Ab initio study of the formation and degradation reactions of semiquinone and phenoxyl radicals

Journal of Molecular Structure: THEOCHEM 848 (2008) 16–23 www.elsevier.com/locate/theochem

Ab initio study of the formation and degradation reactions of semiquinone and phenoxyl radicals Cheri A. McFerrin, Randall W. Hall *, Barry Dellinger Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA Received 8 June 2007; received in revised form 4 September 2007; accepted 11 September 2007 Available online 18 September 2007

Abstract Calculations of the energetics of formation, stability, and reactivity of o-semiquinone, p-semiquinone, and phenoxyl radicals have been performed using B3LYP/6-31G(d,p), BHandHLYP/6-31G(d,p), BHandHLYP/6-311++G(d,p), BHandHLYP/aug-cc-pVDZ, and QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) model chemistries. Formation of these radicals from potential molecular precursors catechol, hydroquinone, and phenol is readily achieved under combustion conditions through unimolecular scission of the phenoxyl–hydrogen bond or abstraction of the phenoxyl hydrogen by a hydrogen atom or hydroxyl radical. The resulting radicals are resonance stabilized and resist decomposition and oxidation. Activation energies for the decomposition of the radicals through concerted elimination of carbon monoxide range from 55 to 75 kcal/mol. Activation energies for the addition of molecular oxygen to the most reactive carbon atom (in all cases at the para-position) range from 8 to 22 kcal/mol. The calculations strongly suggest that combustiongenerated semiquinone and phenoxyl radicals are sufficiently stable and resistant to oxidation to be persistent in the atmospheric environment.  2007 Elsevier B.V. All rights reserved. PACS: 31.15.Ar; 51.30.+i Keywords: Ab initio; Quinone; Free radical

1. Introduction There is increasing experimental evidence that environmentally persistent free radicals are present in airborne fine particulate matter (PM2.5), and their principal source is combustion-generated particles [1–3]. These radicals are biologically active and may lead to DNA damage, pulmonary disease, cardiovascular disease, and liver dysfunction [1,4]. It is suspected that they are semiquinone-type radicals (ortho- and para-hydroxy substituted phenoxyl radicals and various derivatives) that have been reported in cigarette tar [5,6]. These radicals are strong reducing agents in aqueous solution at physiological pHs and reduce dis* Corresponding author. Also at: Department of Physics and Astronomy, Louisiana State University, USA. Tel.: +1 225 578 3472; fax: +1 225 578 3458. E-mail address: [email protected] (R.W. Hall).

0166-1280/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.09.005

solved oxygen to form superoxide and other biologically active, reactive oxygen species (ROS) [1,7]. However, the nature of the radicals in PM2.5 and combustion-generated particulate matter has not been conclusively demonstrated. Furthermore, recent experimental studies suggest that simple phenoxyl-type radicals may be as persistent as semiquinone radicals and be more ubiquitous in the environment [8]. For radicals to have environmentally significant concentrations, they require: • A molecular precursor and favorable route of formation; • Stability, i.e., being resistant to decomposition; and • Low-reactivity, i.e., being resistant to reaction with other molecular or radical species. Semiquinone radicals are thought to be highly stable with low-reactivity due to resonance stabilization including

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Fig. 1. Resonance structures of p-semiquinone radical existing as both carbon-centered and oxygen-centered radicals. Similar schemes can be drawn for o-semiquinone and phenoxyl radicals. In this manuscript, we report calculations concerning the formation, stability, and reactivity of phenoxyl, o-semiquinone, and p-semiquinone radicals under combustion and atmospheric conditions.

both carbon-centered and oxygen-centered radical resonance structures (Fig. 1). Consequently, they have been considered as the most likely candidates for the observed free radicals in combustion and atmospheric PM2.5. However, phenoxyl radicals may have similar chemical properties and exist in higher concentrations. 2. Computational procedures Ab initio calculations were performed using the Gaussian 03 [9] suites of programs. For open-shell systems (including open-shell singlets), unrestricted methods were used. Despite known deficiencies [10,11] density functional theory (DFT) is a common computational procedure. DFT calculations have been used to study radicals [12] and show that the phenoxyl radical decomposition thermodynamics are in good agreement with experiment [13]. A variety of studies have demonstrated that DFT calculations can accurately reproduce the difference in bond dissociation energies between similar compounds [14–19]. Studies of transition state properties have shown that the BHandHLYP [20] and the MPW1K [11] procedures are reasonably accurate. Therefore, we used the B3LYP and BHandHLYP DFT methods, and, as a check on the quality of the DFT energies, the QCISD(T)//BHandHLYP model chemistry. Calculations were performed using the 6-31G(d,p), 6311++G(d,p), and aug-cc-pVDZ basis sets. In this work, we are interested in the relative DErxn and Ea for the reactions of a series of similar molecules and, therefore, we used different model chemistries to assess the accuracy of our calculations. We compared B3LYP/6-31G(d,p), BHandHLYP/6-31G(d,p), BHandHLYP/6-311++G(d,p), BHandHLYP/cc-aug-pVDZ, and QCISD(T)/6-31G(d,p)// BHandHLYP/6-31G(d,p) (see Tables 1–4) and found that all 5 methods gave the same relative ordering of DErxn’s and Ea’s. Full optimizations using CCSD/aug-cc-pVDZ were attempted but proved beyond our computational resources. Stationary points were characterized as either a local minimum structure (no imaginary frequencies) or a transition state (one imaginary frequency) by analytical evaluation of their Hessians. When more than one isomer of a particular species existed, the most stable isomer was used in the calculations. The energies are unscaled and zero-point corrected. All energies are given in kilocalories/mole.

Transition states were located by performing relaxed potential energy surface scans followed by implementation of a Synchronous Transit-guided Quasi-Newton (STQN) method, integrated into the Gaussian suites of programs by Schlegel et al. [21]. Transition states for Channels 4A–6A used the previous work on phenoxyl radical decomposition as a model for the location of the rate limiting structure [13]. All transition states were confirmed using IRC calculations. Reactions between a radical and 3O2 (Channels 4B–6B) were studied in the doublet state based on a previous study of the reaction between phenyl and 3O2 [22]. 3. Results Fig. 2 summarizes the reactions of phenol, catechol, hydroquinone, phenoxyl radical, o-semiquinone radical, and p-semiquinone radical studied in this work. DErxn and Ea for these reactions are presented in Tables 1–4. These reactions can be characterized as follows: 3.1. Radical formation

• Unimolecular decomposition of the parent species via phenoxyl–hydrogen bond rupture (Channels 1A, 2A, and 3A). • Bimolecular reaction of the parent species with a hydrogen atom.  Abstraction of a phenoxyl hydrogen (Channels 1B, 2B, and 3B).  Displacement of a hydroxyl group (Channels 1C, 2C, and 3C). • Bimolecular reaction of the parent species with a hydroxyl radical.  Abstraction of a phenoxyl hydrogen (Channels 1D, 2D, and 2C).

Unimolecular decomposition involving a phenoxyl– hydrogen bond is expected to contribute to radical formation under oxidative and pyrolytic conditions [23]. Hydrogen atoms are the dominant reactive species under pyrolytic conditions and hydroxyl radicals are the dominant reactive species under oxidative conditions, and these

8.8) 7.4) 9.7) 6.8) 7.1)

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species should contribute strongly to radical formation under pyrolytic and oxidative conditions, respectively.

2A

All energies are given in kcal/mol. Numbers in parenthesis give the values of DErxn relative to the phenol reaction (reactions 1A, 1B, 1C, and 1D) for a given level of theory and basis set.

( ( ( ( ( 35.3 34.0 40.1 34.6 35.1

3D

4.5) 4.0) 5.1) 4.2) 4.2) ( ( ( ( ( 31.0 30.6 35.3 32.0 32.2

2D

(0.0) (0.0) (0.0) (0.0) (0.0) 1D

26.5 26.6 30.4 27.9 28.0 5.4 7.3 2.0 9.3 8.3 7.8 9.5 4.0 11.6 10.4 6.2 7.7 2.3 9.7 8.5 25.2 30.4 33.7 27.9 26.0 20.9 27.0 28.9 25.2 23.0 80.7 72.9 71.6 73.9 73.4 I II III IV V

89.4 80.2 81.3 80.6 80.5

(0.0) (0.0) (0.0) (0.0) (0.0)

85.0 76.3 76.4 76.5 76.3

( ( ( ( (

4.4) 3.9) 4.9) 4.3) 4.2)

3A

( ( ( ( (

8.7) 7.3) 9.7) 6.7) 7.1)

16.4 23.1 24.0 21.2 18.8

(0.0) (0.0) (0.0) (0.0) (0.0)

2B 1B 1A

( ( ( ( (

4.5) 3.9) 4.9) 4.0) 4.2)

3B

( ( ( ( (

8.8) 7.3) 9.7) 6.7) 7.2)

1C

(0.0) (0.0) (0.0) (0.0) (0.0)

2C

( ( ( ( (

1.6) 1.8) 1.7) 1.9) 1.9)

3C

(+0.8) (+0.4) (+0.3) (+0.4) (+0.2)

H abstraction by OH OH displacement H 

H abstraction by H Unimolecular decomposition

Table 1 QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) (I), BHandHLYP/6-31G(d,p) (II), B3LYP/6-31G(d,p) (III), BHandHLYP/6-311++G(d,p) (IV), and BHandHLYP/cc-aug-pVDZ (V) values for DErxn for the radical formation reactions

18

3.2. Radical consumption • Thermal decomposition via concerted elimination of carbon monoxide (Channels 4A, 5A, and 6A). • Bimolecular reaction with ground state molecular oxygen (3O2) (Channels 4B, 5B, and 6B). • Unimolecular bond scission of the second hydroxyl hydrogen to form a quinone (Channels 5C and 6C). Unimolecular thermal decomposition via elimination of carbon monoxide may contribute to radical consumption under oxidative or pyrolytic conditions [23]. Hydrogen atom reactions were included in analogy to the reactions of the molecular species in radical formation. Bimolecular reactions with 3O2 were included as 3O2 is the highest concentration reactive species in the atmosphere and the known principal route of consumption of most organic radicals under oxidative conditions [22,24–34]. Activation energies were calculated for the key unimolecular radical decomposition (Channels 4A–6A and 5C– 6C) and bimolecular radical – 3O2 consumption reactions (Channels 4B–6B). 3.3. Radical formation The values of DErxn for radical formation via unimolecular decomposition, H abstraction by H, OH displacement by H, and H abstraction by OH are shown in Table 1. DErxn and DHrxn have been measured for the unimolecular decomposition reaction of phenol. The measured values of DErxn are 90.1 ± 3.1 kcal/mol [35] and 84.0 ± 1.0 kcal/mol [36]. In addition, a previous calculation on DErxn gave a value of 89 kcal/mol [37]. The present calculations give a value of 89.4 kcal/mol using QCISD(T) level of theory, in good agreement with the previous calculation and the higher value from experiment. The results using density functional theory underestimate DErxn by several kcal/mol, but are still acceptable given the variance in experimental values. The measured values of DHrxn are 86.5 ± kcal/mol [38], 86.3 ± 1 kcal/mol [39] and 85.8 ± 2 kcal/mol [40], while our calculated value is 82.7 kcal/mol (B3LYP/631G(d,p)). While different levels of theory give different absolute values for the energetics of the radical forming reactions, the relative energetics are quite similar. This can be seen in relative values of DErxn (relative to the phenol reaction for a given pathway) displayed in Table 1. The consistency between the relative values of DErxn values across basis sets and methods is satisfactory. Unimolecular formation of o- and p-semiquinone radicals require 4–10 kcal/mol less energy than the formation of the phenoxyl radical (Table 1, Channels 1A, 2A, and 3A). The experimentally determined rate constants for H abstraction and OH displacement by H are sim-

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Table 2 QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) (I), BHandHLYP/6-31G(d,p) (II), B3LYP/6-31G(d,p) (III), BHandHLYP/6-311++G(d,p) (IV), and BHandHLYP/cc-aug-pVDZ (V) values for DErxn and Ea for the elimination reactions of the radicals in Fig. 2 H elimination

CO elimination Ea

D Erxn 4A I II III IV V

22.9 29.0 29.6 24.8 26.4

5A (0.0) (0.0) (0.0) (0.0) (0.0)

20.5 27.2 27.8 23.0 24.7

6A ( ( ( ( (

2.4) 1.8) 1.8) 1.8) 1.7)

4A

27.2 32.8 34.6 28.0 29.6

(+4.3) (+3.8) (+5.0) (+3.2) (+3.2)

56.4 62.9 56.4 61.6 61.6

Ea

DErxn 5A (0.0) (0.0) (0.0) (0.0) (0.0)

59.3 65.1 59.9 63.8 63.9

6A (+2.9) (+2.2) (+5.5) (+2.2) (+2.3)

68.3 74.4 70.0 72.0 72.1

5C (+11.9) (+11.5) (+13.6) (+10.4) (+8.2)

54.7 60.9 57.8 59.9 60.3

6C (0.0) (0.0) (0.0) (0.0) (0.0)

5C

68.2 74.3 71.6 73.2 73.9

(+13.5) (+13.4) (+13.8) (+13.3) (+13.6)

71.4 72.9 63.8 73.6 72.5

6C (0.0) (0.0) (0.0) (0.0) (0.0)

79.0 79.8 71.6 80.2 79.2

(+7.6) (+6.9) (+7.8) (+6.6) (+6.7)

All energies are given in kcal/mol. Numbers in parenthesis give the values of DErxn and Ea relative to the phenol reaction (reactions 4A and 5C) for a given level of theory and basis set.

Table 3 QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) (I), BHandHLYP/6-31G(d,p) (II), B3LYP/6-31G(d,p) (III), BHandHLYP/6-311++G(d,p) (IV), and BHandHLYP/cc-aug-pVDZ (V) values for DErxn for the reaction of 3O2 with the radicals in Fig. 2 4B

5B

o-Addition I II III IV V

1.8 12.3 9.3 13.3 12.7

(+3.8) (+3.9) (+3.1) (+4.2) (+4.2)

p-Addition 2.0 8.4 6.2 9.1 8.5

(0.0) (0.0) (0.0) (0.0) (0.0)

6B

o-Addition

p-Addition

o-Addition

p-Addition

3.2 13.7 12.0 14.9 14.5

0.5 11.3 10.7 12.1 11.6

6.9 16.4 15.4 17.5 16.9

2.6 11.8 11.3 12.6 11.9

(+3.7) (+2.4) (+1.3) (+2.8) (+2.9)

(0.0) (0.0) (0.0) (0.0) (0.0)

(+4.3) (+4.6) (+4.1) (+4.9) (+5.0)

(0.0) (0.0) (0.0) (0.0) (0.0)

All energies are given in kcal/mol. Numbers in parenthesis give the values of DErxn relative to the p-addition of 3O2 for a given channel, level of theory, and basis set.

Table 4 QCISD(T)/6-31G(d,p)//BHandHLYP/6-31G(d,p) (I), BHandHLYP/6-31G(d,p) (II), B3LYP/6-31G(d,p) (III), BHandHLYP/6-311++G(d,p) (IV), and BHandHLYP/aug-cc-pVDZ (V) values for Ea for the reaction of 3O2 with the radicals in Fig. 2 4B

I II III IV V

5B

6B

o-Addition

p-Addition

o-Addition

p-Addition

o-Addition

p-Addition

14.6 23.5 13.7 24.3 23.8

11.1 20.7 10.8 21.4 20.9

13.1 22.4 13.1 23.3 23.0

9.3 19.6 10.8 20.0 19.8

18.6 26.4 18.1 27.3 26.7

7.6 21.3 12.5 22.3 21.7

(+3.5) (+2.8) (+2.9) (+2.9) (+2.9)

(0.0) (0.0) (0.0) (0.0) (0.0)

(+3.8) (+2.8) (+2.3) (+3.3) (+3.2)

(0.0) (0.0) (0.0) (0.0) (0.0)

(+11.0) (+5.1) (+5.6) (+5.0) (5.0)

(0.0) (0.0) (0.0) (0.0) (0.0)

All energies are given in kcal/mol. Numbers in parenthesis give the values of Ea relative to the p-addition of 3O2 for a given channel, level of theory, and basis set.

ilar for phenol [43]. Assuming H concentrations [44,45] of 10 13 to 10 11 mol cm 3 predicts rates for these two reactions are greater than the rate for unimolecular decomposition [41] under pyrolytic conditions. Furthermore, the calculated results (Table 1, Channels 1B–3B and 1C–3C) for DErxn show that thermodynamics favors the formation of phenoxyl over the formation of benzene. For o- and p-semiquinone, we expect similar trends in activation energies and reaction rates and our calculations show similar trends in DErxn. Under oxidative conditions, H abstraction by OH should be the dominant pathway for radical formation [46,47]. Simply put, these calculations demonstrate that unimolecular or bimolecular processes at combustion temperatures all readily form the radicals. The radical consumption energetics are more revealing.

3.4. Radical stability Phenoxyl, o-semiquinone, and p-semiquinone radicals (Channels 4A, 5A, and 6A) are all resistant to thermal decomposition by elimination of carbon monoxide to form cyclopentadienyl (from phenoxyl) and hydroxycyclopentadienyl (from p- and o-semiquinone) radicals with activation energies ranging from 55 kcal/mol for phenoxyl to 75 kcal/mol for o-semiquinone (Table 2). The experimental values for DHrxn for the CO elimination reaction is 20 kcal/mol [38,48]. Our calculations using DFT theory give slightly higher values (e.g., 28.2 kcal/mol, BHandHLYP/aug-cc-pVDZ and 31.3, B3LYP/6-31G(d,p)). The values of DErxn are similar to DHrxn and the QCISD(T) values for DErxn are 4–5 kcal/mol less than the DFT level calculations and therefore in better agreement with

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Fig. 2. Reactions of phenol, hydroquinone, catechol, phenoxyl radical, p-semiquinone radical, and o-semiquinone radical. The schemes on the left-handside are the reactions of the parent molecular species, some of which form the radical. The schemes on the right-hand-side include decomposition by elimination of CO and reaction with molecular oxygen of the resulting radical. Unimolecular decomposition (phenoxyl–hydrogen bond rupture in the molecules and concerted elimination of carbon monoxide from the radical), hydrogen atom displacement, hydrogen atom abstraction, and hydroxyl radical abstraction are also included.

experiment. Thus, the agreement between calculation and experiment is similar for the decomposition of the phenoxyl radical via CO elimination and the unimolecular formation of the phenoxyl radical discussed above. The relative values of DErxn and Ea (Table 2) show consistency across the ab initio methods and basis sets. Qualitatively, our calculations do indicate that o-semiquinone is more resistant to decomposition than phenoxyl or p-semiquinone (presumably due to its internal hydrogen bond), which may be significant. o- and p-semiquinone (Channels 5C and 6C, Table 2) may also decompose by the loss of the second phenoxyl hydrogen which is not possible in phenoxyl. These reactions are highly endothermic with calculated activation energies of 70 and 80 kcal/mol for p- and o-semiquinones, respectively (Table 2). The large (>50 kcal/mol, Table 2,

Channels 5C and 6C) activation energies for loss of the second hydrogens are again indicative of the stabilities of the radicals. Since the activation energies for reaction of the radicals with molecular oxygen were similar (10– 20 kcal/mol, Table 4) and the semiquinone radicals have a decomposition channel that phenoxyl radical does not have, our results suggest that phenoxyl may be slightly more stable than the o- and p-semiquinone radicals. 3.5. Radical reactivity Our calculations indicate that the addition of molecular oxygen to the oxygen atom of phenoxyl is energetically unfavored and most likely not possible. A UHF/631G(d,p) model chemistry calculation found a stable ozon-

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ide product (ROOO) with DErxn = 72.8 kcal/mol. However, attempts to optimize the UHF structure with B3LYP/BHandHLYP were unsuccessful; hence, addition to the oxygen atom is not included in the table and not a possibility further considered. This ozonide product suggests addition reactions of molecular oxygen with the oxygen-center of the semiquinones would be highly endothermic as well. Our calculations also indicate 3O2 addition to the ortho-/ para-carbon atoms of phenoxyl (Channel 4B) are slow with calculated activation energies greater than 10 kcal/mol (Table 4). The relative values of DErxn and Ea are again consistent across the ab initio methods (Table 4). The activation energies are lower for reaction with molecular oxygen at the more reactive para-position of each radical. Based on our calculated activation energies, the reaction of 3O2 is slow for any of these radicals. In principle, reaction can also occur at the ortho-position, but the activation energies are 2–11 kcal/mol higher than at the paraposition. Because our calculations indicated that semiquinone and phenoxyl radicals were exceptionally resistant to oxidation, for comparison, we also performed calculations on a phenyl radical that is known to be consumed by reaction with 3O2 [22,24–29]. For the addition of 3O2 to the phenyl radical (Fig. 3), an experimental activation energy of 0.32 kcal/mol has been previously measured [26], and a DErxn of 41 kcal/mol has been calculated using UHF/ 6-31G(d) [27]. Our calculations indicate that this reaction is barrierless (B3LYP/6-31G(d,p)) with a DErxn of 43.4/ 45.3 kcal/mol (BHandHLYP/6-31G(d,p)//B3LYP/631G(d,p)), which is in excellent agreement with experiment. The reaction between the phenoxyl radical and 3O2 (Channel 4B) has also been experimentally studied, and it was found that the room temperature reaction rate was on the order of 10 18 to 10 21 cm3/molecule s [49,50]. This is very slow compared to the reaction of 3O2 with phenyl radical which is 10 11 to 10 13 cm3/molecule s [24–26] and further supports the results of our calculations on o- and p-semiquinone radicals as well as phenoxyl. 3.6. Bader electron densities An additional calculation that is useful for assessment of the reactivity of the radicals is the Bader Valence Electron Density [51]. Using the Laplacian of the electron density,

Fig. 3. The addition reaction of the phenyl radical with molecular oxygen which produces the phenylperoxy radical.

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Table 5 Bader valence electron density analysis (BHandHLYP/6-31G(d,p)) Radical

% Carbon

% Hydrogen

% Oxygen

o-Semiquinone p-Semiquinone Phenoxyl Phenyl

52.2 49.8 63.6 82.2

10.6 13.4 14.8 17.8

37.2 36.8 21.6 na

Values are determined via a Bader analysis of the valence electron density.

Bader’s approach assigns the valence electron density to individual atoms and can be used to determine the amount of valence electron density that is oxygen-centered and carbon-centered. The results presented in Table 5 indicate that o-semiquinone and p-semiquinone are both 37% oxygencentered and phenoxyl is 22% oxygen-centered. Our calculations indicate that molecular oxygen will not react at an oxygen center in the radical to form an ozonide (ROOO), but will react at a carbon-center to form a peroxide (ROO). Since the phenoxyl radical has 25% higher Bader valence electron density on reactive carbon sites, this suggests that the pre-exponential factor for reaction of phenoxyl radical with molecular oxygen will be higher than for either of the semiquinone radicals. This effect, to some extent, counters the effect of higher activation energy for the reaction of molecular oxygen with phenoxyl radical. 4. Discussion The detection of persistent, biologically active free radicals in airborne and combustion-generated particles raises the questions of their origin and nature. The EPR g-values for samples containing these radicals are typically greater than 2.003 and have been observed to be greater than 2.005 [52]. These g-values are characteristic of oxygen-centered radicals or carbon-centered radicals with a nearby oxygen-containing functional group. Because the EPR spectral characteristics are consistent with those of semiquinone radicals and these radicals have been previously reported in cigarette tar, it has been presumed that the observed EPS spectra are due to are semiquinone-type radicals. However, phenoxyl radicals in complex media will yield EPR spectra similar to those of semiquinones and our calculations indicate that the phenoxyl radical is as stable and resistant to oxidation as o- and p-semiquinone. Although formation via unimolecular decomposition from a parent phenol is not as energetically favorable as that of a semiquinone from catechol or hydroquinone, phenol is likely to be present in higher concentrations [53] than catechol or hydroquinone which suggests that phenoxyl may be the higher concentration radical in environmental samples. The highly resonance stabilized o-semiquinone, p-semiquinone, and phenoxyl radical are all resistant to decomposition via the channels examined herein. It was also anticipated that they would be resistant to oxidation, but the activation energies and free energies of addition of 3 O2 to any of the radicals studied herein are surprisingly high, suggesting that they are virtually un-oxidizable by

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molecular oxygen. This result is actually consistent with available experimental evidence and other calculations. As previously mentioned, semiquinone radicals have been reported in aged cigarette tar, supporting their extreme persistence. Recombination of chlorinated phenoxyl radicals have been proposed as a major pathway of formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) in combustion systems which indicates that these radicals survive in an oxidative environment at sufficiently high concentrations to undergo self-recombination to form PCDD/F (though at ppb to ppm concentrations) [54–56]. Elementary reaction kinetic data is not available for semiquinone radicals but has been obtained for phenoxyl radicals. Reported rates for reaction of phenoxyl radical with molecular oxygen are k < 2 · 10 18 and <5 · 10 21 cm3/ molecule s [49,50]. This is in contrast to reaction rates of common and typical organic radicals such as methyl, vinyl, and phenyl which are 10 12, 10 12, and 10 11 to 10 13 cm3/molecule s, respectively, which are 6–10 orders of magnitude faster than phenoxyl radical [24–26,30–34]. While addition of oxygen to common organic radicals such as methyl, vinyl, and phenyl is highly exothermic, it is highly endothermic for phenoxyl and semiquinone radicals, based on our calculations. These calculations indicate that scavenging of phenoxyl and semiquinone radicals is neither a kinetically nor thermodynamically (due to the reduction in entropy due to the reaction stoichiometry) favorable reaction. Our calculations were unable to find any path for molecular oxygen to react with any of these radicals at an oxygencenter, as formation of the ozonide is highly unfavorable. A metastable product could only be found if the reaction occurred at an ortho- or para-carbon. However, the high activation energy of oxidation for semiquinone and phenoxyl radicals appears to ensure that both types of radicals are equally unreactive. In summary, our calculations yield the expected stability and lack of reactivity of semiquinone radicals, but also suggest that phenoxyl radical is just as stable and unreactive. This in turn suggests that phenoxyl radical must be considered as a candidate for the identity of the radicals observed to be associated with airborne fine particles and combustion-generated particles. However, it should be noted that the nature of these radicals may be significantly modified by the surface interaction. Chemisorption of the molecular precursor, followed by electron transfer from the sorbant to the particle surface can result in radical formation, forcing the radical into a carbon-centered or oxygen-centered structure, and partial oxidation of an aromatic hydrocarbon species to a phenoxyl or semiquinone radical [57,58]. Acknowledgements This work was supported by the National Science Foundation through Grant CTS-0317094 and through an allocation of time on the high performance cyberinfrastructure

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