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Isomerization of Unsaturated Radicals. Vi. Isomerization of 3-Cyclopentenyl and Pentamethylene Radicals

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The 184.9 nm photochemistry of gaseous 3-methylcyclopentene and 3-methyl-1,4-pentadiene have been studied. Both photoexcited species decompose mainly through the primary rupture of the C-CH3 bond. Vibrationally excited 3-cyclopentenyl and pentamethylene radicals are formed in the primary decomposition in the former and latter systems respectively. These radicals are connected through isomerization reactions: in the presence of DI, the isomers cyclopentene, and trans-1,3-pentadiene and/or vinylcyclopropane are formed in both systems. The quantum yields depend on the pressure and the starting monomer: cyclopentene and cyclopentadiene are the major products from the photolysis of 3-methylcyclopentene + DI mixtures and only minor quantities of the other C5H8 compounds are formed. Cyclopentadiene is the major product of the photolysis of 3-methyl-1,4-pentadiene + O2 mixtures whereas vinylcyclopropane and trans-1,3-pentadiene are the major C5 products of the photolysis of 3-methyl-1,4-pentadiene + DI mixtures. The geometries of 3-cyclopentenyl and of the structures at the six critical points in the torsional potential energy curve (TPEC) for rotation about the 2- and 3-C-C bonds in the open chain pentamethylene species have been optimized completely by ab initio RHF-SCF gradient methods. For the open-chain structures the bond orders, bond lengths and the free valence (primarily associated with the central carbon atom) all correspond to 1,4-pentadien-3-yl conformations. In the ground state there is a high barrier to formation of 3-cyclopentenyl from 1,4-pentadien-3-yl. The features (relative energies and torsional barriers) of the TPEC for 1,4-pentadien-3-yl explain the ESR observations for the open chain C5H7 radical rotamers.

Affiliations: 1: Laboratoire de Chimie Physique Moléculaire Faculté des Sciences CP. 160 Université Libre de Bruxelles 50, av. F.D. Roosevelt, B-1050 Brussels, Belgium; 2: Département des Sciences Fondamentales Université du Québec à Chicoutimi (UQAC) Chicoutimi, Québec, Canada G7H 2B1


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