The bay-capping mechanism on PAH armchair edges and the kinetics of acetylene addition to 6-6-5 and 5-6-5 bays have been explored by ab initio/RRKM-ME calculations. The bays on the edges were modeled by C21H11 and C20H9 radicals produced by H abstractions from 7H-benzo[c]cyclopenta[e]pyrene and dicyclopenta[cf]pyrene. The C20H9 + C2H2 reaction is shown to have a low entrance barrier and to rapidly form the capped product, indaceno[2,1,8,7-cdefg]pyrene, along with ethynyl substituted dicyclopenta[cf]pyrene at temperatures above 1400 K. The reactivity of C21H11 is shown to be governed by the location of the unpaired electron; the p radical R1 formed by H abstraction from the CH2 group in 7H-benzo[c]cyclopenta[e]pyrene reacts with C2H2 very slowly owing to a high entrance barrier, with the bay-capping rate constant approaching 10-16 cm3 molecule-1 s -1 only at temperatures above 2000 K. This result reaffirms that the growth of p aryl radicals via acetylene addition is inefficient and reflects the generally low reactivity of such radicals where the spin density is highly delocalized over the entire polyaromatic system. Alternatively, the s C21H11 radical R2 produced by H abstraction from the five-membered ring at the bay rapidly reacts with C2H2 forming the bay-capped product, with the rate constant on the order of 10-12 cm3 molecule-1 s -1 at T &gt; 1500 K. Rate constants for the capping reactions at the 6-6-5 and 5-6-5 bays are compared with those at the 6-0-6, 6-6-6, and 6-5-6 bays. The site-specific bay-capping rate constants have been utilized in kMC simulations of the PAH growth and the results showed measurable differences when the 6- 6-5 and 5-6-5 bay-capping reactions are taken into account, including an increase of the growth rate and the formation of closed-shell PAH and a rise of the number of embedded five-membered rings accompanied with a slight decrease of their overall amount.

Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

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