On Prompt Dissociation of ro-vibrationally Hot Radicals Following Exothermic Abstractions of Hydrocarbons

Non-thermal ro-vibrational energy distributions which are often neglected in modeling complex combustion systems can be accessed following exothermic reactions through prompt product channels. However, it can be difficult to isolate these prompt channels (compared to the uncertainty in rate constants) in experiments (particularly at high temperatures) without excellent time resolution. Furthermore, there is uncertainty in estimating the branching fraction to prompt products. The work presented here has focused upon prompt channels from exothermic H abstractions by fluorine, anticipating that high exothermicity could give sufficient isolation of a prompt channel. The present work investigates reactivity by measuring prompt product concentrations. Three different systems have been studied. In the first system, propyl radical isomers (generated from F + propane) have prompt formation of ethylene and propene. In other systems, 1-methyl allyl radical (generated from F + 1-butene and F + cis-2-butene) dissociates promptly to 1,3-butadiene. Experiments involved shock heating nitrogen trifluoride (NF3) + propane in argon and NF3 + butene isomers in argon in two shock tubes. Single pulse shock tube experiments over a temperature range of 900 K < T < 1400 K and pressures between 3 and 4 bar resulted in product concentrations using GC-MS (gas chromatography -mass-spectrometry). High repetition rate shock tube experiments with tunable diode laser absorption spectroscopy over a temperature range of 1000 K < T < 1350 K and pressures between 3 and 4 bar resulted in time resolved HF concentrations. The research presented here showcases the combined power of experimental kinetics measurements and theoretical and modeling efforts which led to the a) assignment of the rate coefficients for the dissociation of NF3 which had large discrepancy, the precursor which acted as a thermal source of F atoms, b) assign H abstraction rate by F atom for propane and butene isomers by measuring time resolved HF absorbance, c) measure stable product concentrations from GC-MS for pyrolysis experiments, d) resolve secondary chemistry (F + alkenes/alkynes) addition-eliminations, and e) estimate branching fractions for F + hydrocarbons including prompt dissociation pathways to distinguish prompt effects from misassigned thermal rate coefficients by analyzing prompt product concentrations from H abstraction experiments of hydrocarbons by F atom.