The Peculiar Kinetics of the Reaction between Acetylene and the Cyclopentadienyl Radical

The cyclopentadienyl radical (cC5H5) is a fascinating molecule characterized by several peculiar properties, such as its high internal symmetry and resonance enhanced stability. This makes cC5H5 one of the most abundant radicals present in high temperature gaseous environments, such as flames. Therefore it is generally considered an interesting candidate as the starting point of reaction pathways leading to the formation of polycyclic aromatic hydrocarbons (PAH) and soot in combustion processes. However, known reaction pathways are not able to explain some recent experimental findings concerning the rapid conversion of cC5H5 into C7H7 and C9H8 in the presence of acetylene. In this work, we used ab initio calculations and quantum Rice−Ramsperger−Kassel (QRRK) theory to investigate the cC5H5 + C2H2 reaction kinetics. We found that cC5H5 can add acetylene to form, through a fast and not previously known reaction, the heptatrienyl radical (cC7H7), which, in many ways, can be considered the superior homologue of cC5H5. The calculated reaction kinetic constant is (2.2 × 1011)exp(−6440/T(K)) cm3 mol-1 s-1 and is in good agreement with experimental data, while that of the inverse process is (4.2 × 1016)T -1 exp(−30 850/T(K)) s-1. In a successive reaction, cC7H7 can add a second acetylene molecule to form indene, cC9H8, and H. The forward and backward kinetic constants are (6.6 × 1011)exp(−10 080/T(K)) and (4.2 × 1014)exp(−27 300/T(K)) cm3 mol-1 s-1, respectively. These two successive reactions, leading from a single C5 cycle to a bicyclic C5−C6 species, represent a new PAH growth mechanism, characterized by a C5−C7 ring enlargement reaction.