The cyclopentadienyl radical (cC(5)H(5)) is a fascinating molecule characterized by several peculiar properties, such as its high internal symmetry and resonance enhanced stability. This makes cC(5)H(5) 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 cC(5)H(5) into C(7)H(7) and C(9)H(8) in the presence of acetylene. In this work, we used ab initio calculations and quantum Rice-Ramsperger-Kassel (QRRK) theory to investigate the cC(5)H(5) + C(2)H(2) reaction kinetics. We found that cC(5)H(5) can add acetylene to form, through a fast and not previously known reaction, the heptatrienyl radical (cC(7)H(7)), which, in many ways, can be considered the superior homologue of cC(5)H(5). The calculated reaction kinetic constant is (2.2 x 10(11))exp(-6440/T(K)) cm(3) mol(-1) s(-1) and is in good agreement with experimental data, while that of the inverse process is (4.2 x 10(16))T(-1) exp(-30 850/T(K)) s(-1). In a successive reaction, cC(7)H(7) can add a second acetylene molecule to form indene, cC(9)H(8), and H. The forward and backward kinetic constants are (6.6 x 10(11))exp(-10 080/T(K)) and (4.2 x 10(14))exp(-27 300/T(K)) cm(3) 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.

Quantum chemistry was used to investigate the kinetics of the reactions of 1,3-C4H6 and 1,3-butadien-1-yl with phenyl and benzene, respectively, with the aim of elucidating mechanisms that might lead to the formation of naphthalene and indene. Kinetic constants for each elementary reaction involved in the reactive processes were calculated with density functional theory and a modified G2MP2 method. Small vibrational frequencies, when necessary, were treated as hindered rotors to calculate their rotational potential energies quantum mechanically and the corresponding partition functions. Global rate constants for the formation of the different products were determined with QRRK theory. The main result of this study is that if 1,3-C4H6 and 1,3-butadien-1-yl are formed in significant amounts then they can contribute significantly to the formation of naphthalene in a flame. It was also found that activation energies and reaction enthalpy changes can be influenced significantly by the level of theory adopted in the calculations, with B3LYP differing from the more accurate G2MP2 calculations by up to 7 kcal/mol. This was attributed to the known problems of DFT in describing radicals having multiple resonance structures. The calculated rates of formation of indene for the investigated reaction channels were all too slow to compete with alternative mechanisms proposed in the literature.

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