cis–transIsomerization of 2-Butenes in the Gas Phase: Benzene Photosensitization at 2537 Å

Abstract

cis–trans Isomerization of 2‐butenes in the gas phase has been studied by the benzene photosensitization method, and the detailed experimental results are given. The ratio of the isomerization rate ${\mathrm{(R}}_{\mathrm{c\to t}}{\mathrm{\hspace{0.17em}/\hspace{0.17em}R}}_{\mathrm{t\to c}})$ and the photostationary‐state ratio [(trans‐2‐butene)/(cis‐2‐butene)]_PSS, have been determined to be 1.02 ± 0.02 and 0.92 ± 0.02, respectively. trans‐2‐Butene has a slightly larger cross section for quenching the triplet benzene than cis‐2‐butene, and its cross section may be as much as 10% greater than that of cis‐2‐butene. Quenching cross sections of various olefins ${\mathrm{(\pi \sigma }}^2)$ vary by two orders of magnitude: ethylene has the smallest value of 0.8 Ų and 1,3‐butadiene has the largest value of 50 Ų found so far. The energy transfer to 1,3‐butadiene appears to occur on almost every collision. The decay rate of the triplet benzene at the benzene pressure range of 0.5–5.7 mm is comparable to the rate of the triplet energy transfer to cis‐2‐butene at 0.028 ± 0.004 mm of cis‐2‐butene. The above rate data are in serious disagreement with the data reported by Cundall et al.