Scission of the Five-Membered Ring in 1--Inden-1-one CHO and Indenyl CH in the Reactions with H and O Atoms.

Quantum chemical G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the CHO potential energy surface were utilized to investigate the mechanism of the 1--inden-1-one (CHO) + H and indenyl (CH) + O reactions and were combined with Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations to predict temperature- and pressure-dependent reaction rate constants and product branching ratios. The most favorable reaction pathways for CHO + H lead to the bimolecular CH + CO products, which are slightly endothermic with respect to the reactants. The reaction begins with H addition to the or C atoms in the five-membered ring, CHO + H → /, and then proceeds by isomerization to , ( →) → . From thereon, the → → → and → → pathways lead to -vinyl phenyl + CO, whereas → → → and CHO + H → → → produce styrenyl + CO. The results of the RRKM-ME calculations showed that only the well-skipping CHO + H → CH (/) + CO mechanism is relevant under combustion conditions. A comparison with a smaller prototype 2,4-cyclopentadienone + H → CH + CO reaction demonstrated that the H atom is a less efficient destroyer of a cyclopentadienone-like moiety when this moiety is linked to an aromatic or a PAH structure. The CH + O reaction begins with highly exothermic barrierless addition of the oxygen atom to the radical site in the five-membered ring of indenyl producing and then, the reaction mostly proceeds by β-scission in the five-membered ring, which may be preceded by H migration to or , and completes by the CO loss forming the highly exothermic CH radical products. Modified Arrhenius expressions for the rate constants of all reactions pertinent to the formation of CH + CO from CHO + H, CH + H, and unimolecular decomposition of benzopyranyl have been generated and suggested for combustion kinetic modeling. It is concluded that the oxidation reactions of a five-membered ring with atomic oxygen remain fast in the presence of attached or surrounding six-membered rings.