A density functional theory study on the kinetics of seven-member ring formation in polyaromatic hydrocarbons

Abstract

In this work, the kinetics of seven-member ring formation in polycyclic aromatic hydrocarbons (PAHs) containing a five-member ring is studied by density functional theory. The pathways studied include integration of a seven-member ring by the hydrogen-abstraction-acetylene-addition (HACA) mechanism for two different PAHs, one closed shell and one resonance-stabilised-radical (RSR) PAH. The pathways were similar in both cases, but the rate of seven-member ring formation by HACA was seen to be faster for the resonance-stabilised-radical PAH. Formation of a seven member ring by bay closure processes facilitated through hydrogen abstraction, hydrogen addition, carbene formation, and direct cyclisation were also studied for two PAHs. In general, the pathways were rather similar for both PAHs, aside from the direct cyclisation route. The rate constants determined for the pathways were then used in kinetic simulations in 0D homogeneous reactors. The results showed that for the RSR PAH, the initial abstraction site is important, with the seven-member ring mainly being formed when abstraction occurs near the five member ring. This was not the case for the closed shell PAH. Additionally, the RSR PAH product was able to undergo an azulene to naphthalene-like transformation at longer timescales. For the bay closures, it was seen for both PAHs that the hydrogen abstraction facilitated bay closures contributes the most to seven-member ring formation at temperatures up to 2000 K, but for very high temperatures of 2500 K, the carbene route becomes the most important contributor. The formation of seven-member rings occurred within

Kinetic Monte Carlo statistics of curvature integration by HACA growth and bay closure reactions for PAH growth in a counterflow diffusion flame

“…1(b) results in a seven-member ring coupled to a partially embedded five-member ring. The process rates were taken from Menon et al [32].…”

“…Figure 1(c) and (d) show new processes for the closure of seven-member bays adjacent to five-member rings. The process rates were taken from Menon et al [32]. Figure 1(e) shows a new process that embeds a fivemember ring at a six-member bay site that includes a partially embedded five-member ring.…”

“…The assumption that this rate can be applied to the process in Fig. 1(e) is made on the grounds that hydrogen abstraction is expected to be the most likely route and is expected to have similar rates in both processes, coupled with the observation [32] that the rates of the processes in Fig. 1(a) and (b) are similar and insensitive to the location of the five-member ring, and likewise for the processes in Fig.…”

“…Recently, Menon et al [32] calculated rates for the formation of seven-member rings on bay sites containing five-member rings using density functional theory at the M06-2X/cc-PVTZ//B3LYP/6-311+G(d,p) level of theory. The ring formation mechanisms included hydrogen-abstraction-facilitated, hydrogenaddition-facilitated, carbene formation, and direct cyclisation bay closure processes and closure via HACA growth.…”

“…The purpose of this paper is to study the development of curvature in PAHs due to HACA growth and bay closure reactions in KMC simulations of a counterflow diffusion flame. The KMC model uses the process rates calculated by Menon et al [32], for first time enabling simulation of the growth of an ensemble of PAHs that include seven-member rings. The model results are consistent with experimental mass spectra and give insight into the relative abundance and location of PAHs containing embedded five-and seven-member rings.…”