Recombination Reactions in the Thermal Decomposition of Anisole: An Investigation of Benzene and Naphthalene Formation*

Thermal decompositions of anisole (C$_{6}$H$_{5}$OCH$_{3})$ and methyl-deuterated anisole (C$_{6}$H$_{5}$OCD$_{3})$ are studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to a supersonic expansion after a residence time of 65 $\mu$s, allowing detection of highly chemically reactive radical species. Anisole decomposes through loss of a methyl group ({\textbullet}CH$_{3})$ to form phenoxy radical (C$_{6}$H$_{5}$O{\textbullet}), followed by ejection of a CO to form cyclopentadienyl radical (c-C$_{5}$H$_{5}$; CPDR). Benzene is generated primarily by thermal decomposition of methylcyclopentadiene (C$_{5}$H$_{5}$CH$_{3}$; MCPD). The MCPD results from methyl radical recombination with CPDR. The MCPD then undergoes two hydrogen atom losses and a ring expansion resulting in benzene. At T$_{wall}$ = 1200 \r{ }C -- 1300 \r{ }C a large amount of propargyl radical (CH$_{2}$CCH) is observed. Propargyl radical recombination accounts for a small fraction of the observed benzene. Naphthalene and its precursor intermediates (C$_{10}$H$_{10}$, C$_{10}$H$_{9})$, resulting from CPDR recombination, are also observed. The presence of benzene and naphthalene is confirmed with resonance-enhanced multiphoton ionization (REMPI).