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New Insights into Thermal Decomposition of Polycyclic Aromatic Hydrocarbon Oxyradicals Peng Liu, He Lin,* Yang Yang, Can Shao, Chen Gu, and Zhen Huang Key Laboratory for Power Machinery and Engineering of Ministry of Education, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China S Supporting Information *

ABSTRACT: Thermal decompositions of polycyclic aromatic hydrocarbon (PAH) oxyradicals on various surface sites including five-membered ring, free-edge, zigzag, and armchair have been systematically investigated by using ab initio density functional theory B3LYP/6-311+G(d,p) basis set. The calculation based on Hückel theory indicates that PAHs (3H-cydopenta[a]anthracene oxyradical) with oxyradicals on a five-membered ring site have high chemical reactivity. The rate coefficients of PAH oxyradical decomposition were evaluated by using Rice−Ramsperger−Kassel−Marcus theory and solving the master equations in the temperature range of 1500−2500 K and the pressure range of 0.1−10 atm. The kinetic calculations revealed that the rate coefficients of PAH oxyradical decomposition are temperature-, pressure-, and surface site-dependent, and the oxyradical on a five-membered ring is easier to decompose than that on a six-membered ring. Fourmembered rings were found in decomposition of the five-membered ring, and a new reaction channel of PAH evolution involving four-membered rings is recommended.

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) formed in incomplete combustion of hydrocarbons are the precursors of soot particles1−3 and were confirmed to be highly carcinogenic and mutagenic.4−7 In the past two decades, a large number of works have revealed the processes of PAH evolution and soot formation in flame.8−20 To our knowledge, experimental findings related to these complex and transient processes is rare because of the lack of experimental method. Therefore, theoretical study is a good way to understand both PAH evolution and soot formation. The soot model first proposed by Frenklach and Wang2 is very popular in predicting the evolution of PAHs in a premixed flame. In the soot model, the oxidation rates of PAHs are roughly considered to be equal to that of phenyl + O2,10,21,22 which have been experimentally determined by Lin et al.21 using shock waves in the temperature range from 1000 to 1800 K and pressure range from 0.4 to 0.9 atm. The estimated oxidation rate is unreasonable when the size of PAHs is larger than that of pyrene;3 for example, the oxidation rate coefficients of multiring aromatic species including soot are significantly lower than the rate coefficients of phenyl + O2 reaction by several orders of magnitude.1 In PAH oxidation, thermal decomposition of PAH oxyradicals is the key step and the thermal decomposition rates are definitely dependent on the type of PAH radical sites, such as free-edge, zigzag, and armchair.10,23−25 Recently, Edwards et al.25 found that the decomposition rate of the oxyradical with O on the outside of the armchair is larger than the rate with O on the inner side of the armchair. You et al.10 revealed that the pentacene oxyradicals with oxygen atom (O) attached to the zigzag are kinetically more stable than the pentacene oxy© 2014 American Chemical Society

radicals with O bonded to the free-edge. To date, thermal decomposition of PAH oxyradicals has received less attention, and a comprehensive investigation on thermal decompositions of PAHs with O atom bonded to different surface sites, including free-edge, zigzag, armchair, bay, and five-membered site types (as shown in Figure 1) should be done to obtain more accurate thermal decomposition rates, especially for large PAHs. In the soot formation mechanisms, PAHs with vinyl are an important intermediate in the growth of PAHs. Recent study found that the reaction of four-membered ring species with H

Figure 1. Principal surface site types of PAHs. Received: July 26, 2014 Revised: November 10, 2014 Published: November 11, 2014 11337

dx.doi.org/10.1021/jp510498j | J. Phys. Chem. A 2014, 118, 11337−11345

The Journal of Physical Chemistry A

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Table 1. Structures and HOMOs and LUMOs of 1,2-Benzanthracene Oxyradical I−IV and 3H-Cydopenta[a]anthracene Oxyradicala

a

The orbits are shown via GaussView software.46

atom is a competitive pathway of forming PAHs with vinyl.17 In the present soot model, however, the four-membered ring species are ignored because the formation of four-membered ring species via hydrogen abstraction-C2H2 addition (HACA) needs to overcome an extremely high energy barrier (nearly 60 kcal/mol). In this study, we attempt to find the possibility of the pathway leading to the formation of four-membered ring species through decomposition of five-membered ring, which extensively exist in flame. Therefore, thermal decompositions of five-membered rings will be paid special attention in this study. In this work, theoretical investigations on thermal decompositions of large PAHs, including 1,2-benzanthracene oxyradicals and 3H-cydopenta[a]anthracene oxyradicals were carried out. First, the thermodynamic stability of various PAH oxyradicals is checked based on simple Hückel theory, followed by exploring the potential energy surfaces (PES) of probable reaction pathways. Then, the rate coefficients of thermal decomposition reactions are determined by solving energytransfer master equations with Rice−Ramsperger−Kassel− Marcus (RRKM) theory. This work focuses on obtaining the decomposition rates of large PAH oxyradicals with different surface site types, including free-edge, zigzag, armchair, and five-membered ring, on both sides of the C−O bond. Furthermore, we also pay great attention to the reaction pathways of large PAH oxyradical decomposition on the basis of kinetic calculation and analysis.

hybrid function26,27 with the 6-311+G(d,p)28 basis set.1,10,29,30 The same method was applied to calculate zero-point energies (ZPE) and vibrational frequencies, which were scaled by a factor of 0.967.10,31 Restricted wave function calculations were employed for singlet, and unrestricted wave function calculations were employed for doublet, triplets, and quartets. All transition states were carefully confirmed by examining the motions corresponding to imaginary modes. The minimumenergy path from each transition state was monitored by intrinsic reaction coordinate (IRC) calculations32 to ensure that the transition states correctly connect with the corresponding energy minima structures. In addition, all of the structures were optimized in their ground states. All calculations were performed using the Gaussian 0933 program package. The rate coefficients of thermal decomposition of PAH oxyradicals were evaluated in the temperature range from 1500 to 2500 K and pressure range from 0.1 to 10 atm by using the MultiWell suite of codes (MultiWell-2013.1).34,35 With Monte Carlo stochastic method, MultiWell can solve the timedependent energy-transfer master equations for multiwell and multichannel reaction system. Microcanonical rate coefficients were calculated by employing RRKM theory, which was also adopted to calculate the high-pressure limit rates. The input parameters including vibrational frequencies, reaction barriers, and moments of inertia were obtained from the quantumchemical results at the B3LYP/6-311+G(d,p) level and are presented in Table S19 in the Supporting Information. The sums and densities of states of local minima structures and transition states were determined by exact count with an energy grain size of 10 cm−1. The maximum energy was 500 000 cm−1. The exponential-down model with ⟨ΔEdown⟩ = 260

2. CALCULATION DETAILS In this study, the geometries of transition states and local minima structures in PAH oxyradicals decomposition were optimized using the density functional theory (DFT) B3LYP 11338



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Table 2. Structure Parameters of PAH Oxyradicalsa

C1−O4 C1−C2 C1−C3 C2−C3 a

1,2-benzanthracene oxyradical I

1,2-benzanthracene oxyradical II

1,2-benzanthracene oxyradical III

1,2-benzanthracene oxyradical IV

3H-cydopenta[a]anthracene oxyradical

1.24415 1.46818 1.44713 2.48099

1.23361 1.47942 1.47706 2.52374

1.24611 1.48419 1.45556 2.51965

1.23540 1.48396 1.48633 2.54573

1.23591 1.45681 1.46051 2.28763

The unit of bond length is angstroms.

cm−1 was employed to describe the collisional energy transfer.36 In addition, detailed discussions on temperature-dependent exponential-down model with ⟨ΔEdown⟩ = 200 × (T/300 K)0.85 cm−137,38 are presented in the Supporting Information. In calculation, argon was chosen as the bath gas collider.39 The Lennard-Jones parameters of PAHs were calculated with the method proposed by Wang et al.,40 and the results are presented in Table S19 in the Supporting Information. The Lennard-Jones parameters σ and ε/kB of argon were equal to 3.47 Å and 114 K, respectively.36 The symmetry number corrections were carried out according to the study of Duncan.41,42 In this study, the real frequencies below 150 cm−1 were paid great attention, and the internal rotation modes were distinguished by graphically visualizing the normal mode vibrations. All internal rotations were treated as one-dimensional (1-D) hindered rotations,43 and the translational and vibrational temperatures were set to be equal. The relaxed potential energy surface scans calculation were carried out to obtain 1-D hindered rotors based on DFT B3LYP with 6-31G basis set. The tunneling correction was ignored because it has almost no influence on the rate coefficient at high temperatures (>1000 K)44 and the calculation of rate coefficients in this study were done in temperature range from 1500 to 2500 K. The number of stochastic trials was changed from 5 × 104 to 1× 107 to keep the statistical fluctuations below 3%. In addition, the uncertainty of MultiWell for determining rate coefficients of the multiwall and multichannel reactions was estimated to be within 1 order of magnitude.10,11,13,45

Table 3. HOMO−LUMO Gap of Reactants species

HOMO−LUMO gap (au)

1,2-benzanthracene oxyradical I 1,2-benzanthracene oxyradical II 1,2-benzanthracene oxyradical III 1,2-benzanthracene oxyradical IV 3H-cydopenta[a]anthracene oxyradical

0.13083 0.12972 0.13602 0.12916 0.0302

atom and C3 atom of 3H-cydopenta[a]anthracene oxyradical is shorter by nearly 11% compared to that of 1,2-benzanthracene oxyradical I−IV because the size of the five-membered ring is smaller than that of the six-membered ring. Therefore, the potential C2−C3 bond of 3H-cydopenta[a]anthracene oxyradical should be stronger, and this has been confirmed by subsequent energetic calculations. The kinetic stability can be quantitatively determined using HOMO−LUMO energy separation based on simple Hückel theory.47−51 A large HOMO−LUMO gap is energetically unfavorable to adding electrons to a high-lying LUMO, as well as unfavorable to extracting electrons from a low-lying HOMO, so as to reduce the probability of forming the activated complexes, which are essential in PAH oxidation reactions.47,48 In this study, the values of HOMO−LUMO gaps increase in the following order: 3H-cydopenta[a]anthracene oxyradical < 1,2-benzanthracene oxyradical IV < 1,2-benzanthracene oxyradical II < 1,2-benzanthracene oxyradical I < 1,2-benzanthracene oxyradical III, as shown in Table 3. The order suggests that 3H-cydopenta[a]anthracene oxyradical has the highest chemical reactivity and the lowest kinetic stability. This trend has been confirmed by subsequent kinetic calculation. It is noticed in Table 3 that the HOMO−LUMO gaps of 1,2benzanthracene oxyradical I−IV are close to each other, and it is difficult to determine the ranks of kinetic stability. 3.2. Potential Energy Surfaces. The potential energy surfaces for the thermal decomposition reactions of investigated PAH oxyradicals were explored at the B3LYP 6-311+G(d,p) level. To make the reaction routes clear, the PAH moiety near the oxyradical are enlarged in the potential energy diagram because the forming and breaking of bonds take place near the oxyradical. In addition, all complete structures are presented in Figures S2−S6 in the Supporting Information. In the system of 1,2-benzanthracene oxyradical I to S1-CS4 as shown in Figure 2, there are two pathways leading to S1-CS3, 1,2benzanthracene oxyradical I → S1-CS5 → S1-CS3 and 1,2benzanthracene oxyradical I → S1-CS2 → S1-CS3. In the first pathway, the process of 1,2-benzanthracene oxyradical I → S1CS5 involving the breaking of C1−C3 bond needs a high energy barrier of 93.3 kcal/mol because of the firm plane sixmembered ring structure. The subsequent process of S1-CS5 → S1-CS3 involves the formation of C2−C3 bond with energy barrier of 9 kcal/mol. In another pathway of 1,2-benzanthracene oxyradical I → S1-CS2→ S1-CS3, a new C2−C3 bond

3. RESULTS AND DISCUSSION In this study, 1,2-benzanthracene oxyradicals I−IV and 3Hcydopenta[a]anthracene oxyradical are investigated, as showed in Table 1. The PAH structures can be classified according to the situation of PAH surface site types on both sides of the C− O bond. In this way, the 1,2-benzanthracene oxyradicals I−IV represent two free-edges on six-membered ring (I), two zigzags on six-membered ring (II), armchair and free-edge on sixmembered ring (III), armchair and zigzag on six-membered ring (IV). The 3H-cydopenta[a]anthracene oxyradical represents the structure that has two free-edges on both sides of the C−O bond on a five-membered ring. 3.1. Comparisons of Structures and Molecular Orbitals of Reactants. The fully optimized structures and the shapes of highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1,2benzanthracene oxyradical I−IV and 3H-cydopenta[a]anthracene oxyradical at the B3LYP 6-311+G(d,p) basis set level are shown in Table 1. To make them comparable, the most important optimized geometrical parameters and HOMO−LUMO gaps are presented in Table 2 and Table 3, respectively. It is notable that the bond lengths of C1−O4, C1−C2, and C1−C3 of five PAH oxyradicals are close and the fluctuation is within 3%. However, the distance between C2 11339



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formation of S3-CS3 with a lower energy barrier of 3.9 kcal/ mol. In the last step, CO separates from S3-CS3 and generates the products of S3-CS4 and CO, the energy barrier of this step is 3.4 kcal/mol. The high-pressure limiting rate of 1,2benzanthracene oxyradical III → S3-CS2 is 6.25 × 108 s−1 at 2500 K, which is lower than that of 1,2-benzanthracene oxyradical I → S1-CS2 as shown in Figure S7 in the Supporting Information. In view of the high-pressure limiting rate, the decomposition of 1,2-benzanthracene oxyradical I is more likely to happen than that of 1,2-benzanthracene oxyradical III, and this was confirmed in the following kinetic analysis. As shown in Figure 4, thermal decompositions of 1,2benzanthracene oxyradical II and 1,2-benzanthracene oxyradical Figure 2. Potential energy diagram showing energies of the chemical species (CS) and transition states (TS) involved in the PAH thermal decomposition from 1,2-benzanthracene oxyradical I to S1-CS4 at 0 K.

is formed and requires a lower energy barrier of 47.2 kcal/mol in the process of 1,2-benzanthracene oxyradical I → S1-CS2. The subsequent process of S1-CS2 → S1-CS3 involves the breaking of existing C1−C3 bond with energy barrier of 17.9 kcal/mol. Elimination of CO from S1-CS3 leads to the products of S1-CS4 and CO, with the energy barrier of 2.6 kcal/mol. Compared with the first pathway, the pathway 1,2benzanthracene oxyradical I → S1-CS2 → S1-CS3→ S1-CS4 is more competitive in terms of the energy barrier, as shown in Figure 2. The contribution of pathway 1,2-benzanthracene oxyradical I → S1-CS5 → S1-CS3 can be ignored because the branching ratio of S1-CS5 to S1-CS2 is close to zero even at 2500 K. In the competitive pathway, the 1,2-benzanthracene oxyradical I → S1-CS2 step is the rate-limiting step because the high-pressure limiting rate of this step is smaller than that of other steps by at least 1 order of magnitude (as showed in Table S10 in the Supporting Information). The mechanistic behavior of 1,2-benzanthracene oxyradical III decomposition is similar to that of 1,2-benzanthracene oxyradical I, regarding the energy barrier and the forming and breaking of bonds, as shown in Figure 3. The pathway of 1,2benzanthracene oxyradical III decomposition is 1,2-benzanthracene oxyradical III → S3-CS2 → S3-CS3 → S3-CS4 + CO. The intermediate of S3-CS2 generates via the formation of C2−C3 bond with the energy barrier of 51.8 kcal/mol, followed by breaking of the C1−C2 bond, leading to the

Figure 4. Potential energy diagram showing energies of the chemical species (CS) and transition states (TS) involved in the PAH thermal decomposition from 1,2-benzanthracene oxyradical II to S2-CS2 and 1,2-benzanthracene oxyradical IV to S4-CS2 at 0 K.

IV are one-step reactions. These reactions generate directly the products of S2-CS2 + CO and S4-CS2 + CO respectively after overcoming the energy barrier of 83.1 and 80.1 kcal/mol. In PES calculation, it was found that no ring-opening pathway is feasible in 1,2-benzanthracene oxyradical II and 1,2-benzanthracene oxyradical IV decompositions because the ringopening intermediates of 1,2-benzanthracene oxyradical II and 1,2-benzanthracene oxyradical IV have severe internal torsional rotations, which will lead to the formation of nonplanar molecular structure without the probability of CO elimination. It was reported that thermal decomposition of pentacene oxyradical is infeasible10 though it has the same surface site as 1,2-benzanthracene oxyradical II with zigzag edges on both sides of the C−O bond. The difference between pentacene oxyradical and 1,2-benzanthracene oxyradical II suggests that the decomposition of such kinds of PAH oxyradicals is significantly sensitive to the structure. The decomposition pathway of 3H-cydopenta[a]anthracene oxyradical is shown as 3H-cydopenta[a]anthracene oxyradical → S5-CS2 → S5-CS3 + CO in Figure 5. The energy barrier of forming S5-CS2 is 9.8 kcal/mol, which is extremely low compared with the energy barrier of similar reaction steps such as 1,2-benzanthracene oxyradical I → S1-CS2 (47.2 kcal/mol). The energy needed for elimination of CO in the step of S5-CS2 → S5-CS3 is as high as 28.4 kcal/mol because of the strengthened C−C bond. This energy barrier is much higher than the energy needed in CO elimination in the thermal decomposition of 1,2-benzanthracene oxyradical I and 1,2benzanthracene oxyradical III, where almost 3 kcal/mol is

Figure 3. Potential energy diagram showing energies of the chemical species (CS) and transition states (TS) involved in the PAH thermal decomposition from 1,2-benzanthracene oxyradical III to S3-CS4 at 0 K. 11340



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fold rate deviation (as shown in Table S1−S4 in the Supporting Information). It is more likely that the deviation arises from stochastic noise or slightly different basis sets. The temperature- and pressure-dependent rate coefficients of decomposition of phenoxy, 1,2-benzanthracene oxyradical I− IV, and 3H-cydopenta[a]anthracene oxyradical are calculated and presented in Figure 6a−f, respectively, which show that all of the decomposition rates increase with the temperature and pressure (see also Table 4). For the thermal decomposition of phenoxy (Figure 6a) and 1,2-benzanthracene oxyradical I−IV (Figure 6b−e), a falloff region occurs only at intermediate to high temperature (1700−2500 K). It can be seen in Figure 6f that the thermal decomposition of 3H-cydopenta[a]anthracene oxyradical is clearly in the falloff region in the entire temperature range of 1500−2500 K. We found that the decomposition rates of 3H-cydopenta[a]anthracene oxyradical converge to the rates of the high-pressure limit at temperatures below 1200 K, as shown in Figure S8 in the Supporting Information. The detailed values of the decomposition rates are supplied in Tables S6−S18 in the Supporting Information. The decomposition rates of PAH oxyradicals with different surface sites are compared with each other and are shown in Figure 7. The rank of the rate coefficients is generally in the following order: 1,2-benzanthracene oxyradical II < 1,2benzanthracene oxyradical IV < 1,2-benzanthracene oxyradical III < 1,2-benzanthracene oxyradical I < 3H-cydopenta[a]anthracene oxyradical, which is consistent with the calculated results of PES. In PES analysis, the decompositions of 1,2benzanthracene oxyradical II and IV need to overcome the energy barriers of 83.1 and 80.1 kcal/mol, respectively, which are much higher than the highest energy barrier in decomposition of 1,2-benzanthracene oxyradical I (62.6 kcal/ mol) and III (54.7 kcal/mol). The highest energy barrier in decomposition of the five-membered ring (3H-cydopenta[a]anthracene oxyradical) is only 37.5 kcal/mol, indicating that five-membered ring is easier to decompose than the sixmembered ring. The decomposition rates of 3H-cydopenta[a]anthracene oxyradical are larger than that of other PAH oxyradicals by more than 1 order of magnitude at the same temperature and pressure. The results also agree with the study of Raj et al.30 who suggested that the formation of CO most likely comes from the oxidation of free-edge sites and 5membered rings. In this study, all of the pressure-dependent rate constants were obtained after the reaction had reached steady state, at which time the vibration energy content of the PAH oxyradicals remain unchanged. It is interesting that the decomposition reactions at high temperature (above 1600 K) reach the steady state only after most of the PAH oxyradicals have been decomposed. For example, the decomposition of 3Hcydopenta[a]anthracene oxyradical at 2000 K and 1 atm reaches the steady state as 99% of the reactant was decomposed, as shown in Figure 8. It is found that most (more than 60%) of 3H-cydopenta[a]anthracene oxyradical is decomposed with the rates larger than 2.13 × 109 s−1, which is close to the high-pressure limit rate (7.04 × 109 s−1). This phenomenon is also observed in the decomposition of other PAH oxyradicals at high temperature. As discussed above, the rate constants of PAH oxyradical decomposition at low temperatures usually converge to the rates of high-pressure limit, as shown in Figure 6. Therefore, we deduce that most of the PAH oxyradicals are decomposed with the rate constants

Figure 5. Potential energy diagram showing energies of the chemical species (CS) and transition states (TS) involved in the PAH thermal decomposition from 3H-cydopenta[a]anthracene oxyradical to S5CS4 at 0 K.

enough. Therefore, the step of CO elimination is the ratelimiting step in the thermal decomposition of 3H-cydopenta[a]anthracene oxyradical, as shown in Table S18 in the Supporting Information. The high-pressure limiting rate of S5CS2 → S5-CS3 is sizable with the value of 1.62 × 1011 s−1 at 2500 K, suggesting that the decomposition of 3H-cydopenta[a]anthracene oxyradical is highly competitive compared with that of other PAH oxyradicals. This result is in accord with the thermodynamic stability analysis, which shows that 3Hcydopenta[a]anthracene oxyradical has the highest chemical reactivity and the lowest kinetic stability among all of the investigated PAH oxyradicals. It is notable that the plane S5CS3 is the PAH with a four-membered ring, and was first found in oxidation of 3H-cydopenta[a]anthracene oxyradical in this study. 3.3. Rate Coefficients. In this study, the rate coefficients are calculated based on PES calculation at DFT B3LYP 6311+G(d,p) level. First, we check the calculation method by comparing the decomposition rates of phenoxy in this study with that in the literature.10,21 The theoretical data in the literature were calculated by You et al.10 at the level of DFT theory with B3LYP hybrid functional 6-311 G(d,p) basis set. The experimental data were measured by Lin et al.21 using shock waves and are effective in the temperature range from 1000 to 1800 K and the pressure range from 0.4 to 0.9 atm. As shown in Figure 6a, the calculated decomposition rates of phenoxy at 1 atm agree with that of You et al.10 in the whole temperature range, and are close to the experimentally measured values. At 0.1 atm, the rates calculated in this study are close to that of You et al.10 in the temperature range of 1400−2500 K, and slightly lower than that of You et al. in the low-temperature range of 1000−1400 K. To explore the reason for the deviation of rates at 0.1 atm, the comparison of highpressure limit rates is carried out as shown in Figure 6a. The high-pressure limit rates calculated in this study are slightly lower than that of You et al, and the deviation is no more than 100%. We speculate that the rate deviation does not result from the parameters setting (external symmetry number, optical isomers), which may influence the reaction path degeneracy and cause the rate constants of high-pressure limit differing in multiples of 2. The sensitive analysis of external symmetry number, optical isomers, maximum energy, and collisional energy model also confirms our speculation because the changes of these parameters will produce the more than 211341



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Figure 6. Rate coefficients of decomposition reaction of PAH oxyradicals in pressure range from 0.1 to 10 atm: (a) phenoxy, (b) 1,2-benzanthracene oxyradical I, (c) 1,2-benzanthracene oxyradical II, (d) 1,2-benzanthracene oxyradical III, (e) 1,2-benzanthracene oxyradical IV, and (f) 3Hcydopenta[a]anthracene oxyradical.

In this channel, the formed four-membered ring can react with a H atom to produce PAHs with vinyl, and this has been confirmed by Richter.17 The addition reaction of four-

obtained at nonsteady state and are close to the high-pressure limit rates in some cases. In summary, the formation of a four-membered ring via thermal decomposition of five-membered ring is feasible through PES and kinetic analysis in this study. On the basis

H

membered ring → PAHs with vinyl is an exothermal reaction (nearly 60 kcal/mol), and the energy barrier of it is no more than 6.5 kcal/mol, as shown in Figure S1 in the Supporting Information. The produced PAHs with vinyl will grow up to be either PAHs with a six-membered ring or PAHs with a fivemembered ring via HACA.

O

of this, a new channel of PAH evolution, six-membered ring → O

H

five-membered ring → four-membered ring → PAHs with vinyl HACA

⎯⎯⎯⎯⎯⎯→ six-membered ring or five-membered ring, is proposed. 11342



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Table 4. Rate Coefficients in the Form ATn exp(−E/RT)a no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

A

reaction phenoxy → C5H5 (cyclopentadienyl radical) + CO 1,2-benzanthracene oxyradical I → S1-CS4 + CO 1,2-benzanthracene oxyradical II → S2-CS2 + CO 1,2-benzanthracene oxyradical III → S3-CS4 + CO 1,2-benzanthracene oxyradical IV → S4-CS4 + CO 3H-cydopenta[a]anthracene oxyradical → S5-CS3 + phenoxy → C5H5 + CO 1,2-benzanthracene oxyradical I → S1-CS4 + CO 1,2-benzanthracene oxyradical II → S2-CS2 + CO 1,2-benzanthracene oxyradical III → S3-CS4 + CO 1,2-benzanthracene oxyradical IV → S4-CS4 + CO 3H-cydopenta[a]anthracene oxyradical → S5-CS3 + 1,2-benzanthracene oxyradical I → S1-CS4 + CO 1,2-benzanthracene oxyradical II → S2-CS2 + CO 1,2-benzanthracene oxyradical III → S3-CS4 + CO 1,2-benzanthracene oxyradical IV → S4-CS4 + CO 3H-cydopenta[a]anthracene oxyradical → S5-CS3 + phenoxy → C5H5 (cyclopentadienyl radical) + CO 1,2-benzanthracene oxyradical I → S1-CS4 + CO 1,2-benzanthracene oxyradical II → S2-CS2 + CO 1,2-benzanthracene oxyradical III → S3-CS4 + CO 1,2-benzanthracene oxyradical IV → S4-CS4 + CO 3H-cydopenta[a]anthracene oxyradical → S5-CS3 +

3.57 1.20 1.06 3.87 5.79 4.13 3.61 8.82 2.19 1.76 1.44 3.27 7.64 4.57 7.62 5.69 4.85 9.25 5.31 6.21 4.03 5.09 1.03

CO

CO

CO

CO

× × × × × × × × × × × × × × × × × × × × × × ×

49

10 1015 1058 1029 109 1023 1045 109 1067 1023 1011 106 106 107 1019 1012 108 107 1016 1013 1014 1013 1016

n

E

pressureb

−10.74 −1.83 −13.11 −5.775 −0.108 −3.939 −9.363 −0.063 −14.96 −3.681 −0.1 1.156 1.131 1.33 −2.44 0.004 0.7312 1.96 −0.633 0.121 −0.51 0.096 −0.529

72.56 32.04 101.62 46.14 50.32 30.06 72.56 33.01 125.12 48.00 59.36 21.41 36.00 69.32 50.18 72.16 24.68 51.68 66.6 85.64 55.74 82.66 40.70

0.1 0.1 0.1 0.1 0.1 0.1 1 1 1 1 1 1 10 10 10 10 10 ∞ ∞ ∞ ∞ ∞ ∞

The units are kcal/mol, K, and s−1. The effective temperature range is from 1500 to 2500 K, and the unit of pressure is atm. b∞ indicates highpressure limit, and the high-pressure limit decomposition rates are calculated using steady-state approximation. Specific formulas are presented in Table 21 in the Supporting Information.

a

Figure 7. Comparison of decomposition rates of different PAH oxyradicals at 1 atm.

Figure 8. Decomposition rates of 3H-cydopenta[a]anthracene oxyradical at 2000 K and 1 atm.

4. CONCLUSIONS In this work, the thermal decompositions of large 1,2benzanthracene oxyradical I−IV and 3H-cydopenta[a]anthracene oxyradical were systematically investigated at the DFT B3LYP 6-311+G(d,p) level. We calculated the values of HOMO−LUMO gaps and found that the 3H-cydopenta[a]anthracene oxyradical has the highest chemical reactivity and lowest kinetic stability. The rate coefficients were calculated by solving energy-transfer master equations with RRKM theory in the temperature range of 1500−2500 K and the pressure range of 0.1−10 atm. The results indicated that the decomposition rates of PAH

oxyradicals are temperature-, pressure-, and surface sitedependent. Thermal decomposition rates of five PAH oxyradicals increase in the order of 1,2-benzanthracene oxyradical II < 1,2-benzanthracene oxyradical IV < 1,2-benzanthracene oxyradical III < 1,2-benzanthracene oxyradical I < 3Hcydopenta[a]anthracene oxyradical. The calculation of rate coefficient indicated that the five-membered ring is easier to decompose than the six-membered ring. It was noticed that most of the PAH oxyradicals were decomposed before the reaction reaches steady state, which means the rates for decomposing the bulk of PAH oxyradicals are larger than that obtained at steady state. In view of the calculated data and the kinetic analysis, we speculate that the thermal decomposition of the five-membered 11343



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ring is likely to generate the PAHs with four-membered ring, and a new channel of PAH evolution, namely six-membered O

O

H

ring → five-membered ring → four-membered ring → PAHs HACA

with vinyl ⎯⎯⎯⎯⎯⎯→ six-membered ring or five-membered ring, is highly possible. In this way, the five-membered ring plays a key role in PAH evolution, and more attention should be paid to four-membered ring species in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Potential energy surface of reaction C6H5 + C2H2 (Figure S1); all structures involving 1,2-benzanthracene oxyradicals I−IV and 3H-cydopenta[a]anthracene oxyradicals decomposition (Figures S2−S6); rate coefficients of thermal decomposition of PAH oxyradicals (Tables S5−S18); relative zero-point energies, values of S2 operator, rotational coefficients, and vibrational frequencies (Table S19); Cartesian coordinates for optimized structures at the B3LYP/6-311+G(d,p) level of theory (Table S20); the derived steady-state approximation formula for the high-pressure limit rates (Table S21); details of computational methodology, discussion of ⟨ΔE⟩down, external symmetry number, optical isomers, and maximum energy. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51210010) and the National Basic Research Program of China (973 Program) (2013CB228502).

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REFERENCES

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