Evaluation of OH Radical Reaction Positions in 3-Ring PAHs Using Transition State Energy and Atomic Charge Calculations

Abstract

In this study, transition state energy and atomic charge were calculated using the Gaussian 09 program with focus on three-ring PAHs, such as acenaphthylene and anthracene, which are most likely found in contaminated sites. The calculation results were then compared with the radical reaction positions reported in the existing literature. Because the energy difference between the reactant and the transition state according to the reaction position was very small, no distinct correlation was obtained when results were compared with those of the OH radical test findings reported in the literature. It was also found that the charge calculation makes it possible to accurately predict the radical reaction position of the target material. In addition, MK and HLY charges were found to be more accurate than CHelpG charges in predicting the radical reaction positions. The charge calculation can also be applied in predicting radical reaction positions for hazardous materials with different molecular structures.

1. Introduction

Polycyclic aromatic hydrocarbons (PAHs) are compounds that contain two or more aromatic rings, such as naphthalene with two benzene rings, benzo[a]pyrene with five benzene rings, and benzo[ghi]perylene with six benzene rings. The US EPA has designated and manages 16 types of PAHs as high-priority substances. In the case of environmental media (soil, etc.) contaminated with PAHs, several types of PAHs are mixed in most cases. Due to their high carcinogenicity, recalcitrance, and bioaccumulation, PAHs have been subject to much research since the early 20th century in relation to their behaviors and treatment technologies [1]. Most of the PAHs present in the environment are generated by the incomplete combustion of organic materials like dioxins, and some are caused by natural factors such as forest fires [2]. In recent years, a chemical method has commonly been used as a treatment method for PAHs owing to its low cost and fast treatment process. Typical chemical treatment methods include Fenton treatment [3], ozone oxidation [4], and H₂O₂ + UV (Ultra Violet) [5]. These chemical treatment methods have a mechanism that decomposes recalcitrant materials such as PAHs using radicals (OH·) or UV light with strong energy. The kinds of radicals generated by the radical reactions also vary. Among them, the OH radical is the most reactive. Radicals are produced by various radical-generating reactions, causing chain reaction [6]. Many published previous studies have so far mainly dealt with the decomposition rates and pathways of PAHS during chemical treatment using radicals. The combined O₃/H₂O₂ treatment resulted in high removal rates of the selected PAHs: 89% for fluorene, 66% for phenanthrene, 71% for anthracene and 81% for dibenz[a,h]anthracene [7]. However, there is a lack of research on which positions of the PAHs are decomposed or oxidized by attacks of radicals. Recently, in some research papers, attempts have been made to interpret the reaction positions of various compounds based on radical reaction analysis using the DFT method [8], the calculation of bond dissociation energy [9], the method of ab initio transition-state theory calculation [10], and radical reaction position calculation using the semi-empirical method [11]. In this study, the transition state energy and atomic charge of OH radicals and PAHs were calculated using the DFT method. The accuracy of prediction of the radical reaction positions was evaluated by comparing with the radical products of PAHs reported in the literature.

2. Calculation Methods

The radical reaction results of PAHs used in this study were obtained by referring to the existing literature [9]. For the transition state energy calculation, the structures of PAH and H₂O₂ as reactants were optimized. A structure calculated using the structure combined with PAH and OH radicals at the same level was also used for the final product.

To predict the radical reaction position of acenaphthylene (ACEL) and anthracene (ANT), the plane figures of all the molecules in this study were drawn and the three-dimensional features of these figures were confirmed with the Gauss View program [12]. With this Gauss View program, the Gaussian input files were created to run the Gaussian-09W program [13].

In this study we carried out the density functional theory calculations with the Becke 3-parameter Lee-Yang-Parr exchange correlation functional (B3LYP) [14] to obtain the optimized geometrical parameters of the molecule at the level of B3LYP/6-31G(d,p).

In order to obtain the activation energy of the transition state, the structure of the transition state was searched at the same level using the QST2 method, which utilizes the optimized structures of reactants and products. With respect to the transition state structure obtained from the QST2 calculation result, the zero point energy was calculated by performing TS and FREQ calculations, and it was confirmed to be a transition state (i.e., one imaginary frequency).

Meanwhile to determine the carbon in the benzene ring showing a high minus charge, the atomic charge distributions were calculated using the CHelpG [15], MK (Merz-Kollman/Singh) [16], and HLY [17] methods with these optimized structures of molecules.

3. Results and Discussion

3.1. Literature Research on the Radical Reaction Product

In this study, ACEL and ANT, which have three rings among 16 PAHs, were selected as research subjects.

Table 1. Chemical properties of ACEL and ANT [18].

Structure	PAH	Molecular Weight	Boiling Point (°C)	Log Kow *	Vapor Pressure at 25 °C (mmHg)
	ACEL	152.2	280	4.05	NA **
	ANT	178.2	400	4.45	1.95 × 10−4

*: n-Octanol/Water Partition Coefficient. **: not available.

summarizes the chemical properties of ACEL and ANT. As shown in Table 1, both ACEL and ANT have relatively low vapor pressure and high octanol/water partition coefficients, which indicate high bioaccumulation and toxicity.

Table 2. ACEL, ANT, and their OH radical products.

PAH	Reaction Product
ACEL	1,8-naphthalic anhydride
ANT	9,10-anthracenedione

shows ACEL, ANT, and their OH radical products cited in the literature [11]. The results shown in Table 2 reveal that in general, OH is bound by radical attack, and hydrogen is subsequently dissociated to form a final product in the form of quinone.

3.2. Transition State Calculation

The transition state structures of ACEL and ANT with OH radicals are shown in Figure 1 and Figure 2. The values in parentheses show the distance between each carbon and oxygen of the radical. These occur when the distance between the carbon 2 of ACEL and the oxygen of OH radical is 3.22 Å, and the distance between the carbon 9 of ANT and the oxygen of OH radical is 3.21 Å.

The transition state energies of ACEL, ANT, and OH radicals were calculated to determine why oxygen is bound only to a specific reaction position among the various reaction positions of ACEL and ANT. Since these two molecules have C_2v symmetry, calculation of the symmetric position was not conducted. The results are summarized in

Table 3. Activation energy (E_a) from reactants to transition state *.

Reaction Position	ACEL	ANT
	Ea **
1	- ***	117.922
2	45.436	116.426
3	45.037	116.435
4	45.297	-
5	51.571	-
6	-	-
7	-	-
8	-
9		118.189
10		-

* Sum of electronic and zero point energies; kcal/mol. ** (Etransition state + EOH radical) − (EPAH + EH2O2). *** Symmetric position.

.

The transition state theory states that a radical reaction occurs at the position where the energy difference (activation energy, E_a) between the reactant and the transition state is the smallest. Because ACEL has a symmetrical structure, E_a was 45.037 kcal/mol at positions 3 and 8. This result was not consistent with that at positions 1 and 2 as reported in the literature. However, the comparison of E_a shows that the difference was only 0.399 kcal/mol, and was also not significant at other reaction positions. Even in the case of ANT, the energy difference (E_a) between the reactant and the transition state was the smallest at positions 2 and 7 (116.426 kcal/mol). In the existing literature, positions 9 and 10 were reported as reaction positions, showing inconsistent results. However, the difference of E_a at positions 2 and 7, and positions 9 and 10 was not significant at 1.763 kcal/mol. In addition, the difference of E_a was not large at other reaction positions. These results suggest that contrary to what is generally known, a method of calculating transition state energies that requires a large calculation cost and a complicated formula is not the most accurate method for predicting radical reaction positions.

3.3. Atomic Charge Calculation

The results of the atomic charge calculation, which uses a simple calculation method and entails a relatively low calculation cost, are summarized in

Table 4. Calculated atomic charge distributions of ACEL and ANT *.

Position			ACEL	ANT
	Charge
	MK	CHelpG	HLY	MK	CHelpG	HLY
1	−0.295	−0.215	−0.259	−0.254	−0.223	−0.247
2	−0.295	−0.215	−0.259	−0.127	−0.060	−0.107
3	−0.256	−0.249	−0.251	−0.127	−0.060	−0.107
4	−0.092	0.029	−0.050	−0.253	−0.223	−0.247
5	−0.305	−0.317	−0.315	−0.253	−0.223	−0.247
6	−0.305	−0.317	−0.315	−0.127	−0.060	−0.107
7	−0.092	0.029	−0.050	−0.127	−0.060	−0.107
8	−0.256	−0.249	−0.251	−0.254	−0.223	−0.247
9				−0.353	−0.440	−0.466
10				−0.353	−0.440	−0.466

* Charges in electron.

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The reaction between ACEL and ANT due to ·OH radical oxidation starts as ·OH, which has strong electrophilicity and attacks carbon because of the electron deficiency of the ring system. This indicates that examination of the electron density of carbon constituting ACEL and ANT is effective in analyzing the radical reaction. In other words, the electrical attraction with the electrophilic ·OH radical increases with higher negative charge values of the carbon forming the ring of ACEL and ANT. Therefore, in this study, MK, CHelpG, and HLY charges were calculated with respect to the molecular structure obtained by optimization at the B3LYP/6-31G(d,p) level. As shown in Table 4, the charge values were the same at the symmetrical positions of ACEL and ANT, which are symmetrical structures, as expected. Looking at the charge value of ACEL, the charge values at positions 5 and 6 were smaller (the largest negative charge value) than MK, CHelpG, and HLY charges at other positions. In addition, MK and HLY charge values were the second smallest at positions 1 and 2. Since the MK and HLY charge values at positions 1 and 2 did not differ significantly from those at positions 5 and 6, it is possible to predict the radical reaction position with the MK and HLY charge calculations. However, in the case of CHelpG charge, the charge of carbon at positions 3 and 8 was found to be the second smallest. In the existing literature, it is reported that the reaction occurs at positions 1 and 2, showing a difference from the calculation results. The MK and HLY charge values are in good agreement with those of the actual radical decomposition products. The charge values of ANT at positions 9 and 10 were found to be very small compared to those of MK, CHelpG, and HLY at other positions. Existing literature reports show that reactions occur at positions 9 and 10. Therefore, it can be confirmed that the contents of the literature are in exact agreement with the MK, HLY, and CHelpG charge values. In addition, oxygen OH radicals react with a specific carbon in ACEL and ANT, resulting in a quinone-type final product.

4. Conclusions

The transition state energy and atomic charge values of ACEL and ANT, which are three-ring PAHs, were calculated and compared with those reported in the actual literature. The following results were obtained.

• The comparison with the test results confirmed that the OH radical reacts at the carbon position with a large negative charge distribution in the carbon ring of the target compound.
• In the case of the target compound, the MK and HLY charges were very accurate in predicting the OH radical reaction position, but the CHelpG charge showed a slight difference.
• In the case of ANT, the carbons at positions 9 and 10 have significantly smaller charge values because they are affected by the benzene rings on both sides. Therefore, it is predicted that the radical reaction at positions 9 and 10 is relatively more favorable than the reaction at other positions.
• The difference between the transition state energy values calculated at all reaction positions of ACEL and ANT was not large, and inconsistent results were obtained even when compared with those of the test results. Therefore, it can be confirmed that the radical reaction position can be sufficiently predicted with the atomic charge calculation only, at a low calculation cost and using a simple calculation method.

The atomic charge calculation results of ACEL and ANT were compared with the experimental values from the existing literature, and the comparison results suggest that the calculation of MK and HLY charge values can be useful in predicting the radical reaction positions. In the future, it is expected that charge calculation can be sufficiently utilized in the decomposition treatment of recalcitrant organic pollutants (dioxins, PCBs, POPs, etc.) and polymerization using radicals.