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Isomerization of unsaturated radicals. VI. Isomerization of 3-cyclopenlenyl and pentamethylene radicals

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Abstract

The 184.9 nm photochemistry 0f gaseous 3-methylcyclopentene and 3-methyl-1,4-pentadiene have been studied. Both photoexcited species decompose mainly through the primayy rupture of the C-CH3 bond. Vibrationally excited 3-cyclopenennyl and pentamethylene radicals are formed in the primayy decomposition in the former and latter systems respectively. These radicals are connected through isomerization reactions: in the presence of DI, the isomers cyclopenten,, and trans-1,3-pentadtene 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 producss 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 torsionll barriers) of the TPEC for 1,4-pentadien-3-yl explain the ESR observations for the open chain C5H7 radical rotamers.

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Authors and 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

    George R. De Maré

  2. Département des Sciencss Fondamentales, Université du Québec à Chicoutimi (UQAC), G7H 2B1, Chicoutimi, Québec, Canada

    Hélène Deslauriers & Guy J. Collin

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  1. George R. De Maré
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  2. Hélène Deslauriers
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  3. Guy J. Collin
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Maré, G.R.D., Deslauriers, H. & Collin, G.J. Isomerization of unsaturated radicals. VI. Isomerization of 3-cyclopenlenyl and pentamethylene radicals. Res Chem Intermed 14, 133–159 (1990). https://doi.org/10.1163/156856790X00229

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