Cyclodehydrogenation Reactions to Cyclopentafused Polycyclic Aromatic Hydrocarbons Angela Violi* Department of Mechanical Engineering, UniVersity of Michigan, Ann Arbor, MI 48109 ReceiVed: May 8, 2005 B3LYP/6-31G(d,p) electronic structure calculations are employed to elucidate the reaction mechanisms for the conversion of the alternant C 18 H 12 polycyclic aromatic hydrocarbon benzo[c]phenanthrene into the nonalternant C 18 H 10 PAHs cyclopenta[cd]pyrene and benzo[ghi]fluoranthene. Isomerization reactions such as 5/6-ring switching and hydrogen atom scrambling are analyzed. Bay region chemistry, involving the rupture of one benzene ring followed by the formation of a new five-membered ring, is also studied, together with the mechanism for the formation of an aryne. The rearrangement of the latter yields annelated cyclopenta- dienylidenecarbene, which is then trapped intramolecularly. Introduction Polycyclic aromatic hydrocarbons (PAHs) containing fully unsaturated five-membered rings as integral components of their trigonal carbon networks (CP-PAHs) have attracted considerable attention in different fields. 1-3 Some of these nonalternant hydrocarbons exhibit unusual photophysical behavior 4,5 and biological activity. 6 Toxicity studies provide strong evidence that CP-PAHs are primarily responsible for the genotoxicity of combustion mixtures. 7-9 Relative to other PAHs, CP-PAHs demonstrate a greater facility to undergo certain kinds of reactions, such as isomer- ization involving intramolecular rearrangement. 7,10 This is due to the fact that fusion of the cyclopenta ring alters the electronic properties of the PAH, as demonstrated by differences in the resonance energy 11 and measured differences in ultraviolet- visible (UV-vis) absorption 12 and fluorescence. 4,5 The presence of five- and six-membered rings may also provide the structural property of nonplanarity to CP-PAHs. Bowl-shaped CP-PAHs such as corannulene constitute building blocks for fullerenes. 13-17 Recently, CP-PAHs have been widely found in combustion systems (e.g., Lafleur et al. 18 ); they have been observed as pyrolysis products of anthracene, 19,20 pyrene, 21 and benzene 22 and as combustion products of benzene, 23 ethylene, and ethyl- ene-naphthalene 24 mixtures. Wornat et al. 25 pyrolyzed brown coal and, through the analysis of the product tar, identified several different CP-PAHs. Consequently, the elucidation of CP- PAH formation processes under high-temperature conditions is an important research topic. According to Badger’s free radical mechanism, 26,27 simple aromatic hydrocarbons first lose hydrogen through a C-H bond cleavage, producing aryl radicals. Recombination of these radicals yields biaryls, which in turn are subject to further dehydrogenation. The energy of the C-H bond in simple PAHs is determined mostly by steric effects. 28 Consequently, the loss of hydrogen is expected to occur preferentially at sterically congested sites, where new C-C bonds may readily form, implying the propensity of aryls to undergo, whenever possible, cyclodehydrogenation rather than dehydrogenative polymeri- zation. Little is known about the basic mechanism of thermally induced cyclodehydrogenation reactions. Two hydrogen atoms are lost, and a new ring is created by the formation of a new transannular C-C bond. The details concerning the order of the steps and the nature of the reactive intermediates are still missing, and most of the reaction mechanisms for cyclodehy- drogenation do not account for the presence of CP-PAHs. The importance of cyclodehydrogenation reactions and in- terconversions of PAHs to combustion chemistry, combined with the lack of definite experimental evidence for the mech- anisms, has emphasized the need to carry out a comprehensive state-of-the-art electronic structure study of cyclodehydrogena- tion of benzo[c]phenanthrene (1) (Figure 1). This compound has been proposed to play an important role in competing PAH growth pathways, i.e., the buildup of planar versus nonplanar CP-PAHs. 29,30 Thermolysis of 1 produces not only benzo[ghi]- fluoranthene but also cyclopenta[cd]pyrene. Evidence for the viability of this process has been obtained from flash vacuum thermolysis (FVT) experiments. 31-36 These reactions are be- lieved to proceed through several stages, 29,37 including dehy- drogenation as well as rearrangements such as intramolecular trapping, H scrambling, and five- to six-membered interconver- sion. 9,38-40 A detailed analysis of these reactions together with the elucidation of efficient high-temperature routes is reported in this paper. After the first two sections, which describe the formation pathways from benzo[c]phenanthrene to benzo[ghi]- fluoranthene and cyclopenta[cd]pyrene, respectively, the inter- conversion of the latter two nonalternant C 18 H 10 PAHs is reported. Reaction rates are computed for the pathways analyzed together with the thermodynamic data of all the intermediates involved in the reaction steps. Since benzo[c]phenanthrene may be regarded as a rigid analogue of 1-phenylnaphthalene, and Cioslowsky et al. 41 reported a detailed analysis of its thermally induced cyclode- hydrogenation, some of the results presented in this study for benzo[c]phenanthrene are compared with those obtained for 1-phenylnaphthalene. Details of the Calculations All calculations in this study were carried out with the GAUSSIAN G03 suite of programs. 42 Geometries of all species * E-mail: violi@eng.utah.edu; until Jan 2006 at the University of Utah, Department of Chemical Engineering and Department of Chemistry. 7781 J. Phys. Chem. A 2005, 109, 7781-7787 10.1021/jp052384r CCC: $30.25 © 2005 American Chemical Society Published on Web 08/02/2005