A review on reactions of polycyclic aromatic

REVIEW

Progress in Reaction Kinetics and Mechanism, 2017, 42(3), 201–220 https://doi.org/10.3184/146867817X14821527549293 Paper: 1600580

A review on reactions of polycyclic aromatic hydrocarbons with the most abundant atmospheric chemical fragments: theoretical and experimental data Maryam Nayebzadeh* and Morteza Vahedpour Department of Chemistry, University of Zanjan, PO Box 45371-38791, Zanjan, Iran *E-mail: [email protected]

ABSTRACT Aerosols are ubiquitous in the atmosphere and have strong effects on climate and public health due to the importance of reactions of polycyclic aromatic hydrocarbon (PAH) compounds in air. Over the last decade, study of the reactions of PAHs and their derivatives in the atmosphere has become a key topic to find an effective way to decrease the impact of this spontaneous reaction and so reduce air pollution. This article aims to pool the majority of research on the reactions of PAHs with atmospheric agents such as oxygen, hydrogen and ozone and compare the theoretical and experimental results. In examining theoretical research, the number of aromatic rings is very important in calculating the rate constants and determining the main pathway of the reaction. So, while there are weak theoretical data, several papers issued in this field have concurred with key experimental results. For reactants with more than six aromatic rings, small basis sets have good conformity with experimental outcomes. Due to the abundance of OH in the atmosphere, much research has been done to find the best reaction pathway and calculate the associated rate constants experimentally and theoretically. In future, the opportunity exists for new researchers to detect the main intermediates, most important pathways, rate constants and the products of reactions with more than six aromatic rings and to detect PAHs in a dense atmosphere. Product identification will help to reduce air pollution. KEYWORDS: polycyclic aromatic compound, atmosphere, rate constant, kinetics, density functional theory, potential energy surface, hydroxyl radical 1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) [1], shown in Figure 1 with their IUPAC nomenclature, are a type of environmental contaminant that have attracted researchers in the fields of organic chemistry, theoretical chemistry, physical chemistry, environmental science, toxicology, cancer research and energy sciences [2–7]. PAHs and their derivatives are one of the most prominent groups of toxic air pollutants which are produced and emitted into the atmosphere from chemical reactions, human activity processes, biomass burning and other activities on the Earth and also from the incomplete combustion of fuel, fossil fuels and mobile combustion sources [8–10]. They reside in significant amounts in fine air particulate matter that can penetrate deep into human lungs and play an important role in the formation of combustion-generated particles such as soot. Their presence in atmospheric aerosols has been widely shown [11]. PAHs are ubiquitous environmental pollutants and many of them are potentially genotoxic, mutagenic or carcinogenic [9,12]. PAHs are chemically reactive to tropospheric gases such as hydrogen, oxygen, ozone, NO2, NO3 and OH radicals. The most 201

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important sink reaction of organic compounds in the troposphere is that with OH radicals [13– 16]. In the last decade, researchers have found that the gas-phase atmospheric chemistry of PAHs has a serious impact on the mutagenic activity [17] of ambient atmospheres [18–21] and, due to their carcinogenic character which increases with the number of aromatic rings [22], growing attention is being paid to volatile organic compounds (VOCs), particularly to PAHs [23-27]. Nitrated PAHs (NPAHs) are one of the main pollutants in air; these are the products of the atmospheric reaction with aerosol compounds so, due to their capacity to induce direct mutagenic activity of inhalable suspended particles in polluted areas, they are the most important derivatives of PAHs. Although several NPAHs have been detected in the atmosphere as well as in emission particulates, nitrated pyrenes and fluoranthenes seem to be responsible for the main effect on human health [28]. As air pollution has been positively associated with death from lung cancer [29,30], researchers launched investigations about the mechanism and pathway of reaction of PAHs and the main air components [31–33]. It is expected that air quality would be improved rapidly [34] if the emission of these PAHs was controlled. For many reaction systems, kinetic analyses often consist of hundreds of elementary reactions. However, it is difficult to calculate the thermal rate for every reaction. Among the available methods, transition state theory (TST) is the easiest and it needs only the geometries, energies and vibrational frequencies of the reactants and transition state (TS) [35]. There are some ways to use this to predict the rate constants and other kinetic parameters, for example, linear free energy [36,37], reaction class transition state theory (RC-TST), the linear energy relationship (LER), the relationship between the activation energies and both dissociation energies or heats of reaction and in the case where some limited rate information is available, it is suggested to use the thermo-chemical kinetics TST (TK-TST) method developed by Benson [38]. Recently, researchers have found the best pathway of reaction of cyclic and polycyclic compounds with air components both experimentally and theoretically. Nevertheless, the large size of PAH molecules limits the use of quantum chemistry methods to obtain essential information. A better approach is by comparing the experimental data and estimating the rate coefficients for a similar reaction [39]. In this review, we have tried to collate the most applicable approaches to investigating the reaction of molecules such as pyrene, anthracene, naphthalene and coronene and especially

Figure 1 Chemical structures and ring numbering systems for representative PAHs.

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those compounds with five rings such as benzo[a]pyrene (BaP) and dibenzo[a,h]anthracene due to their high carcinogenic power, with those air components which constitute the major part of the atmosphere. There are several experimental data that demonstrate the accuracy of the theoretical results. Thus, the most important reactants are reported which have a large presence in air and will have the most significant effect on human health. 2. REACTION OF POLYCYCLIC AROMATIC HYDROCARBONS WITH H ATOMS The reactions of different cyclic PAHs with H atoms have been studied by Violi et al. [31]. To predict thermal rate constants, they applied the RC-TST/LER method. Two classes of reactions were considered (see Table 1), consisting of six-membered ring compounds and five-membered ring compounds respectively and 22 reactions were studied to develop the RC-TST/LER parameters [40]. Potential energy surfaces for these reactions were calculated at the B3LYP and BH&HLYP levels of theory using the 6-31G(d,p) basis set. All calculations were performed using the Gaussian 98 program [41]. By using TST, thermal rate constants were calculated. The calculated rate constants for the principal reactions [42,43] and other PAHs [44] were in excellent agreement with available experimental data, although there are few experimental data for five-ring PAHs. An important conclusion is that the rate constants for any reaction in these two classes can be approximated by those of its corresponding principal reaction corrected by Table 1 List of reaction classes. Reprinted (adapted) with permission from ref. [31]. Copyright 2004 American Chemical Society Class 1 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16

C6H6 + H = C6H5 + H2 C10H8 + H = 1*C10H7 + H2 C10H8 + H = 2* C10H7 + H2 Phenanthrene + H = 1*phenanthrene + H2 Phenanthrene + H = 9*phenanthrene + H2 Anthracene + H = 1* anthracene + H2 Anthracene + H = 2* anthracene + H2 Anthracene + H = 10* anthracene + H2 C16H10 + H = C16H9 + H2 C12H8 + H = 3*C12H7 + H2 C12H8 + H = 5* C12H7 + H2 C12H8 + H = 5* C12H7 + H2 Aceanthrylene + H =9* aceanthrylene + H2 C18H10 + H = 6*C18H9 + H2 C24H12 + H = 3*C24H11 + H2 C20H10 + H = 2*C20H9 + H2

Class 2 R17 R18 R19 R20 R21 R22

C12H8 + H = 1*C12H7 + H2 Acephenanthrylene + H = 5*acephenanthrylene H2 Aceanthrylene + H = 2* aceanthrylene + H2 C18H10 + H = 4* C18H9 + H2 C24H12 + H = 9* C24H11 + H2 C20H10 + H = 4* C20H9 + H2

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Table 2 Reaction barriers (ΔV #), reaction energies (ΔE) and deviations between calculated barriers and LER barriers. Units are kcal mol–1. Reprinted (adapted) with permission from ref [31]. Copyright 2004 American Chemical Society Reaction

DEa

DV#b

DV#cLER

DV# –DV# LER

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22

6.72 6.88 6.86 6.88 6.83 6.85 6.82 7.04 7.20 6.48 7.15 6.75 6.65 6.94 7.14 6.65 10.88 10.71 11.12 11.03 10.87 10.57

15.07 15.34 15.19 15.38 15.34 15.23 15.08 15.49 15.53 15.15 15.31 15.33 15.02 15.39 15.45 15.12 17.73 17.69 17.84 17.84 17.84 17.58

15.19 15.29 15.28 15.29 15.25 15.27 15.25 15.39 15.49 15.04 15.45 15.20 15.14 15.32 15.45 15.14 17.77 17.66 17.91 17.85 17.76 17.57

0.12 0.054 0.184 0.087 0.089 0.038 0.16 0.096 0.046 0.10 0.14 0.12 0.12 0.070 0.0087 0.022 0.033 0.034 0.074 0.012 0.086 0.0053

aCalculated

at B3LYP/6-31G (d,p) level of theory. at BH&HLYP//B3LYP level of theory. c ΔV LER was calculated from the LER expression by substituting B3LYP/6-31G(d,p) reaction energies. bCalculated

the reaction symmetry factor and no additional calculation is needed. The calculated reaction energies and the reaction barrier heights for all the reactions analysed are listed in Table 2. 3. REACTION OF POLYCYCLIC AROMATIC HYDROCARBONS WITH OH 3.1 Reaction of phenanthrene and anthracene with OH Rate constants for the reaction of OH with some PAHs (Figure 2) in the gas-phase were measured experimentally [45]. We should note that studies of the interaction of phenanthrene, anthracene and fluoranthene were much more difficult experimentally, relative to the other PAHs. A comparison of the individual PAH structures (Figure 2) suggests some differences between their OH reaction rate constants. Naphthalene and acenaphthene both contain two aromatic rings, but H-abstraction plays the main role in the OH reaction rate of acenaphthene. Such a comparison among the three-ring PAHs is not as easy. Studies of the reaction products of OH with fluorene and phenanthrene [46,47] indicate that OH radical attack most likely occurs at the 9-position for fluorene. These results, however, do not explain the difference between PAHs which only differ structurally by the relevant position of their aromatic rings. The calculated net atomic charges for phenanthrene and anthracene [48] indicate that the most favourable position for OH reaction in each PAH is different. However, for phenanthrene, the carbons with the most positive net charges were those at the 11- and 12-positions and for anthracene the 9- and 10-positions have the same net charge and are thus the best positions for reaction. However, the 11- and 12-positions in www.prkm.co.uk


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Figure 2 Structures of PAHs and numbered positions discussed in the text.

phenanthrene are strictly unfavourable given their hindered positions. Therefore, phenanthrene may not react as efficiently as anthracene with OH, due to the latter having unhindered carbons with the most positive charges. 3.2 Reaction of naphthalene with OH radicals The most important reactions of organic compounds in the atmosphere (troposphere) are those with OH radicals [14,15]. Naphthalene and its nitro derivatives are the most abundant PAHs in polluted urban areas and are reactive in the atmosphere [49,50]. The oxidation mechanism of naphthalene by OH radicals in the gas phase has been studied computationally using density functional theory (DFT). The purpose was related to the reaction steps involved in the so-called OH radical addition pathway [13]. The two pathways are shown in Figures 3 and 4. Due to the formation of a pre-reactive van der Waals molecular complex [C10H8...OH]+, the corresponding TS lies below the reactant, so the most abundant product resulting from the oxidation of naphthalene by OH radicals must be 1-naphthol rather than 2-naphthol. The TST and Rice–Ramsperger–Kassel–Marcus (RRKM) theory data for the first bimolecular reaction step (R → IM2a) in pathway 1 are all negatively dependent on the temperature. All the energetic and thermodynamic parameters such as activation energy and rate constant are in good agreement with the experimental data [45,51,52]. These parameters are extremely dependent on temperature, which means that at temperatures lower than 410 K, by increasing the temperature the rate constant is increased. In another paper [52], the kinetics of the gas-phase reaction of naphthalene, 2-methylnaphthalene and 2,3-dimethylnaphthalene with OH radicals were investigated experimentally and it was found that rate constants of (2.59 ± 0.24) × 10–11, (5.23 ± 0.42) × 10–11 and (7.68 ± 0.48) × 10–11 cm3 molecule–1 s–1 were determined for naphthalene, 2-methylnaphthalene and 2,3-dimethylnaphthalene respectively. By comparison, from the data in Table 3, methyl as an electron donor group to naphthalene plays an important role in increasing the rate of reaction and its position influences the rate of reaction [53,54]. www.prkm.co.uk

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Figure 3 Energy profile for reaction pathway 1 characterising the oxidation by OH radicals of naphthalene into 1-naphthol. Reprinted (adapted) with permission from ref. [13]. Copyright 2014 American Chemical Society.

Figure 4 Energy profile for reaction pathway 2 characterising the oxidation by OH radicals of naphthalene into 2-naphthol. Reprinted (adapted) with permission from ref. [13]. Copyright 2014 American Chemical Society.

Table 3 Room temperature rate constants k (cm3 molecule –1 s–1) for the reaction of OH radicals with naphthalene and ≤ C2 alkylnaphthalenes. Reprinted with permission from Taylor and Francis Ltd [53], http://www.informaworld.com. PAH Naphthalene 1-Methylnaphthalene 2- Methylnaphthalene 1- Ethylnaphthalene 2- Ethylnaphthalene 1,2-Dimethylnaphthalene 1,3-Dimethylnaphthalene 1,4-Dimethylnaphthalene 1,5-Dimethylnaphthalene 1,6-Dimethylnaphthalene 1,7-Dimethylnaphthalene 1,8-Dimethylnaphthalene 2,3-Dimethylnaphthalene 2,6-Dimethylnaphthalene 2,7-Dimethylnaphthalene cFrom

Reaction with OH 1012 × k (cm3 molecule –1 s–1) 23.9c 40.9c 48.6c 36.4c 40.2c 59.6c 74.9c 57.9c 60.1c 63.4c 67.9c 62.7c 61.5c 66.5c 68.7c

ref. [54].

3.3 Kinetics of the reaction of naphthalene with OH The reaction of OH with naphthalene is faster by more than an order of magnitude compared to the reaction of OH with benzene. This reaction shows a dependency on pressure and temperature. Reaction at low temperature depends on pressure, with the rate constant increasing with pressure [51]. This reaction has two positions for addition with a 1:1 ratio. www.prkm.co.uk


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Scheme 1

Scheme 2

Generally, the main product is the stable one as shown in Scheme 1. However, there is the possibility that the vibrational excited adduct may decompose to yield naphthol + H according to Scheme 2. Naphthalene, a relatively non-toxic but very abundant PAH [55], produces a series of compounds in the troposphere through reaction with OH; some of the lateral products are nitro derivatives that are mutagenic [56] and significantly more persistent in the atmosphere than the parent hydrocarbons. These products might partition from the gas phase and become adsorbed on aerial component particles, where they would be further fixed against reaction. This would increase their lifetimes relative to their gas-phase counterparts and might significantly increase their risk to health. 3.4 Oxidation mechanism of naphthalene initiated by OH radicals The oxidation mechanism of naphthalene initiated by OH radicals was studied [57] using DFT. For unimolecular processes, high-pressure-limit rate constants (kUni) were estimated using traditional TST [58]. The C1-adduct (C10H8-1-OH) was considered and the main pathway is shown in Figure 5. 3.5 Reaction of benzo[a]pyrene with OH radicals OH radicals were generated by the photolysis of methyl nitrite in air [52] as shown in reactions 1a, 1b and 1c. CH3ONO + hν → CH3 + NO

(1a)

CH3O + O2 → HCHO + HO2

(1b)

HO2+ NO → OH + NO2

(1c)

In a review [59], aromatic compounds are considered to react with the OH radical by OH radical addition to the aromatic ring and by H–atom abstraction from the ring. The reactivity www.prkm.co.uk

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Figure 5 Predicted main paths for the atmospheric oxidation of 1-naphthol. Adapted from [57] with permission of the PCCP Owner Societies.

of the aromatic ring towards OH radical addition is dependent upon the number, identity and position of the substituent group(s). For four aromatic compounds, namely acenaphthene, acenaphthylene, tetralin and styrene, both processes are expected to occur and contribute to the overall reaction rate constant [59]. The presence of C–H and C=H bonds in the molecular structure of BaP, which contains five aromatic rings, offers several pathways for its reaction with OH. Its mechanistic pathway and kinetic properties were studied by Zhang and co-workers and the rate constants were calculated using the RRKM method [60]. The calculated rate constant of OH with phenanthrene (3.98 × 10–11 cm3 molecule–1 s–1) gave good agreement with the corresponding experimental value of 2.42 × 10–11 cm3 molecule–1 s–1. Due to the lack of any experimental data to compare with the theoretical value of the rate constant of OH with BaP, the experiment with phenanthrene is seen as justifying the value of the overall rate constant for the addition reaction of BaP with OH of 2.29 × 10–10 cm3 molecule–1 s–1 [60]. Twelve H abstraction pathways were identified and 12 transition states were located (Figure 6). At the BB1K/6–311+G(3df,2p) level, the potential energy barriers were 4.50–5.45 kcal mol–1. All H abstraction pathways are exothermic and the reaction heats are from –5.92 to –3.42 kcal mol–1. 3.6 Kinetics of reaction of anthracene with OH radicals Atkinson and co-workers used a relative rate technique employing gas chromatography and long path-length differential optical spectroscopy to determine rate constants for gas-phase reactions of OH radicals with naphthalene, phenanthrene and anthracene [61]. These three PAHs are among the most abundant emitted from automobiles [62] and wood-burning stoves [63,64]. These gas-phase reactions of OH with the two- and three-ring PAHs are rapid, with the room temperature rate constants increasing from naphthalene to phenanthrene to anthracene in the ratio 1.0:1.4:4.7; by comparing the data with that of the monocyclic aromatic hydrocarbons [65,66] and kinetic information [67], it is evident that these reactions proceed via initial OH radical addition to the aromatic rings. There are several studies of the reaction of OH with anthracene at different temperatures. The reported value at 325 K was (11 ± 0.9) × 10–11 cm3 molecule–1 s–1 [61]. Kwok et al. reported www.prkm.co.uk

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Figure 6 OH addition reaction scheme of BaP embedded with the potential barrier ΔE (in kcal mol–1) and heat of reaction ΔH (in kcal mol–1). ΔH is calculated at 0 K. Reprinted from ref. [60] with permission from Elsevier.

values at ca. 296 K of (13 ± 7) and (17 ± 6) × 10–12 cm3 molecule–1 s–1 in two studies [68,69]. Goulay et al. determined values over an extended temperature range of 58–470 K [70]. Ananthula et al. investigated the reaction between these two components over the temperature range 373–1200 K [71]. Two reactions occur, of which one is OH-addition and the second is H-abstraction. In fact, there is no clear separation of these two mechanisms below 1000 K, as is evident for benzene and naphthalene. To summarise, increase in molecular size has a significant positive effect on the OH–addition rate, whereas the rate of H-abstraction does not appear to change. 4. REACTION OF POLYCYCLIC AROMATIC HYDROCARBONS WITH O2 AND O3 4.1 Oxidation mechanism and dynamics of zigzag graphene nanoribbon by O2 and O3 The mechanisms and pathways of oxidation by O2/O3 were investigated using the PAH C28H13 as a model [72]. By varying the temperature range between 400 and 1200 K and the O2 pressure between 0.01 and 5 Torr, the geometric parameters of the stationary points on the oxidation reaction potential-energy surfaces of the zigzag graphene nanoribbon (ZGNR) edge with O2/ O3 were identified by DFT [73], being optimised by the Gaussian 09 program [74] utilising the B3LYP functional with 6-311G(d,p) and 3-21G(d) basis sets. The oxidation reaction rate constants were calculated using statistical mechanics and TST rate theory with the VariFlex program [75]. There are several products, intermediates and TSs in the reaction (Figures 7 and 8). The C28H13(2A′) + O2(3Σ−g) model reaction (2a,2b) (Figure 9) is a consecutive two-step reaction and the C28H12(1A′) + O3(1A1) model reaction (3a,3b,3c) (Figure 9) involves a consecutive threestep reaction. Reactions (2a) and (2b), as shown in Figure 9, contain two low energy steps: C28H13 (2A′) + O2 (3Σ−g) → P1 + CO www.prkm.co.uk

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Figure 7 Main geometric parameters (bond lengths in Å) of intermediates (INT), transition states (TS), products (P) and C28H13 (2A′) as model for reaction (2) of ZNGR with O2. Reprinted (adapted) with permission from ref. [72]. Copyright 2014 American Chemical Society.

Figure 8 Main geometric parameters (bond lengths in Å) of intermediates (int), transition states (ts), products (p), and C28H12 (1A′) as model for reaction (3) of ZGNR with O3. Reprinted (adapted) with permission from ref. [72]. Copyright 2014 American Chemical Society.

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C24H9O5 (2A′′) + O2 (3Σ−g) → P2 + CO

211 (2b)

Reactions (3a), (3b) and (3c), also shown in Figure 9, contain three low energy steps C28H12(1A′) + O3 (1A1 ) → p1 + CO

(3a)

C26H10O4(1A′) + O3 (1A1 ) → p2 + CO

(3b)

C25H11O6 → p3 + CO2

(3c)

In the first reaction step, the exothermicity of reaction (2) is estimated to be about 114.8 kcal mol–1 and for reaction (3) it is 233.7 kcal mol–1. The second step for both reactions is the ratedetermining step. The oxidation time shortens significantly with increasing O2/O3 pressure. For reaction (2), it shortens rapidly when the temperature increases from 400 to 700 K and lengthens slowly at temperatures higher than 700 K. For reaction (3), the reaction time at 400 K is very long, while at 500 K it shortens dramatically. At temperatures over 500 K, the reaction time continues to drop, but the change is much less significant. Therefore, the etching processes of ZGNR by oxidation by O2 and O3 can be precisely controlled by temperature and pressure. Also, this theoretical result predicts that the O3 etching process can act as an excellent alternative to the existing O2 process. In Figure 9, the intermediates, transition states and products of reaction (3) are all designated in lower case (int, ts, and p respectively), in order to distinguish them from the intermediates, transition states and products in reaction (2) which are shown in upper case (INT, TS, P). 4.2 Reaction mechanism of naphthyl radicals with molecular oxygen The reactions of 1-C10H7 + O2 and 2-C10H7 + O2 have been investigated [32] and one of the most important intermediates which is found is C10H7O2. 4.2.1 Potential energy surface of the 1-C10H7 + O2 reaction system The reaction starts with addition of the oxygen molecule to the radical site (like the basic reaction C6H5 + O2 [76]) of 1-C10H7. There are several pathways. Three different isomers of the 1-C10H7OO species can be produced, a1 and a1′, as well as a8 containing a CO2 three-

Figure 9 Potential energy surfaces of graphene edge reaction with O2 (3Σ–g) and O3 (1A1) in kcal mol–1 for the initial oxidation of the outermost row carbon atoms of ZGNR edge by O2 (3Σ- g) [reaction (2)]. (3a-c) represent the potential energy surface of the three reaction steps for the ozonolysis process of the ZGNR edge [reaction (3)]. Reprinted (adapted) with permission from ref. [72]. Copyright 2014 American Chemical Society.

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membered ring (Figure 10). The energy of a1 is 3.7 kcal mol–1 lower than that of a1′ and the two conformations can easily rearrange to each other by rotation around the C−O bond; the a1′ intermediate is not likely to exist and would rearrange to a1 spontaneously. Alternatively, addition of O2 to the reactant leads to the formation of a8 as the other intermediate. After formation of 1-naphthoxy radical a1 has occurred, it can undergo a partition of the O–O bond and decompose to the 1-naphthoxy radical (1-C10H7O, a3) and the ground-state oxygen atom, O (3P). Then, one of the oxygens inserts into the ring to produce intermediates a10 or a10′, in which the six-membered ring linked with O2 expands to form a seven-membered ring

Figure 10 Potential energy diagram for the initial channels of the 1-naphthyl radical + O2 reaction: formation of C10H7O2 adducts, their decomposition to 1-C10H7O + O and oxygen atom insertion into the C6 ring. The numbers show relative energies in kcal mol–1 calculated at the G3(MP2,CC)//B3LYP/6311G** + ZPE (in brackets) level of theory. Reprinted (adapted) with permission from ref. [32]. Copyright 2012 American Chemical Society.

Figure 11 Potential energy diagram for the initial channels of the 2-naphthyl radical + O2 reaction: formation of C10H7O2 adducts, their decomposition to 2- C10H7O + O and oxygen atom insertion into the C6 ring. The numbers show relative energies in kcal mol–1 calculated at the G3(MP2,CC)//B3LYP/6311G** + ZPE (in brackets) level of theory. Reprinted (adapted) with permission from ref. [32]. Copyright 2012 American Chemical Society.

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containing six carbons and one oxygen atom and the second O atom remains attached to the attacked carbon through a C=O bond. 4.2.2 Potential energy surface of the 2-C10H7 + O2 reaction system The 2-naphthyl + O2 reaction starts with addition of the oxygen molecule to the radical site [32] of 2-C10H7. This can lead to three different isomers of the 2-C10H7OO species, two conformations of the 2-naphthylperoxy radical, b1 and b1′ and a tricyclic b8 adduct with a CO2 ring (Figure 11). As the b1 and b1’ energies are close to each other, they can isomerise to one another by rotation around the C–O bond, with a low barrier of 2.5–2.6 kcal mol–1. Next, one of the oxygen atoms can insert into the ring in two distinct positions to form two different intermediates, b10 and b10′. After the 2-naphthylperoxy radical b1 is produced, it can undergo a cleavage of the O−O bond and decompose to the 2-naphthoxy radical plus oxygen atom, 2-C10H7O (b3) + O (3P). Thus, the potential energy surfaces of various pathways in the reaction of 1- and 2-naphthyl radicals with molecular oxygen have been investigated at the G3(MP2,CC)//B3LYP level of theory. Both reactions are started by addition of O2 to the radical sites of naphthyl radicals and both reactions are exothermic by about 45-46 kcal mol–1. Next, the 1- and 2-C10H7OO radicals can eliminate an oxygen atom, leading to the formation of 1- and 2-naphthoxy radical products. The O-loss process from C10H7OO producing C10H7O is predicted to be favourable at high temperatures. Upper limit rate constants for the reaction of naphthalene, 2-methylnaphthalene and 2,3dimethylnaphthalene with O3 were obtained at < 3 × 10–19, < 4 × 10–19 and < 4 × 10–19 cm3 molecule–1 s–1 respectively [52]. 5. REACTIONS OF NO2 WITH POLYCYCLIC AROMATIC HYDROCARBONS Diesel motor vehicles are an important source of fine carbonaceous particles in air [77]. The diesel particulate matter is Standard Reference Material from the U.S. National Institute of Standards and Technology (NIST) and the concentrations of several used PAHs are certified by NIST. Experiments on the reaction of PAHs with NO2 were performed using a fast flow reactor at room temperature and (2.4 ± 0.3) mbar total pressure [78]. It is important to note that, on the same time scale and for the same NO2 concentration, the adsorption of PAHs on diesel particles is less important than the adsorption of graphite on diesel particles. Some experimental decays corresponding to the reaction of adsorbed PAHs with NO2 are presented in Figure 12. From the rate constants shown in Table 4, phenanthrene, pyrene and BaP are the most reactive PAHs with NO2. These result suggest that the heterogeneous reactivity of PAHs depends on the nature (carbon content, surface area, porosity etc.) and the mode of formation (temperature, pressure, atmosphere etc.) of the particulate layer with which they are affiliated. 6. REACTIONS OF NO3 WITH POLYCYCLIC AROMATIC HYDROCARBONS Understanding the reactions of alkylnaphthalenes with NO3 radicals is important because they may lead to the formation of potentially toxic nitro-derivatives [81,82]. NO3 radicals are generated from the thermal decomposition of N2O5 [83]. N2O5 → NO3 + NO2 www.prkm.co.uk

(4)

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Figure 12 Degradation decays versus time, corresponding to the reaction of NO2 with some diesel particulate PAHs (phenanthrene, fluoranthene, pyrene, benzo[a]anthracene). Uncertainties (1σ) were extracted from three experimental replicates. Reprinted from ref. [78] with permission from Elsevier.

Table 4 Pseudo-first-order rate constants (s–1) relative to the heterogeneous reaction of NO2 with PAHs adsorbed on diesel particles [78] (NIST SRM 1650a) ([NO2] = 8 × 1013 molecule cm–3, T =298 K) compared to those measured when PAHs are adsorbed on graphite particles [79] or on natural residential wood stove particles [80]

PAH Phenanthrene Anthracene Fluoranthene Pyrene Chrysene Benz(a)anthracene Benzo(k)fluoranthene Benzo(b+k+j)fluoranthene Benzo(a)pyrene Benzo(e)pyrene Perylene Indenol(1,2,3-cd)pyrene Benzo(ghi)perylene

PAHs adsorbed on diesel particles [78] Pseudo-first-order rate constants (s–1) (2.1±0.9) × 10 –3 – (1.9±0.8) × 10 –3 (2.9±1.2) × 10 –3 (1.7±0.8) × 10 –3 (1.7±1.0) × 10 –3 – (1.6±0.9) × 10 –3 (3.2±1.2) × 10 –3 (1.9±0.6) × 10 –3 – (1.7±0.6) × 10 –3 –

PAHs adsorbed on graphite PAHs naturally adsorbed particles [79] on wood stove [80] Pseudo-first-order rate Second-order rate constants constants (s–1) (cm3 molecule –1 s–1) (2.8±0.8) × 10 –3 – (5.5±1.2) × 10 –3 – (2.3±1.0) × 10 –3 – (4.1±3.0) × 10 –3 – –3 (3.0±1.1) × 10 3.88 × 10 –18 (2.6±0.6) × 10 –3 3.88 × 10 –18 (2.0±0.6) × 10 –3 3.25 × 10 –18 – – (6.2±0.1) × 10 –3 3.88 × 10 –18 (2.8±1.1) × 10 –3 – – (8.4±0.8) × 10 –3 – – (3.0±0.4) × 10 –3 –

Reprinted from ref. [78] with permission from Elsevier.

Alkyl substitution enhances the reactivity, with the disubstituted naphthalenes being more reactive than the mono substituted methyl- or ethyl-naphthalenes. The NO3 radical reaction appears more sensitive to steric effects. In 1,8-dimethylnaphthalene the methyl groups are in peri positions and in 1,2-dimethylnaphthalene the methyl groups are ortho to one www.prkm.co.uk


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Figure 13 NO3 radical reaction with naphthalene. (From: Phousongphouang, P.T. and Arey, J. (2003) Environ. Sci. Technol., 37, 308–313).

Figure 14 Structural formulae of PAHs (a) coronene (C24H12) and (b) hexabenzocoronene C42H18.

another with a hydrogen peri to one of the methyl groups (Figure 13). The influence of steric crowding in naphthalene substitution, at the 1,8- and 1,2- positions was seen in the photolysis rates of methylnitronaphthalenes [53]. Measurements of the gas-phase photolysis of methylnitronaphthalenes indicate that photolysis of 1-methyl-8-nitronaphthalene proceeds most rapidly, followed by photolysis of 2-methyl-1-nitronaphthalene. NO3 + alkylnaphthalene → products (1, obs)

(5)

NO3 + naphthalene → products (2, obs)

(6)

The rate constants at room temperature increase with the number of alkyl substituent groups. The concentrations of NO2 and O2 in the daytime and night-time have an extremely important effect on the reaction of PAHs and NO3. Analytical techniques have been utilised for the efficient investigation of oxygenated and nitrated derivatives of large PAHs, including liquid chromatography–mass spectrometry [84]. For example, coronene and hexabenzocoronene (Figure 14) were exposed to nitrogen dioxide under simulated diesel exhaust conditions [84]; it was found that the main products of coronene oxidation included odd numbers of nitrogen atoms, whilst one of the main products from hexabenzocoronene included even numbers of nitrogen atoms. Thus far, most studies on PAHs have focused on gas-phase homogeneous chemistry, with only a limited amount of work reported on heterogeneous reactions. The heterogeneous reaction of PAHs with different atmospheric particles has been investigated and the available data suggest that, in general, heterogeneous reactions can differ dramatically from homogeneous reactions www.prkm.co.uk

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in rates, mechanisms and products [85–88]. Heterogeneous reactions between condensed phase PAHs and the following gas-phase oxidants have been explored: OH [78,79], NO [79], NO2 [89–92,78,79], HNO3 [93], N2O5 [81,94] and NO3 [81]. Gross and Bertram found that, under certain atmospheric conditions, NO3 radicals can be a more important reactant for PAHs than NO2, HNO3, N2O5, or O3 and have a good impact on the tropospheric lifetimes of PAHs [95]. 7. REACTION OF ANTHRACENE WITH CH RADICALS The rate coefficient has been determined experimentally over a wide temperature range (58-470 K, Table 5) for the reaction of anthracene with CH [96]. The methylidine radical is produced in the atmosphere by methane photolysis, so in daytime this reaction is more conceivable. In an experiment with anthracene, the uncertainty on the anthracene density led the authors to evaluate the uncertainty on the rate constant to be greater than in previous studies [97,98]. This is because, in the flow system, the anthracene density was harder to evaluate than that of gaseous species at room temperature. The general equation for such a reaction is CH + CnH2n → Cn+1 + H2n + H

(7)

As regards the reaction of CH with aromatic compounds [99], these reaction are always exothermic as in the reaction of CH with benzene and toluene. The main product of the CH reaction with anthracene is expected to be the stable norcaratriene-like compound: cyclopropa[b] anthracene (Scheme 3). Due to the lack of theoretical calculations for CH reactions with unsaturated hydrocarbons, to confirm the mechanism of this reaction and the formation of cyclopropa[b]anthracene, ab initio calculations are needed. 8. REACTION OF POLYCYCLIC AROMATIC HYDROCARBONS AND OZONE There is little quantitative information about the products of the heterogeneous reaction of PAHs and ozone. The only study that investigated the products of the heterogeneous reaction between anthracene and ozone [100] involved a thin film of anthracene in a Langmuir trough. Anthracene was adsorbed on a stearic acid–aqueous surface and on dry Teflon and monitored by laserinduced fluorescence. Although anthracene may react with adsorbed ozone, several reaction steps may occur to produce an intermediate that then reacts with ozone to form anthraquinone (Figure 15). The reaction between this intermediate and ozone will be the rate-limiting step in anthraquinone product formation. The rate-determining steps for anthracene loss and the Table 5 Rate constants for CH + anthracene obtained with the CRESU apparatus. Reprinted (adapted) with permission from ref. [96]. Copyright 2006 American Chemical Society T (K)

Carrier gas

58 58 58

He He He/N2a (100:3.2) N2 N2 N2

119 235 470 a

Total number density (1016 molecules cm–3) 4.01 4.01 4.01

[Anthracene] (1012 cm–3) 9.6–124.1 10.9–39.8 10.9–39.8

Rate coefficient (10 –11 cm3 s–1) 13.2 12.6 13.2

1.96 2.06 2.60

25.7–83.0 14.7–110.2 17.6–48.9

23.5 28.1 41.8

Flow rates: Q N2 = 2.7 SLM, Q He = 84.07 SLM. SLM is standard litre per minute

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Scheme 3

Figure 15 Reaction mechanism for the ozonation of anthracene to yield anthraquinone (adapted from Bailey [102]). The mechanism outlined with bold arrows was proposed by Bailey for the direct formation of anthraquinone and is suggested here to govern the heterogeneous reaction of surfacebound anthracene and ozone. Reprinted (adapted) from ref. [101]. Copyright 2006 American Chemical Society.

anthraquinone yield have a similar dependence on the level of ozone concentration remaining on the surface [101]. The reaction kinetics followed the Langmuir–Hinshelwood mechanism, which suggests a surface reaction. Heterogeneous reaction rates of gas-phase ozone with naphthalene, anthracene, fluoranthene, phenanthrene, pyrene and BaP were measured over a range of ozone concentrations from 3.5 × 1014 to 2.3 × 1016 molecule cm–3 at room temperature. The relative rates of reaction www.prkm.co.uk

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were: fluoranthene < phenanthrene < pyrene < naphthalene < anthracene < BaP. The slowest reaction is related to the reaction of fluoranthene with ozone [88]. In order to improve our understanding of how the aerosol substrate influences the heterogeneous reaction of ozone with anthracene, Ma et al. investigated the process of interaction of PAH with ozone using different substrates such as mineral oxides and a Teflon disc [103]. It was also found that the nature of different substrates had a strong influence on the heterogeneous reactions between ozone and anthracene. Published online: 22 August 2017 9. REFERENCES Mastral, A.M and Callén. M.S. (2000) Environ. Sci. Technol., 34, 3051–3057. Gelboin, H. (1976) In: Free radicals in biology, Vol. 1, pp. 45–70. Academic Press, New York. Connell, D.W. (1997) Basic concepts of environmental chemistry, pp. 205–217. CRC-Press. Shaw G.R. and Connell D.W. (1994) Rev. Environ. Contam. Toxicol., 135, 1–62. Harvey, R.G. (1991) Polycyclic aromatic hydrocarbons: chemistry and carcinogenicity. Cambridge University Press, Cambridge. [6] Public Health Service (1998) National Toxicology Program. US Department of Health and Human Services Integrated Laboratory Systems, Inc., Research Triangle Park, NC, 178–181. [7] Menzie, C.A., Potocki, B.B. and Santodonato, J. (1992) Environ. Sci. Technol., 26, 1278–1284. [8] Nikolaou, K., Masclet, P. and Mouvier, G. (1984) Sci. Total Environ., 32, 103–132. [9] Tokiwa, H., Ohnishi, Y. and Rosenkranz, H.S. (1986) Crit. Rev. Toxicol., 17, 23–58. [10] Smith, I.M. (1984) PAH from coal utilisation: emissions and effects, p. 18. IEA Coal Research, London. [11] Allen, J.O., Dookeran, N.M., Smith, K.A., Sarofim, A.F., Taghizadeh, K. and Lafleur, A.L. (1996) Environ. Sci. Technol, 30, 1023. [12] Josephy, P.D., Mannervik, B. and Ortiz de Montellano, P.R. (1997) Molecular toxicology, pp. 152–186. Oxford University Press, New York. [13] Shiroudi, A., Deleuze, M.S. and Canneaux, S. (2014) J. Phys. Chem. A, 118, 4593−4610. [14] Bunce, N.J., Liu, L., Zhu, J. and Lane, D.A. (1997) Environ. Sci. Technol, 31, 2252–2259. [15] Atkinson, R. and Arey, J. (1994) Environ. Health Perspect., 102(Suppl. 4), 117–126. [16] Keyte, I.J., Harrison, R.M. and Lammel, G. (2013) Chem. Soc. Rev., 42, 9333–9391. [17] Hongtao, Y. (2002) J. Environ. Sci. Health, Part C: Environ. Carcinog. Ecotoxicol. Rev., 20, 149–183. [18] Schuetzle, D. (1983) Environ. Health Perspect., 47, 65–80. [19] Salmeen, I.T., Pero, A.M., Zator, R., Schuetzle, D. and Riley, T.L. (1984) Environ, Sci, Technol., 18, 375–382. [20] Bjorseth, A. and Ramdahl, T. (1985) Handbook of polycyclic aromatic hydrocarbons. Volume 2: emission sources and recent progress in analytical chemistry, pp. 1–20. Marcel Dekker, New York. [21] Siegmann, K. and Siegmann, H.C. (1998) In: Móran-López, J.L. (ed.), Current problems in condensed matter, pp. 143–160. Plenum Press, New York. [22] Boström, C.E., Gerde, P., Hanberg, A., Jernström, B., Johansson, C., Kyrklund, T., Rannug, A., Törnqvist, M., Victorin, K. and Westerholm, R. (2002) Environ. Health Perspect., 110(Suppl. 3), 451–488. [23] International Agency for Research on Cancer. (1983) Polynuclear aromatic compounds, part 1, chemical, environmental and experimental data, IARC Monogr. Eval. Carcinog. Risks Hum., Vol. 32. Lyon, France. [24] Lee, M.L., Novotny, M.V. and Bartle, K.D. (1981) Analytical chemistry of polycyclic aromatic compounds. Academic Press, New York. [25] Badger, G.M. (1962) Natl. Cancer Inst. Monogr., 9, 1–16. [26] Carcinogen Assessment Group (1985) Office of Health and Environmental Assessment, Office of Research and Development. U.S. EPA, Washington, DC. [27] Hazard Identification and Dose-response Assessment (1997) Ministry of Environment and Energy, Toronto, Ontario, Canada. [28] Marino, F., Cecinato, A. and Siskos, P.A. (2000) Chemosphere, 40, 533–537. [29] Dockery, D.W., Pope, C.A., Xu, X., Spengler, J.D. , Ware, J.H., Fay, M.E., Ferris Jr, B.G. and Speizer, F.E. (1993) N. Engl. J. Med., 329, 1753–1759. [30] Abedi-Ardenkani, B., Kamangar, F., Hewitt, S.M., Hainaut, P., Sotoudeh, M., Abnet, C.C., Taylor, P.R., Boffetta, P., Malekzadeh, R. and Dawsey, S.M. (2010) Gut, 59, 1178–1183. [31] Violi, A., Truong, T.N. and Sarofim, A.F. (2004) J. Phys. Chem. A, 108, 4846–4852. [32] Zhou, C.-W., Kislov, V.V. and Mebel, A.M. (2012) J. Phys. Chem. A, 116, 1571−1585. [33] Unterreiner, B.V., Sierka, M. and Ahlrichs, R. (2004) Phys. Chem. Chem. Phys., 6, 4377–4384. [34] McKay, J., Molyneux, M., Pizzella, G., Radojcic, V, Baverstock, S.J. and Schipper, H.W. (1999) Environmental levels of benzene at the boundaries of three European refineries, p. 78. CONCAWE Air Quality Management Group, Brussels. [1] [2] [3] [4] [5]

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