Dissociation/Isomerization of Selected Radicals: allyl, 2-propenyl, 1-propenyl

These experiments use a technique, introduced in spring 2000 with our experiments on 2-propenyl radicals, to probe the competing unimolecular dissociation channels of isomerically-selected hydrocarbon radicals as a function of internal energy in the radical. Radical intermediates play a key role in a wide range of chemical processes, yet even after decades of the finest research the reactions of many key isomeric radical intermediates elude direct experimental probes. We photolytically produce from an appropriate precursor a selected radical isomer and disperse the radicals by their neutral velocity imparted in the photolysis, thus dispersing them by internal energy in the neutral time-of-flight spectrum. For the unstable radicals, we then measure the branching between C-C and C-H fission products via tunable VUV photoionization of products dispersed by their arrival time at the detector. For the C-H fission channels this offers the unprecedented ability to measure the branching between isomeric product channels as a function of internal energy in the dissociating radical isomer on the ground state potential energy surface. Our first experiments in 2000-2001, on C₃H₅ radical isomers, contrast the branching between the C-C and two C-H fission product channels from allyl vs. 2-propenyl vs. 1-propenyl radical isomers. They measure the product branching between the allene + H and the propyne+H product channels as a function of internal energy from 0 to 20 kcal/mol above the allene+H product asymptote (see energy diagram of the C₃H₅ radical isomers). We are continuing these studies with the unimolecular reactions of six different C₄H₇ radical isomers, probing the C-C and C-H fission reactions of each isomer with VUV photoionization. We are also using the same methodology to investigate radical intermediates in bimolecular reactions. For instance producing rotationally excited unstable CH₃O radicals dispersed by internal energy allows us to study the importance of large impact parameters in the O + CH₃ bimolecular reaction, particularly the influence of angular momentum in branching to the H₂ + HCO product channel.