The formation of polycyclic aromatic hydrocarbons (PAHs) and carbon-based nanoparticles in combustion environments is driven by the pyrolysis and oxidation of abundant fuel molecules. The subsequent growth of these pyrolytic byproducts is then propagated by reactions with small free radical species like OH, CH, O atoms, and C2H. Cyclopentadiene (C5H6) is a significant five-member combustion intermediate that has been detected in many high temperature reactions, such as in the pyrolysis of jet fuels like JP-10. Thorough investigations into the reactions of cyclopentadiene with these small free radical species are necessary towards understanding the initial rate-limiting steps in the overall PAH growth scheme. Experimental kinetic rate constants are obtained for the CH radical reaction with cyclopentadiene under pseudo-first order conditions in a quasi-static reaction cell. The CH radical is generated by pulsed laser photolysis (PLP) of bromoform (CHBr3) at 266 nm and the concentration is monitored using laser-induced fluorescence (LIF) at an excitation wavelength of 430 nm. Isomeric products and associated branching ratios are measured for CH and OH radical reactions with cyclopentadiene using multiplexed photoionization time-of-flight mass spectrometry coupled to synchrotron radiation at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratories in Berkeley, California. Density functional theory (DFT) and composite basis set (CBS) methods are used to calculate the potential energy surface for mechanistic discussions of the previous reactions as well as the Franck-Condon Factors used for determining the branching ratios of the products from the ALS data. Master Equation calculations using the MESMER open-source program are performed to infer mechanistic information and product branching ratios about the respective products for comparison with the experiment. The evidence supports a fast, barrierless cycloaddition of the CH radical along the pi bonds of the five-member ring to form C6H6 isomers (benzene, fulvene, among others) and a H atom. Preliminary results from the OH radical reactions suggest the main pathways as association/substitution to form isomers of C5H5OH + H or abstraction of a H atom to form resontantly-stabilized cyclopentadienyl radicals (C5H5).

Investigations into small free radical reactions with cyclopentadiene: Pathways to polycyclic aromatic hydrocarbon growth

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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