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

Abstract

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. 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.