The mechanism and kinetic studies on oxidation reaction of acetofenate initiated by HO_x, NO₃, O₃, and Cl radicals

Abstract

Acetofenate (AF) is a widely used pesticide. The mechanism of HO_x, NO₃, O₃, and Cl initiated oxidation reactions of AF was investigated with density functional theory. For each of the OH, NO₃, and Cl, both addition and hydrogen abstraction were investigated, for each of the HO₂ radicals and O₃, addition reactions were investigated. The cycloaddition reactions of O₃ were considered, including the exploration of isomerization. Based on the potential energy surface, the rate constants were calculated with the transition state theory method over a temperature range of 200–400 K and fitted with the Arrhenius formulas. The rate constants of the AF reaction with OH, HO₂, NO₃, O₃, and Cl, are 4.04 × 10⁻¹³, 7.02 × 10⁻³³, 6.93 × 10⁻²⁰, 1.45 × 10⁻²⁵, and 5.07 × 10⁻¹² cm³ per molecule per s at 298.15 K, respectively. The OH-initiated reactions are dominant according to the branching ratio of reaction constants. The atmospheric lifetime of the reaction species was estimated according to rate constants.

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Introduction

Acetofenate (AF), also known as benzethazet and plifenate, is used worldwide to control pests.¹ As a commercial organochlorine pesticide, AF was developed as an analogue of DDT since the 1970s.² Due to its low toxicity, selective insecticidal activity, and high efficacy, AF is considered a good alternative to combat pests, especially health pests.^3,4 However, its popularity has led to the direct release of pesticides into the environment for a long time.⁵

In the troposphere, radicals have great effects on the removal or transformation of organic compounds through controlling the oxidative capacity of the atmosphere.⁶ Once released into the atmosphere, AF could be oxidized by nucleophilic active species, HO_x radicals, NO₃ radical, O₃, Cl atom, and other species, and generate a series of oxidation products.^7–11

The HO_x radicals (OH and HO₂ radical), whose photochemical lifetimes are very short, are responsible for the majority of oxidation processes in the troposphere and control concentrations of many species.^12,13 The degradation pathway of AF initiated by OH is important for the related chemistry features of OH, such as the inherent reactivity. However, the formation of OH mainly takes place in daytime through photolysis, and its concentration decreases rapidly after sunset.^8,9,14 As for HO₂, the principal sources are of photolysis and oxidation of HCHO, and the tropospheric abundances are about 100 times higher than that of OH during the daytime in clean air.^15,16 On the whole, reactions with HO_x radicals mainly occur during the day.

It is worth mentioning that not all atmospheric chemistry is initiated in the daytime. In the troposphere, the NO₃ radical is thought to be the main night-time oxidant.^17,18 During the daytime, the concentration of NO₃ is very low. However, in the night-time atmosphere NO₃ radicals were measured at typical concentrations of 10–100 parts per trillion.¹⁹ Clearly, the reactions of AF with NO₃ pathway during nighttime cannot be ignored. In addition, in polluted environment, NO₃ radical is also an important oxidant and may contribute to the removal of AF.^20,21

Ozone is another powerful oxidant that cannot be ignored in atmospheric environment. Its strong oxidizing power and participation in the production of other radicals make ozone a key role in the oxidation of organic matters during both day and night.^22,23 Most notably, the ozone arises from a variety of sources, and the concentration of O₃ (7.0 × 10¹¹ molecule per cm³) is much higher than that of OH radical (9.7 × 10⁵ molecule per cm³).^24,25

Moreover, in the marine atmosphere where the concentration of Cl atoms can reach a peak value of 10⁵ molecule per cm³, the Cl atom-initiated chemistry can be an important removal process for AF.²⁶ As for H-abstraction reactions, the rate constant of Cl atom is 10³ times more active than that of OH radical.²⁷ Thus, Cl atom also should be considered for the oxidation of AF in the atmosphere.

In this work, quantum chemistry method was applied to reveal the reaction mechanisms of AF. The initial reactions of HO_x radicals, NO₃ radical, O₃, and Cl atom with AF were investigated, and thermodynamic parameters are included too. Based on quantum chemistry information, the rate constants are calculated using the transition state theory method. Arrhenius equations of the rate constants with a temperature range of 200–400 K are fitted, and the lifetimes of the reaction species in troposphere are estimated according to the rate constants.

Computational methods

Mechanism study

It is well known that the density functional theory (DFT) is a calculation tool to deal with atmospheric chemical reaction.²⁸ In previous studies DFT have successfully applied to medium or even larger scale molecular system.^29,30 The MPWB1K method is a hybrid DFT model with performance for thermochemistry, thermochemical dynamics, weak interactions, and hydrogen bonding.³¹ In this paper, all the work is performed using the Gaussian 03 programs and SGI workstation.³² The geometrical parameters of reactants, transition states (TS), intermediates (IM), and products were fully optimized at the MPWB1K/6-31+G(d,p) level. The vibrational frequencies have been calculated at the same level in order to determine the nature of stationary points. The single point energy is calculated at the basis sets 6-311+G(3df,2p).

Kinetic calculation

The transition state theory (TST) is a model based on the interaction potential between reactants and products with a statistical representation of the dynamics to determine rate constants.³³ It has been widely employed to discuss of rate processes in previous explorations. In this paper, the rate constants calculation of all elementary reactions have been carried out with the method over a suitable temperature range. Taking a bimolecular reaction as example:

A + BC → AB + C (1)

The rate constant is calculated using the following equation:

The definition of each parameter can be referred to literature.³⁴ The numerical method for linear equation is used to fit the Arrhenius formula of rate constants with temperature.

Results and discussion

OH-Initiated reaction

Under the atmospheric condition, the chemical reactions of AF can be initiated by the OH radical. Two possible reaction pathways are taken into consideration: OH radical addition and H atom abstraction. For convenience, the atom labels are marked in the structure of AF in Fig. 1. The reaction schemes with the potential barriers (ΔE_b) and reaction heats (ΔE_r) are depicted in Fig. 1, S1, and S2,† respectively.

Fig. 1 OH initiated addition pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

Because of its molecule asymmetry, AF has two kinds of different addition positions, that is, the same and opposite side of the benzene ring (the cis-addition and the trans-addition) relative to branched chain substituents. This paper mainly presents the cis-addition reactions. It is obvious that all of cis-addition channels can easily happen for their low potential barriers (3.70–9.94 kcal mol⁻¹). As for trans-addition reactions, the channels are also easily happen, for the ΔE_b values are less than 10.00 kcal mol⁻¹. All these processes are exothermic (11.81–23.06 kcal mol⁻¹), with less than 24.00 kcal mol⁻¹ of energy released. Taken ΔE_b values (<9.94 kcal mol⁻¹) and ΔE_r values (<23.06 kcal mol⁻¹) into consideration, every OH addition reaction can occur easily. Among these pathways, the C₃-cis-addition and C₁-trans-addition are thermodynamically favorable since the barriers are low, 3.70 kcal mol⁻¹ and 2.49 kcal mol⁻¹.

Due to its nucleophilicity, OH radical can abstract the H atom from AF. There are five kinds of H atoms in AF molecule, suggesting that there are five possible pathways: three in the aromatic ring, one in the methylene group, and one in the methyl group. All these pathways can easily happen for their low potential barriers (0.59–9.10 kcal mol⁻¹). Compared with other three exothermic (1.70–2.84 kcal mol⁻¹) processes, pathway A11 (29.05 kcal mol⁻¹) and A12 (17.62 kcal mol⁻¹) are highly exothermic. Among the abstraction pathways, barriers are lower in the reaction of the H atom abstracted from the methylene group and methyl group than those from the aromatic ring.

HO₂-Initiated reaction

HO₂-Initiated reaction schemes of AF embedded with the potential barriers and reaction heats are showed in Fig. 2 and S3,† respectively.

Fig. 2 HO₂ initiated pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

Unlike the OH radical, HO₂ radical cannot abstract H atom from AF, and all channels are addition reactions. Clearly, HO₂ radicals react with AF via TS to form products. All reactions are endothermic, and the ΔE_r varies over the range of 3.51–17.43 kcal mol⁻¹. The rate constant at 298.15 K is 7.02 × 10⁻³³ cm³ per molecule per s, which is smaller than that of OH-initiated reaction.

NO₃-Initiated reaction

NO₃ radical can be added to the double bond and abstract H atom from AF which is similar to the reactions of AF and OH. The diagrams of NO₃-initiated degradation mechanism of AF are illustrated in Fig. 3, 4, and S4,† respectively.

Fig. 3 NO₃ initiated addition pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

Fig. 4 NO₃ initiated H atom abstraction pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

As for AF, NO₃ can attract C=C or C=O double bond to form two different kinds of NO₃-adducts. It can be seen that in each of the pathways NO₃ radicals are added to the benzene. There exists an intermediate AF–NO₃, whose energy is lower than that of AF + NO₃ by 5.89 kcal mol⁻¹ of the cis-addition and by 4.69 kcal mol⁻¹ of the trans-addition. Except the processes AF react with NO₃ radical in the C₂ and C₇-trans-site are endothermic, the rest processes are all slightly exothermic with less than 8.67 kcal mol⁻¹ of energy released. As shown from the cis-addition the calculated potential barriers and reaction heats, NO₃ addition to C₄ is the most favorable reaction with the lowest barrier (5.18 kcal mol⁻¹) and releases the most heat (6.06 kcal mol⁻¹). As for the trans-addition reactions, pathway C1′ and C3′ occur more early for the low potential barriers less than 2.26 kcal mol⁻¹. Then followed by pathway C4′, the potential barriers is 4.22 kcal mol⁻¹, giving out 7.52 kcal mol⁻¹ of energy.

The hydrogen on the AF can be abstracted by NO₃ to produce nitric acid (HNO₃). Noticeably, the pathway C11 and C12 are exothermic, while pathway C8, C9, and C10 are endothermic. The potential barrier of pathway C11 (5.68 kcal mol⁻¹) is lower than that of other pathways. At the same time, pathway C11 (21.29 kcal mol⁻¹) is more exothermic than pathway C12 (9.95 kcal mol⁻¹). Thus, the pathway C11, i.e., NO₃ abstracts H atom from the methylene group, occurs more easily. What's more, at 298.15 K the total rate constant of addition reactions (4.59 × 10⁻²⁰ cm³ per molecule per s) is larger than that of abstraction reactions (2.34 × 10⁻²⁰ cm³ per molecule per s) with NO₃ at 298.15 K.

O₃-Initiated reaction

The ozonolysis reactions are initiated by addition of O₃ to the double bond of AF to produce primary ozonide. The reaction channels are provided in Fig. 5 and S5,† respectively.

Fig. 5 O₃ initiated pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

The twelve elementary reactions all take place from AF with O₃ via TS to form the ozonide. All processes are highly exothermic with released heat of 27.33–35.72 kcal mol⁻¹, and the barrier varies over the range of 11.84–21.50 kcal mol⁻¹. The smaller energy barrier comes from the trans-addition pathways that O₃ addition to C₂–C₃ products IM2′D. The high reaction energy is retained as the internal energy of the adduct, the excited ozonide subsequently undergoes unimolecular decomposition due to highly exothermic.³⁵ Then the C₂–C₃ and O–O of the added O₃ bonds break, respectively. In consideration of the large system of reactants and IM2′D is obtained more easily, and only IM2′D is chosen to study the mechanism of isomerization. The reaction scheme is described as follows:

Calculations indicate that in the two processes there are apparent potential barriers of 36.30 kcal mol⁻¹ and 32.25 kcal mol⁻¹, absorbing 0.05 kcal mol⁻¹ and 1.23 kcal mol⁻¹ of heat.

Cl-Initiated reaction

Similar to the above reaction pathways, the Cl atom can attack the double bonds of AF, and abstract hydrogen to generate hydrochloric acid (HCl). The probable pathways for the reactions are depicted in Fig. 6, 7 and S6,† respectively.

Fig. 6 Cl initiated addition pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

Fig. 7 Cl initiated H atom abstraction pathways with the potential barriers ΔE_b and reaction heats ΔE_r (kcal mol⁻¹).

The cis-addition reactions are analysed in details. When Cl atom is added to the benzene ring to react with AF, they first form the intermediate AF–Cl-1, with the energy lower than that of AF + Cl by 6.14 kcal mol⁻¹, and then generate IM via TS. The barriers of these processes vary over the range of 0.01–11.27 kcal mol⁻¹. Among the six pathways, the barrierless process comes from pathway E4 (0.01 kcal mol⁻¹), and the reaction is slightly exothermic, giving out 2.58 kcal mol⁻¹ of energy while others are endothermic. When the Cl is added to C=O double bond, this process without the formation of AF–Cl, so the process is endoergic, absorbing 6.91 kcal mol⁻¹ of heat, and the potential barrier is 8.08 kcal mol⁻¹.

The trans-addition reactions are similar with the cis-addition reactions. When Cl atom is added to the benzene ring, there also exists an intermediate AF–Cl-1′, and then generate IM via TS. The barriers of these processes vary over the range of 0.16–8.76 kcal mol⁻¹. The process of Cl is added to C=O double bond without the formation of AF–Cl, and it is endoergic, absorbing 8.24 kcal mol⁻¹ of heat, and the potential barrier is 8.76 kcal mol⁻¹. Among these reactions, pathway E1′, E3′, and E4′ could easily occur for low potential barriers (<1.18 kcal mol⁻¹) and the reaction pathway E2′ is slightly endothermic, absorbing 1.39 kcal mol⁻¹ of energy while others are exothermic.

The H atom abstraction pathways of AF with Cl atoms are all with positive barriers. The potential barriers of pathway E11 and E12 are lower than that of pathway E8, E9, and E10. Compared with other three endothermic (2.78–14.19 kcal mol⁻¹) processes, pathway E11 (4.54 kcal mol⁻¹) and E12 (1.82 kcal mol⁻¹) are slight exothermic. In addition, the rate constants of pathway E11 and E12 are larger than those of other pathways. So taken the thermodynamic and kinetic into consideration, it is obvious that pathway E11 and E12 are favored in H abstraction pathways.

Subsequent reactions

In general, the radicals formed from additions and abstractions could further be decomposed in the presence of O₂ and NO/H₂O in the atmosphere. Evidently, the addition reactions in which HO₂, NO₃, and Cl are added to C₄-cis and C₁-trans sites are the most favorable pathways with the lowest barriers in each group, which can further lose the H atom that connected with the C atom and degraded in the atmosphere. In the H atom abstraction pathways, different radicals obtain five similar intermediates. The barrierless and slight exothermic processes where H atom is abstracted from the methylene group and methyl group are more dominant. Some subsequent reactions for this part have been explored by our group.³⁶

Kinetic analysis

With TST method, the rate constants of OH, HO₂, NO₃, O₃, and Cl initiated reactions of AF at a suitable temperature range of 200–400 K are listed in Tables S1A–S8A,† respectively. The Arrhenius equations i.e. k(T) = A exp(−E_a/RT)³⁷ for rate constants of element reactions are listed in Tables S1B–S5B,† respectively. The total Arrhenius equations and rate constants at 298.15 K are shown in Table 1. Obviously, most of the calculated rate constants can be fitted by Arrhenius equations. The correlation coefficient R² is above 0.9. The branching ratios of addition and abstraction reactions among different initiated reaction from 200 to 400 K are shown in Fig. S7–S9.†

Table 1 Arrhenius formulas of rate constants k for elementary reactions at 200–400 K (cm³ per molecule per s)

Reactions	ktotal (T = 298.15)	Arrhenius formulas	R^2
AF + OH˙ → IMA	4.04 × 10^−13	k = 7.71 × 10^−12 exp(−865.03/T)	0.9941
AF + HO2˙ → IMB	7.02 × 10^−33	k = 2.91 × 10^−15 exp(−8133.1/T)	0.9999
AF + NO3 → IMC	6.93 × 10^−20	k = 8.85 × 10^−17 exp(−2067.6/T)	0.9799
AF + O3 → IMD	1.45 × 10^−25	k = 5.38 × 10^−16 exp(−6555.2/T)	0.9999
AF + Cl˙ → IME	5.07 × 10^−12	k = 7.84 × 10^−10 exp(−947.77/T)	0.9326

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The rate constants at 298.15 K and 1 atom in the routes have been chosen for discussion. Take OH-initiated reaction for example. The individual rate constants for the addition pathway A1–A7 and A1′–A7′ are noted k_A1–k_A7 and as k_A1′–k_A7′ respectively, and rate constants for the H atom abstraction pathway 8–12 are noted as k_A8–k_A12 respectively. The rate constants of overall OH radical addition reaction and H atom abstraction are defined as k^add_OH = k_A1 + k_A2 + k_A3 + k_A4 + k_A5 + k_A6 + k_A7 + k_A1′ + k_A2′ + k_A3′ + k_A4′ + k_A5′ + k_A6′ + k_A7′, and k^abs_OH = k_A8 + k_A9 + k_A10 + k_A11 + k_A12, respectively. The overall rate constant for the AF with OH reaction is labeled as k^total_OH = k^add_OH + k^abs_OH. At 298.15 K, the overall calculated rate constants for reactions of AF with OH, HO₂, NO₃, O₃, and Cl, are 4.04 × 10⁻¹³, 7.02 × 10⁻³³, 6.93 × 10⁻²⁰, 1.45 × 10⁻²⁵, and 5.07 × 10⁻¹² cm³ per molecule per s.

The degradation rate can be expressed by the formula r = k_X[X][AF]. Once the concentrations of AF and X radical in atmosphere are determined the degradation rate can be calculated. The rate branching ratio can be presented by

(X = OH, HO₂, NO₃, O₃, Cl). In calculations, the 24 h average oxidant concentrations^13,24,38,39 are considered: OH radical concentration of 9.7 × 10⁵ molecule per cm³, HO₂ radical concentration of 1.0 × 10⁸ molecule per cm³, NO₃ concentration of 1.2 × 10⁷ molecule per cm³, O₃ concentration of 7 × 10¹¹ molecule per cm³, and Cl concentration of 1 × 10³ molecule per cm³. Fig. S10† indicates the rate branching ratio of different rate constants of initiated reactions to the total rate. It can be observed that the removal of AF in the atmosphere is dominated by its reactions with OH radical (>98%), followed by minor reactions with Cl-atom (<2%) in the temperature range of 200–400 K. On the contrary, HO₂, O₃, and NO₃ initiated reactions take little part in this range of temperature. When the temperature below 300 K, the rate branching ratio of OH-initiated reactions increases slowly and the rate branching ratio of Cl-initiated reactions decreases as temperature rises. After 300 K, the former increases and the later decrease. Generally speaking, the branching ratios change little with the various temperatures. In the whole reactions, OH initiated reactions make more contribution.

Atmospheric lifetime

To assess its impact on the environment, it is critical to know the atmospheric lifetime of AF. The atmospheric lifetimes (τ) of AF, with respect to reactions with OH, HO₂, NO₃, O₃, and Cl, can be calculated and analyzed on the basis of the rate constants in the elementary reaction, using the formula . In which [X] term is the concentration of radical and can be taken as a constant in the troposphere. The calculated lifetimes at the range of 200–400 K are listed in Tables S1C–S8C.† Overall, the higher the temperature is, the shorter the lifetime.

The total atmospheric lifetime can be calculated with the formula . Total atmospheric lifetimes of AF with different oxidants are listed in Table SD.† According to the results, the τ_total for all OH, HO₂, NO₃, O₃, and Cl initiated oxidation reactions is 2.14 days. And the τ for the reaction of OH radical is around 2.18 days, and 135 days for Cl. Evidently, AF is more likely to be removed quickly by the reaction with ozone near their emission sources. However, both the concentration of radicals and the rate constant of reactions with radicals determine the atmospheric lifetime of AF. In some places, the abundant OH radicals may play an important role in the degradation of AF. At night or in seriously polluted regions, the 12 h averaged NO₃ radical concentration is about 5 × 10⁸ cm³ per molecule per s, the reaction with NO₃ may contribute to controlling AF.⁴⁰ Moreover, in some coastal areas where the concentration of Cl atoms can reach 1 × 10⁵ molecule per cm³, the reaction of AF with Cl atom may contribute great portion to the removal of AF in the atmosphere.²⁶

Conclusions

The degradation mechanism of AF initiated by OH radical, HO₂ radical, NO₃ radical, O₃, and Cl atom were investigated at the MPWB1K/6-31+G(d,p) level. The kinetic calculations were carried out with TST method. Some valuable conclusions can be drawn as follows:

The mechanism of OH, HO₂, NO₃, O₃, and Cl initiated reactions of AF includes H abstraction pathways and the addition pathways.

The O₃-adducts can undergo unimolecular decomposition, leading to C–C/C–O bond cleavage. For cis-addition HO₂, NO₃, and Cl added to C₄ are the most favorable reaction pathways with the lowest barrier and release the more heat, while they added C₁ are the most favorable reaction pathways among the trans-addition, the adducts can further leave the H atom connect with C₄ or C₁ and be degraded in the atmosphere.

At 298.15 K, the total rate constants of AF with OH, HO₂, NO₃, O₃, and Cl, are 4.04 × 10⁻¹³, 7.02 × 10⁻³³, 6.93 × 10⁻²⁰, 1.45 × 10⁻²⁵, and 5.07 × 10⁻¹² cm³ per molecule per s, respectively. The rate branching ratio of OH initiated reactions are dominant. The HO₂, NO₃, O₃ and Cl initiated reactions make little contribution to the reaction rate of AF from 200 to 400 K compared with the OH initiated reactions.

The τ for all OH, HO₂, NO₃, O₃, and Cl initiated oxidation reactions is 2.14 days.

Acknowledgements

This work is supported by National Natural Science Foundation of China (21277082, 21337001), the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2012HZ009), Natural Science Foundation of Shandong Province (No. ZR2014BP012), Program for New Century Excellent Talents in University (NCET-13-0349), Marie Curle International Research Staff Exchange Scheme Fellowship within the 7th European Community Framework Program (No. 295132), Project for science and technology development of Shandong province (2014GSF117028), Beijing National Laboratory for Molecular Science (20140160), Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB05010200), and the Fundamental Research Funds of Shandong University (2015JC020).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11453e

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