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

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.