed.