Scission of the Five-Membered Ring in 1-H-Inden-1-one C₉H₆O and Indenyl C₉H₇ in the Reactions with H and O Atoms

Quantum chemical G3(MP2,CC)//B3LYP/6-311G(d,p) calculations of the C₉H₇O potential energy surface were utilized to investigate the mechanism of the 1-H-inden-1-one (C₉H₆O) + H and indenyl (C₉H₇) + 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 C₉H₆O + H lead to the bimolecular C₈H₇ + CO products, which are slightly endothermic with respect to the reactants. The reaction begins with H addition to the ortho or meta C atoms in the five-membered ring, C₉H₆O + H → w2/w3, and then proceeds by isomerization to w1, (w3 →) w2 → w1. From thereon, the w1 → w9 → w8 → p1 and w2 → w8 → p1 pathways lead to ortho-vinyl phenyl + CO, whereas w1 → w10 → w11 → p2 and C₉H₆O + H → w10 → w11 → p2 produce styrenyl + CO. The results of the RRKM-ME calculations showed that only the well-skipping C₉H₆O + H → C₈H₇ (p1/p2) + CO mechanism is relevant under combustion conditions. A comparison with a smaller prototype 2,4-cyclopentadienone + H → C₄H₅ + 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 C₉H₇ + O reaction begins with highly exothermic barrierless addition of the oxygen atom to the radical site in the five-membered ring of indenyl producing w1 and then, the reaction mostly proceeds by β-scission in the five-membered ring, which may be preceded by H migration to w2 or w10, and completes by the CO loss forming the highly exothermic C₈H₇ radical products. Modified Arrhenius expressions for the rate constants of all reactions pertinent to the formation of C₈H₇ + CO from C₉H₆O + H, C₉H₇ + H, and unimolecular decomposition of benzopyranyl have been generated and suggested for combustion kinetic modeling. Furthermore, 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.