Indene Formation from Alkylated Aromatics: Kinetics and Products of the Fulvenallene plus Acetylene Reaction

A novel reaction is described for formation of the polyaromatic hydrocarbon (PAH) indene in aromatic flames, via the reaction of fulvenallene with acetylene (C2H2). Fulvenallene has been recently identified as the major decomposition product of the benzyl radical, the dominant intermediate in the oxidation of alkylated aromatic hydrocarbons, yet it is not presently included in kinetic models for aromatic oxidation or PAH/soot formation. Ab initio calculations with the G3B3 theoretical method show that acetylene adds to fulvenallene with a barrier of around 27 kcal mol(-1). This forms an activated C9H8 adduct that can rearrange to indene and dissociate to 1-indenyl + H with energy barriers below that of the entrance channel, Master equation Simulations across a range of temperature and pressure conditions demonstrate that for temperatures relevant to combustion indene is the dominant product at high pressures while 1-indenyl + H dominate at lower pressures. At low to moderate temperatures, the production of collision stabilized cyclopentadiene-fulvene intermediates is also significant. The results presented in this study provide a new pathway to cyclopenta-fused PAHs in aromatic combustion and are expected to improve modeling of PAH and soot formation. The formation of cyclopenta-fused C-5-C-6 structures is required to describe the flame synthesis of carbon nanoparticles like fullerenes and buckybowls (corannulene). Improved rate expressions are also reported for the 1-indenyl + H -> indene association reaction, and for the reverse dissociation, from variational transition state theory calculations. The new rate constants are significantly different than Current estimates, primarily due to a re-evaluation of the indene C-H bond dissociation energy.