Photosensibilisation (253,7 nm) et photolyse directe (184,9 nm) du cis-pentène-2 gazeux

Abstract

The major primary process in the interaction of Hg(6 3P 1) with cis-pentene-2 is energy transfer to yield vibrationally excited pentene-2 molecules in the first triplet state (φ ⩾ 0.98). The quantum yield for paraffinic-type quenching is less than or equal to 0.02. At low pressures fragmentation of the vibrationally excited molecules yields many products, including the isoemrs 3-methylbutene-1 and pentene-1. At pressures greater than 40 Torr, the quantum yield for cis → trans isomerization is 0.50 ± 0.02. An isomer (probably ethylcyclopropane) as well as some of the pentene-1 are formed by intramolecular hydrogen-atom migration, followed by collissional stabilization. The steady-state ratio [ trans-pentene-2]/[ cis-pentene-2] is 1.04 ± 0.04. The direct photolysis of cis-pentene-2 at 184.9 nm yields an excited molecule which decomposes at low pressures to give methyl and α-methallyl radicals (φ ≈ 0.9). The effect of adding DI, D 2S or O 2 as a radical trap has been studied. The excited pentene-2 molecules (or excited isomers formed by intramolecular hydrogen atom migration) can be deactivated collisionally: addition of propane (at 2 atm or higher) leads to φ( cis-pentene-2 → isomers) = 0.44 ± 0.05. Since φ(trans → cis) = 0.38 ± 0.03 under these conditions, the total quantum yield for deactivation is φ(C 5H 10) = 0.82 ± 0.09.