Molecular beam studies of selected radical isomers and photolytic precursors

The experiments on allyl chloride and cis-1-bromopropene examining the primary photodissociation channels and the unimolecular dissociation reactions of the nascent allyl and 1-propenyl radicals provide valuable insight into chemical dynamics. In allyl chloride, four competing primary channels were seen, two C-Cl bond fission channels and two HCl elimination channels. One C-Cl bond fission pathway and one HCl elimination pathway showed a large energy partitioning to product recoil, consistent with direct excited state dissociation/elimination reactions. The other two channels formed high internal energy, slow products and likely occur after internal conversion of the allyl chloride after the initial photo-excitation. The high internal energy allyl radicals formed in the low kinetic energy C-Cl bond fission pathway undergo C-H bond fission to yield primarily allene instead of isomerizing to the 2-propenyl radical isomer.; The investigation of cis-1-bromopropene excited at 193 nm provides the first experimental determination of the lowest energy dissociation barrier of the 1-propenyl radical. It was found to be 31.5 +/- 2 kcal/mol. These experiments also demonstrate that 1 this C-C bond fission channel turns on first, before isomerization or H atom loss. At energies where isomerization and C-H bond fission effectively compete with C-C fission, formation of propyne + H from H atom loss and isomerization to allyl followed by H atom loss to yield allene + H both become important. This indicates that the barriers for these two processes are likely more similar in energy than predicted by recent published calculations. The primary channels of cis-1-bromopropene excited at 193 nm include HBr elimination, two C-Br bond fission channels, and a product channel described by the total reaction C3H 5Br → Br + H2 + C3H3; we can not definitively determine whether H2 elimination occurs from the C 3H5Br molecule (followed by swift C-Br bond fission) or from hot mass 41 radicals formed after the C-Br bond breaks in the parent molecule. We also show evidence that the Br atom is formed in the excited (2 P1/2) spin-orbit state from the C-Br bond fission product channels.