Kinetic models of combustion and pyrolysis processes are often highly sensitive to bimolecular rate coefficients and product branching ratios for radical-molecule reactions. Experimental measurements of such reactions are scarce due to the transience of reaction intermediates, particularly under combustion-relevant conditions. While transient absorption spectroscopy is a reliable method to determine the rates of radical reactions, no information is obtained about product branching or secondary chemistry. The Chemical Dynamics Laboratory at MIT has developed a unique apparatus for the simultaneous detection of 1) photolytic generation and decay of the radical reactant by laser absorption spectroscopy and 2) mass- and time-resolved products by VUV-PIMS. This apparatus was used to investigate the addition of acetylene molecules to vinyl radical. The VUV-PIMS product measurements show evidence of temperature and pressure dependent branching to the hydrogen loss and adduct stabilization channels, as well as formation of benzene following a second acetylene addition to the 1,3-butadiene-1-yl radical adduct. These results indicate that stable products of secondary reactions can be detected and quantified using our apparatus. The hydrogen-abstraction-acetylene-addition (HACA) mechanism, of key importance in the formation of polycyclic aromatic hydrocarbons (PAH), can lead to ring closing via sequential acetylene additions; quantitative measurements of the HACA and ring-closing steps for aromatic radicals are thus invaluable for validating combustion models. VUV-PIMS results for bimolecular reactions of acetylene and aromatic radicals (phenyl, naphthyl) and comparison with the predicted product branching from ab initio calculations will be discussed.

Acetylene addition to vinyl and aryl radicals: Experimental investigation of HACA and PAH formation pathways

256th National Meeting (2018)

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