J Mol Model (2014) 20:2280 DOI 10.1007/s00894-014-2280-y
ORIGINAL PAPER
Theoretical studies on degradation mechanism for OH-initiated reactions with diuron in water system Xiaohua Ren & Zhaojie Cui & Youmin Sun
Received: 12 February 2014 / Accepted: 25 April 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Diuron, a chlorine-substituted dimethyl herbicide, is widely used in agriculture. Though the degradation of diuron in water has been studied much with experiments, little is known about the detailed degradation mechanism from the molecular level. In this work, the degradation mechanisms for OH-induced reactions of diuron in water phase are investigated at the MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+ G(d,p) level with polarizable continuum model (PCM) calculation. Three reaction types including H-atom abstraction, addition, and substitution are identified. For H-atom abstraction reactions, the calculation results show that the reaction abstracting H atom from the methyl group has the lowest energy barrier; the potential barrier of ortho- H (H1’) abstraction is higher than the meta- H abstraction, and the reason is possibly that part of the potential energy is to overcome the side chain torsion for the H1’ abstraction reaction. For addition pathways, the ortho- site (C (2) atom) is the most favorable site that OH may first attack; the potential barriers for OH additions to the ortho- sites (pathways R7 and R8) and the chloro-substituted para- site (R10) are lower than other sites, indicating the ortho- and para- sites are more favorable to be attacked, matching well with the -NHCO- group as an ortho-para directing group. Keywords Addition . H-atom abstraction . OH radical . Reaction mechanism . Substitution X. Ren (*) Weifang University of Science and Technology, Weifang 262700, Peoples Republic of China e-mail: [email protected] Z. Cui School of Environmental Science and Engineering, Shandong University, Jinan 250100, Peoples Republic of China Y. Sun School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, Peoples Republic of China
Introduction As a representative of the most commonly used phenylurea herbicides, diuron (N-(3,4-dichlorophenyl)-N, Ndimethylurea), a chlorine-substituted dimethyl derivation, is widely applied for controlling annual grasses and broad-leaf weeds in many crops [1]. Due to the extensive dispersion in agriculture and the solubility in water for diuron, the surface water and ground water have been polluted from its surface runoffs and soil leaching [2], and it has been detected in these waters [3]. Diuron is reported to have a toxic effect on plant photosynthesis by inhibiting the photoreduction behavior of photosystem II (PSII) [4]. In addition, diuron is considered to be ordinarily dangerous for aquatic life and flora, and has a potential of raising congenital malformations for human being [5]. Therefore, it has been classified as a “known/likely” carcinogen since 1997 by US Environmental Protection Agency (EPA), and has been regarded as an approval priority substance by the European Union Water Framework Directive (Directive 2000/60/EC) [6]. The structure formula of diuron is shown in Scheme 1. The degradation of diuron has been studied using various methods, such as biological [7], photocatalytic [8, 9], electrochemical [10, 11] approaches, and some other oxidation methods [12, 13]. The biodegradation of diuron has been investigated a lot, and some degradation mechanisms have been proposed, which report that the main metabolites initiate from the methyls attached to the terminal N atom and the intermediates are more toxic than the parent [7, 14, 15]. However, the biodegradation method is a little ineffective and time-consuming. Therefore, as the alternatives, these above promising advanced oxidation processes (AOPs) have been applied to remove diuron in a variety of water bodies, based on the generation of powerful oxidant (·OH). Some degradation pathways for diuron and OH reactions (such as photocatalytic method [8, 9] and electrochemical method [10, 11]) have been proposed according to the previous
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J Mol Model (2014) 20:2280
Cl O Cl
H N
C
CH3 N CH3
Scheme 1 Chemical structure of diuron
experimental results. However, the detailed reaction mechanism including exhaustive transformation processes from reactants to intermediates and activation energies have not been studied with quantum chemistry. In this work, we will simulate the reaction mechanism for OH and diuron reactions in water with polarizable continuum model (PCM) calculation, which is more accurate to reflect the real reaction conditions. Our aims are to investigate possible H-atom abstraction, addition, and substitution mechanisms for·OH and diuron reactions in water, and to identify the initial reaction site at which the reaction occurs. The theoretical study can provide us a chance to comprehend the detail degradation mechanism at the molecular level and support the experimental results.
Computational methods The quantum chemical calculations are carried out using the Gaussian 03 package [16] for possible reactions of OH and diuron. The structures of reactants, transition states, intermediates and products are optimized employing MPWB1K [17] method with a standard 6–31+G(d,p) basis set, which was revealed to be an excellent method for the degradation mechanism of organic compounds [17–21]. The polarizable continuum model (PCM) with water solvent is performed for all calculations to estimate the solvent dependence. The vibrational frequencies are also calculated at the same level for obtaining the nature of the stationary points and the zero point energies (ZPEs). Every transition state is verified to connect the designed reactants with the corresponding products by performing intrinsic reaction coordinate (IRC) [22] calculation. A more flexible basis set, 6–311+G(3df,2p), is used for single-point energy calculations to obtain more accurate energy parameters. Additionally, one of the reaction channels is studied with ab initio G2MP2 [23] to verify the accuracy of the MPWB1K results.
Results and discussion The structure and atom labels of diuron at MPWB1K/6–31+ G(d,p) level are shown in Fig. 1a, from which it can been seen that it consists of a chloro-substituted benzene ring and a -NHCON (CH3) 2 group. All atoms are almost in a plane except hydrogen atoms in methyl groups. In this work, three
Fig. 1 a The structure, atom labels and bond lengths of diuron at MPWB1K/6-31+g(d,p) level. b The structure, atom labels and bond lengths of diuron at MP2/6-31+g (d,p) level
reaction patterns including H-atom abstraction, addition, and substitution are considered, and the schematic reaction pathways are illustrated in Scheme 2. Theoretically, all reaction channels for OH abstracting hydrogen atoms in methyl groups should be studied. The study results of Oturan et al. [11], Feng et al. [13], and Zhou et al. [24] reveal that the hydrogen atoms in methyl groups are considered as the same, and the main aim is to compare the reaction characteristics of OH abstracting hydrogen atom in methyl groups and the hydrogen atom attached to the benzene ring. Therefore, that OH abstracting one of hydrogen atoms in methyl groups as a representative is discussed. That OH abstracting chlorine atoms on the benzene ring should also be considered. However, previous research works [25, 26] have shown that the abstraction of halogen atoms on the benzene ring hardly occurs. Additionally, the attempt to find the pathway for OH abstracting H4 atom adjacent to N1 atom failed. Therefore, the chlorine abstractions and H4 abstraction are not discussed in this work. All optimized geometries of the intermediates, transition states, and products are shown in Fig. 2, and profiles of the potential energy surface with zero point energy (ZPE) correction are displayed in Fig. 4. The relative energies including reaction barrier heights, reaction heats and free energy of reactions are summarized in Table 1. (The reaction barrier heights is the relative energy of the transition state with respect to the corresponding pre-complex if the pathway contains the corresponding reactants.)
J Mol Model (2014) 20:2280
Page 3 of 12, 2280 OH
Cl
H
Cl
Cl
Abstraction reactions
H
O
N
C
CH3 N
Cl
H
O
N
C
CH3 N
CH3 H
H
H
Cl H
O
N
C
CH3
H
Cl
OH H
O
N
C
H
+
H2O
R2
+
H2O
R3
+
H2O
R4
H
Cl
N
H
O
N
C
CH3 N CH3
CH3 Cl
R1
P1
H
Cl
H2O
H
TS1 Cl
+ CH3
H
H
CH3 N
TS2
P2
CH3 H
H
+ OH
Cl
H
Cl
Cl
H
O
N
C
H
CH3 N
Cl
H
O
N
C
CH3 N
CH3 H
CH3 H
H OH
P3
TS3 OH Cl
H
Cl
Cl
H H
O
N
C
H
CH2 N
Cl
H
O
N
C
CH3 H
H
CH3 H
TS4
CH2 N
H P4
(a)
Scheme 2 Reaction pathways of diuron and OH: a H-atom abstraction reactions; b OH addition reactions; c OH substitution reaction
First, to verify the accuracy of the MPWB1K results, one reaction channel R4 has been studied with G2MP2. The structures of reactants, transition state, and product included in the channel R4 are optimized at UMP2/6–31+G(d,p) level, and the vibrational frequencies are also calculated at the same level. On the basis of the optimized structures, a series of single point energies are calculated at UMP2/6–311G(d,p), UQCISD (T)//6–311G(d,p), and UMP2/6–311+G(3df,2p) levels. The G2MP2 total energy is: EðG2MP2Þ ¼ E½UQCISDðTÞ=6−311Gðd; pÞ
The optimized structure of diuron at MP2/6–31+g(d,p) level is shown in Fig. 1b, and the geometries of the transition state and product are illustrated in Fig. 3. The result shows that the calculated energy barrier is 4.70 kcal mol−1, and the reaction heat is −23.52 kcal mol−1. The energy barrier is only 0.28 kcal mol−1 lower than that with MPWB1K. It can be seen that the structures of TS4’ and P4’ in Fig. 3 are similar with the structures of TS4 and P4 in Fig. 2a, respectively. Though the little differences exist, the reaction mechanism is the same. Therefore, the results with the MPWB1K method could be reliable.
þ ΔMP2 þ HLC þ EðZPEÞ; H-atom abstraction mechanism where ΔMP2 =E [UMP2/6–311+G(3df,2p)]–E [UMP2/6– 311G(d,p)]. HLC is a higher level correction, and HLC = −0.00019 nα -0.0048 nβ, in which nβ and nα are the number of α and β valence electrons, respectively.
As illustrated in Scheme 2a, four direct H-atom abstraction pathways denoted by R1-R4 have been identified, and hydrogen atoms abstracted can be divided into two kinds: aromatic
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J Mol Model (2014) 20:2280 Cl TS5
Addition reactions
H
Cl H
H
O
N OH
C
CH3 N
R5
CH3
H P5
Cl
H
Cl Cl
H
O
N
C
CH3 N
TS6
Cl
CH3 H
H
O C
H
CH3 N
R6
Cl
H
O
N
C
CH3 N
R7 CH3
H OH P7
CH3 N
OH
Cl
CH3 H
C
H
H
N
N
P6 Cl
H H
O
H
OH
TS7
Cl
H
CH3
H
OH IM1
Cl
H
+
Cl HO
H
Cl
H
O
N
C
CH3 N
TS8
H
Cl
H
O
N
C
CH3 N
H
H
H
H
IM2
P8
OH
Cl TS9 OH
Cl
H
Cl
H
O
N
C
CH3 N
H
O
N
C
H
CH3
H
N
P9 CH3
H
R9
CH3
H
Cl
R8 CH3
CH3
OH
Cl
H
H
Cl
IM3
TS10
H
O
N
C
CH3 N
HO
R10
CH3 H
H P10
(b) Substitution reactions Cl
H
Cl H
Cl
N
O N
CH3 H
H
+
OH
Cl
H
CH3 Cl
H
O
N
C
TS11
H
OH
H
(c)
H O
Cl
N
CH3 IM4
(continued)
CH3 N
H
OH H
H P11
C
CH3 N CH3
R11
J Mol Model (2014) 20:2280
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(a)
TS1
P1
TS2
P2
TS3
P3
TS4
P4
Fig. 2 Optimized geometries of the intermediates and transition states for the reactions of OH and diuron at MPWB1K/6–31+G(d,p) level (The gray–, white–, red–, and green–balls denote carbon, hydrogen, oxygen,
and chlorine atoms, respectively, and bond lengths are in angstroms): a Hatom abstraction reactions; b addition reactions; c substitution reactions
H atoms and aliphatic H atoms. The detailed reaction mechanisms will be discussed in the following. First, that OH abstracting the ortho- H atoms (H1’ atom and H3’ atom) is considered, and pathways R1 and R3 are identified. It can be seen from Scheme 2a that two transition states TS1 and TS3 are firstly found for R1 and R3, respectively. As shown in Fig. 2a, in TS1, the C2-H1’ bond length with 1.270 Å is elongated by 18.2 % compared with the equilibrium value of 1.074 Å in diuron, and the distance of O-H1’ is 1.191 Å, indicating that the C2-H1’ bond is being broken and the O-H1’ is being formed. Similarly, in TS3, the C6-H3’
bond is elongated by 15.7 % and the O…H3’ distance is 1.222 Å, meaning that H3’ is being abstracted and the OH3’ bond is being formed. Then, P1 and H2O are produced via the transition state TS1, and P3 and H2O are formed via the transition state TS3. As illustrated in Table 1, the energy barriers for pathways R1 and R3 are 15.32 kcal mol−1 and 11.62 kcal mol−1, respectively, and the two pathways are exothermic by 2.67 kcal mol −1 and 6.18 kcal mol −1 . Interestingly, the energy barrier of the pathway R3 is 3.70 kcal mol−1 lower than that of the pathway R1, indicating that the H3’ atom is a little easier to be abstracted than the H1’ atom. It
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Fig. 2 (continued)
can been noticed from Fig. 2a that the -CON (CH3) 2 group deviates from the benzene ring with a dihedral angle of 45.9° in TS1. This torsion could consume part of energy barrier for the pathway R1, which probably leads to the above result. Second, the meta-H atom (H2’) is abstracted by OH. A transition state TS2 is first located on the potential energy surface. As shown in Fig. 2a, in TS2, the C5-H2’ bond is being broken denoted by that the C5-H2’ bond is longer than the equilibrium value of 1.078 Å by 15.7 %. The direct abstraction results in the formation of P2 and H2O. This reaction has an energy barrier of 12.74 kcal mol−1 and an enthalpy of −4.28 kcal mol−1. This indicates that the meto-H atom abstraction reaction can occur readily.
Finally, H atom abstraction from the methyl group is taken into account. As depicted in Fig. 4a, a transition state denoted by TS4 is initially located on the potential energy surface. It can be seen from the transition state TS4 in Fig. 2a that the H9’ atom is obviously leaving the C9 atom marked by the lengthened C9-H9’ bond from 1.081 Å to 1.167 Å. Then, a radical P4 and H2O are yielded from the reactants via the transition state TS4. This pathway has the lowest energy barrier of 4.98 kcal mol−1 among the four abstraction reactions, which indicates that the H atoms in the methyl group are most kinetically and favorably abstracted by OH. This result is in satisfactory agreement with P4 as the main intermediate proposed by Macounová et al. [27], Oturan et al. [11], and Feng et al. [13].
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Page 7 of 12, 2280
TS8
P8
IM3
TS9
P9
TS10
P10
(c)
IM4
P11 Fig. 2 (continued)
TS11
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Table 1 Calculated relative energies (kcal mol−1) of all pathways for the diuron and OH reactions in the water (MPWB1K/6–311+G(3df,2p)// MPWB1K/6–31+G(d,p) and MPWB1K/6–31+G d,p), respectively, 298.15 K)
H-atom abstractions
Addition reactions
Substitution reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11
MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+G(d,p)
MPWB1K/6–31+G(d,p)
ΔE# 15.32 12.74 11.62
ΔH −2.67 −4.28 −6.18
ΔEs# 15.06 12.56 14.71
ΔHs −2.62 −4.01 −6.72
ΔG 32.45 27.29 27.94
4.98 23.16 10.61 7.82 1.30 12.89 10.12 68.85
−28.69 −14.69 −16.46 −20.80 −13.78 −23.12 −28.09 48.92
3.31 12.49 11.01 8.12 1.44 12.84 10.10 69.83
−41.44 −11.27 −16.65 −26.18 −15.47 −11.31 −16.54 49.14
25.11 93.28 90.77 87.49 5.21 88.54 86.41 69.79
Note: ΔE# denotes the reaction barrier height ΔH denotes the reaction heat ΔEs# denotes the reaction barrier height with smaller basis set ΔHs denotes the reaction heat with smaller basis set ΔG denotes the reaction free energy
Comparing the four direct H abstraction reactions, the energy barrier of the pathway R4 is lowest. This fact means that the H atoms in the methyl group are easier to be abstracted than the H atoms attached to the benzene ring, which matches well with the previous studies [28]. The energy barrier for the pathway R2 is lower than the pathway R1 but higher than the pathway R3. This result indicates that the meta-H (H2’) is easier to be attacked by OH than the ortho-H (H1’) and more difficult than the ortho- H (H3’). Generally, the meta-H abstraction reaction harder to occur than the ortho- H abstraction reaction, which is in accordance with the above calculation that the barrier height of the meta-H abstraction R2 is higher than that of the ortho-H abstraction R3. However, the ortho-H
TS4’
(H1’) is more difficult to be abstracted than the meta-H atom (H2’). The probable reason can be seen in Fig. 2a: for the pathway R1, a part of the energy barrier is used to overcome the torsion of the side chain from the benzene ring, which can be signified by the C2-C1-N1-C7 dihedral of 45.9° in TS1. The formations of P1 + H2O, P2 + H2O, and P3 + H2O are slightly exothermic by 2.67 kcal mol−1, 4.28 kcal mol−1, and 6.18 kcal mol−1, respectively, while the reaction R4 yielding P4 + H2O is strongly exothermic by 28.69 kcal mol−1. Additionally, it can be seen from Table 1 that the free energy of the reaction R4 is negative and smallest of the four Habstraction channels, which matches well with the above results.
P4’
Fig. 3 Optimized geometries of the transition state and product for ·OH abstracting H9’ atom (R4) at MP2/6-31+g (d,p) level (The grey-, white-, red-, and green-balls denote carbon, hydrogen, oxygen, and chlorine atoms, respectively, and bond lengths are in angstroms)
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Erel kcal mol-1
TS1 15.32 TS2 12.74
Diuron + OH
TS3 11.62
R1
TS4 4.98
R3 R4
R2
-2.67 P1 + H O 2
0.00
P2 + H2O -4.28 P3 + H2O -6.18
(a) Erel kcal mol-1
-28.69
P4 + H2O
TS5 21.37
R5 R6 R7 R8
TS9 11.25
R9 R10
TS6 8.83 TS10 8.48 TS8 6.36
IM2 5.05 Diuron + OH 0.00
TS7 6.03 IM3 -1.64 IM1 -1.79 567
-13.78
P8
-14.69 P5 -16.46 P6 -20.80 P7 -23.12 P9 -28.09 P10
(b) Fig. 4 Calculated potential energy surface profiles for all reaction pathways: a H-atom abstraction reactions; b addition reactions; c substitution reactions
OH addition mechanism Six positions which OH add to are considered, including three types: the ipso- C atom (C (1) in Fig. 1), the C atoms connected to H atoms (C (2), C (5), and C (6) in Fig. 1) and the chloro-substituted C atoms (C (3) and C (4) in Fig. 1) . The six addition pathways denoted by R5-R10 will be discussed in the following. Each pathway proceeds with the following steps: first, one pre-complex is formed; second, a transition state is found; finally, the products are yielded through the corresponding transition states. First, the addition of OH to the ipso- C atom (C1) is considered, and a channel denoted R5 is found. IM1 is the
formed pre-complex, in which the C1…O distance is 3.032 Å. The energy of IM1 is 1.79 kcal mol−1 more stable than that of original reactants. TS5 is found to be the transition state for OH adding to the C1 site. In the structure of TS5, the distance between C1 and O is decreased from 3.032 Å in IM1 to 1.987 Å. P5 is the product of this process, in which the C1O bond has been formed indicated by its bond length of 1.422 Å, and the C1-N1 bond is stretched by 0.059 Å compared to that in the corresponding transition state TS5. The energy barrier of this addition reaction is 23.16 kcal mol−1, and the reaction heat is −14.69 kcal mol−1. Second, that OH attacking the C (5), C (6), and C (2) atoms attached to H atoms are considered, and three pathways R6,
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J Mol Model (2014) 20:2280 Erel kcal mol-1
TS11 74.70 R11
P11 48.92
Diuron + OH
IM4 5.85
0.00
(c) Fig. 4 (continued)
R7, and R8 are identified, respectively. Interestingly, both the pathways R6 and R7 share the same pre-complex IM1, in which the C5…O distance is 3.516 Å, and the C6…O distance is 2.697 Å. This means that OH is closer to the C6 atom, so that OH is probably easier to add to the C6 site than the C5 site, which matches well with that the calculated energy barrier of R7 is lower than that of R6. Then, two transition states TS6 and TS7 are found on the potential energy surfaces for the pathways R6 and R7, respectively. In the structure of TS6, the distance of C5…O has been significantly decreased from 3.516 Å in IM1 to 1.994 Å, and the C4-C5 bond and C5C6 bond are slightly elongated by 0.127 Å and 0.122 Å, respectively. While in the structure of TS7, the C6…O distance has been only reduced by 0.626 Å, and the C5-C6 bond and C6-C1 bond are slightly lengthened by 0.017 Å and 0.018 Å, respectively. Finally, adducts P6 and P7 are produced. Compared with TS6, in P6, the C5…O distance has been shortened by 0.574 Å, meaning that the C5-O bond is being formed. The C5-H2’ bond has been a little elongated by 0.015 Å. The C4-C5 bond and C5-C6 bond are stretched by 0.085 Å and 0.082 Å, respectively. In P7, the C6-O bond has been formed indicated by its bond length from 2.071 Å in TS7 to 1.418 Å, which is expected for sp3 hybridized carbon [29]. The C6-H5’, C6-C5 and C6-C1 bonds are elongated by 0.018 Å, 0.087 Å and 0.088 Å compared with those in TS7, respectively. The energy barriers of the reactions R6 and R7 are 10.61 kcal mol−1 and 7.82 kcal mol−1, respectively, and exothermic by 16.46 kcal mol−1 and 20.80 kcal mol−1, respectively. For the pathway R8, a pre-complex IM2, which lies 5.05 kcal mol−1 above the reactants, is first formed. Then, a transition state TS8 is found on the potential energy surface, in
which OH is nearly perpendicularly adding to the C2 atom with an O-C2-H1’ angle of 86.9° and the C2…O distance is decreased from 2.530 Å in IM2 to 2.058 Å. Finally, P8 is obtained from IM2 via the transition state TS8. Compared with TS8, the C2-O bond has been formed with a length of 1.433 Å and the C2-H1’ bond is elongated by 0.009 Å in P8. Additional changes in P8 have been seen in the distances of C2-C3 and C2-C1 bonds, which have been stretched by 0.087 Å and 0.086 Å compared with those in TS8, respectively. The barrier height of the pathway R8 is 1.30 kcal mol−1, the lowest of all the addition reactions. This result suggests that OH adding to the C2 site is the most energetic and kinetic channel. Finally, the meto- C (C3) and para- C (C4) atoms attached to Cl atoms are attacked by OH, and two channels R9 and R10 are identified, respectively. First, a structure IM3 is found to be the pre-complex for both pathways R9 and R10. The energy of IM3 is 1.64 kcal mol−1 lower than the original reactants, and in IM3, the C3…O and C4…O distances are 3.154 Å and 2.944 Å, respectively. It can be noted that OH is 0.210 Å nearer to the C4 atom than the C3 atom, indicating that OH is slightly easier to add to the C4 site than the C3 site. This fact is in good agreement with that the barrier height of R10 is a little lower than R9. Then, two transition states TS9 and TS10 are found on the potential energy surfaces, in which OH is almost vertically adding to the C3 and C4 atoms, and the C3…O and C4…O distances have been shortened by 1.166 Å and 0.903 Å, respectively. Additionally, the C3-C2, C3-C4, and C3-Cl1’ bonds in TS9 and the C4-C3, C4-C5, and C4-Cl2’ bonds in TS10 are all elongated compared to those in IM3. Finally, adducts P9 and P10 are obtained. In P9, the C3-
J Mol Model (2014) 20:2280
O bond has been formed indicated by the C3-O bond decrease from 1.988 Å in TS7 to 1.361 Å, and compared to TS9, the C3-Cl1’, C3-C2, C3-C4 bonds are elongated by 0.197 Å, 0.059 Å, and 0.059 Å, respectively. Similar changes have been seen in the adduct P10. The activation energies of the pathways R9 and R10 are 12.89 kcal mol−1 and 10.12 kcal mol−1, respectively, and the corresponding products P9 and P10 lie 23.12 kcal mol−1 and 28.09 kcal mol−1 below the reactants (diuron and OH), respectively. Additions of OH to C sites of the phenyl ring except the ipso- C site are found to be the import processes by many researches: Oturan et al. [11, 30] take OH addition on the benzene ring as one principle oxidation route and Katsumata et al. [31] obtain that one product results from OH adding to the aromatic ring. Comparing the six addition pathways, the potential energy of the pathway R8 is the lowest, indicating that C2 atom is easiest to be attacked. From Table 1, it also can be obtained that the free energy of the channel R8 is smallest of all addition reactions, indicating that the channel R8 is easiest to occur. However, the relative energy of transition states minus reactants for the two ortho- sites addition reactions R7 and R8, are similar, both lower than those of other pathways. This result suggests that the orthosites are more active than other sites. Additionally, the potential barrier of the channel R10 (·OH adding to the chloro-substituted para- C atom) is lower than those of channels R5, R6, and R9. This fact is in good accordance with that the -NHCO- group is an ortho-para directing group, meaning that the ortho- and para- sites are more easily attacked than other sites. The potential energy of the path R5 is the highest among all the addition reactions. This result indicates that the ipso- site is most difficult to be attacked, which is observed to agree with that the side chain substituted is not found in the beginning of the experiments [11, 13, 30]. Substitution mechanism Only one channel, R11, is found for the substitution reaction where OH substituting H4’ atom attached to the N1 atom. The channel R11 starts from the formation of a pre-complex, IM4, in which the distance between N1 and O is 2.503 Å and N1H4’ bond length is 1.003 Å. The energy of IM4 is 5.85 kcal mol−1 higher than the original reactants. TS11 is found to be the transition state structure for this OH attacking N1 atom process, the energy of which lies 68.85 kcal mol−1 above that of IM4. In TS11, the N1-O bond will be formed denoted by that the N1…O distance decreased from 2.503 Å in IM4 to 1.430 Å, and the N1-H4’ bond is being broken indicated by that the bond is stretched from 1.003 Å in IM4 to 1.170 Å. P11 and H radical are the products of the substitution reaction. It can be noted that the energy barrier of the channel R11 is the highest of all considered reactions, meaning that this channel
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is difficult to occur and can be ignored. The pathway R11 is strongly endothermic by 48.92 kcal mol−1. Conclusions Three reaction types for OH attacking diuron with water solvent have been investigated employing MPWB1K method with polarizable continuum model (PCM) calculation. For Hatom abstractions, four pathways are identified, and the calculation results display that the pathway R4 (·OH abstracting H atom in methyl group) has the lowest energy barrier of 4.98 kcal mol−1, indicating that this reaction could be the first step for H-atom abstraction reactions; moreover, the side chain torsion for the H1’ abstraction reaction (R1) possibly consumes part of the energy, leading to that the ortho- H (H1’) is more difficult to be abstracted than the meta- H. For addition reactions, the potential barriers for OH additions to the orthosites (pathways R7 and R8) and the chloro-substituted parasite (R10) are lower than other sites, suggesting the ortho- and para- sites are more active, which is in good agreement with that the -NHCO- group is an ortho-para directing group. ·OH addition to the ipso- site (R5) has the highest energy barrier, meaning that this reaction is most difficult to occur. For the only found substitution reaction R11, OH substituting H4’ atom attached to the N1 atom has the highest energy barrier of 68.85 kcal mol−1, meaning that this reaction is the most difficult to occur among all considered reactions and not expected to be important. Acknowledgments This work is supported by the Scientific Research Funds for Yong Scholars of Weifang University of Science and Technology (No sdlgy2013w003) and the Postgraduate Independent Innovation Program of Shandong University (No. 11,440,072,613,118). We also thank University Institutes Innovation Program of Jinan Science and Technology Bureau (No.201102046).
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ORIGINAL PAPER
Theoretical studies on degradation mechanism for OH-initiated reactions with diuron in water system Xiaohua Ren & Zhaojie Cui & Youmin Sun
Received: 12 February 2014 / Accepted: 25 April 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract Diuron, a chlorine-substituted dimethyl herbicide, is widely used in agriculture. Though the degradation of diuron in water has been studied much with experiments, little is known about the detailed degradation mechanism from the molecular level. In this work, the degradation mechanisms for OH-induced reactions of diuron in water phase are investigated at the MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+ G(d,p) level with polarizable continuum model (PCM) calculation. Three reaction types including H-atom abstraction, addition, and substitution are identified. For H-atom abstraction reactions, the calculation results show that the reaction abstracting H atom from the methyl group has the lowest energy barrier; the potential barrier of ortho- H (H1’) abstraction is higher than the meta- H abstraction, and the reason is possibly that part of the potential energy is to overcome the side chain torsion for the H1’ abstraction reaction. For addition pathways, the ortho- site (C (2) atom) is the most favorable site that OH may first attack; the potential barriers for OH additions to the ortho- sites (pathways R7 and R8) and the chloro-substituted para- site (R10) are lower than other sites, indicating the ortho- and para- sites are more favorable to be attacked, matching well with the -NHCO- group as an ortho-para directing group. Keywords Addition . H-atom abstraction . OH radical . Reaction mechanism . Substitution X. Ren (*) Weifang University of Science and Technology, Weifang 262700, Peoples Republic of China e-mail: [email protected] Z. Cui School of Environmental Science and Engineering, Shandong University, Jinan 250100, Peoples Republic of China Y. Sun School of Municipal and Environmental Engineering, Shandong Jianzhu University, Jinan 250101, Peoples Republic of China
Introduction As a representative of the most commonly used phenylurea herbicides, diuron (N-(3,4-dichlorophenyl)-N, Ndimethylurea), a chlorine-substituted dimethyl derivation, is widely applied for controlling annual grasses and broad-leaf weeds in many crops [1]. Due to the extensive dispersion in agriculture and the solubility in water for diuron, the surface water and ground water have been polluted from its surface runoffs and soil leaching [2], and it has been detected in these waters [3]. Diuron is reported to have a toxic effect on plant photosynthesis by inhibiting the photoreduction behavior of photosystem II (PSII) [4]. In addition, diuron is considered to be ordinarily dangerous for aquatic life and flora, and has a potential of raising congenital malformations for human being [5]. Therefore, it has been classified as a “known/likely” carcinogen since 1997 by US Environmental Protection Agency (EPA), and has been regarded as an approval priority substance by the European Union Water Framework Directive (Directive 2000/60/EC) [6]. The structure formula of diuron is shown in Scheme 1. The degradation of diuron has been studied using various methods, such as biological [7], photocatalytic [8, 9], electrochemical [10, 11] approaches, and some other oxidation methods [12, 13]. The biodegradation of diuron has been investigated a lot, and some degradation mechanisms have been proposed, which report that the main metabolites initiate from the methyls attached to the terminal N atom and the intermediates are more toxic than the parent [7, 14, 15]. However, the biodegradation method is a little ineffective and time-consuming. Therefore, as the alternatives, these above promising advanced oxidation processes (AOPs) have been applied to remove diuron in a variety of water bodies, based on the generation of powerful oxidant (·OH). Some degradation pathways for diuron and OH reactions (such as photocatalytic method [8, 9] and electrochemical method [10, 11]) have been proposed according to the previous
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Cl O Cl
H N
C
CH3 N CH3
Scheme 1 Chemical structure of diuron
experimental results. However, the detailed reaction mechanism including exhaustive transformation processes from reactants to intermediates and activation energies have not been studied with quantum chemistry. In this work, we will simulate the reaction mechanism for OH and diuron reactions in water with polarizable continuum model (PCM) calculation, which is more accurate to reflect the real reaction conditions. Our aims are to investigate possible H-atom abstraction, addition, and substitution mechanisms for·OH and diuron reactions in water, and to identify the initial reaction site at which the reaction occurs. The theoretical study can provide us a chance to comprehend the detail degradation mechanism at the molecular level and support the experimental results.
Computational methods The quantum chemical calculations are carried out using the Gaussian 03 package [16] for possible reactions of OH and diuron. The structures of reactants, transition states, intermediates and products are optimized employing MPWB1K [17] method with a standard 6–31+G(d,p) basis set, which was revealed to be an excellent method for the degradation mechanism of organic compounds [17–21]. The polarizable continuum model (PCM) with water solvent is performed for all calculations to estimate the solvent dependence. The vibrational frequencies are also calculated at the same level for obtaining the nature of the stationary points and the zero point energies (ZPEs). Every transition state is verified to connect the designed reactants with the corresponding products by performing intrinsic reaction coordinate (IRC) [22] calculation. A more flexible basis set, 6–311+G(3df,2p), is used for single-point energy calculations to obtain more accurate energy parameters. Additionally, one of the reaction channels is studied with ab initio G2MP2 [23] to verify the accuracy of the MPWB1K results.
Results and discussion The structure and atom labels of diuron at MPWB1K/6–31+ G(d,p) level are shown in Fig. 1a, from which it can been seen that it consists of a chloro-substituted benzene ring and a -NHCON (CH3) 2 group. All atoms are almost in a plane except hydrogen atoms in methyl groups. In this work, three
Fig. 1 a The structure, atom labels and bond lengths of diuron at MPWB1K/6-31+g(d,p) level. b The structure, atom labels and bond lengths of diuron at MP2/6-31+g (d,p) level
reaction patterns including H-atom abstraction, addition, and substitution are considered, and the schematic reaction pathways are illustrated in Scheme 2. Theoretically, all reaction channels for OH abstracting hydrogen atoms in methyl groups should be studied. The study results of Oturan et al. [11], Feng et al. [13], and Zhou et al. [24] reveal that the hydrogen atoms in methyl groups are considered as the same, and the main aim is to compare the reaction characteristics of OH abstracting hydrogen atom in methyl groups and the hydrogen atom attached to the benzene ring. Therefore, that OH abstracting one of hydrogen atoms in methyl groups as a representative is discussed. That OH abstracting chlorine atoms on the benzene ring should also be considered. However, previous research works [25, 26] have shown that the abstraction of halogen atoms on the benzene ring hardly occurs. Additionally, the attempt to find the pathway for OH abstracting H4 atom adjacent to N1 atom failed. Therefore, the chlorine abstractions and H4 abstraction are not discussed in this work. All optimized geometries of the intermediates, transition states, and products are shown in Fig. 2, and profiles of the potential energy surface with zero point energy (ZPE) correction are displayed in Fig. 4. The relative energies including reaction barrier heights, reaction heats and free energy of reactions are summarized in Table 1. (The reaction barrier heights is the relative energy of the transition state with respect to the corresponding pre-complex if the pathway contains the corresponding reactants.)
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Page 3 of 12, 2280 OH
Cl
H
Cl
Cl
Abstraction reactions
H
O
N
C
CH3 N
Cl
H
O
N
C
CH3 N
CH3 H
H
H
Cl H
O
N
C
CH3
H
Cl
OH H
O
N
C
H
+
H2O
R2
+
H2O
R3
+
H2O
R4
H
Cl
N
H
O
N
C
CH3 N CH3
CH3 Cl
R1
P1
H
Cl
H2O
H
TS1 Cl
+ CH3
H
H
CH3 N
TS2
P2
CH3 H
H
+ OH
Cl
H
Cl
Cl
H
O
N
C
H
CH3 N
Cl
H
O
N
C
CH3 N
CH3 H
CH3 H
H OH
P3
TS3 OH Cl
H
Cl
Cl
H H
O
N
C
H
CH2 N
Cl
H
O
N
C
CH3 H
H
CH3 H
TS4
CH2 N
H P4
(a)
Scheme 2 Reaction pathways of diuron and OH: a H-atom abstraction reactions; b OH addition reactions; c OH substitution reaction
First, to verify the accuracy of the MPWB1K results, one reaction channel R4 has been studied with G2MP2. The structures of reactants, transition state, and product included in the channel R4 are optimized at UMP2/6–31+G(d,p) level, and the vibrational frequencies are also calculated at the same level. On the basis of the optimized structures, a series of single point energies are calculated at UMP2/6–311G(d,p), UQCISD (T)//6–311G(d,p), and UMP2/6–311+G(3df,2p) levels. The G2MP2 total energy is: EðG2MP2Þ ¼ E½UQCISDðTÞ=6−311Gðd; pÞ
The optimized structure of diuron at MP2/6–31+g(d,p) level is shown in Fig. 1b, and the geometries of the transition state and product are illustrated in Fig. 3. The result shows that the calculated energy barrier is 4.70 kcal mol−1, and the reaction heat is −23.52 kcal mol−1. The energy barrier is only 0.28 kcal mol−1 lower than that with MPWB1K. It can be seen that the structures of TS4’ and P4’ in Fig. 3 are similar with the structures of TS4 and P4 in Fig. 2a, respectively. Though the little differences exist, the reaction mechanism is the same. Therefore, the results with the MPWB1K method could be reliable.
þ ΔMP2 þ HLC þ EðZPEÞ; H-atom abstraction mechanism where ΔMP2 =E [UMP2/6–311+G(3df,2p)]–E [UMP2/6– 311G(d,p)]. HLC is a higher level correction, and HLC = −0.00019 nα -0.0048 nβ, in which nβ and nα are the number of α and β valence electrons, respectively.
As illustrated in Scheme 2a, four direct H-atom abstraction pathways denoted by R1-R4 have been identified, and hydrogen atoms abstracted can be divided into two kinds: aromatic
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J Mol Model (2014) 20:2280 Cl TS5
Addition reactions
H
Cl H
H
O
N OH
C
CH3 N
R5
CH3
H P5
Cl
H
Cl Cl
H
O
N
C
CH3 N
TS6
Cl
CH3 H
H
O C
H
CH3 N
R6
Cl
H
O
N
C
CH3 N
R7 CH3
H OH P7
CH3 N
OH
Cl
CH3 H
C
H
H
N
N
P6 Cl
H H
O
H
OH
TS7
Cl
H
CH3
H
OH IM1
Cl
H
+
Cl HO
H
Cl
H
O
N
C
CH3 N
TS8
H
Cl
H
O
N
C
CH3 N
H
H
H
H
IM2
P8
OH
Cl TS9 OH
Cl
H
Cl
H
O
N
C
CH3 N
H
O
N
C
H
CH3
H
N
P9 CH3
H
R9
CH3
H
Cl
R8 CH3
CH3
OH
Cl
H
H
Cl
IM3
TS10
H
O
N
C
CH3 N
HO
R10
CH3 H
H P10
(b) Substitution reactions Cl
H
Cl H
Cl
N
O N
CH3 H
H
+
OH
Cl
H
CH3 Cl
H
O
N
C
TS11
H
OH
H
(c)
H O
Cl
N
CH3 IM4
(continued)
CH3 N
H
OH H
H P11
C
CH3 N CH3
R11
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(a)
TS1
P1
TS2
P2
TS3
P3
TS4
P4
Fig. 2 Optimized geometries of the intermediates and transition states for the reactions of OH and diuron at MPWB1K/6–31+G(d,p) level (The gray–, white–, red–, and green–balls denote carbon, hydrogen, oxygen,
and chlorine atoms, respectively, and bond lengths are in angstroms): a Hatom abstraction reactions; b addition reactions; c substitution reactions
H atoms and aliphatic H atoms. The detailed reaction mechanisms will be discussed in the following. First, that OH abstracting the ortho- H atoms (H1’ atom and H3’ atom) is considered, and pathways R1 and R3 are identified. It can be seen from Scheme 2a that two transition states TS1 and TS3 are firstly found for R1 and R3, respectively. As shown in Fig. 2a, in TS1, the C2-H1’ bond length with 1.270 Å is elongated by 18.2 % compared with the equilibrium value of 1.074 Å in diuron, and the distance of O-H1’ is 1.191 Å, indicating that the C2-H1’ bond is being broken and the O-H1’ is being formed. Similarly, in TS3, the C6-H3’
bond is elongated by 15.7 % and the O…H3’ distance is 1.222 Å, meaning that H3’ is being abstracted and the OH3’ bond is being formed. Then, P1 and H2O are produced via the transition state TS1, and P3 and H2O are formed via the transition state TS3. As illustrated in Table 1, the energy barriers for pathways R1 and R3 are 15.32 kcal mol−1 and 11.62 kcal mol−1, respectively, and the two pathways are exothermic by 2.67 kcal mol −1 and 6.18 kcal mol −1 . Interestingly, the energy barrier of the pathway R3 is 3.70 kcal mol−1 lower than that of the pathway R1, indicating that the H3’ atom is a little easier to be abstracted than the H1’ atom. It
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Fig. 2 (continued)
can been noticed from Fig. 2a that the -CON (CH3) 2 group deviates from the benzene ring with a dihedral angle of 45.9° in TS1. This torsion could consume part of energy barrier for the pathway R1, which probably leads to the above result. Second, the meta-H atom (H2’) is abstracted by OH. A transition state TS2 is first located on the potential energy surface. As shown in Fig. 2a, in TS2, the C5-H2’ bond is being broken denoted by that the C5-H2’ bond is longer than the equilibrium value of 1.078 Å by 15.7 %. The direct abstraction results in the formation of P2 and H2O. This reaction has an energy barrier of 12.74 kcal mol−1 and an enthalpy of −4.28 kcal mol−1. This indicates that the meto-H atom abstraction reaction can occur readily.
Finally, H atom abstraction from the methyl group is taken into account. As depicted in Fig. 4a, a transition state denoted by TS4 is initially located on the potential energy surface. It can be seen from the transition state TS4 in Fig. 2a that the H9’ atom is obviously leaving the C9 atom marked by the lengthened C9-H9’ bond from 1.081 Å to 1.167 Å. Then, a radical P4 and H2O are yielded from the reactants via the transition state TS4. This pathway has the lowest energy barrier of 4.98 kcal mol−1 among the four abstraction reactions, which indicates that the H atoms in the methyl group are most kinetically and favorably abstracted by OH. This result is in satisfactory agreement with P4 as the main intermediate proposed by Macounová et al. [27], Oturan et al. [11], and Feng et al. [13].
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TS8
P8
IM3
TS9
P9
TS10
P10
(c)
IM4
P11 Fig. 2 (continued)
TS11
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Table 1 Calculated relative energies (kcal mol−1) of all pathways for the diuron and OH reactions in the water (MPWB1K/6–311+G(3df,2p)// MPWB1K/6–31+G(d,p) and MPWB1K/6–31+G d,p), respectively, 298.15 K)
H-atom abstractions
Addition reactions
Substitution reaction
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11
MPWB1K/6–311+G(3df,2p)//MPWB1K/6–31+G(d,p)
MPWB1K/6–31+G(d,p)
ΔE# 15.32 12.74 11.62
ΔH −2.67 −4.28 −6.18
ΔEs# 15.06 12.56 14.71
ΔHs −2.62 −4.01 −6.72
ΔG 32.45 27.29 27.94
4.98 23.16 10.61 7.82 1.30 12.89 10.12 68.85
−28.69 −14.69 −16.46 −20.80 −13.78 −23.12 −28.09 48.92
3.31 12.49 11.01 8.12 1.44 12.84 10.10 69.83
−41.44 −11.27 −16.65 −26.18 −15.47 −11.31 −16.54 49.14
25.11 93.28 90.77 87.49 5.21 88.54 86.41 69.79
Note: ΔE# denotes the reaction barrier height ΔH denotes the reaction heat ΔEs# denotes the reaction barrier height with smaller basis set ΔHs denotes the reaction heat with smaller basis set ΔG denotes the reaction free energy
Comparing the four direct H abstraction reactions, the energy barrier of the pathway R4 is lowest. This fact means that the H atoms in the methyl group are easier to be abstracted than the H atoms attached to the benzene ring, which matches well with the previous studies [28]. The energy barrier for the pathway R2 is lower than the pathway R1 but higher than the pathway R3. This result indicates that the meta-H (H2’) is easier to be attacked by OH than the ortho-H (H1’) and more difficult than the ortho- H (H3’). Generally, the meta-H abstraction reaction harder to occur than the ortho- H abstraction reaction, which is in accordance with the above calculation that the barrier height of the meta-H abstraction R2 is higher than that of the ortho-H abstraction R3. However, the ortho-H
TS4’
(H1’) is more difficult to be abstracted than the meta-H atom (H2’). The probable reason can be seen in Fig. 2a: for the pathway R1, a part of the energy barrier is used to overcome the torsion of the side chain from the benzene ring, which can be signified by the C2-C1-N1-C7 dihedral of 45.9° in TS1. The formations of P1 + H2O, P2 + H2O, and P3 + H2O are slightly exothermic by 2.67 kcal mol−1, 4.28 kcal mol−1, and 6.18 kcal mol−1, respectively, while the reaction R4 yielding P4 + H2O is strongly exothermic by 28.69 kcal mol−1. Additionally, it can be seen from Table 1 that the free energy of the reaction R4 is negative and smallest of the four Habstraction channels, which matches well with the above results.
P4’
Fig. 3 Optimized geometries of the transition state and product for ·OH abstracting H9’ atom (R4) at MP2/6-31+g (d,p) level (The grey-, white-, red-, and green-balls denote carbon, hydrogen, oxygen, and chlorine atoms, respectively, and bond lengths are in angstroms)
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Erel kcal mol-1
TS1 15.32 TS2 12.74
Diuron + OH
TS3 11.62
R1
TS4 4.98
R3 R4
R2
-2.67 P1 + H O 2
0.00
P2 + H2O -4.28 P3 + H2O -6.18
(a) Erel kcal mol-1
-28.69
P4 + H2O
TS5 21.37
R5 R6 R7 R8
TS9 11.25
R9 R10
TS6 8.83 TS10 8.48 TS8 6.36
IM2 5.05 Diuron + OH 0.00
TS7 6.03 IM3 -1.64 IM1 -1.79 567
-13.78
P8
-14.69 P5 -16.46 P6 -20.80 P7 -23.12 P9 -28.09 P10
(b) Fig. 4 Calculated potential energy surface profiles for all reaction pathways: a H-atom abstraction reactions; b addition reactions; c substitution reactions
OH addition mechanism Six positions which OH add to are considered, including three types: the ipso- C atom (C (1) in Fig. 1), the C atoms connected to H atoms (C (2), C (5), and C (6) in Fig. 1) and the chloro-substituted C atoms (C (3) and C (4) in Fig. 1) . The six addition pathways denoted by R5-R10 will be discussed in the following. Each pathway proceeds with the following steps: first, one pre-complex is formed; second, a transition state is found; finally, the products are yielded through the corresponding transition states. First, the addition of OH to the ipso- C atom (C1) is considered, and a channel denoted R5 is found. IM1 is the
formed pre-complex, in which the C1…O distance is 3.032 Å. The energy of IM1 is 1.79 kcal mol−1 more stable than that of original reactants. TS5 is found to be the transition state for OH adding to the C1 site. In the structure of TS5, the distance between C1 and O is decreased from 3.032 Å in IM1 to 1.987 Å. P5 is the product of this process, in which the C1O bond has been formed indicated by its bond length of 1.422 Å, and the C1-N1 bond is stretched by 0.059 Å compared to that in the corresponding transition state TS5. The energy barrier of this addition reaction is 23.16 kcal mol−1, and the reaction heat is −14.69 kcal mol−1. Second, that OH attacking the C (5), C (6), and C (2) atoms attached to H atoms are considered, and three pathways R6,
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J Mol Model (2014) 20:2280 Erel kcal mol-1
TS11 74.70 R11
P11 48.92
Diuron + OH
IM4 5.85
0.00
(c) Fig. 4 (continued)
R7, and R8 are identified, respectively. Interestingly, both the pathways R6 and R7 share the same pre-complex IM1, in which the C5…O distance is 3.516 Å, and the C6…O distance is 2.697 Å. This means that OH is closer to the C6 atom, so that OH is probably easier to add to the C6 site than the C5 site, which matches well with that the calculated energy barrier of R7 is lower than that of R6. Then, two transition states TS6 and TS7 are found on the potential energy surfaces for the pathways R6 and R7, respectively. In the structure of TS6, the distance of C5…O has been significantly decreased from 3.516 Å in IM1 to 1.994 Å, and the C4-C5 bond and C5C6 bond are slightly elongated by 0.127 Å and 0.122 Å, respectively. While in the structure of TS7, the C6…O distance has been only reduced by 0.626 Å, and the C5-C6 bond and C6-C1 bond are slightly lengthened by 0.017 Å and 0.018 Å, respectively. Finally, adducts P6 and P7 are produced. Compared with TS6, in P6, the C5…O distance has been shortened by 0.574 Å, meaning that the C5-O bond is being formed. The C5-H2’ bond has been a little elongated by 0.015 Å. The C4-C5 bond and C5-C6 bond are stretched by 0.085 Å and 0.082 Å, respectively. In P7, the C6-O bond has been formed indicated by its bond length from 2.071 Å in TS7 to 1.418 Å, which is expected for sp3 hybridized carbon [29]. The C6-H5’, C6-C5 and C6-C1 bonds are elongated by 0.018 Å, 0.087 Å and 0.088 Å compared with those in TS7, respectively. The energy barriers of the reactions R6 and R7 are 10.61 kcal mol−1 and 7.82 kcal mol−1, respectively, and exothermic by 16.46 kcal mol−1 and 20.80 kcal mol−1, respectively. For the pathway R8, a pre-complex IM2, which lies 5.05 kcal mol−1 above the reactants, is first formed. Then, a transition state TS8 is found on the potential energy surface, in
which OH is nearly perpendicularly adding to the C2 atom with an O-C2-H1’ angle of 86.9° and the C2…O distance is decreased from 2.530 Å in IM2 to 2.058 Å. Finally, P8 is obtained from IM2 via the transition state TS8. Compared with TS8, the C2-O bond has been formed with a length of 1.433 Å and the C2-H1’ bond is elongated by 0.009 Å in P8. Additional changes in P8 have been seen in the distances of C2-C3 and C2-C1 bonds, which have been stretched by 0.087 Å and 0.086 Å compared with those in TS8, respectively. The barrier height of the pathway R8 is 1.30 kcal mol−1, the lowest of all the addition reactions. This result suggests that OH adding to the C2 site is the most energetic and kinetic channel. Finally, the meto- C (C3) and para- C (C4) atoms attached to Cl atoms are attacked by OH, and two channels R9 and R10 are identified, respectively. First, a structure IM3 is found to be the pre-complex for both pathways R9 and R10. The energy of IM3 is 1.64 kcal mol−1 lower than the original reactants, and in IM3, the C3…O and C4…O distances are 3.154 Å and 2.944 Å, respectively. It can be noted that OH is 0.210 Å nearer to the C4 atom than the C3 atom, indicating that OH is slightly easier to add to the C4 site than the C3 site. This fact is in good agreement with that the barrier height of R10 is a little lower than R9. Then, two transition states TS9 and TS10 are found on the potential energy surfaces, in which OH is almost vertically adding to the C3 and C4 atoms, and the C3…O and C4…O distances have been shortened by 1.166 Å and 0.903 Å, respectively. Additionally, the C3-C2, C3-C4, and C3-Cl1’ bonds in TS9 and the C4-C3, C4-C5, and C4-Cl2’ bonds in TS10 are all elongated compared to those in IM3. Finally, adducts P9 and P10 are obtained. In P9, the C3-
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O bond has been formed indicated by the C3-O bond decrease from 1.988 Å in TS7 to 1.361 Å, and compared to TS9, the C3-Cl1’, C3-C2, C3-C4 bonds are elongated by 0.197 Å, 0.059 Å, and 0.059 Å, respectively. Similar changes have been seen in the adduct P10. The activation energies of the pathways R9 and R10 are 12.89 kcal mol−1 and 10.12 kcal mol−1, respectively, and the corresponding products P9 and P10 lie 23.12 kcal mol−1 and 28.09 kcal mol−1 below the reactants (diuron and OH), respectively. Additions of OH to C sites of the phenyl ring except the ipso- C site are found to be the import processes by many researches: Oturan et al. [11, 30] take OH addition on the benzene ring as one principle oxidation route and Katsumata et al. [31] obtain that one product results from OH adding to the aromatic ring. Comparing the six addition pathways, the potential energy of the pathway R8 is the lowest, indicating that C2 atom is easiest to be attacked. From Table 1, it also can be obtained that the free energy of the channel R8 is smallest of all addition reactions, indicating that the channel R8 is easiest to occur. However, the relative energy of transition states minus reactants for the two ortho- sites addition reactions R7 and R8, are similar, both lower than those of other pathways. This result suggests that the orthosites are more active than other sites. Additionally, the potential barrier of the channel R10 (·OH adding to the chloro-substituted para- C atom) is lower than those of channels R5, R6, and R9. This fact is in good accordance with that the -NHCO- group is an ortho-para directing group, meaning that the ortho- and para- sites are more easily attacked than other sites. The potential energy of the path R5 is the highest among all the addition reactions. This result indicates that the ipso- site is most difficult to be attacked, which is observed to agree with that the side chain substituted is not found in the beginning of the experiments [11, 13, 30]. Substitution mechanism Only one channel, R11, is found for the substitution reaction where OH substituting H4’ atom attached to the N1 atom. The channel R11 starts from the formation of a pre-complex, IM4, in which the distance between N1 and O is 2.503 Å and N1H4’ bond length is 1.003 Å. The energy of IM4 is 5.85 kcal mol−1 higher than the original reactants. TS11 is found to be the transition state structure for this OH attacking N1 atom process, the energy of which lies 68.85 kcal mol−1 above that of IM4. In TS11, the N1-O bond will be formed denoted by that the N1…O distance decreased from 2.503 Å in IM4 to 1.430 Å, and the N1-H4’ bond is being broken indicated by that the bond is stretched from 1.003 Å in IM4 to 1.170 Å. P11 and H radical are the products of the substitution reaction. It can be noted that the energy barrier of the channel R11 is the highest of all considered reactions, meaning that this channel
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is difficult to occur and can be ignored. The pathway R11 is strongly endothermic by 48.92 kcal mol−1. Conclusions Three reaction types for OH attacking diuron with water solvent have been investigated employing MPWB1K method with polarizable continuum model (PCM) calculation. For Hatom abstractions, four pathways are identified, and the calculation results display that the pathway R4 (·OH abstracting H atom in methyl group) has the lowest energy barrier of 4.98 kcal mol−1, indicating that this reaction could be the first step for H-atom abstraction reactions; moreover, the side chain torsion for the H1’ abstraction reaction (R1) possibly consumes part of the energy, leading to that the ortho- H (H1’) is more difficult to be abstracted than the meta- H. For addition reactions, the potential barriers for OH additions to the orthosites (pathways R7 and R8) and the chloro-substituted parasite (R10) are lower than other sites, suggesting the ortho- and para- sites are more active, which is in good agreement with that the -NHCO- group is an ortho-para directing group. ·OH addition to the ipso- site (R5) has the highest energy barrier, meaning that this reaction is most difficult to occur. For the only found substitution reaction R11, OH substituting H4’ atom attached to the N1 atom has the highest energy barrier of 68.85 kcal mol−1, meaning that this reaction is the most difficult to occur among all considered reactions and not expected to be important. Acknowledgments This work is supported by the Scientific Research Funds for Yong Scholars of Weifang University of Science and Technology (No sdlgy2013w003) and the Postgraduate Independent Innovation Program of Shandong University (No. 11,440,072,613,118). We also thank University Institutes Innovation Program of Jinan Science and Technology Bureau (No.201102046).
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