Chemosphere 122 (2015) 62–69
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Elucidating triplet-sensitized photolysis mechanisms of sulfadiazine and metal ions effects by quantum chemical calculations Se Wang a, Xuedan Song a, Ce Hao a,⇑, Zhanxian Gao a, Jingwen Chen b, Jieshan Qiu a a b
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
h i g h l i g h t s Triplet-sensitized photolysis mechanism of SDZ was electron transfer or H transfer. 0
SDZ and SDZ
showed different triplet-sensitized photolysis routs. 2+ 2+ promoted triplet-sensitized photolysis of SDZ0. 2+ 2+ 2+ Presence of Mg , Ca , or Zn inhibited triplet-sensitized photolysis of SDZ . 2+
Presence of Mg , Ca , or Zn
a r t i c l e
i n f o
Article history: Received 22 May 2014 Accepted 2 November 2014 Available online 12 December 2014 Handling Editor: Klaus Kümmerer Keywords: Sulfadiazine Triplet-sensitized photolysis Mechanism DFT Metal ions
a b s t r a c t Sulfadiazine (SDZ) mainly proceeds triplet-sensitized photolysis with dissolved organic matter (DOM) in the aquatic environment. However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully worked out. In this study, we investigated the mechanisms of triplet-sensitized photolysis of SDZ0 (neutral form) and SDZ (anionic form) with four DOM analogues, i.e., fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP), and three metal ions (i.e., Mg2+, Ca2+, and Zn2+) effects using quantum chemical calculations. Results indicated that the triplet-sensitized photolysis mechanism of SDZ0 with FL, TX, and 2-AN was hydrogen transfer, and with CBBP was electron transfer along with proton transfer (for complex SDZ0-CBBP2) and hydrogen transfer (for complex SDZ0-CBBP1). The triplet-sensitized photolysis mechanisms of SDZ with FL, TX, and CBBP was electron transfer along with proton transfer, and with 2-AN was hydrogen transfer. The triplet-sensitized photolysis product of both SDZ0 and SDZ was a sulfur dioxide extrusion product (4(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ0 and SDZ were different. In addition, effects of the metal ions on the triplet-sensitized photolysis of SDZ0 and SDZ were different. The metal ions promoted the triplet-sensitized photolysis of SDZ0, but inhibited the triplet-sensitized photolysis of SDZ . Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Antibiotics that fight bacterial infections are widely applied in aquaculture, animal husbandry, and medical treatment (Kümmerer, 2009). Due to their abuse, antibiotics are largely released into and frequently detected in the aquatic environment (Kümmerer, 2009; Zuccato et al., 2010). To date, antibiotics known as emerging pollutants are of particular concern due to their adverse effects such as inducing bacterial resistance (Yu et al., 2013) and the emergence of ‘‘superbugs’’ (Woodward, 2010). Sulfadiazine (SDZ) was found to be one of the most frequently detected ⇑ Corresponding author. Tel./fax: +86 411 84986335. E-mail address: [email protected] (C. Hao). http://dx.doi.org/10.1016/j.chemosphere.2014.11.007 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
antibiotics in surface water (Wei et al., 2011; Na et al., 2013; Bayen et al., 2014). Recently a lot of attention has also been drawn to the studies on the behavior and fate of SDZ in the aquatic environment (Sukul et al., 2008; Anskjaer et al., 2013; Fang et al., 2014). Photolysis is found to be an important factor of determining the fate and behavior of organic pollutants including SDZ (Sabljic and Peijnenburg, 2001; Boreen et al., 2004, 2005; Sukul et al., 2008; Ge et al., 2009, 2010; Li et al., 2011; Wammer et al., 2011; Challis et al., 2013; Wei et al., 2013; Batchu et al., 2014). The wavelength at maximum electronic absorbance of SDZ is 263.6 nm in water (Premakumari et al., 2011). SDZ mainly proceeds indirect photolysis in the aquatic environment due to the fact that only sunlight with a wavelength >290 nm reaches the surface of the earth. Indirect photolysis may include reaction with transient
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excited species such as singlet oxygen (1O2), hydroxyl radical (OH), triplet excited state dissolved organic matter (3DOM), or other reactive species (Boreen et al., 2005). Boreen et al. (2005) reported that SDZ in natural water proceeds triplet-sensitized photolysis with DOM, and the primary product formed in the triplet-sensitized photolysis was identified as a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline). However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully understood. In recent years, the issues on the combined pollution of organic pollutants and metal ions has been drawing a lot of attention. Mg2+ and Ca2+ occur at a high concentration and Zn2+ is as a metal ion pollutant in natural water environment (Hafuka et al., 2014). Contemporary studies suggest that metal ions e.g. Mg2+ and Ca2+ can have an effect on the photochemical behavior of organic pollutants. Werner et al. (2006) found that the direct photolysis rate constant of tetracycline in the presence of Ca2+ or Mg2+ is greater by a magnitude than that of tetracycline in the absence of Ca2+ and Mg2+. Martínez et al. (1996) also found that Ca2+ and Mg2+ had an impact on the photochemical properties of norfloxacin. Following environmental release there will be interactions between metal ions and SDZ. Therefore, effects of metal ions on the photolysis of SDZ are urgently needed to be investigated. Obtaining experimental data can be laborious, costly, and timeconsuming. Quantum chemistry calculations have been found to be efficient alternatives for predicting the environmental behavior and fate of organic pollutants, and for providing an important information on reaction intermediates or reactive species involved in chemical reactions that are difficult to be detected experimentally (Sabljic, 2001; Wang et al., 2012, 2014a,b; Kovacevic and Sabljic, 2013a,b). It was the purpose of this study to investigate the triplet-sensitized photolysis mechanisms of SDZ in water and three metal ions (Mg2+, Ca2+, and Zn2+) effects. Based on density functional theory (DFT), NBO charge and electron spin densities were calculated to study the triplet-sensitized photolysis mechanisms (electron transfer or hydrogen transfer) of SDZ with four DOM analogues, and the formation routes of photolysis products and the metal ions effects were calculated.
2. Computational methods SDZ was selected as a model compound (Fig. 1). The geometry optimization of all structures in solvent water was carried out using DFT (Kohn et al., 1996) and Becke’s three-parameter hybrid exchange function with Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) (Becke, 1993) with 6-311+G(d,p) basis set. The polarized continuum model (PCM) was employed to consider the solvent effects in water. The UV absorbance spectra of SDZ in water was calculated using the time-dependent density functional
+
theory (TDDFT) at B3LYP/6-311++G(d,p) level (Burke et al., 2005; Zhao and Han, 2009, 2012). The atom charge was examined based on the natural bond orbital (NBO) scheme at B3LYP/6-311++G(d,p) level. Sensitizers fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP) were selected as DOM analogues to investigate the triplet-sensitized photolysis mechanisms of SDZ. The energies of the lowest triplet excited state of FL, TX, 2-AN, and CBBP were calculated at TDDFT/B3LYP/6311++G(d,p) level. The triplet-sensitized photolysis pathways of SDZ with DOM analogues in solvent water were calculated employing DFT method at B3LYP/6-311+G(d,p) level of theory. Frequency calculations were done at the same level to confirm all the stationary points. Transition states were characterized with one imaginary vibrational frequency. Intrinsic reaction coordinate (IRC) calculations were performed to confirm that transition states do connect with the corresponding reactions and products. Zeropoint energy correction was considered for the estimated reaction activation energy. All the calculations were carried out using the Gaussian 09 software package (Frisch et al., 2009). 3. Results and discussion 3.1. Geometries of two dissociation species (SDZ0 and SDZ ) of SDZ in water As depicted in Fig. 1, SDZ has three dissociation species with pKa1 1.8 and pKa2 6.5 (S ß anli et al., 2010; Białk-Bielin´ska et al., 2012). Both the neutral (SDZ0) and the anionic (SDZ ) form are present in the natural water environment. The optimized geometries of SDZ0 and SDZ were presented in the Fig. 2. There are some differences between the geometries of SDZ0 and SDZ . For example, the bond length of C1AS in SDZ0 (1.77 Å) is a little shorter than that in SDZ (1.80 Å), and the bond lengths of SAN1 (1.70 Å) and N1AC2 (1.39 Å) in SDZ0 are a little longer than those (1.61 and 1.36 Å) in SDZ . The largest difference of dihedral angles C1ASAN1AC2, C3AC1ASAN1, and SAN1AC2AN4 between SDZ0 and SDZ reaches to 18.3°. The computed wavelengths at maximum electronic absorbance of SDZ0 and SDZ in water are 263.0 nm and 250.0 nm at TDDFT/B3LYP/6-311++G(d,p) level, respectively, which are in good agreement with the experimental data 263.6 nm in water (pH = 6.5) (Premakumari et al., 2011). 3.2. Triplet-sensitized photolysis mechanisms of SDZ0 Due to the fact that the energy of the lowest triplet excited state of DOM (57.4 kcal mol 1) (Zepp et al., 1985)/FL (54.7 kcal mol 1)/TX (62.0 kcal mol 1)/2-AN (59.4 kcal mol 1)/CBBP (68.8 kcal mol 1) is lower than that of SDZ (80.0 kcal mol 1 for SDZ0 and 77.7 kcal mol 1
NH2
NH2
NH3
pKa1
pKa2 N
N SO2
SO2
N H
SDZ+
N
N H
SO2
-N
N
SDZ0 Fig. 1. Dissociation species of sulfadiazine (SDZ) at different pH values.
N
N
SDZ-
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Fig. 2. Optimized geometries of SDZ0 and SDZ along with selected bond lengths (Å) and dihedral angles (°).
for SDZ at TDDFT/B3LYP/6-311++G(d,p) level), the energy transfer (from 3DOM/3FL/3TX/32-AN/3CBBP to SDZ) mechanism is impossible for the triplet-sensitized photolysis of SDZ with DOM or DOM analogues. Both the electron transfer along with proton transfer mechanism and the hydrogen transfer mechanism are possible. DOM molecules have many C@O groups (Diallo et al., 2003; Niederer and Goss, 2007). Hydrogen/proton from the NAH groups of SDZ0 may be abstracted by the C@O groups of DOM via the intermolecular hydrogen bonds formed between the C@O groups of DOM and NAH groups of SDZ0. In this study, FL, TX, 2-AN, and CBBP, which all have C@O groups, are selected as DOM analogues (Canonica et al., 1995; Lathioor and Leigh, 2006). As depicted in the Fig. 3, the C@O groups of these sensitizers can form intermolecular hydrogen bonds with NAH (N1AH1/N2AH2) groups of SDZ0. Two hydrogen-bonded complexes SDZ0-sensitizer1 and SDZ0-sensitizer2 (FL/TX/2-AN/CBBP) can be formed. As can be seen from Table 1, for complexes SDZ0-FL1/FL2/TX1/TX2/2-AN1/2-AN2/ CBBP1, there is no electron transfer between SDZ0 and sensitizer from ground state to the lowest triplet state, and the NBO charge of both SDZ0 and sensitizer is approximately 0. Then, in the lowest triplet state, the H1/H2 of SDZ0 is abstracted by the C@O group of sensitizer. In the products of hydrogen transfer reaction, the charges of both SDZ0-H and sensitizer+H are approximately 0, and the electron spin densities of both SDZ0-H and sensitizer+H are about 1. Hence, the products of hydrogen transfer reaction are radicals (SDZ0-H (a)/(b) and sensitizer+H) (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ0 with these sensitizers is hydrogen transfer, but not electron transfer. However, for the complex SDZ0-CBBP2, from ground state to the lowest triplet state electron transfer is occurring, and the electron is transferred from SDZ0 to CBBP (Table 1). The charge of CBBP is about 1 in the lowest triplet state, following with that the H2 proton transfer is occurring from SDZ0 to CBBP. In the products of H2 proton transfer reaction, the charges of SDZ0-H and CBBP+H are approximately 0, and the electron spin densities of SDZ0-H and CBBP+H are about 1. The products of H2 proton transfer reaction are SDZ0-H (b) radical and CBBP+H radical (Fig. 4). Hence, for complex SDZ0-CBBP2, the mechanism of triplet-sensitized photolysis of SDZ0 with CBBP is electron transfer along with proton transfer. In addition, the intermolecular hydrogen bond O H2 formed between C@O group of CBBP and N2AH2 of SDZ0 is shortened in the lowest triplet state (1.42 Å) compared with that in ground state (2.01 Å) (Fig. S1). Therefore, the intermolecular hydrogen bond O H2 is strengthened in the lowest triplet state. Zhao et al. reported that the strengthening of intermolecular hydrogen bond could facilitate the intermolecular electron transfer (Zhao et al., 2007). As can be seen in Fig. 4, the lowest and highest computed activation energies of the H1 transfer from SDZ0 to sensitizer are 0.8 kcal mol 1 (CBBP) and 12.8 kcal mol 1 (2-AN), respectively.
Radical SDZ0-H(a) is one of the two products of H1 transfer (Fig. 4). In addition, the computed activation energy of the H2 transfer from SDZ0 to 2-AN is 10.7 kcal mol 1, but the H2 proton transfer from SDZ0 to CBBP is a barrierless process. This may be due to the fact that in the lowest triplet state the proton is more easier to be abstracted by CBBP with a negative charge. Radical SDZ0-H(b) is one of the two products of H2 transfer (Fig. 4). In addition, the geometries of transition states are presented in Fig. S2, and the potential energy profiles of H/proton transfer are presented in Fig. S3. The formation of sulfur dioxide extrusion product 4-(2-iminopyrimidin-1(2H)-yl)aniline from SDZ0-H (a) and SDZ0-H (b) precursors are depicted in Fig. 5. The geometries of reaction intermediates and transition states are presented in Fig. S4. The formation of sulfur dioxide extrusion product from SDZ0-H (a) precursor has three pathways. Pathway 1 has two steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved; step 2, H in a water molecule (H donor) is abstracted by N1, and SAN1 bond is cleaved. The computed activation energies of step 1 and 2 are 9.3 and 20.1 kcal mol 1, respectively (Fig. 5). Thus, the step 2 is the rate-determining step for pathway 1. Pathways 2 and 3 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 65.9 and 35.3 kcal mol 1 (Fig. 5), respectively. Thus, pathways 2 and 3 are difficult to carry out. The formation of sulfur dioxide extrusion product from SDZ0-H (b) precursor has three pathways (Fig. 5). Pathway 4 has two steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved. N1AS bond is lengthened to 2.3 Å; step 2, H from a water molecule (H donor) is abstracted by N2 atom, and the sulfur dioxide extrusion product is formed. The computed activation energies of step 1 and 2 are 35.6 and 24.4 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the rate-determining step for pathway 4. Pathways 5 and 6 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 70.0 and 44.2 kcal mol 1, respectively (Fig. 5). Due to the high activation energies of pathways 4, 5 and 6, it is difficult for the formation of sulfur dioxide extrusion product from SDZ0-H (b) precursor. To sum up, pathway 1 is the main pathway of the formation of sulfur dioxide extrusion product from SDZ0-H (a)/(b). In addition, the computed activation energy of the rate-determining step (step 2) of pathway 1 (20.1 kcal mol 1) is higher than those of H/proton transfer reactions from SDZ0 to sensitizer (FL/TX/2-AN/CBBP) (the highest one is 12.8 kcal mol 1). Therefore, the rate-determining step of the triplet-sensitized photolysis of SDZ0 is the step 2 of pathway 1. 3.3. Effects of metal ions on triplet-sensitized photolysis of SDZ0 As discussed above, the rate-determining step of tripletsensitized photolysis of SDZ0 is the step 2 of pathway 1. The effects
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Fig. 3. Structural formulas of four DOM analogues and geometries of complexes SDZ0/SDZ -sensitizer (FL/TX/2-AN/CBBP) in ground state.
Table 1 NBO charge of SDZ0/SDZ -sensitizer (FL, TX, 2-AN, and CBBP) complexes in ground state and reactant and product of H or proton transfer reaction in the lowest triplet state, and electron spin density of product in parentheses. Complexes
Ground state SDZ
SDZ0-FL1 SDZ0-FL2 SDZ0-TX1 SDZ0-TX2 SDZ0-(2-AN1) SDZ0-(2-AN2) SDZ0-CBBP1 SDZ0-CBBP2 SDZ -FL SDZ -TX SDZ -(2-AN) SDZ -CBBP
0.01 0.02 0.05 0.01 0.03 0.01 0.01 0.02 1.03 1.01 1.01 1.02
Sensitizer 0.01 0.02 0.05 0.01 0.03 0.01 0.01 0.02 0.03 0.01 0.01 0.02
Reactant (lowest triplet state)
Product (lowest triplet state)
SDZ
SDZ-H
0.02 0.02 0.08 0.01 0.01 0.02 0.20 0.94 0.04 0.07 1.02 0.03
of metal ions on the rate-determining step (step 2 of pathway 1) were investigated. The step 2 of pathway 1 is the H transfer reaction from the water molecule to N1 of SDZ0. When the metal ion Mg2+/Ca2+/Zn2+ is added to the reactant of step 2 of pathway 1, three geometries (SDZ0-Mg2+/Ca2+/Zn2+(a), SDZ0-Mg2+/Ca2+/Zn2+(b), and
Sensitizer 0.02 0.02 0.08 0.01 0.01 0.02 0.2 0.94 0.96 0.93 0.02 0.97
0.01 0.00 0.03 0.01 0.03 0.01 0.01 0.02 1.01 0.98 1.00 0.98
Sensitizer+H (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.02)
0.01 0.00 0.03 0.01 0.03 0.01 0.01 0.02 0.01 0.02 0.00 0.02
(0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.98)
SDZ0-Mg2+/Ca2+/Zn2+(c)) are formed (Fig. S5). As seen in Fig. S5, the energy of SDZ0-Mg2+/Ca2+/Zn2+(a) is the lowest. Thus, SDZ0-Mg2+/ Ca2+/Zn2+(a) is as the reactant of step 2 with metal ion. The activation energy of the step 2 of pathway 1 without metal ion is 20.1 kcal mol 1 (Fig. 5), but the activation energy of the step 2 of
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Fig. 4. Hydrogen/proton transfer of triplet-sensitized photolysis of SDZ0/SDZ with sensitizers (FL, TX, 2-AN, and CBBP).
pathway 1with Mg2+/Ca2+/Zn2+ is reduced to 5.8/6.9/0.2 kcal mol 1 (Table 2). In addition, the effects of hydration of metal ion in water are also investigated. Our calculations indicated that the first shell of Mg2+/Ca2+/Zn2+ in complex SDZ0-Mg2+/Ca2+/Zn2+(a) can be fully occupied with five water molecules (Fig. S5). One of the five water molecules is the hydrogen donor for the H transfer reactions. As can be seen in Table 2, the activation energy of the step 2 of pathway 1 with Mg2+/Ca2+/Zn2+(H2O)5 is 9.3/12.8/3.6 kcal mol 1, respectively, which is slightly higher than that of with Mg2+/Ca2+/Zn2+ but is still lower than that of without metal ions. Therefore, the metal ion (Mg2+/Ca2+/Zn2+) reduced the reaction activation energy of the rate-determining step (step 2 of pathway 1) of triplet-sensitized photolysis of SDZ0. That is to say, the metal ion (Mg2+/Ca2+/Zn2+) promotes the triplet-sensitized photolysis of SDZ0. In addition, the geometries of the transition states of step 2 in pathway 1 with metal (Mg2+, Ca2+, and Zn2+) are presented in Fig. S6. 3.4. Triplet-sensitized photolysis mechanisms of SDZ As depicted in the Fig. 3, the C@O group of sensitizer (FL/TX/2-AN/ CBBP) can form intermolecular hydrogen bond with N2AH2 group of SDZ . A hydrogen-bonded complex SDZ -sensitizer (FL/TX/2-AN/CBBP) can be formed. As can be seen from Table 1, for complex SDZ -(2-AN), there is no electron transfer between SDZ and 2-AN from ground state to the lowest triplet state, and the charges of SDZ and 2-AN are approximately 1 and 0, respectively. In the lowest triplet state, the H2 atom of SDZ is abstracted by the C@O group of 2-AN. In the products of H2 transfer reaction,
the charges of SDZ -H and (2-AN)+H are approximately 1 and 0, respectively, and the electron spin densities of both SDZ -H and (2-AN)+H are about 1. Hence, the products of H2 transfer reaction are SDZ -H radical and (2-AN)+H radical (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ with 2-AN sensitizer is hydrogen transfer, but not electron transfer. However, for the complex SDZ -(FL/TX/CBBP) from ground state to the lowest triplet state electron transfer is occurring, and the electron is transferred from SDZ to FL/TX/CBBP (Table 1). The charge of FL/TX/ CBBP is about 1 in the lowest triplet state, following with that the H2 proton transfer is occurring from SDZ to FL/TX/CBBP. In the products of H2 proton transfer reaction, the charges of SDZ -H and FL/TX/CBBP+H are approximately 1 and 0, respectively, and the electron spin densities of SDZ -H and FL/TX/CBBP+H are about 1. Hence, the products of H2 proton transfer reaction are SDZ -H radical and sensitizer+H radical (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ with FL/TX/CBBP is electron transfer along with proton transfer. As can be seen in Fig. 4, the computed activation energy of the H2 atom transfer from SDZ to 2-AN is 11.4 kcal mol 1, and the computed activation energy of the H2 proton transfer from SDZ to CBBP is 0.8 kcal mol 1. In addition, the H2 proton transfer from SDZ to FL/TX is a barrierless process. This may be due to the fact that in the lowest triplet state the proton is more easier to be abstracted by FL/TX/CBBP with a negative charge. The product of H2 transfer is SDZ -H radical (Fig. 4). In addition, the potential energy profiles of H/proton transfer are presented in Fig. S3.
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Fig. 5. Formation routes of photolysis product 4-(2-iminopyrimidine-1(2H)-yl)aniline from the precursors SDZ0-H (a), SDZ0-H (b), and SDZ -H.
Table 2 Computed activation energies (Ea, kcal mol 1) for the step 2 of pathway 1 with metals (Mg2+, Ca2+, and Zn2+). Computed activation energies (Ea, kcal mol 1) for the step 1 of pathways 7 and 8 with metals (Mg2+, Ca2+, and Zn2+). Complexes
Ea
Ea (SDZ0/ -Metal2+(H2O)n)
SDZ0-Mg2+(a) SDZ0-Ca2+(a) SDZ0-Zn2+(a) SDZ -Mg2+(a) SDZ -Mg2+(b) SDZ -Ca2+(b) SDZ -Ca2+(c) SDZ -Zn2+(b)
5.8 6.9 0.2 26.8 37.5 33.1 32.7 38.4
9.3 (5H2O) 12.8 (5H2O) 3.6 (5H2O) 30.8 (4H2O) 30.9 (5H2O) 33.8 (5H2O) 31.2 (5H2O) 31.2 (5H2O)
The four pathways (pathway 7, 8, 9, and 10) of the formation of sulfur dioxide extrusion anion product from SDZ -H precursor are depicted in Fig. 5. The geometries of reaction intermediates and transition states are presented in Fig. S4. Pathway 7 has three steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved; step 2, H from a water molecule (H donor) is abstracted by N2; step 3, SAN1 bond is cleaved. The computed activation energies of step 1, 2, and 3 are 25.7, 25.1, and 8.1 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the rate-determining step for pathway 7. Pathway 8 also has three steps: step 1, C1AN4 bond is formed, and
C1AS bond is cleaved; step 2, SAN1 bond is cleaved; step 3, H from a water molecule (H donor) is abstracted by N2. The computed activation energies of step 1, 2, and 3 are 25.7, 25.1, and 9.9 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the ratedetermining step for pathway 8. Pathways 9 and 10 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 64.8 and 52.5 kcal mol 1 (Figs. 5 and S2), respectively. Thus, pathways 9 and 10 are difficult to carry out. In summary, pathways 7 and 8 are the main pathways of the formation of sulfur dioxide extrusion anion product from SDZ -H. Step 1 is the rate-determining step of pathway 7 and 8 (Fig. 5). In addition, the computed activation energies of the ratedetermining steps of pathway 7 and 8 (25.7 kcal mol 1) are higher than those of H/proton transfer reactions from SDZ to sensitizer (FL/TX/2-AN/CBBP) (the highest one is 11.4 kcal mol 1). Therefore, the rate-determining step of triplet-sensitized photolysis of SDZ is the step 1 of pathway 7/8.
3.5. Effects of metal ions on triplet-sensitized photolysis of SDZ As aforementioned, the rate-determining step of triplet-sensitized photolysis of SDZ is the step 1 of pathway 7/8. Hence, the effects of metal ions (Mg2+/Ca2+/Zn2+) on the rate-determining step (step 1 of pathway 7/8) were investigated. The step 1 of pathway
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7/8 is the formation of C1AN4 bond and the cleavage of C1AS bond. When the metal ion Mg2+/Ca2+/Zn2+ is added to the reactant of step 1 of pathway 7/8, five geometries (SDZ -Mg2+/Ca2+/Zn2+(a), SDZ -Mg2+/ Ca2+/Zn2+(b), SDZ -Mg2+/Ca2+/Zn2+(c), SDZ -Mg2+/Ca2+/Zn2+(d), and SDZ -Mg2+/Ca2+/Zn2+(e)) are formed (Fig. S7). As seen in Fig. S7, the energies of geometries (a) and (b) for SDZ -Mg2+, (b) and (c) for SDZ -Ca2+, and (b) for Zn2+ are the lowest. Thus, SDZ -Mg2+(a), SDZ -Mg2+(b), SDZ -Ca2+(b), SDZ -Ca2+(c), and SDZ -Zn2+(b) are as the reactants of step 1 of pathway 7/8 with metal ion. The activation energy of the step 1 of pathway 7/8 without metal ion is 25.7 kcal mol 1 (Fig. 5), but the activation energies of the step 1 of pathway 7/8 with Mg2+ are increased to 26.8 kcal mol 1 (a) and 37.5 kcal mol 1 (b) (Table 2). The activation energies of the step 1 of pathway 7/8 with Ca2+ are increased to 33.1 kcal mol 1 (b) and 32.7 kcal mol 1 (c) (Table 2). The activation energy of the step 1 of pathway 7/8 with Zn2+ is increased to 38.4 kcal mol 1 (b) (Table 2). In addition, the effects of hydration of metal ion in water are also investigated. As can be seen in Table 2, the activation energies of the step 2 of pathway 1 with Mg2+(a)(H2O)4, Mg2+(b)(H2O)5, Ca2+(b)(H2O)5, Ca2+(c)(H2O)5, and Zn2+(b)(H2O)5 are 30.8, 30.9, 33.8, 31.2, and 31.2 kcal mol 1, respectively, which are still higher than that (25.7 kcal mol 1) of without metal ions. Therefore, the metal ion (Mg2+/Ca2+/Zn2+) increased the reaction activation energy of the rate-determining step (step 1 of pathway 7/8) of triplet-sensitized photolysis of SDZ . That is to say, the metal ion (Mg2+/Ca2+/Zn2+) inhibits the triplet-sensitized photolysis of SDZ . In addition, the geometries of the transition states of step 1 in pathway 7/8 with metal (Mg2+, Ca2+, and Zn2+) are presented in Fig. S8.
4. Conclusion In this study, we provided an insight into the mechanisms of triplet-sensitized photolysis of SDZ at different dissociation forms with four DOM analogues (FL, TX, 2-AN, and CBBP) and metal ions effects. The triplet-sensitized photolysis mechanisms of SDZ0 with FL, TX, and 2-AN were hydrogen transfer; The triplet-sensitized photolysis mechanisms of SDZ0 with CBBP were electron transfer along with proton transfer (for complex SDZ0-CBBP2) and hydrogen transfer (for complex SDZ0-CBBP1); The triplet-sensitized photolysis mechanisms of SDZ with FL, TX, and CBBP were electron transfer along with proton transfer; and the triplet-sensitized photolysis mechanism of SDZ with 2-AN was hydrogen transfer. SDZ0 and SDZ had the same triplet-sensitized photolysis product, namely a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the product formation routs of SDZ0 and SDZ were different. In addition, the three metal ions (Mg2+/ Ca2+/Zn2+) promoted the triplet-sensitized photolysis of SDZ0, but inhibited the triplet-sensitized photolysis of SDZ .
Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant Nos. 21036006, 21137001, and 21373042) and the financial support of the Fundamental Research Funds for the Central Universities of China (Grant No. DUT13RC(3)013).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2014. 11.007.
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Elucidating triplet-sensitized photolysis mechanisms of sulfadiazine and metal ions effects by quantum chemical calculations Se Wang a, Xuedan Song a, Ce Hao a,⇑, Zhanxian Gao a, Jingwen Chen b, Jieshan Qiu a a b
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China
h i g h l i g h t s Triplet-sensitized photolysis mechanism of SDZ was electron transfer or H transfer. 0
SDZ and SDZ
showed different triplet-sensitized photolysis routs. 2+ 2+ promoted triplet-sensitized photolysis of SDZ0. 2+ 2+ 2+ Presence of Mg , Ca , or Zn inhibited triplet-sensitized photolysis of SDZ . 2+
Presence of Mg , Ca , or Zn
a r t i c l e
i n f o
Article history: Received 22 May 2014 Accepted 2 November 2014 Available online 12 December 2014 Handling Editor: Klaus Kümmerer Keywords: Sulfadiazine Triplet-sensitized photolysis Mechanism DFT Metal ions
a b s t r a c t Sulfadiazine (SDZ) mainly proceeds triplet-sensitized photolysis with dissolved organic matter (DOM) in the aquatic environment. However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully worked out. In this study, we investigated the mechanisms of triplet-sensitized photolysis of SDZ0 (neutral form) and SDZ (anionic form) with four DOM analogues, i.e., fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP), and three metal ions (i.e., Mg2+, Ca2+, and Zn2+) effects using quantum chemical calculations. Results indicated that the triplet-sensitized photolysis mechanism of SDZ0 with FL, TX, and 2-AN was hydrogen transfer, and with CBBP was electron transfer along with proton transfer (for complex SDZ0-CBBP2) and hydrogen transfer (for complex SDZ0-CBBP1). The triplet-sensitized photolysis mechanisms of SDZ with FL, TX, and CBBP was electron transfer along with proton transfer, and with 2-AN was hydrogen transfer. The triplet-sensitized photolysis product of both SDZ0 and SDZ was a sulfur dioxide extrusion product (4(2-iminopyrimidine-1(2H)-yl)aniline), but the formation routs of the products for SDZ0 and SDZ were different. In addition, effects of the metal ions on the triplet-sensitized photolysis of SDZ0 and SDZ were different. The metal ions promoted the triplet-sensitized photolysis of SDZ0, but inhibited the triplet-sensitized photolysis of SDZ . Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Antibiotics that fight bacterial infections are widely applied in aquaculture, animal husbandry, and medical treatment (Kümmerer, 2009). Due to their abuse, antibiotics are largely released into and frequently detected in the aquatic environment (Kümmerer, 2009; Zuccato et al., 2010). To date, antibiotics known as emerging pollutants are of particular concern due to their adverse effects such as inducing bacterial resistance (Yu et al., 2013) and the emergence of ‘‘superbugs’’ (Woodward, 2010). Sulfadiazine (SDZ) was found to be one of the most frequently detected ⇑ Corresponding author. Tel./fax: +86 411 84986335. E-mail address: [email protected] (C. Hao). http://dx.doi.org/10.1016/j.chemosphere.2014.11.007 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.
antibiotics in surface water (Wei et al., 2011; Na et al., 2013; Bayen et al., 2014). Recently a lot of attention has also been drawn to the studies on the behavior and fate of SDZ in the aquatic environment (Sukul et al., 2008; Anskjaer et al., 2013; Fang et al., 2014). Photolysis is found to be an important factor of determining the fate and behavior of organic pollutants including SDZ (Sabljic and Peijnenburg, 2001; Boreen et al., 2004, 2005; Sukul et al., 2008; Ge et al., 2009, 2010; Li et al., 2011; Wammer et al., 2011; Challis et al., 2013; Wei et al., 2013; Batchu et al., 2014). The wavelength at maximum electronic absorbance of SDZ is 263.6 nm in water (Premakumari et al., 2011). SDZ mainly proceeds indirect photolysis in the aquatic environment due to the fact that only sunlight with a wavelength >290 nm reaches the surface of the earth. Indirect photolysis may include reaction with transient
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excited species such as singlet oxygen (1O2), hydroxyl radical (OH), triplet excited state dissolved organic matter (3DOM), or other reactive species (Boreen et al., 2005). Boreen et al. (2005) reported that SDZ in natural water proceeds triplet-sensitized photolysis with DOM, and the primary product formed in the triplet-sensitized photolysis was identified as a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline). However, the mechanisms underlying the triplet-sensitized photolysis of SDZ with DOM have not been fully understood. In recent years, the issues on the combined pollution of organic pollutants and metal ions has been drawing a lot of attention. Mg2+ and Ca2+ occur at a high concentration and Zn2+ is as a metal ion pollutant in natural water environment (Hafuka et al., 2014). Contemporary studies suggest that metal ions e.g. Mg2+ and Ca2+ can have an effect on the photochemical behavior of organic pollutants. Werner et al. (2006) found that the direct photolysis rate constant of tetracycline in the presence of Ca2+ or Mg2+ is greater by a magnitude than that of tetracycline in the absence of Ca2+ and Mg2+. Martínez et al. (1996) also found that Ca2+ and Mg2+ had an impact on the photochemical properties of norfloxacin. Following environmental release there will be interactions between metal ions and SDZ. Therefore, effects of metal ions on the photolysis of SDZ are urgently needed to be investigated. Obtaining experimental data can be laborious, costly, and timeconsuming. Quantum chemistry calculations have been found to be efficient alternatives for predicting the environmental behavior and fate of organic pollutants, and for providing an important information on reaction intermediates or reactive species involved in chemical reactions that are difficult to be detected experimentally (Sabljic, 2001; Wang et al., 2012, 2014a,b; Kovacevic and Sabljic, 2013a,b). It was the purpose of this study to investigate the triplet-sensitized photolysis mechanisms of SDZ in water and three metal ions (Mg2+, Ca2+, and Zn2+) effects. Based on density functional theory (DFT), NBO charge and electron spin densities were calculated to study the triplet-sensitized photolysis mechanisms (electron transfer or hydrogen transfer) of SDZ with four DOM analogues, and the formation routes of photolysis products and the metal ions effects were calculated.
2. Computational methods SDZ was selected as a model compound (Fig. 1). The geometry optimization of all structures in solvent water was carried out using DFT (Kohn et al., 1996) and Becke’s three-parameter hybrid exchange function with Lee–Yang–Parr gradient-corrected correlation functional (B3LYP) (Becke, 1993) with 6-311+G(d,p) basis set. The polarized continuum model (PCM) was employed to consider the solvent effects in water. The UV absorbance spectra of SDZ in water was calculated using the time-dependent density functional
+
theory (TDDFT) at B3LYP/6-311++G(d,p) level (Burke et al., 2005; Zhao and Han, 2009, 2012). The atom charge was examined based on the natural bond orbital (NBO) scheme at B3LYP/6-311++G(d,p) level. Sensitizers fluorenone (FL), thioxanthone (TX), 2-acetonaphthone (2-AN), and 4-benzoylbenzoic acid (CBBP) were selected as DOM analogues to investigate the triplet-sensitized photolysis mechanisms of SDZ. The energies of the lowest triplet excited state of FL, TX, 2-AN, and CBBP were calculated at TDDFT/B3LYP/6311++G(d,p) level. The triplet-sensitized photolysis pathways of SDZ with DOM analogues in solvent water were calculated employing DFT method at B3LYP/6-311+G(d,p) level of theory. Frequency calculations were done at the same level to confirm all the stationary points. Transition states were characterized with one imaginary vibrational frequency. Intrinsic reaction coordinate (IRC) calculations were performed to confirm that transition states do connect with the corresponding reactions and products. Zeropoint energy correction was considered for the estimated reaction activation energy. All the calculations were carried out using the Gaussian 09 software package (Frisch et al., 2009). 3. Results and discussion 3.1. Geometries of two dissociation species (SDZ0 and SDZ ) of SDZ in water As depicted in Fig. 1, SDZ has three dissociation species with pKa1 1.8 and pKa2 6.5 (S ß anli et al., 2010; Białk-Bielin´ska et al., 2012). Both the neutral (SDZ0) and the anionic (SDZ ) form are present in the natural water environment. The optimized geometries of SDZ0 and SDZ were presented in the Fig. 2. There are some differences between the geometries of SDZ0 and SDZ . For example, the bond length of C1AS in SDZ0 (1.77 Å) is a little shorter than that in SDZ (1.80 Å), and the bond lengths of SAN1 (1.70 Å) and N1AC2 (1.39 Å) in SDZ0 are a little longer than those (1.61 and 1.36 Å) in SDZ . The largest difference of dihedral angles C1ASAN1AC2, C3AC1ASAN1, and SAN1AC2AN4 between SDZ0 and SDZ reaches to 18.3°. The computed wavelengths at maximum electronic absorbance of SDZ0 and SDZ in water are 263.0 nm and 250.0 nm at TDDFT/B3LYP/6-311++G(d,p) level, respectively, which are in good agreement with the experimental data 263.6 nm in water (pH = 6.5) (Premakumari et al., 2011). 3.2. Triplet-sensitized photolysis mechanisms of SDZ0 Due to the fact that the energy of the lowest triplet excited state of DOM (57.4 kcal mol 1) (Zepp et al., 1985)/FL (54.7 kcal mol 1)/TX (62.0 kcal mol 1)/2-AN (59.4 kcal mol 1)/CBBP (68.8 kcal mol 1) is lower than that of SDZ (80.0 kcal mol 1 for SDZ0 and 77.7 kcal mol 1
NH2
NH2
NH3
pKa1
pKa2 N
N SO2
SO2
N H
SDZ+
N
N H
SO2
-N
N
SDZ0 Fig. 1. Dissociation species of sulfadiazine (SDZ) at different pH values.
N
N
SDZ-
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Fig. 2. Optimized geometries of SDZ0 and SDZ along with selected bond lengths (Å) and dihedral angles (°).
for SDZ at TDDFT/B3LYP/6-311++G(d,p) level), the energy transfer (from 3DOM/3FL/3TX/32-AN/3CBBP to SDZ) mechanism is impossible for the triplet-sensitized photolysis of SDZ with DOM or DOM analogues. Both the electron transfer along with proton transfer mechanism and the hydrogen transfer mechanism are possible. DOM molecules have many C@O groups (Diallo et al., 2003; Niederer and Goss, 2007). Hydrogen/proton from the NAH groups of SDZ0 may be abstracted by the C@O groups of DOM via the intermolecular hydrogen bonds formed between the C@O groups of DOM and NAH groups of SDZ0. In this study, FL, TX, 2-AN, and CBBP, which all have C@O groups, are selected as DOM analogues (Canonica et al., 1995; Lathioor and Leigh, 2006). As depicted in the Fig. 3, the C@O groups of these sensitizers can form intermolecular hydrogen bonds with NAH (N1AH1/N2AH2) groups of SDZ0. Two hydrogen-bonded complexes SDZ0-sensitizer1 and SDZ0-sensitizer2 (FL/TX/2-AN/CBBP) can be formed. As can be seen from Table 1, for complexes SDZ0-FL1/FL2/TX1/TX2/2-AN1/2-AN2/ CBBP1, there is no electron transfer between SDZ0 and sensitizer from ground state to the lowest triplet state, and the NBO charge of both SDZ0 and sensitizer is approximately 0. Then, in the lowest triplet state, the H1/H2 of SDZ0 is abstracted by the C@O group of sensitizer. In the products of hydrogen transfer reaction, the charges of both SDZ0-H and sensitizer+H are approximately 0, and the electron spin densities of both SDZ0-H and sensitizer+H are about 1. Hence, the products of hydrogen transfer reaction are radicals (SDZ0-H (a)/(b) and sensitizer+H) (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ0 with these sensitizers is hydrogen transfer, but not electron transfer. However, for the complex SDZ0-CBBP2, from ground state to the lowest triplet state electron transfer is occurring, and the electron is transferred from SDZ0 to CBBP (Table 1). The charge of CBBP is about 1 in the lowest triplet state, following with that the H2 proton transfer is occurring from SDZ0 to CBBP. In the products of H2 proton transfer reaction, the charges of SDZ0-H and CBBP+H are approximately 0, and the electron spin densities of SDZ0-H and CBBP+H are about 1. The products of H2 proton transfer reaction are SDZ0-H (b) radical and CBBP+H radical (Fig. 4). Hence, for complex SDZ0-CBBP2, the mechanism of triplet-sensitized photolysis of SDZ0 with CBBP is electron transfer along with proton transfer. In addition, the intermolecular hydrogen bond O H2 formed between C@O group of CBBP and N2AH2 of SDZ0 is shortened in the lowest triplet state (1.42 Å) compared with that in ground state (2.01 Å) (Fig. S1). Therefore, the intermolecular hydrogen bond O H2 is strengthened in the lowest triplet state. Zhao et al. reported that the strengthening of intermolecular hydrogen bond could facilitate the intermolecular electron transfer (Zhao et al., 2007). As can be seen in Fig. 4, the lowest and highest computed activation energies of the H1 transfer from SDZ0 to sensitizer are 0.8 kcal mol 1 (CBBP) and 12.8 kcal mol 1 (2-AN), respectively.
Radical SDZ0-H(a) is one of the two products of H1 transfer (Fig. 4). In addition, the computed activation energy of the H2 transfer from SDZ0 to 2-AN is 10.7 kcal mol 1, but the H2 proton transfer from SDZ0 to CBBP is a barrierless process. This may be due to the fact that in the lowest triplet state the proton is more easier to be abstracted by CBBP with a negative charge. Radical SDZ0-H(b) is one of the two products of H2 transfer (Fig. 4). In addition, the geometries of transition states are presented in Fig. S2, and the potential energy profiles of H/proton transfer are presented in Fig. S3. The formation of sulfur dioxide extrusion product 4-(2-iminopyrimidin-1(2H)-yl)aniline from SDZ0-H (a) and SDZ0-H (b) precursors are depicted in Fig. 5. The geometries of reaction intermediates and transition states are presented in Fig. S4. The formation of sulfur dioxide extrusion product from SDZ0-H (a) precursor has three pathways. Pathway 1 has two steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved; step 2, H in a water molecule (H donor) is abstracted by N1, and SAN1 bond is cleaved. The computed activation energies of step 1 and 2 are 9.3 and 20.1 kcal mol 1, respectively (Fig. 5). Thus, the step 2 is the rate-determining step for pathway 1. Pathways 2 and 3 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 65.9 and 35.3 kcal mol 1 (Fig. 5), respectively. Thus, pathways 2 and 3 are difficult to carry out. The formation of sulfur dioxide extrusion product from SDZ0-H (b) precursor has three pathways (Fig. 5). Pathway 4 has two steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved. N1AS bond is lengthened to 2.3 Å; step 2, H from a water molecule (H donor) is abstracted by N2 atom, and the sulfur dioxide extrusion product is formed. The computed activation energies of step 1 and 2 are 35.6 and 24.4 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the rate-determining step for pathway 4. Pathways 5 and 6 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 70.0 and 44.2 kcal mol 1, respectively (Fig. 5). Due to the high activation energies of pathways 4, 5 and 6, it is difficult for the formation of sulfur dioxide extrusion product from SDZ0-H (b) precursor. To sum up, pathway 1 is the main pathway of the formation of sulfur dioxide extrusion product from SDZ0-H (a)/(b). In addition, the computed activation energy of the rate-determining step (step 2) of pathway 1 (20.1 kcal mol 1) is higher than those of H/proton transfer reactions from SDZ0 to sensitizer (FL/TX/2-AN/CBBP) (the highest one is 12.8 kcal mol 1). Therefore, the rate-determining step of the triplet-sensitized photolysis of SDZ0 is the step 2 of pathway 1. 3.3. Effects of metal ions on triplet-sensitized photolysis of SDZ0 As discussed above, the rate-determining step of tripletsensitized photolysis of SDZ0 is the step 2 of pathway 1. The effects
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Fig. 3. Structural formulas of four DOM analogues and geometries of complexes SDZ0/SDZ -sensitizer (FL/TX/2-AN/CBBP) in ground state.
Table 1 NBO charge of SDZ0/SDZ -sensitizer (FL, TX, 2-AN, and CBBP) complexes in ground state and reactant and product of H or proton transfer reaction in the lowest triplet state, and electron spin density of product in parentheses. Complexes
Ground state SDZ
SDZ0-FL1 SDZ0-FL2 SDZ0-TX1 SDZ0-TX2 SDZ0-(2-AN1) SDZ0-(2-AN2) SDZ0-CBBP1 SDZ0-CBBP2 SDZ -FL SDZ -TX SDZ -(2-AN) SDZ -CBBP
0.01 0.02 0.05 0.01 0.03 0.01 0.01 0.02 1.03 1.01 1.01 1.02
Sensitizer 0.01 0.02 0.05 0.01 0.03 0.01 0.01 0.02 0.03 0.01 0.01 0.02
Reactant (lowest triplet state)
Product (lowest triplet state)
SDZ
SDZ-H
0.02 0.02 0.08 0.01 0.01 0.02 0.20 0.94 0.04 0.07 1.02 0.03
of metal ions on the rate-determining step (step 2 of pathway 1) were investigated. The step 2 of pathway 1 is the H transfer reaction from the water molecule to N1 of SDZ0. When the metal ion Mg2+/Ca2+/Zn2+ is added to the reactant of step 2 of pathway 1, three geometries (SDZ0-Mg2+/Ca2+/Zn2+(a), SDZ0-Mg2+/Ca2+/Zn2+(b), and
Sensitizer 0.02 0.02 0.08 0.01 0.01 0.02 0.2 0.94 0.96 0.93 0.02 0.97
0.01 0.00 0.03 0.01 0.03 0.01 0.01 0.02 1.01 0.98 1.00 0.98
Sensitizer+H (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.01) (1.02)
0.01 0.00 0.03 0.01 0.03 0.01 0.01 0.02 0.01 0.02 0.00 0.02
(0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.99) (0.98)
SDZ0-Mg2+/Ca2+/Zn2+(c)) are formed (Fig. S5). As seen in Fig. S5, the energy of SDZ0-Mg2+/Ca2+/Zn2+(a) is the lowest. Thus, SDZ0-Mg2+/ Ca2+/Zn2+(a) is as the reactant of step 2 with metal ion. The activation energy of the step 2 of pathway 1 without metal ion is 20.1 kcal mol 1 (Fig. 5), but the activation energy of the step 2 of
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Fig. 4. Hydrogen/proton transfer of triplet-sensitized photolysis of SDZ0/SDZ with sensitizers (FL, TX, 2-AN, and CBBP).
pathway 1with Mg2+/Ca2+/Zn2+ is reduced to 5.8/6.9/0.2 kcal mol 1 (Table 2). In addition, the effects of hydration of metal ion in water are also investigated. Our calculations indicated that the first shell of Mg2+/Ca2+/Zn2+ in complex SDZ0-Mg2+/Ca2+/Zn2+(a) can be fully occupied with five water molecules (Fig. S5). One of the five water molecules is the hydrogen donor for the H transfer reactions. As can be seen in Table 2, the activation energy of the step 2 of pathway 1 with Mg2+/Ca2+/Zn2+(H2O)5 is 9.3/12.8/3.6 kcal mol 1, respectively, which is slightly higher than that of with Mg2+/Ca2+/Zn2+ but is still lower than that of without metal ions. Therefore, the metal ion (Mg2+/Ca2+/Zn2+) reduced the reaction activation energy of the rate-determining step (step 2 of pathway 1) of triplet-sensitized photolysis of SDZ0. That is to say, the metal ion (Mg2+/Ca2+/Zn2+) promotes the triplet-sensitized photolysis of SDZ0. In addition, the geometries of the transition states of step 2 in pathway 1 with metal (Mg2+, Ca2+, and Zn2+) are presented in Fig. S6. 3.4. Triplet-sensitized photolysis mechanisms of SDZ As depicted in the Fig. 3, the C@O group of sensitizer (FL/TX/2-AN/ CBBP) can form intermolecular hydrogen bond with N2AH2 group of SDZ . A hydrogen-bonded complex SDZ -sensitizer (FL/TX/2-AN/CBBP) can be formed. As can be seen from Table 1, for complex SDZ -(2-AN), there is no electron transfer between SDZ and 2-AN from ground state to the lowest triplet state, and the charges of SDZ and 2-AN are approximately 1 and 0, respectively. In the lowest triplet state, the H2 atom of SDZ is abstracted by the C@O group of 2-AN. In the products of H2 transfer reaction,
the charges of SDZ -H and (2-AN)+H are approximately 1 and 0, respectively, and the electron spin densities of both SDZ -H and (2-AN)+H are about 1. Hence, the products of H2 transfer reaction are SDZ -H radical and (2-AN)+H radical (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ with 2-AN sensitizer is hydrogen transfer, but not electron transfer. However, for the complex SDZ -(FL/TX/CBBP) from ground state to the lowest triplet state electron transfer is occurring, and the electron is transferred from SDZ to FL/TX/CBBP (Table 1). The charge of FL/TX/ CBBP is about 1 in the lowest triplet state, following with that the H2 proton transfer is occurring from SDZ to FL/TX/CBBP. In the products of H2 proton transfer reaction, the charges of SDZ -H and FL/TX/CBBP+H are approximately 1 and 0, respectively, and the electron spin densities of SDZ -H and FL/TX/CBBP+H are about 1. Hence, the products of H2 proton transfer reaction are SDZ -H radical and sensitizer+H radical (Fig. 4). That is to say, the mechanism of triplet-sensitized photolysis of SDZ with FL/TX/CBBP is electron transfer along with proton transfer. As can be seen in Fig. 4, the computed activation energy of the H2 atom transfer from SDZ to 2-AN is 11.4 kcal mol 1, and the computed activation energy of the H2 proton transfer from SDZ to CBBP is 0.8 kcal mol 1. In addition, the H2 proton transfer from SDZ to FL/TX is a barrierless process. This may be due to the fact that in the lowest triplet state the proton is more easier to be abstracted by FL/TX/CBBP with a negative charge. The product of H2 transfer is SDZ -H radical (Fig. 4). In addition, the potential energy profiles of H/proton transfer are presented in Fig. S3.
S. Wang et al. / Chemosphere 122 (2015) 62–69
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Fig. 5. Formation routes of photolysis product 4-(2-iminopyrimidine-1(2H)-yl)aniline from the precursors SDZ0-H (a), SDZ0-H (b), and SDZ -H.
Table 2 Computed activation energies (Ea, kcal mol 1) for the step 2 of pathway 1 with metals (Mg2+, Ca2+, and Zn2+). Computed activation energies (Ea, kcal mol 1) for the step 1 of pathways 7 and 8 with metals (Mg2+, Ca2+, and Zn2+). Complexes
Ea
Ea (SDZ0/ -Metal2+(H2O)n)
SDZ0-Mg2+(a) SDZ0-Ca2+(a) SDZ0-Zn2+(a) SDZ -Mg2+(a) SDZ -Mg2+(b) SDZ -Ca2+(b) SDZ -Ca2+(c) SDZ -Zn2+(b)
5.8 6.9 0.2 26.8 37.5 33.1 32.7 38.4
9.3 (5H2O) 12.8 (5H2O) 3.6 (5H2O) 30.8 (4H2O) 30.9 (5H2O) 33.8 (5H2O) 31.2 (5H2O) 31.2 (5H2O)
The four pathways (pathway 7, 8, 9, and 10) of the formation of sulfur dioxide extrusion anion product from SDZ -H precursor are depicted in Fig. 5. The geometries of reaction intermediates and transition states are presented in Fig. S4. Pathway 7 has three steps: step 1, C1AN4 bond is formed, and C1AS bond is cleaved; step 2, H from a water molecule (H donor) is abstracted by N2; step 3, SAN1 bond is cleaved. The computed activation energies of step 1, 2, and 3 are 25.7, 25.1, and 8.1 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the rate-determining step for pathway 7. Pathway 8 also has three steps: step 1, C1AN4 bond is formed, and
C1AS bond is cleaved; step 2, SAN1 bond is cleaved; step 3, H from a water molecule (H donor) is abstracted by N2. The computed activation energies of step 1, 2, and 3 are 25.7, 25.1, and 9.9 kcal mol 1, respectively (Fig. 5). Thus, the step 1 is the ratedetermining step for pathway 8. Pathways 9 and 10 are the cleavages of C1AS bond and SAN1 bond, of which the computed activation energies are 64.8 and 52.5 kcal mol 1 (Figs. 5 and S2), respectively. Thus, pathways 9 and 10 are difficult to carry out. In summary, pathways 7 and 8 are the main pathways of the formation of sulfur dioxide extrusion anion product from SDZ -H. Step 1 is the rate-determining step of pathway 7 and 8 (Fig. 5). In addition, the computed activation energies of the ratedetermining steps of pathway 7 and 8 (25.7 kcal mol 1) are higher than those of H/proton transfer reactions from SDZ to sensitizer (FL/TX/2-AN/CBBP) (the highest one is 11.4 kcal mol 1). Therefore, the rate-determining step of triplet-sensitized photolysis of SDZ is the step 1 of pathway 7/8.
3.5. Effects of metal ions on triplet-sensitized photolysis of SDZ As aforementioned, the rate-determining step of triplet-sensitized photolysis of SDZ is the step 1 of pathway 7/8. Hence, the effects of metal ions (Mg2+/Ca2+/Zn2+) on the rate-determining step (step 1 of pathway 7/8) were investigated. The step 1 of pathway
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7/8 is the formation of C1AN4 bond and the cleavage of C1AS bond. When the metal ion Mg2+/Ca2+/Zn2+ is added to the reactant of step 1 of pathway 7/8, five geometries (SDZ -Mg2+/Ca2+/Zn2+(a), SDZ -Mg2+/ Ca2+/Zn2+(b), SDZ -Mg2+/Ca2+/Zn2+(c), SDZ -Mg2+/Ca2+/Zn2+(d), and SDZ -Mg2+/Ca2+/Zn2+(e)) are formed (Fig. S7). As seen in Fig. S7, the energies of geometries (a) and (b) for SDZ -Mg2+, (b) and (c) for SDZ -Ca2+, and (b) for Zn2+ are the lowest. Thus, SDZ -Mg2+(a), SDZ -Mg2+(b), SDZ -Ca2+(b), SDZ -Ca2+(c), and SDZ -Zn2+(b) are as the reactants of step 1 of pathway 7/8 with metal ion. The activation energy of the step 1 of pathway 7/8 without metal ion is 25.7 kcal mol 1 (Fig. 5), but the activation energies of the step 1 of pathway 7/8 with Mg2+ are increased to 26.8 kcal mol 1 (a) and 37.5 kcal mol 1 (b) (Table 2). The activation energies of the step 1 of pathway 7/8 with Ca2+ are increased to 33.1 kcal mol 1 (b) and 32.7 kcal mol 1 (c) (Table 2). The activation energy of the step 1 of pathway 7/8 with Zn2+ is increased to 38.4 kcal mol 1 (b) (Table 2). In addition, the effects of hydration of metal ion in water are also investigated. As can be seen in Table 2, the activation energies of the step 2 of pathway 1 with Mg2+(a)(H2O)4, Mg2+(b)(H2O)5, Ca2+(b)(H2O)5, Ca2+(c)(H2O)5, and Zn2+(b)(H2O)5 are 30.8, 30.9, 33.8, 31.2, and 31.2 kcal mol 1, respectively, which are still higher than that (25.7 kcal mol 1) of without metal ions. Therefore, the metal ion (Mg2+/Ca2+/Zn2+) increased the reaction activation energy of the rate-determining step (step 1 of pathway 7/8) of triplet-sensitized photolysis of SDZ . That is to say, the metal ion (Mg2+/Ca2+/Zn2+) inhibits the triplet-sensitized photolysis of SDZ . In addition, the geometries of the transition states of step 1 in pathway 7/8 with metal (Mg2+, Ca2+, and Zn2+) are presented in Fig. S8.
4. Conclusion In this study, we provided an insight into the mechanisms of triplet-sensitized photolysis of SDZ at different dissociation forms with four DOM analogues (FL, TX, 2-AN, and CBBP) and metal ions effects. The triplet-sensitized photolysis mechanisms of SDZ0 with FL, TX, and 2-AN were hydrogen transfer; The triplet-sensitized photolysis mechanisms of SDZ0 with CBBP were electron transfer along with proton transfer (for complex SDZ0-CBBP2) and hydrogen transfer (for complex SDZ0-CBBP1); The triplet-sensitized photolysis mechanisms of SDZ with FL, TX, and CBBP were electron transfer along with proton transfer; and the triplet-sensitized photolysis mechanism of SDZ with 2-AN was hydrogen transfer. SDZ0 and SDZ had the same triplet-sensitized photolysis product, namely a sulfur dioxide extrusion product (4-(2-iminopyrimidine-1(2H)-yl)aniline), but the product formation routs of SDZ0 and SDZ were different. In addition, the three metal ions (Mg2+/ Ca2+/Zn2+) promoted the triplet-sensitized photolysis of SDZ0, but inhibited the triplet-sensitized photolysis of SDZ .
Acknowledgements This study was supported by the National Natural Science Foundation of China (Grant Nos. 21036006, 21137001, and 21373042) and the financial support of the Fundamental Research Funds for the Central Universities of China (Grant No. DUT13RC(3)013).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere.2014. 11.007.
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