COS Photoinitiated cis–trans Isomerization of 2-Butenes in the Gas Phase: A Chain Process

It is found that 2-butenes cleanly undergo cis–trans isomerization by a chain process when the gaseous mixtures of COS and cis-2-butene (or trans-2-butene) is photolyzed at 2537 Å; no intramolecular doublebond migration in 2-butenes is observed and the process is largely homogeneous. The catalytic chain initiator is proposed to be the ground-electronic-state S atom (3P) which is produced by the photodecomposition of COS. The quantum yield of trans-2-butene resulting from the gaseous mixture of COS (1.0 torr) and cis-2-butene (2.5 torr) is (2.3 ± 0.5) × 102 at room temperature, and the chain length is estimated to be (1 − 3) × 103. The photostationary-state ratio of trans to cis, [(t-B) / (c-B)]Pss, and the initial isomerizationrate ratio, [Rc→t / Rt→c], have been determined to be 2.83 ± 0.06 and 2.8 ± 0.2, respectively. A competitive quenching study has been carried out to determine the efficiencies of various chain terminators. 1,3-Butadiene is found to be the most efficient terminator, more efficient than O2 or mono-olefins by 3 orders of magnitude.