Hydroxyl radical induced transformation of phenylurea herbicides: A theoretical study

Radiation Physics and Chemistry 132 (2017) 16–21

Contents lists available at ScienceDirect

Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Hydroxyl radical induced transformation of phenylurea herbicides: A theoretical study

crossmark

⁎

Viktória Milea, Ildikó Harsányia, Krisztina Kovácsa, , Tamás Földesb, Erzsébet Takácsa, László Wojnárovitsa a b

Institute for Energy Security and Environmental Safety, Centre for Energy Research, Hungarian Academy of Sciences, Budapest, Hungary Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Budapest, Hungary

A R T I C L E I N F O

A BS T RAC T

Keywords: Phenylurea herbicide Hydroxyl radical Dehalogenation Advanced oxidation processes Radiolysis DFT calculations

Aromatic ring hydroxylation reactions occurring during radiolysis of aqueous solutions are studied on the example of phenylurea herbicides by Density Functional Theory calculations. The effect of the aqueous media is taken into account by using the Solvation Model Based on Density model. Hydroxyl radical adds to the ring because the activation free energies (0.4–47.2 kJ mol−1) are low and also the Gibbs free energies have high negative values ((−27.4) to (−5.9) kJ mol−1). According to the calculations in most of cases the ortho- and paraaddition is preferred in agreement with the experimental results. In these reactions hydroxycyclohexadienyl type radicals form. In a second type reaction, when loss of chlorine atom takes place, OH/Cl substitution occurs without cyclohexadienyl type intermediate.

1. Introduction Phenylurea herbicides are photosynthesis inhibitors, and are applied to control broadleaf weeds. These herbicides are mostly Ndimethyl derivatives bearing various substituents on the aromatic ring. They are highly persistent in the environment with half-lives of several months in the soil. The compounds are regularly detected in surface waters in concentrations up to several µg dm−3. Persistent organic pollutants in water matrix may be eliminated by using advanced oxidation processes (AOP). These techniques use aggressive radicals, mainly hydroxyl radicals (•OH), to degrade the harmful organic pollutants. The degradation of phenylurea herbicides was investigated by several AOP, e.g. Fenton, photo-Fenton, H2O2/UV or by high energy ionizing radiation treatments (Mazellier and Sulzberger, 2001CanleLopez et al., 2005; Bobu et al., 2006; Zhang et al., 2008; Oturan et al., 2010; Kovács et al., 2014, 2015). As it was shown by time resolved pulse radiolysis and pulse photolysis experiments •OH reacts with the aromatic ring resulting in hydroxycyclohexadienyl radicals, which transform to stable products hydroxylated in the aromatic ring (phenol type compounds) (Canle-Lopez et al., 2001, 2005; Zhang et al., 2008; Kovács et al., 2014, 2015). •OH attack on the methyl groups was found less important than the reaction with the aromatic ring (e.g. Oturan et al., 2010). However, Mazellier and Sulzberger (2001) observed only one product in the heterogeneous photo-Fenton degradation of diuron (3-(3,4-dichlorophenyl)-1-formyl-1-methylurea), which was formed in

⁎

the •OH attack on a methyl group. During the complex AOP reactions of the halogen containing phenylureas (e.g. monuron and diuron, Fig. 1) dehalogenation was also observed. In general, dehalogenation is attributed to reductive processes, e.g. in radiolytic reactions to the reactions of hydrated electrons. The most striking result of diuron and monuron investigations is the fact that the dehalogenation is also observed in the reactions of hydroxyl radicals (Tahmasseb et al., 2002; Zhang et al., 2008; Oturan et al., 2010; Kovács et al., 2015, 2016). For example in monuron solution about 40% of •OH reactions may induce chloride release (Kovács et al., 2016). The details of these reactions have not been investigated yet. Two reaction pathways can be visualized: (i) •OH addition to the carbon atom when hydroxycyclohexadienyl type radical intermediate is forming, followed by HCl elimination, and (ii) direct •OH/Cl• exchange. Tahmasseb et al. (2002) suggested even more complicated mechanisms for the Cl/OH substitution: (i) the dissociation of a C-Cl bond in the pesticide gives a phenylurea radical, which step is followed by an •OH addition to this radical, or (ii) a C-Cl reductive cleavage occurs on a previously hydroxylated molecule. Under the usual conditions such two-step reactions should have low probabilities. The present work focuses on the aromatic ring hydroxylation by applying quantum chemical calculations. The purpose was to clarify the site of attack on the aromatic ring and the mechanism of chlorine elimination. Fenuron (1,1-dimethyl-3-phenylurea), monuron (3-(4-

Corresponding author. E-mail address: [email protected] (K. Kovács).

http://dx.doi.org/10.1016/j.radphyschem.2016.11.003 Received 24 August 2016; Received in revised form 21 October 2016; Accepted 5 November 2016 Available online 15 November 2016 0969-806X/ © 2016 Published by Elsevier Ltd.

Radiation Physics and Chemistry 132 (2017) 16–21

V. Mile et al.

of the degradation has been studied theoretically in cases of diuron by several researchers and all studies agreed that the ortho and para directions are preferred (Carrier et al., 2009; Ren et al., 2014; Mendoza-Huizar, 2015). However, it is not known yet whether all the reactions have barrier, or some of them proceeds with barrierless mechanism. For the fenuron + hydroxyl radical reaction the detailed calculations of Zeng et al. (2015) suggest hydroxylations in the following order: Cortho-1 > Cpara > Cipso > Cmeta-1 > Cmeta-2 according to the Gibbs free energy of the reaction. In case of activation free energy the order is: Cmeta-2 > Cmeta-1 > Cpara≅Cortho-1 > Cmeta-2. 2. Methods The computational analysis was carried out using Density Functional Theory (DFT) methods. Becke's three parameter hybrid functional has been applied with the Lee-Yang-Parr correlation extension, generally known as B3LYP (Becke, 1993). The standard 6–311+ +G(d,p) (McLean and Chandler, 1980; Krishnan et al., 1980; Clark et al., 1983; Frisch et al., 1984) basis set was applied for geometry optimizations. Frequency calculations were also performed for establishing the stationary points (i.e. ground state: frequencies are all positive, transition state: there is one imaginary frequency). The electronic energy was refined by single-point energy calculations at the B3LYP/6-311++G(3df,3pd) level (McLean et al., 1980; Krishnan et al., 1980; Clark et al., 1983; Frisch et al., 1984). The Gibbs free energies are reported for T=298.15 K and corrected for c=1 mol dm−3. The calculations were performed with the Gaussian program package (Frisch et al., 2009). The Solvation Model Based on Density model (SMD) (Marenich et al., 2009) was applied in order to model the aqueous media. The geometries were also reoptimized under the effect of the solvation model. Based on Gibbs free energies of the reactions and activation free energies we make suggestions for the most probable sites of •OH attack, and also we clarify the details of the dechlorination reaction.

Fig. 1. Structures of the studied molecules: aniline, phenol, fenuron, monuron, diuron-1 and diuron-2. The nomination of the reaction direction (i.e. addition of •OH to a given carbon atom) is according to the arene substitution pattern. The two ortho (ortho-1 and ortho-2) and the two meta (meta-1 and meta-2) positions at herbicides are distinguished with 1 and 2 shown in the case of fenuron. The order starting from the phenyl-urea group is the following (counter clockwise): ipso, ortho-1, meta-1, para, meta-2, ortho-2.

3. Results and discussion

chlorophenyl)-1,1-dimethylurea) and diuron (3-(3,4-dichlorophenyl)1,1-dimethylurea) were chosen from the class of phenylurea herbicides for this study (Fig. 1). Aniline and phenol were also involved as model compounds for the more complicated phenylureas. The hydroxyl radical reactions with aniline have been previously studied by pulse radiolysis combined with computer optimization procedures. Solar et al., (1986) suggested 54% and 10% •OH attacks to ortho- and para-carbon atom, respectively. •OH radical additions to the aromatic ring of phenol were studied both experimentally (e.g. Albarran and Schuler, 2007) and theoretically (Lundqvist and Eriksson, 2000; Lee et al., 2009; Wu et al., 2012; Jayathilaka and et al., 2014). Catechol and hydroquinone are produced in the catalytic oxidation reaction of phenol (Norena-Franco et al., 2002; Parida et al., 2008). The frequencies of additions to different positions are reported to be: ipso: 8%, ortho: 50 (2×25)%, meta: 8 (2×4)%, para: 34% (Albarran and Schuler, 2007). Wu et al. (2012) concluded that the ortho adduct is the most stable thermodynamically, followed by the ipso, para and meta ones. According to the charge distribution calculations on phenol the two ortho carbon atoms have the highest negative charges (Lundqvist and Eriksson, 2000). The ipso and meta positions of phenol are not preferred due to a positive charge or a small negative one (Lundqvist and Eriksson, 2000). The results of Wu and co-workers (2012) indicate that the ortho addition pathway is dominant, the same result was obtained by Jayathilaka et al. (2014). Hydroxylated phenylurea herbicides are created during the degradation of pesticides in advanced oxidation processes, e.g. in radiolysis (Kovács et al., 2014, 2015). The directions of the hydroxyl radical addition to the ring are not known from the experiments. The first step

The dimethylurea-group in the molecule is relatively rigid because of the delocalized electrons. Hence, two possible places of attack for • OH can be distinguished on the ring at the ortho and the meta-ring positions. The difference between ortho-1 and ortho-2 and between meta-1 and meta-2 is the orientation of the C=O group within the dimethylurea moiety. In case of diuron there are two chlorine atoms in the molecule. The possible conformations depend on the direction of C=O group. Diuron-2 is denoted when the meta chlorine atom is on the opposite side of C=O group. The names and the possible reaction pathways for the sake of clarity are shown in Fig. 1. The optimized bond length values are provided in Supplementary Material (Table 1S). The bond lengths are in good agreement with results of former calculations (Ren et al., 2014; Mendoza-Huizar, 2015). The typical Cring-Cring, Cring -H and Cring-N next to the ring and Cring-Cl bond lengths are 1.39, 1.08, 1.41 and 1.76 Å, respectively. Table 1 contains the values of the Gibbs free energies (ΔrG) and the activation free energies (ΔG‡) relative to the reactants. All modelled reactions are found to be exergonic. Calculations suggest larger stabilization in the case of chlorine substituted aromatic rings (monuron: para; diuron-1: meta-1, para: diuron-2: meta-2, para). •OH attacks at the para-positions are energetically the most favorable reactions for the phenylureas. Based on the calculated Gibbs free energy and activation free energy data the ortho and para reactions take place with almost equal probability for aniline, while the ipso and the meta-reactions are energetically less favorable as it was suggested by Solar et al. (1986) based on pulse and gamma-radiolysis experiments. Similarly to aniline for phenol the ortho and para reactions are energetically favored, however, because of the low Gibbs free energy 17

Radiation Physics and Chemistry 132 (2017) 16–21

V. Mile et al.

Table 1 Thermodynamic properties of •OH reactions with the aromatic rings of molecules at 298.15 K. ΔrG: Gibbs free energy, ΔG‡: activation free energy (relative to reactants). The energy data were adjusted to c =1 mol dm−3. The Table includes data without using water solvent (i.e. gas phase) and applying SMD water solvent (aqueous media), too. Values marked with asterisks are rough estimates (see later) assuming the same Cring…Oradical distances for ortho-2 cases, 2.05 Å for para case of fenuron and 2.0 Å for ortho case of aniline (in gas phase). Aniline

– Unit – ipso ortho meta para Position – Unit – ipso ortho-1 ortho-2 meta-1 meta-2 para Position – Unit – ipso ortho-1 ortho-2 meta-1 meta-2 para

ΔrG kJ mol−1 in gas phase −7 −34.1 −11.1 −24.7 Fenuron ΔrG kJ mol−1 in gas phase −16.3 −28.1 −31.6 −5.4 −12.2 −23.2 Diuron-1 ΔrG kJ mol−1 in gas phase 20.8 −30.2 −35 −72.7 −78 −85.8

Rel. E. referred to separeted diuron-2 + °OH / kJ/mol

Position

Phenol ΔG‡

ΔrG

ΔG‡

35.1 21.1* 40.1 24.5

in aqueous media −5.9 −27.4 −7.5 −23.7

21.7 3.2 29.1 0.4

ΔG‡

ΔrG

ΔG‡

52.2 37.5 21.8 41.7 39.4 29.7

in aqueous media −9.3 −23.9 −21.9 −7.5 −7.6 −23

38.4 23.9 18.1* 33.3 32.8 13.5*

ΔG‡

ΔrG

ΔG‡

in gas phase −22.2 −27.5 −32.4 −11.3 −12.9 −148 Diuron-2 ΔrG

52.4 38.8 23.1 59.7 42.1 49.3

in aqueous media −6.5 −23.8 −23.9 −114.3 −13.7 −126.2

47.9 29.8 24.9* 51.4 36.3 42.8

in gas phase −20.3 −30.2 −34.2 −14 −66.1 −154.1

-18 -20 -22 -24 -26 -28 -30 -32 -34 -36 -38 -40 -42 -44 -46 -48 -50 -52 -54 -56 1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

in gas phase −20.3 −16.8 −4.9 −10.4 Monuron ΔrG

ΔG‡

ΔrG

ΔG‡

40 39.8 41.4 33.3

in aqueous media −18.1 −15.1 −6.4 −12.4

33 24.4 34.5 22.9

ΔG‡

ΔrG

ΔG‡

50.2 39.4 24 46.7 40 47.4

in aqueous media −8.8 −22.4 −28.6 −11 −8.2 −171.7

37.6 27.5 23.6* 38 37.8 35.5

ΔG‡

ΔrG

ΔG‡

52.9 38.4 26.2 42.4 60 47.9

in aqueous media −8.3 −25.9 −27.5 −11.1 −110.5 −166.6

45.4 29 22.6* 36.6 58 35.3

case of fenuron. In gas phase the activation free energies of the reaction at ortho-2 positions are somewhat smaller (at least by 7.9 kJ mol−1) than at other positions. Comparing the activation free energies in aqueous and non-aqueous medium smaller ΔG‡ (by 2–24 kJ mol−1) can be obtained when using SMD water model. This systematic decrease of the activation free energies might lead to a barrierless transformation in the ortho-2 case. This behavior also emphasizes the importance of the application of a solvation model. In all other cases when the aqueous media model is applied, the barriers vary between 0.4 and 58.0 kJ mol–1 suggesting very fast reactions. The Gibbs free energies of the reactions for phenylureas have smaller negative values in water, except for those reactions in which a chlorine atom is bonded to the attacked carbon. There are two other exceptions, the meta- and the para-addition to phenol, where the ΔrG values are somewhat (by 1.5–2.4 kJ mol−1) smaller in the gas phase reactions. Ortho-para directing is suggested by the calculations with all three groups, –NH2, –OH and –NHCON(CH3)2. Let us examine the two types of ortho reactions. In order to understand the unequal probabilities of the ortho-1 and ortho-2 reactions further calculations were carried out for the diuron-2 molecule as a suitable model system. In both cases the interatomic distance between Cring and Oradical were changed in 0.025 Å steps from 1.7 Å up to 2.6 Å and constrained geometry optimizations (i.e. relaxed PES scan) were carried out at B3LYP/6–311++G(3df, 3pd)//B3LYP/ 6–311++G(d,p) level at each point. The calculated energies were analyzed with respect to the separated molecule and hydroxyl radical (Fig. 2). A clear difference is found between the obtained potential energy curves, which is large at larger Cring-Oradical distances, but shrinks towards the product states (smaller Cring-Oradical distances). The potential energy curve is quite flat especially in the case of the ortho-1 reaction, which indicates that there is a shallow minimum before the TS might be caused by weak secondary interactions (e.g. hydrogen bonding between radical and sidechain). The shapes of the energy curves in Fig. 2 are quite similar to each other up to 2.1 Å so a rough estimate can be given for activation free energy assuming the same Cring…Oradical distances for ortho-2 cases.

ortho-1 ortho-2

1.7

ΔrG

2.6

dC(ring) - O (radical) / Angstrom Fig. 2. Constrained partial optimization calculations of diuron-2 + •OH reaction in kJ/ mol.

the ipso reaction can also take place. Nevertheless, the relatively high barrier to ipso reaction decreases the probability. Experimental results do not indicate significant formation of the ipso-product either (Albarran and Schuler, 2007). By our calculations local maxima for ortho-2 for herbicides and of para for fenuron could not been found on the potential energy surface at the applied level using SMD water model. It should be noted that Ren et al. (2014) located transition states in all cases for diuron at MPWB1K/63+G(d,p) level applying polarizable continuum model with water solvent. In their DFT calculation using DZP basis without application of the water solvation model Carrier and co-workers (2009) reported barrierless •OH addition on each atom of the aromatic ring of diuron. Zeng et al. (2015) did not mention the ortho-2 case in their study, but for the para reaction they located a transition state in 18

Radiation Physics and Chemistry 132 (2017) 16–21

V. Mile et al.

Table 2 Characteristics of the hydrogen bonds of diuron-2: distances (in Å) of the Ocarbonyl···•O; Ocarbonyl···H; Namino···•O; Namino···H are shown and the angles (in °) of Ocarbonyl···H-O•; Namino···O-H• for ortho-1 TS and estimated ortho-2 TS. H-bond Ocarbonyl···H Ocarbonyl···•O Ocarbonyl···H-O•

H-bond Namino···H Namino···•O Namino···O-H•

ortho-1 2.0 2.9 148°

ortho-2 3.4 3.2 97°

This is illustrated in Fig. 3, where the products along with the removed chlorine atoms are depicted. In these cases the hydroxyl groups are attached to meta or para positioned carbon atoms. The •O-H bonds were found to be longer than 1 Å in para-cases of monuron and diuron-2, where the released chlorine atoms attract the hydrogen atoms of the hydroxyl groups. In a subsequent reaction, a phenoxyl type radical may form (H elimination) as it was reported in pulse radiolysis experiments (Kovács et al., 2015, 2016). Based on the calculations it is presumable that the first step is OH/Cl substitution, then the leaving chlorine atom extracts the hydrogen atom. The reactions also affect the geometry of the aromatic ring. The bond lengths change by (−0.01)-(+0.12) Å. For phenol our results are in good agreement with results of other calculations (Lundqvist and Eriksson, 2000; Jayathilaka et al., 2014). In cases when chlorine atom is not attached to the carbon atom the bond lengths are longer by 0.09– 0.12 Å. The bond lengths increase on average by 0.03 Å for reactions affecting the chlorine atoms. For diuron-1 meta-1 and diuron-2 meta-2 (where the chlorine atom bonds to a carbon atom) one of the bond lengths decreases slightly (caused by the nearby released chlorine atom), the other increases by 0.04 Å. In the literature there are suggestions about pre-formation of a loose •OH-aromatic ring “π-complex”, in which •OH is attached to the aromatic ring electron sextet (e.g. Ashton et al., 1995). This π-complex may dissociate or transform to a so-called σ-complex, where the hydroxyl group is located on a carbon atom. The possibility of •OH-π type pre-complexes have also been studied here. Geometry optimizations were carried out starting from a structure when the hydroxyl radical is above the center of ring. In these calculations we observed the migration of the hydroxyl radical towards the ortho carbon atom for

The resulting values are collected in the Table 1 with asterisks, they are 3.8–5.7 kJ mol−1 less than the ΔG‡ of ortho-2 cases. Considering the estimation and the small energy difference we cannot tell which reaction is more probable, but we can conclude that the secondary interactions (e.g intermolecular H-bonds) and the consideration of solvation are important factors. For the diuron-2 molecule the TS of ortho-1 and the analogous structure (partially optimized with the same Cring-Oradical distance) of ortho-2 are depicted in Table 2. In the case of ortho-1 the formation of an intermolecular H-bond is possible. In the identified transition states the Cgiven position-•O distances vary between 2.00 Å and 2.21 Å (details in Table 2S). For phenol the distances of Cipso-•O, Cortho-•O, Cmeta-•O, Cpara-•O are 2.06 (2.020), 2.09 (2.073), 2.06 (2.015) and 2.06 (2.050) Å, respectively. The distances obtained by Wu et al. (2012) using B3LYP/6311+ +G(2d,2p) method are given in parentheses. The results of the two calculations are in reasonable agreement. •O-H distances are found to be 0.97 Å in all TS cases. In the products the Cgiven position-•O distances vary between 1.44 Å and 1.49 Å during •OH attacks resulting in hydroxycyclohexadienyl type radicals. In cases when these bond lengths change between 1.31 Å and 1.34 Å chlorine atom release takes place. The loss of a chlorine atom leads to non-radical products, in contrast to the other attacks. The values of •O-H bonds lengths are mainly 0.97 Å as they were in the TS. During these hydroxylation processes weak hydrogen bonds develop between •O-H and the carbonyl oxygens of the dimethylurea moiety. Characteristics of the hydrogen bonds in products are summarized in Table 3. In para-reactions of monuron and diuron-2 the bond lengths of •OH are significantly longer (1.02 Å and 1.03 Å) than in other reactions.

Table 3 Characteristics of the hydrogen bonds: distances (in Å) of the Ocarbonyl···•O; Ocarbonyl···H; Namino···•O; Namino···H are shown and the angles (in °) of Ocarbonyl···H-O•; Namino···O-H•. H bond Ocarbonyl···H Ocarbonyl···•O Ocarbonyl···H-O• H bond Ocarbonyl···H Ocarbonyl···•O Ocarbonyl···H-O•

Fenuron ipso 1.91 2.72 138 ° Monuron ipso 1.90 2.70 138 °

ortho-1 1.81 2.68 147 ° ortho-1 1.80 2.67 147 °

Diuron-1 ipso 1.91 2.71 137 ° Diuron-2 ipso 1.91 2.71 137 °

Monuron ipso ortho-1 1.80 2.67 146 °

ortho-2 Namino···H Namino···•O Namino···O-H•

3.23 2.88 102 °

ortho-1 1.81 2.67 145 °

ortho-2 Namino···H Namino···•O Namino···O-H•

3.42 2.82 121 °

19

Radiation Physics and Chemistry 132 (2017) 16–21

V. Mile et al.

Fig. 3. The release of chlorine atoms in the hydroxyl radical + monuron and diuron reaction. Left to right: monuron (para); diuron-1 (meta-1); diuron-1 (para); diuron-2 (meta-2); diuron-2 (para).

Appendix A. Supporting information

aniline and phenol and the ortho-1 carbon atom for fenuron, monuron, diuron-1 and diuron-2. These structures are very similar to the ortho or ortho-1 transition states discussed previously, but the distances of the Cring and Oradical are longer by 0.2–0.3 Å. Based on the energy data the resulting complex structures are relatively less favored with respect to the starting system. Longliving and stable pre-complexes could not been found.

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.radphyschem.2016.11.003. References Albarran, G., Schuler, R.H., 2007. Hydroxyl radical as a probe of the charge distribution in aromatics: phenol. J. Phys. Chem. A 111, 2507–2510. Ashton, L., Buxton, G.V., Stuart, C.R., 1995. Temperature dependence of the rate of reaction of OH with some aromatic compounds in aqueous solution. Evidence for the formation of a π-complex intermediate? J. Chem. Soc. Faraday Trans. 91, 1631–1633. Becke, A.D., 1993. Density‐functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 98, 5648–5652. Bobu, M., Wilson, S., Greibrokk, T., Lundanes, E., Siminiceanu, I., 2006. Comparison of advanced oxidation processes and identification of monuron photodegradation products in aqueous solution. Chemosphere 63, 1718–1727. Canle-Lopez, M., Rodrígez, S., Rodrígez Vazques, L.F., Santaballa, J.A., Steenken, S., 2001. First stages of photodegradation of the urea herbicides Fenuron, Monuron and Diuron. J. Mol. Struct. 565−566, pp. 133−139 Canle-Lopez, M., Fernandez, M.I., Rodrígez, S., Santaballa, J.A., Steenken, S., Vulliet, E., 2005. Mechanisms of direct and TiO2-photocatalised UV degradation of phenylurea herbicides. ChemPhysChem, 6, pp. 2064−2074 Carrier, M., Guillard, C., Besson, M., Bordes, C., Chermette, H., 2009. Photocatalytic degradation of diuron: experimental analyses and simulation of HO• radical attacks by Density functional theory calculations. J. Phys. Chem. A 113, 6365–6374. Clark, T., Chandrasekhar, J., Spitznagel, G.W., v. Schleyer, P.R., 1983. Efficient diffuse function-augmented basis-sets for anion calculations. 3. The 3-21+G basis set for 1st-row elements, Li-F. J. Comput. Chem. 4, 294–301. Frisch, M.J., Pople, J.A., Binkley, J.S., 1984. Self-consistent molecular orbital methods. 25. Supplementary functions for Gaussian basis sets. J. Chem. Phys. 80, 3265–3269. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G.A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H.P., Izmaylov, A.F., Bloino, J., Zheng, G., Sonnenberg, J.L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery, J.A., Jr., Peralta, J.E., Ogliaro, F., Bearpark, M., Heyd, J.J., Brothers, E., Kudin, K.N., Staroverov, V.N., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A., Burant, J. C., Iyengar, S. S., Tomasi, J., Cossi, M., Rega, N., Millam, J.M., Klene, M., Knox, J.E., Cross, J.B., Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R. E., Yazyev, O., Austin, A.J., Cammi, R., Pomelli, C., Ochterski, J.W., Martin, R.L., Morokuma, K., Zakrzewski, V.G., Voth, G.A., Salvador, P., Dannenberg, J.J., Dapprich, S., Daniels, A.D., Farkas, O., Foresman, J.B., Ortiz, J.V., Cioslowski, J., Fox, D.J., 2009. Gaussian 09, Revision A.02. Gaussian, Inc., Wallingford CT. Jayathilaka, P.B., Pathiraja, G.C., Bandara, A., Subasinghe, N.D., Nanayakkara, N., 2014. Theoretical study of phenol and hydroxyl radical reaction mechanism in aqueous medium by the DFT/B3LYP/6-31+G(d,p)/CPCM model. Can. J. Chem. 92, 809–813. Kovács, K., Mile, V., Csay, T., Takács, E., Wojnárovits, L., 2014. Hydroxyl radical-induced degradation of fenuron in pulse and gamma radiolysis: kinetics and product analysis. Environ. Sci. Pollut. Res. 21, 12693–12700.

4. Summary The attack of •OH to aniline, phenol, fenuron, monuron, diuron has been studied by using B3LYP/6–311++G(d,p) DFT simulation technique, applying corrections of the single point energy at the level of B3LYP/6–311++G(3df, 3dp). Both gas and aqueous phase (Solvation Model Based on Density) calculations were conducted. Equilibrium geometries and energies of all stationary points had been obtained. Applying the water model at the addition of •OH to ortho-2 position no transition states could be found due to the secondary interaction in the transition states. The H-bond contributions are higher in ortho-1 product than in ortho-2. When chlorine atoms are not relevant in the reaction, a simple •OH addition takes place to the ring by forming a hydroxycyclohexadienyl type radical. According to the energy data the hydroxyl radical addition is probable to any ring positions but the ortho- and para-reactions are energetically more favorable in good agreement with the experimental results obtained in radiolysis. As the results of calculations also show, this selectivity is connected to the effect of electron releasing –OH, –NH2 and –NH-CO-N(CH3)2 groups in the molecule. The •OH/Cl substitution in monuron and diuron occurs without cyclohexadienyl type intermediate in cases where chlorine is released. The solvation energy of Cl may also contribute to the high probability of this reaction.

Acknowledgments The authors thank Hungarian Science Foundation (OTKA, NK 105802) for the support and Imre Pápai for his valuable suggestions. (Additional Supporting Information is available in the online version of this article). 20

Radiation Physics and Chemistry 132 (2017) 16–21

V. Mile et al.

hydroxylation of phenol employing Cu–MCM-41 catalysts. Catal. Today 75, 189–195. Oturan, M.A., Edelahi, M.C., Oturan, N., El Kacemi, K., Aaron, J.-J., 2010. Kinetics of oxidative degradation/mineralization pathways of the phenylurea herbicides diuron, monuron and fenuron in water during application of the electro-Fenton process. Appl. Catal. B 97, pp. 82−89. Parida, K.M., Mallick, S., 2008. Hydroxylation of phenol over molybdovanadophosphoric acid modified zirconia. J. Mol. Catal. A: Chem. 279, 104–111. Ren, X., Cui, Z., Sun, Y., 2014. Theoretical studies on degradation mechanism for OHinitiated reactions with diuron in water system. J. Mol. Model., 2280, 2280. Solar, S., Solar, W., Getoff, N., 1986. Resolved multisite OH-attack on aqueous aniline studied by pulse radiolysis. Radiat. Phys. Chem. 28, 229–234. Tahmasseb, L.A., Nélieu, S., Kerhoas, L., Einhorn, J., 2002. Ozonation of chlorophenylurea pesticides in water: reaction monitoring and degradation pathways. Sci. Total Environ. 291, pp. 33−44. Wu, P.Z., Li, J., Li, S.J., Tao, F.M., 2012. Theoretical study of mechanism and kinetics for the addition of hydroxyl radical to phenol. Sci. China Chem. 55, 70–276. Zeng, J., Yang, H., Deng, J., Liu, H.J., Yi, X., Yang, L.P., Yi, B., 2015. Common characteristic assessments of transformation mechanism for substituted phenylurea herbicides by reactive oxygen species (ROSs) during photocatalytic process. Chem. Eng. J. 273, 519–526. Zhang, J., Zheng, Z., Zhao, T., Zhao, Y., Wang, L., Zhong, Y., Xu Y., 2008. Radiationinduced reduction of diuron by gamma-ray irradiation. J. Hazard. Mater. 151, pp. 465−472.

Kovács, K., He, S., Mile, V., Csay, T., Takács, E., Wojnárovits, L., 2015. Ionizing radiation induced degradation of diuron in dilute aqueous solution. Chem. Cent. J. 9, 21. Kovács, K., He, S., Mile, V., Csay, T., Takács, E., Wojnárovits, L., 2016. Ionizing radiation induced degradation of monuron in dilute aqueous solution. Radiat. Phys. Chem. 124, 191–202. Krishnan, R., Binkley, J.S., Seeger, R., Pople, J.A., 1980. Self‐consistent molecular orbital methods. XX. A basis set for correlated wave functions. J. Chem. Phys. 72, 650–654. Lee, B.D., Lee, M.J., 2009. Prediction of hydroxyl substitution site(s) of phenol, monochlorophenols and 4-chloronitrobenzene by atomic charge distribution calculations. Bull. Korean Chem. Soc. 30, 787–790. Lundqvist, M.J., Eriksson, L.A., 2000. Hydroxyl radical reactions with phenol as a model for generation of biologically reactive tyrosyl radicals. J. Phys. Chem. B 104, 848–855. Marenich, A.V., Cramer, C.J., Truhlar, D.G., 2009. Universal solvation model based on solute electron density and a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions. J. Phys. Chem. B 113, 6378–6396. Mazellier, P., Sulzberger, B., 2001. Diuron degradation in irradiated, heterogeneous iron/ oxalate systems: the rate determining step. Environ. Sci. Technol. 35, pp. 3314−3320. McLean, A.D., Chandler, G.S., 1980. Contracted Gaussian basis sets for molecular calculations. I. Second row atoms, Z=11–18. J. Chem. Phys. 72, pp. 5639−5648. Mendoza-Huizar, H.L., 2015. Chemical reactivity of isoproturon, diuron, linuron, and chlorotoluron herbicides in aqueous phase: a theoretical quantum study employing global and local reactivity descriptors. J. Chem., 751527. Norena-Franco, L., Hernandez-Perez, I., Aguilar, P., Maubert-Franco, A., 2002. Selective

21