Atmospheric oxidation mechanism of chlorobenzene

Chemosphere 111 (2014) 537–544

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Atmospheric oxidation mechanism of chlorobenzene Runrun Wu a, Sainan Wang a, Liming Wang a,b,⇑ a b

School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou 510006, China

h i g h l i g h t s  OH addition to chlorobenzene occurs at o-, m-, and p-sites.  Reaction of chlorobenzene-o-OH adduct with O2 proceeds mainly by H-abstraction.  Chlorophenol yields in chlorobenzene oxidation are high.  Chlorophenol yields in chlorobenzene oxidation are pressure-dependent.  Chlorinated nitrobenzenes could be formed in small yields.

a r t i c l e

i n f o

Article history: Received 25 November 2013 Received in revised form 8 April 2014 Accepted 18 April 2014

Handling Editor: X. Cao Keywords: Chlorobenzene Atmospheric oxidation mechanism Chlorophenols OH radical

a b s t r a c t The atmospheric oxidation mechanism of chlorobenzene (CB) initiated by the OH radicals is investigated at M06-2X/6-311++G(2df, 2p) and ROCBS-QB3 levels. The oxidation is initiated by OH addition to the ortho (50%), para (33%) and meta (17%) positions, forming CB-OH adducts as R2, R3, and R4; while the ipso-addition is negligible (0.2%). The reactions of the CB-OH adducts with the atmospheric oxygen are further investigated in detail by coupling the unimolecular reaction rate theory calculations with master-equation (RRKM-ME). The CB-OH adducts react with O2 either by irreversible H-abstraction to form chlorophenol and HO2 or by reversible additions to form CB-OH–O2 radicals, which subsequently cyclize to bicyclic radicals. RRKM-ME calculations show that the addition reactions of CB-OH and O2 at the atmospheric pressure are close to but not yet reach their high-pressure-limits. The RRKM-ME simulations predict the yields of 93%, 38%, and 74% for ortho-, meta- and para-chlorophenols from the reactions of O2 with R2, R3 and R4, being lower than their high-pressure-limit yields of 95%, 48%, an 80%, respectively. Overall, the yield of chlorophenols is determined as 72% at the atmospheric pressure. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Chlorobenzene (CB) is used mainly as intermediate in the synthesis of pesticides and other chemicals, and as solvent carrier for pesticides, rubber polymers and textiles dyes. CB is released into the environment upon its production and usage (EPA, 1995). With a vapor pressure of 1665 Pa at 298 K, CB exists mainly as gaseous form in the atmosphere. Concentrations of chlorobenzene in the atmosphere range typically from tens of ppt in remote areas to 1 ppb in cities, although high concentrations at tens of ppb could be detected near the sources such as the solid waste incineration sites (Chen et al., 2007; Yan et al., 2010). Chlorobenzene is possibly removed from the atmosphere by its reactions with OH, NO3, and O3, photolysis and deposition. Atkinson et al. have measured the rate constants for reactions ⇑ Corresponding author at: School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China. Tel.: +86 2787112900. E-mail address: [email protected] (L. Wang). http://dx.doi.org/10.1016/j.chemosphere.2014.04.067 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

and photolysis, and found that CB would be removed mainly by its reaction with the OH radicals, while photolysis and reactions with ozone or nitrate radicals are of negligible importance (Atkinson et al., 1985; Edney et al., 1986). Rate constant measurements (Mulder and Louw, 1987; Wallington et al., 1987; Bryukov et al., 2009) and theoretical study (Bryukov et al., 2009) found that the reaction of CB with OH would proceed via OH addition to the benzene ring at the atmospheric temperatures, while the Habstraction and Cl-displacement become dominant only at hightemperatures. Theoretical calculations at different levels reached the same conclusion that the ortho- and para-additions would dominate over the meta- and ispo-additions, of which the barrier heights of ispo-addition is 12 kJ/mol higher than others at all levels of calculations. This is different from the additions of OH to toluene (Suh et al., 2002; Wu et al., 2014), where the barrier height of ispo-addition is close to the others. The atmospheric lifetime of chlorobenzene due to its reaction with the OH radicals can be estimated as 13 days with the average OH concentration of 106 molecule cm3 (Atkinson et al., 1985);

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however, little information is available on the ultimate fate of CB in the atmosphere. In current study, the fate of CB in the atmosphere is investigated. 2. Theoretical methods All the molecular structures, rotational constants and vibrational frequencies (in Supplementary Data) are calculated using density functional method of M06-2X/6-311++G(2df, 2p), which is suitable for structure optimizations and kinetic calculations (Zhao and Truhlar, 2008). The M06-2X structures are subject to electronic energy calculations by using the restricted-open-shell complete-basis-set model chemistry (ROCBS-QB3) (Wood et al., 2006) for reliable energies in kinetics calculations. High-pressurelimit rate constants for unimolecular (kuni, in s1) and bimolecular (kb, in cm3 molecule1 s1) reactions are estimated by traditional transition state theory (TST) as (Pilling and Seakins, 1999):

kb ¼ r  j 

  kB T Dr G– RT 106  exp   o h RT P NA

kuni ¼ j  kB T=h  expðDr G– =RTÞ where kB is the Boltzmann constant, DrG– the Gibbs energy barrier,

r the reaction-path symmetry number, j the tunneling correction factor using the asymmetric Eckart model (Johnson and Heicklen, 1962). All the quantum chemistry calculations are performed using Gaussian 09 package (Frisch et al., 2009). RRKM-ME calculations are carried out to investigate the reactions between CB-OH adducts and O2 using the Mesmer program (Glowacki et al., 2012). The E-resolved microcanonical rate constants and collisional exponential-down model (hDEid = 250 cm1) are employed to construct the master equation. The unimolecular rate constants are evaluated by the unimolecular RRKM theory (Holbrook et al., 1996):

kðEÞ ¼

rm– g –e WðE  E0 Þ  hqðEÞ r– mg e

where r and r– are the external rotational symmetry numbers for the reactants and transition state, m and m– the numbers of optical isomer, ge and g – e the electronic state degeneracy, E0 the reaction critical energy, W(E  E0) the sum of states of the transition state, and q(E) the density of states of the reactants. The collisional parameters are estimated by the method of (Gilbert and Smith, 1990) as e = 720 K and r = 6.7 Å for CB-OH–O2 radicals. The RRKM-ME results at high pressures are equivalent to the results from traditional transition state theory. 3. Results and discussion 3.1. The initial OH addition Four adducts Rn (n = 1–4) are possibly formed in the OH addition to chlorobenzene:

0.7 kJ/mol at ROCBS-QB3 level are systematically lower than the M06-2X values of 23.6, 8.5, 12.8 and 8.3 kJ/mol for OH additions to C1–C4 positions, respectively; while the reaction energies from M06-2X and ROCBS-QB3 agree within 4 kJ/mol. Our ROCBSQB3 barrier heights are also 2–3 kJ/mol lower than the CBS-QB3 values of 14.1, 2.6, 4.6, and 2.2 kJ/mol and 12–13 kJ/mol lower than the G3//B3LYP values of 24.0, 12.1, 14.9, and 12.4 kJ/mol by (Bryukov et al., 2009). The high barriers by G3//B3LYP are most likely due to the heavy spin contamination in the post-Hartree– Fock calculations based on the unrestricted wavefunction in G3; while CBS-QB3 contains the correction term for spin-contamination (Montgomery et al., 2000), therefore the barrier heights by CBS-QB3 are only slightly higher than those by ROCBS-QB3. The estimated high-pressure-limit rate constant for the OH addition (kb,298K) is 5.56  1013 cm3 molecule1 s1 if using the ROCBS-QB3 barrier heights and 1.26  1014 cm3 molecule1 s1 if using the M06-2X ones. The former agrees well with the experimental values of (8.8 ± 1.1)  1013 cm3 molecule1 s1 at atmospheric pressure (Atkinson et al., 1985), (7.41 ± 0.94)  1013 cm3 molecule1 s1 at 50 Torr (Wallington et al., 1987), and (6.02 ± 0.34)  1013 cm3 molecule1 s1 at 100 Torr (Bryukov et al., 2009). M06-2X greatly overestimates the barrier heights and therefore underestimates the rate constants. Nevertheless, ROCBS-QB3 and M06-2X predict similar branching ratios for R1–R4, e.g., 0.00, 0.50, 0.17, and 0.33 with ROCBS-QB3 versus 0.00, 0.49, 0.11, and 0.40 with M06-2X. At both levels of theory, addition to ispo-site is negligible, agreeing with the previous theoretical result (Bryukov et al., 2009). In the following, the fate of R2, R3, and R4 are examined on the basis of the reaction energies and barrier heights at ROCBS-QB3 level unless otherwise specified. 3.2. Fate of chlorobenzene-OH adducts In the atmosphere, CB-OH adducts (R2, R3 and R4) can be sufficiently thermalized by collision, and are subject to reactions with O2, NOx, and HOx, etc. In analogy to the atmospheric oxidation mechanism of benzene and toluene (Suh et al., 2003; Glowacki et al., 2009; Wu et al., 2014), the reactions of CB-OH adducts with O2 can proceed as direct H-abstraction to form (o-, m- and p-) chlorophenol and HO2 irreversibly or as addition to form peroxyl radicals reversibly. Besides back-decomposition to R2 + O2, the available unimolecular pathways for the peroxyl radicals include ring-closure to bicyclic radicals, intramolecular H-shift from –OH to –OO, or concerted HO2 elimination to chlorophenols. The peroxyl radicals might also react with the atmospheric trace radicals such as NOx, HO2, and RO2 to form the alkoxy radicals and other products. In the example of R2, its reaction with O2 forms o-chlorophenol irreversibly by direct H-abstraction and R2-iOO-a/s (i = 1, 3 and 5) by reversible addition to C1, C3 and C5, where i denotes the position of O2 addition and a/s (anti/syn) represents the O2 approaching from the opposite or same direction of the ring with respect to the –OH group: Cl

Cl 1

Cl HO

6

2

5

3

Cl

Cl

Cl

OH

OH

Cl 1

+ OH 6

OH

4 R1

R2

R3

OH R4

Transition states for the four additions are located at M06-2X level. Table 1 lists the calculated reaction energies and barrier heights, along with the estimated rate constants using transition state theory. The barrier heights ðDE– 0K Þ of 11.2, 0.9, 2.3 and

OH

+ O2 − HO2 o-chlorophenol

2 3

5 4 R2

Cl + O2

Cl

OO OH

R2-1OO-a/s

Cl OH

OO R2-3OO-a/s

OH

OO R2-5OO-a/s

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Table 1 Calculated relative energies (DE0K and DG298K, all in kJ/mol) for transition states and adducts at M06-2X and ROCBS-QB3 levels and the high-pressure-limit rate constants (kforward in cm3 molecule1 s1 and kreverse in s1) and branching ratios (C).a Species

M06-2X

DE0K TS-R1 TS-R2 TS-R3 TS-R4 kb (Total) kb (Expt.) R1 R2 R3 R4 a b c d e

ROCBS-QB3 kforwardb

DG298K

23.6 8.5 12.8 8.3

60.4 45.2 48.9 44.0

80.2 74.6 69.4 74.2

42.0 36.7 31.8 36.6

kreverse 18

6.63  10 6.20  1015 1.35  1015 5.01  1015 1.26  1014 (8.8 ± 1.1)  1013c,

DE0K

C 6

7.1  10 2.9  102 4.5  102 4.7  102

0.00 0.49 0.11 0.40

DG298K

11.2 0.9 2.3 0.7

47.9 35.7 38.4 35.0

kforwardb

kreverse

C

1.00  1015 2.78  1013 9.37  1013 1.84  1013 5.56  1013

2.5  104 4.9  101 1.3  100 6.6  101

0.00 0.50 0.17 0.33

(7.41 ± 0.94)  1013d, (6.02 ± 0.34)  1013e 83.8 45.6 77.8 39.1 71.5 33.9 76.6 39.0

Ground state of OH as 2P3/2. Path symmetry number s = 1, 2, 2, and 1 for ipso-, ortho-, meta-, and para-additions. From Atkinson et al. (1985). From Wallington et al. (1987). From Bryukov et al. (2009).

It is noticed that the O2 additions to R2 radicals are reversible. The reversibility diminishes the role of R2-5OO-a/s radicals

The available unimolecular pathways for R2-iOO-a/s include, in the example of R2-1OO-a/s:

Cl

OOH OH

H-shift

Only possible for R2-1OO-a

Cl OH

− HO2

Only possible for R2-1OO-a

Cl 6 5

OO 1

OH

o-chlorophenol

2 3

4 R2-1OO-a/s

O

OOH O

Cl H-shift

Cl

O

− OH

R2-1OOH-a/s Cl Ring closure

Cl

Cl OH

O O R2-13OO-a/s

where the bicyclic radicals are denoted as R2-ijOO-a/s (i and j: the numbering of the carbon atoms connected by –O–O–). Similar reaction pathways are expected for R2-3OO-a/s and R2-5OO-a/s. Transition states are located for all these possible reaction pathways at M06-2X level, and their reaction energies and barrier heights at M06-2X level are listed in Table S1 of Supplementary Material. A large number of reaction pathways can be safely removed from the reaction mechanism by simply referring to their much high energy barriers at M06-2X level, e.g., the cyclization of R2-1OO-s to R2-15OO-s with M06-2X barrier of 152.3 kJ/mol would be negligible if being compared to the cyclization to R2-13OO-s with barrier of 63.2 kJ/mol. For the ‘important’ pathways thus obtained, ROCBSQB3 calculations are carried out for more reliable reaction energies. Table 2 lists the reaction energies for these pathways, along with the high-pressure-limit rate constants, and Fig. 1 shows the potential energy diagram. Negative barriers may arise from the formation of pre-reactive complexes formed between adducts and O2.

O O R2-14OO-a/s

O OH

OH

O

Cl OH

O O R2-15OO-a/s

R2-16OO-a/s

because their forward ring-closures and intramolecular H-shift are hindered by the high Gibbs barriers of >120 kJ/mol (M06-2X, relative to R2 + O2), which are much higher than the barriers of 66.0 and 59.8 kJ/mol for their backward decomposition to R2 + O2. Furthermore, formation of R2-5OO-a/s from R2 and O2 is endothermic by 15.0 and 15.6 kJ/mol (DG298K, ROCBS-QB3). Therefore, the fate of R2-5OO-a/s is to decompose back to R2 + O2 at rates of 2.15  107 s1 and 1.92  108 s1, and R2-5OO-a/s play negligible roles in oxidation of atmospheric chlorobenzene, even though the barriers for O2 additions to C5 of R2 (46.2 and 41.4 kJ/ mol for a-/s-additions) are lower than those to C1 (49.7 and 50.9 kJ/mol) and C3 (53.0 and 55.4 kJ/mol, DG– 298K by ROCBS-QB3). This is similar to the cases of alkyl benzenes and naphthalenes. The additions of O2 to C1 and C3 of R2 can be then modeled as: in which, the ring-closures are considered as being irreversible because of their exothermicity of at least 35 kJ/mol (DE0K).

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Cl

Cl

OO

OH O O

6

R2-1OO-a − HO2

OH

− HO2

o-chlorophenol

Cl

Cl

+ O2

O O

Cl

+ O2

OH

− HO2

+ O2 Cl

OO

OH

R2-1OO-s

4

+ O2 OH

R2-3OO-a

OH 2 3

5 Cl

OH

Cl 1

+ O2

R2-13OO-a

OO

Cl

R2

OH

R2-3OO-s

OH

R2-13OO-s

OO

o-chlorophenol

High-pressure-limit rate constants of all the steps can be found in Table 2. Time evolution of each species is obtained by solving a coupled differential equation set analytically using a MATLAB code (Fig. 2). Simulations found that the fractions of o-chlorophenol (o-CP), R2-13OO-a and R2-13OO-s are 0.95, 0.00 and 0.05. However, it is unclear whether the reactions are at their highpressure-limit under the ambient atmospheric pressures. In the study of benzene oxidation, Glowacki et al. (2009) argued that some unimolecular reactions may not be at their high-pressure limits under typical atmospheric conditions. To examine the validity of the high-pressure-limit approximation in chlorobenzene oxidation, RRKM-ME calculations are carried out here for the additions of O2 to R2, R3, and R4 and O2 by using MESMER code. Fig. 2 shows the timedependent profiles under pseudo first order conditions of

yields of chlorophenols as a function of pressure. For reaction of R2 and O2, the RRKM-ME calculations give a yield of 0.93 for o-CP, which is slightly different from the high-pressure-limit value of 0.95; but for R3 and R4, the high-pressure- limit approximation overestimates the yields of m- and p-CP significantly. For adduct R3, the reactions with O2 form m-chlorophenol by direct H-abstraction and R3-2OO-a/s and R3-4OO-a/s by addition, while the role of R3-6OO-a/s is negligible. For R3-2OO-s and R34OO-s, their isomerization to R3-24OO-s would dominate over the H-shift and other cyclization processes; while for R3-2OO-a and R3-4OO-a, the concerted HO2-eliminations to form m-chlorophenol are favored over their cyclization to R3-24OO-a by 8 kJ/ mol in barrier heights (Table 2). Thus, the reaction mechanism of R3 with O2 can be simplified as:

Cl

Cl

Cl

R3

OO O O

6

R3-2OO-a

2 3

5 Cl

− HO2

+ O2

1

+ O2

OH

OH R3-24OO-a Cl

OO

Cl

OH

4

+ O2

+ O2

OH R3-2OO-s

O O

Cl

OH R3-24OO-s

+ O2 − HO2 OH m-chlorophenol

Cl

Cl

− HO2

OH

OH

OO R3-4OO-a

OO R3-4OO-s OH m-chlorophenol

[O2]  [R2], and Fig. 3 shows several phenomenological rate coefficients as a function of pressure. Obviously, all the elementary reactions have not reached their high-pressure limits yet at 760 Torr. The product yields changes with the reaction pressures. Fig. 4 shows the

in which, the products formed from the reaction of R3 and O2 are R3-24OO-s and m-CP, with the branching ratios of 0.62 and 0.38 by RRKM-ME simulations, or 0.52 and 0.48 by high-pressure-limit approximation.

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Similarly for adduct R4, the reaction with O2 can be modeled as:

Cl

R4

O O

Cl

Cl

Cl

Cl

Cl

1

OH R4-35OO-a

+ O2 OO

− HO2 OH R4-3OO-a

6

2

5

3

+ O2 O O OO

4 OH + O2 − HO2

OH R4-3OO-s

OH R4-35OO-s

OH p-chlorophenol

in which p-CP formation from HO2 elimination of R4-3OO-a is included because its Gibbs barrier (58.6 kJ/mol, relative to R4 + O2) is close to that for of cyclization of R4-3OO-s to R435OO-s (59.6 kJ/mol, Table S1). The overall yield of p-CP is 0.73, of which 0.53 is from direct H-abstraction and 0.20 from concerted HO2-elimination from R4-3OO-a at 760 Torr from RRKM-ME calculations; while the yield of p-CP is predicted as 0.84 at high-pressure-limit. The effective removal rates of R2, R3, and R4 in the atmosphere can be obtained from the time profiles shown in Fig. 2, as 980, 1100, and 363 s1 for their reactions with O2, and the effectively rate constants of 2.0  1016, 2.2  1016, and 7.3  1017 cm3 molecule1 s1, respectively. The detailed splitting of the effective rate constants into direct H-abstraction, the bicyclic radical routes Rn + O2 ? Rn-iOO-a/s ? Rn-ijOO-a/s, and the HO2-elimination routes Rn + O2 ? Rn-iOO-a ? chlorophe-

nol + HO2 is listed in Table 3. The slow reactions of R2/R3/R4 with O2 lead to certain importance of the reactions of Rn with the atmospheric NO2, for which the rate constants are 1011 cm3 molecule1 s1 by referring to the reactions of NO2 with benzene-OH or toluene-OH (Koch et al., 2007). Assuming [NO2] of 100 ppb, the bimolecular rates for Rn with NO2 can reach 25 s1, resulting in the estimated yields of 2.5%, 2.2%, and 6.4% for chlorinated nitrobenzene from R2, R3, and R4, respectively. On the other hand, the possible reaction between Rn-iOO-s and NO/NO2 can be ignored for which the rates of cyclization at 760 Torr are 3.2  104, 5.0  104, 7.7  104, 2.2  104, and 2.0  104 s1 for R2-1OO-s, R2-3OO-s, R3-2OO-s, R3-4OO-s, and R4-3OO-s, respectively, all being much faster than their possible reactions with the atmospheric NOx. The competition between H-abstraction and addition was also observed for the reactions of O2 with benzene-OH and toluene-OH

60

40

20

0

−20

Fig. 1. Gibbs energy diagram for reaction between C6H5Cl-o-OH and O2 at ROCBS-QB3 level with M06-2X geometries (in kJ/mol).

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0.99

1.0

(a) R2

760 Torr

o-CP

0.95

0.93 0.8

0.66

0.79

Branching Ratio

0.33 R2-13OO-s

0.00

Species Profiles

0.99

(b) R3

0.66

R3-24OO-s

0.33

m-CP

0.73 0.6

0.48

0.4 0.38 0.2

Ortho-CP Meta-CP Para-CP

0.0 0

0.99

1

10

0.00

2

10

10

3

4

10

5

10

10

Pressure (Torr) (c) R4 Fig. 4. Dependence of yield of chlorophenols from reactions of R2, R3, and R4 on reaction pressures.

p-CP

0.66 0.33

R4-35OO-s

Cl

Cl OH

0.00

0.50

0.000

0.002

0.004

0.006

0.008

(0.93)

0.010

Time (/s) Fig. 2. Time-dependent species profile of R2/R3/R4 and their products for in their reactions with O2 at 298 K and [O2] of 5  1018 molecule cm3 (solid lines: highpressure-limits, dash lines: RRKM-ME results at 760 Torr).

Cl 1 6

Cl 2

Cl

(0.38) OH Cl

−1

k (s )

760 Torr 6

10

5

10

4

10

3

10

2

10

1

10

0

10

-1

10

-2

O O

OH Cl

0.33

OH

(0.07)

(0.62) OH

Cl

(0.73)

OH

OH O O Cl

0.17

3

5 4

10

Cl OH

O O

(0.27)

OH

Scheme 1.

77%, being much higher than the yield of 20% for cresols in the oxidation of toluene. R2---R2-1OO-a R2---R2-1OO-s R2---Ortho-CP R2-1OO-a---R2 R2-1OO-s---R2 R2-1OO-a---R2-13OO-a R2-1OO-a---R2-13OO-s

0

1000

2000

3000

4000

Pressure (Torr) Fig. 3. Phenomenological rate constants for several elementary steps at 298 K and [O2] of 5  1018 molecule cm3.

adducts. Table 3 summarizes the barrier heights and the resulted effective rate constants for CB-OH, benzene-OH, and toluene-OH, and the predicted rate constants at the same level of theory. From toluene to benzene to CB, the barriers for both the H-abstraction and the cyclization processes increases slowly, resulting in slowly decreased rate coefficients. The high barriers in the cyclization of R2-1OO-s and R4-3OO-s lead to much reduced formation of the bicyclic radical intermediates, i.e., the branching ratio of only 7% in R2 reaction with O2. Consequently, the yield of chlorophenols in OH-initiated oxidation of chlorobenzene in the atmosphere is

4. Concluding remarks The atmospheric oxidation mechanism of chlorobenzene initiated by the OH radicals is investigated at M06-2X/6-311++G (2df,2p) and ROCBS-QB3 levels. The mechanism at the typical atmospheric condition (298 K, 760 Torr) is summarized in Scheme 1. The oxidation starts as the OH addition, and the adducts formed would react with O2 to form (o-, m- and p-) chlorophenols and bicyclic intermediates. Overall, 77% of chlorophenols are formed from the oxidation of chlorobenzene, and the high yield of chlorophenols may impose different environmental impact (Czaplicka, 2004).

Acknowledgments We acknowledge the financial support from NSF China (No. 21177041) and the service support from SCUTGrid from Information Network Engineering and Research Center of South China University of Technology.

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Table 2 Calculated energy changes (DG– 298K and DG298K, all in kJ/mol) for additions of chlorobenzene-OH adducts (R2, R3, and R4) and O2 at M06-2X and ROCBS-QB3 levels, and the rate constants at 1 atm pressure and at high-pressure limit (HPL) (bimolecular reactions in cm3 molecule1 s1 and unimolecular reactions in s1). Reactions

M06-2X

R2 + O2 ? R2-1OO-a R2 + O2 ? R2-1OO-s R2 + O2 ? R2-3OO-a R2 + O2 ? R2-3OO-s R2 + O2 ? R2-5OO-a R2 + O2 ? R2-5OO-s R2 + O2 ? o-chlorophenol + HO2 R2-1OO-a ? R2-13OO-a R2-1OO-a ? o-chlorophenol + HO2 R2-1OO-s ? R2-13OO-s R2-3OO-a ? R2-13OO-a R2-3OO-a ? o-chlorophenol + HO2 R2-3OO-s ? R2-13OO-s R3 + O2 ? R3-2OO-a R3 + O2 ? R3-2OO-s R3 + O2 ? R3-4OO-a R3 + O2 ? R3-4OO-s R3 + O2 ? m-chlorophenol + HO2 R3-2OO-a ? R3-24OO-a R3-2OO-a ? m-chlorophenol + HO2 R3-2OO-s ? R3-24OO-s R3-4OO-a ? R3-24OO-a R3-4OO-a ? m-chlorophenol + HO2 R3-4OO-s ? R3-24OO-s R4 + O2 ? R4-3OO-a R4 + O2 ? R4-3OO-s R4 + O2 ? p-chlorophenol + HO2 R4-3OO-a ? R4-35OO-a R4-3OO-a ? p-chlorophenol + HO2 R4-3OO-s ? R4-35OO-s

ROCBS-QB3

DG298K

DG– 298K

DE0K

DG298K

DE– 0K

DG– 298K

kforward (1 atm/HPL)

kreverse (1 atm/HPL)

7.5 13.9 5.8 9.6 11.1 12.6 – 14.9 – 30.2 13.2 – 25.9 3.1 5.3 2.1 4.9 – 24.8  39.2 19.6 – 38.8 2.6 7.7 – 11.3 – 30.1

74.9 73.4 75.4 75.0 66.0 59.8 70.6 83.4 79.2 63.2 87.0 78.5 67.5 81.5 71.7 80.6 73.2 73.7 82.3 77.6 61.9 92.3 84.7 65.8 80.9 72.7 72.7 89.8 75.9 71.0

36.7 32.5 30.9 29.3 27.6 30.2 – 35.7 – 50.0 41.5 – 53.2 35.5 35.4 39.7 36.2 – 47.8 – 61.5 43.6 – 60.8 35.4 33.6 – 36.1 – 52.6

9.9 14.4 12.2 14.5 15.0 15.6 – 32.7 – 47.2 35.0 – 37.3 8.6 8.9 3.8 8.3 – 43.5 – 57.1 38.7 – 56.5 8.7 11.6 – 31.2 – 48.7

4.4 5.3 9.2 10.7 2.2 3.5 9.4 60.4 58.1 42.6 56.9 54.1 39.6 13.3 4.6 13.6 7.0 11.7 57.7 53.0 39.9 65.3 60.0 43.1 15.3 8.4 13.3 64.0 52.2 43.7

49.7 50.9 53.0 55.4 46.2 41.4 51.4 63.6 57.7 46.0 63.3 56.1 45.7 57.0 49.4 57.0 52.1 53.4 62.4 54.7 44.7 70.8 61.8 47.8 59.0 53.3 55.0 69.1 49.9 48.0

3.5/5.0  1016 2.2/3.0  1016 1.2/1.3  1016 4.7/5.0  1017 2.0  1015 1.4  1014 1.8/2.5  1016 1.6/4.4  101 4.8  102 3.2/5.6  104 2.6/5.0  101 9.4  102 5.0/6.0  104 2.6/2.6  1017 5.1/5.5  1016 2.6/2.6  1017 1.9/1.9  1016 0.8/1.0  1016 6.0/7.1  101 1.6  103 7.7/9.2  104 2.2/2.4  100 9.3  101 2.2/2.5  104 2.4/2.4  1017 2.1/2.3  1016 3.8/5.8  1017 3.5/4.8  100 1.0/1.1  104 2.0/2.5  104

4.2/6.6  105 1.6/2.5  106 3.5/4.3  105 3.5/4.2  105 2.1  107 1.9  108 – 8.3  105 – 3.0  104 3.7  105 – 3.2  104 1.9/2.1  104 4.0/4.9  105 2.9/2.9  103 1.1/1.3  105  1.7  106 – 9.0  106 4.0  107 – 3.2  106 8.5/9.7  103 2.5/3.1  105 – 1.7  105 – 7.2  105

Table 3 3 1 1 Calculated barrier heights (DG– s ) for reactions between O2 and OH-adducts of 298K , in kJ/mol, relative to aromatics-OH + O2) and effective rate constants (in cm molecule toluene, benzene, and chlorobenzene: H-abstraction, O2 syn-addition  cyclization, and O2 anti-addition  HO2-elimination. OH-Adducts

H-Abs

Toluene-o-OH

49.2

Benzene-OH CB-o-OH

50.9 51.4

CB-m-OH

CB-p-OH

53.4

55.0

O2 Addition C1-s: 45.2 C3-s: 50.9 C2-s: 50.7 C1-s: 50.9 C3-s: 55.4 C1-a: 49.7 C3-a: 53.0 C2-s: 49.4 C4-s: 52.1 C2-a: 53.4 C4-a: 57.0 C3-s: 53.3 C3-a: 59.0

Cyclization C1C3: C1C3: C2C6: C1C3: C1C3:

HO2-Elimi.

47.1 48.8 53.7 60.4 60.2

kH-Abs 3.7  10

kEff:HO2 -Elimination 16

3.6  1019 2.8  1019

67.6 68.3 7.9  1017

3.8  1017

C3C5: 59.6

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9.6  1017 3.2  1017 7.90  1018 7.86  1019

63.3 65.6

Appendix A. Supplementary material

13.6  1016 2.1  1016 1.5  1016 7.3  1018 3.4  1018

3.0  1016 1.8  1016

C2C4: 53.6 C2C4: 56.1

58.6

kEff.Cyclization

1.9  1017 1.4  1017

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