Transformation of Triclosan by Fe (III)-Saturated Montmorillonite

Dec 14, 2009 - Abiotic transformation of triclosan (TCS) was investigated by incubating TCS with Fe(III)r and Narmontmorillonite at 40% relative humid...
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Environ. Sci. Technol. 2010, 44, 668–674

Transformation of Triclosan by Fe(III)-Saturated Montmorillonite CHAMINDU LIYANAPATIRANA,† STEVEN R. GWALTNEY,† AND K A N G X I A * ,†,‡ Department of Chemistry and Mississippi State Chemical Laboratory, P.O. Box CR, Mississippi State University, Mississippi State, Mississippi 39762

Received July 6, 2009. Revised manuscript received December 1, 2009. Accepted December 2, 2009.

Abiotic transformation of triclosan (TCS) was investigated by incubating TCS with Fe(III)- and Na-montmorillonite at 40% relative humidity and room temperature for up to 100 days. The TCS transformation products were characterized using LC/ MS, GC/MS, and computational modeling and quantified using HPLC/UV and GC/MS. Within 1-5 days, depending on the initial TCS concentrations, about 55% of the TCS was rapidly transformed in the presence of Fe(III)-montmorillonite, producing 2,4-dichlorophenol, 3-chlorophenol, 2,4-dichlorophenol dimer, chlorophenoxy phenols, and TCS dimers and trimers. Computational modeling based on density functional theory confirmed the formation of four TCS dimer conformers and six TCS trimer conformers. The TCS phenoxy radicals, produced by Fe(III) oxidation of TCS, react with other TCS molecules to form TCS dimers. The TCS trimers were formed by attachment of TCS dimer phenoxy radicals, produced by Fe(III) oxidation of TCS dimers, with TCS molecules. Significantly smaller quantities of TCS transformation products were detected in the reactions with Na-montmorillonite compared to the reactions with Fe(III)-montmorillonite. Formation of a significant amount of 2,4dichlorophenol, especially in reaction with Fe(III)-montmorillonite, may have negative impact on the environment because of its toxicity. However, mineral-facilitated TCS polymerization may reduce its mobility and bioavailability in soils.

Introduction Triclosan (TCS) is an antibacterial agent that is widely added at as high as 0.5-5 wt % in many consumer products (1). It has been frequently detected in water, sediments, soils, aquatic organisms, and even in human milk and urine (2-4). The main sources of TCS in the environment are effluents and biosolids generated by wastewater treatment plants (5). TCS tends to bioaccumulate and is potentially toxic toward aquatic organisms such as algae, frogs, and fish (6-8). TCS can be transformed via photolysis, oxidation, and biodegradation (9-11). Under natural sunlight or UV light in aqueous condition, TCS can be transformed to form 2,7/ 2,8-dichlorodibenzo-p-dioxins (12), 2,4-diclorophenol (13), 2,4,6-trichlorophenol, chloroform (14), and dimers and trimers (15). Methyltriclosan, which is more lipophilic and * Corresponding author phone: 662-325-5896; fax: 662-325-7807; e-mail: [email protected]. † Department of Chemistry. ‡ Mississippi State Chemical Laboratory. 668

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more bioaccumulative than TCS, is the main biodegradation product of TCS (16). An early report has shown oxidative transformation of TCS in aqueous medium by manganese oxides into p-quinine, p-hydroquinone, and trace amounts of 2,4-dichlorophenols and TCS dimers (17). However, photocatalytic oxidation of TCS with TiO2 in aqueous phase only yielded p-quinine, p-hydroquinone, and a trace amount of 2,4-dichlorophenols but no dimers (18). Besides the metal oxides, ozone can react with TCS in aqueous phase via direct oxidation of the TCS molecule by O3, yielding oxygen addition to its phenol ring or phenol ring-opening (5). Once entering the environment via biosolids application, its reactions with clay minerals in soils may significantly impact the environmental fate of TCS. Although much research has been done on TCS transformation in water, very little is known about its abiotic transformation in soils. Our previous investigation has suggested the importance of abiotic transformation for TCS in biosolids-amended soils (19) Montmorillonite, a layered 2:1 aluminosilicate clay mineral, is widely distributed in soils and sediments of temperate and cold climates (20). Due to isomorphic substitution, montmorillonite tends to possess negative charges in its layered structures, resulting in its high cation exchange capacity, excellent swelling capacity, and large specific surface area (20). Gu and co-workers demonstrated that under relatively dehydrated conditions (5-18% relative humidity) a single electron could be transferred from a pentachlorophenol (PCP) molecule to an interlayer Fe(III) cation in montmorillonite, thereby forming a PCP radical cation (21). The formation of PCP radical cations resulted in subsequent dimerization, dechlorination, and ring-closure reactions and eventually the formation of octachlorodibenzop-dioxin. Similar radical formation and polymerization under dehydrated condition were also reported for a variety of aromatic molecules, including PCP in the presence of montmorillonite saturated with Cu(II) as interlayer cations (22). The objective of this study was to understand the natural and Fe(III)-modified montmorillonite-facilitated abiotic transformation of TCS at ambient room temperature and relative humidity.

Materials and Methods Experimental Setup. Detailed information about the chemicals used for this experiment and procedures for preparing the Fe(III)-saturated montmorillonite can be found in section S1 of the Supporting Information. Two milliliters of acetone containing an appropriate amount of TCS was added separately to 20 mL clear glass vials containing 500 mg of Na-montmorillonite, 500 mg of Fe(III)-montmorillonite, 0.0433 g of FeCl3 [the amount of Fe(III) equivalent to that sorbed in the montmorillonite interlayer], and empty control vials. Two initial TCS concentrations (0.3 and 0.01 mmol TCS/g of mineral) were used for the experiment. A concentration of 0.01 mmol TCS g-1 mineral would provide approximately 50% surface coverage, assuming that the TCS molecules are aligned parallel to the surface. A concentration of 0.3 mmol TCS g-1 mineral would saturate the mineral surface with TCS. Those two concentration levels are several orders of magnitude higher than that in the biosolids and biosolids-amended soils. However, concentrations higher than what can be found for a target compound in the environment are often used in the literature for this type of mechanistic investigation on transformation products and pathways (21). The same amount of TCS was added to the vials containing FeCl3 and the empty control vials. Immediately after the addition of TCS to each vial, the contents 10.1021/es902003f

 2010 American Chemical Society

Published on Web 12/14/2009

in each vial were homogenized before the acetone was completely evaporated under a fume hood. There were three replicates for each treatment. Three 20 mL glass vials containing 500 mg of Fe(III)-montmorillonite without TCS were set up as an additional control. The samples were kept in darkness at room temperature and relative humidity of 40% until sacrificed for analysis of TCS and its transformation products at 1, 3, 12, 30, and 100 d. Detailed information on analytes extraction and transformation products identification and quantification using GC/MS, LC/MS, and HPLC/ UV can be found in the Supporting Information. Computational Study. Geometry optimizations with the B3LYP (23, 24) density functional theory and the 6-31G** basis set (25, 26) were performed for TCS and for some of its transformation products. For each system an AM1 conformational search was performed, and the lowest energy conformer was chosen for the DFT calculations. The surface area and the volume were also calculated. On the basis of these data, relative energies and enthalpies of reactions were calculated for the various products. Q-Chem 3.0 (27) was used to perform the calculations. Gas-phase models were used in our modeling. It is important to point out that although gas-phase models would be inappropriate to describe the surface reactions themselves, our calculations are not intended to be accurate representations of the mechanisms of the reactions. Instead, we were modeling the thermodynamic stability of the various possible TCS transformation products. In this case, solvation is unlikely to drastically modify the relative energies of the TCS transformation products considered, and relative energies were being used to evaluate the stability of each product.

Results and Discussion TCS Transformation. In the absence of montmorillonite, the TCS transformation was not significant (data not shown). At the higher initial TCS concentration (0.3 mmol TCS g-1 mineral), with the presence of Fe(III)-montmorillonite, a rapid TCS transformation (45% reduction) was detected within the first 5 days, followed by a slower transformation up to 100 days (Figure 1). At the same initial TCS loading, less than 5% of the initially added TCS was transformed in the presence of Na-montmorillonite. In the presence of Fe(III)-montmorillonite, a faster initial TCS transformation (58% reduction within 1 day) was observed at the lower initial TCS concentration compared to the higher TCS level (Figure 1). After the initial rapid TCS transformation, the loss of TCS progressed slowly with time. Significantly higher TCS transformation was observed in the presence of Na-montmorillonite at the lower initial TCS concentration compared to the treatment with higher initial TCS level. However, the TCS transformation in the presence of Na-montmorillonite within the first 5 days was significantly slower compared to the treatment with Fe(III)-montmorillonite (Figure 1). After 100 days of reaction, there was about 80% TCS loss in reactions treated with Na-montmorillonite or Fe(III)-montmorillonite. The increasing Fe(II) concentration (Figure S2-1, Supporting Information) with time in reactions with Fe(III)-montmorillonite suggests that TCS was oxidized by Fe(III). Previous research has suggested that oxidation of an aromatic molecule at mineral surfaces containing metal oxidants such as Fe(III), Cu(II), and Mn(IV) occurs via electron transfer within a precursor complex of the aromatic reductant and the metal oxidant, followed by the release of organic oxidative transformation products and reduced metal ions or further reactions, producing dimers and trimers (17, 21, 22, 28-30). Those studies have also suggested that the precursor complex formation and electron transfer are mainly facilitated and enhanced by mineral surface chemistry. In our study only approximately 10% of the added TCS was transformed within

FIGURE 1. TCS transformation with time for treatments with initial TCS concentrations of 0.3 and 0.01 mmol TCS g-1 mineral. TCS was reacted with Fe(III)- and Na-montmorillonite. Cf is the final TCS concentration at each time interval, and Ci is the initial TCS concentration. 100 days when it was mixed with FeCl3 without the presence of montmorillonite. It was apparent that oxidation of TCS by Fe(III) was significantly enhanced at the interlayer surface of montmorillonite. The Na-montnorillonite (SWy-2) naturally contains structural Fe(III) evenly distributed in the octahedral layers of the mineral at a concentration of approximately 1.2 mmol Fe(III) g-1 mineral (31). Our results have suggested that the structural Fe(III) in the Na-montnorillonite was also capable of oxidizing TCS. This was more obvious at the initial TCS concentration of 0.01 mmol TCS g-1 mineral than at the higher initial TCS concentration (Figure 1). The observed significant TCS transformation rate dependence on initial TCS concentration in the presence of Na-montmorillonite might be due to the limited quantity of Fe(III) as oxidants at the higher initial TCS concentration (0.3 mmol TCS g-1 mineral). The TCS transformation rate in the reaction with Na-montnorillonite was much slower compared to the reaction with Fe(III)-saturated montmorillonite, which contains a higher amount of easily accessible Fe(III) (0.53 mmol Fe(III) g-1 mineral) at the mineral interlayer space. TCS Transformation Products. The 2,4-dichlorophenol was detected for TCS reactions with Fe(III)- and Na-montmorillonite (Figure S2-2, Supporting Information). Twelve distinct TCS product peaks appeared in the HPLC chromatograms of extractants from reactions with an initial concentration of 0.3 mmol TCS g-1 Fe(III)-montmorillonite (Figure 2a). A significant peak at 13.6 min (peak A) was also observed in addition to the above-mentioned 12 peaks for treatments with 0.01 mmol TCS g-1 mineral (Figure 2b). Compared to reactions with Fe(III)-montmorillonite, fewer product peaks were observed for reactions with Na-montmorillonite (Figure 2). The LC/MS molecular ion stable isotope cluster patterns, the number of estimated Cl atoms/ molecule, and the molecular mass of those products indicate the formation of TCS dimers and trimers, 2,4-dichlorophenol dimer, and two dichlorophenoxy phenol compounds (Table 1). The product peak A (Figure 2b) is 2,4-dichlorophenol dimer with a molecular mass of 322 (Table 1). Ukrainczyk VOL. 44, NO. 2, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. HPLC chromatograms of 30-day reactions at initial TCS concentrations of (a) 0.3 mmol TCS g-1 mineral and (b) 0.01 mmol TCS g-1 mineral. TCS was reacted with Fe(III)- and Na-montmorillonite.

TABLE 1. TCS Transformation Products Analyzed by LC/MS molecular ion stable isotope clusterb (m/z increment of 2 starting with [M - H]-) peaka

m/z

A 1 2 3-6 7-12

321, 323, 325, 327, 447. 449. 451, 453, 413, 415, 417, 419, 573, 575, 577, 579, 859, 861, 863, 865, 873, 875, 877

329 455, 456.6 421 581, 583, 585 867, 869, 871,

stable isotope ratio

number of Cl atoms

molecular mass

compoundsc

71:100:37:1:0.1 59:100:48:7:2:0.3 66:100:48:6:0.6 47:100:77:29:11:2:0.2 19:36:100:61:33:10:2:3:2:2

4 5 4 6 9

322 448 414 574 860

product A product B product C TCS dimers TCS trimers

a Peak numbers correspond to the peaks listed in Figure 2. b Mass spectra are shown in Figure S2-3 (Supporting Information). c See Figure 3 for chemical structures and Figure S2-5 (Supporting Information) for 3-D structures. Product A, 2,4-dichlorophenol dimer; product B, 3-chloro-2-(2,3-dichlorophenoxy)-6-(2,4-dichlorophenoxy)phenol; product C, 3-chloro2-(3-chlorophenoxy)-6-(2,4-dichlorophenoxy)phenol.

and McBride observed formation of 2,4-dichlorophenol dimer when 2,4-dichlorophenol was oxidized by manganese oxide (32). At higher TCS concentrations, product A only appeared at day 100 in the reactions with Fe(III)-montmorillonite. Products B and C shown as HPLC peaks 1 and 2 in Figure 2 have molecular masses of 448 and 414, respectively (Table 1). These two products were formed by reactions between TCS and 2,4-dichlorophenol or 3-chlorophenol; both are ether bond cleavage products of a TCS molecule, resulting in 3-chloro-2-(2,3-dichlorophenoxy)-6-(2,4-dichlorophenoxy)phenol and 3-chloro-2-(3-chlorophenoxy)-6-(2,4-dichlorophenoxy)phenol. This reaction did not occur in the presence of Na-montmorillonite (Figure 2a,b), likely a result of less cleavage of TCS ether bond compared to reactions with Fe(III)-montmorillonite. Product peaks 3-6 (Figure 2a,b) were four TCS dimer conformers with molecular masses of 574 (Table 1). TCS dimer formation was also observed in earlier photolysis 670

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and oxidation studies (15, 29, 33). The TCS dimer formation might occur via addition of a TCS phenoxy radical to each of the six nonsubstituted carbons in a second TCS molecule (Table 2), creating theoretically six possible conformers. However, only four TCS dimers were detected (Figure 2, Table 1). Assuming the loss of the phenolic hydrogen after the TCS was oxidize by Fe(III) to form free radical, computational modeling suggested that the unpaired electron was delocalized over the phenolic oxygen, the ortho and para carbons, and the ether oxygen (Figure S24, Supporting Information). The strong electron resonance within the molecule predicted by the modeling confirms the stability of the phenoxy radical. The computeroptimized energy and geometry of each possible TCS dimer conformer show that conformers 1 and 2 have the lowest energies, followed by conformers 4 and 5, with slightly higher energies (Table 2). Due to steric and resonance effects, the energies of conformers 3 and 6 were significantly

FIGURE 3. Transformation pathways of TCS reaction with Fe(III)-montmorillonite. Products A, B, C, and TCS dimers and trimers correspond to HPLC peaks A, 1, 2, 3-6, and 7-12, respectively, as shown in Figure 2. higher than the other four conformers. Therefore, TCS dimer conformers 1, 2, 4, and 5 were more likely to form than conformers 3 and 6 (Table 2). Our calculations agree with the HPLC chromatograms, which show two high intensity dimer peaks (peaks 4 and 5) and two lower intensity dimer peaks (peaks 3 and 6). Theoretically, a conformer with lower energy would be more likely to form and, therefore, produce a HPLC peak with higher intensity, because all the conformer molecules should have very similar UV absorbance. However, because of the inherent uncertainties in the calculated energies, it is impossible to differentiate between conformers 1 and 2, with an energy difference of 0.39 kcal mol-1, and between conformers 4 and 5, with an energy difference of 0.34 kcal mol-1 (Table 2). The energies of conformers 4 and 5 are approximately

1 kcal mol-1 higher than those of conformers 1 and 2. It is therefore reasonable to speculate that the two TCS dimers appearing at peaks 4 and 5 and the two appearing at peaks 3 and 6 (Figure 2) are conformers 1 or 2 and 4 or 5 (Table 2, Figure S2-5, Supporting Information), respectively. Significantly more product peaks (7-12) were observed for reactions with Fe(III)-montmorillonite compared to Na-montmorillonite (Figure 2a,b). Product peaks 7-12 had 10 molecular ion stable isotope mass peaks, indicating the existence of nine chlorine atoms per molecule (Table 1). The molecular ion stable isotope cluster patterns for those six products were similar and suggest the formation of six TCS trimer conformers with molecular masses of 860. Dioxin trimer formation was previously observed on Cu(II)smectite (29). Our computational modeling predicted that VOL. 44, NO. 2, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Molecular Energies for TCS, Six TCS Dimer Conformers, Phenoxy Radical of TCS Dimer Conformer 2, and One TCS Trimera

a Coordinates for TCS, all six dimer conformers, and the lowest energy TCS trimer are listed in Table S3-1 of the Supporting Information. * The numbers on the TCS molecule correspond to the conformer numbers listed in the first column. # The numbers on the TCS molecule correspond to the conformer numbers listed in the first column. $ Lowest energy TCS trimer compared to other TCS trimers.

the TCS trimer formation occurred via addition of a TCS dimer phenoxy radical to a neutral TCS molecule because the energy required for this reaction was estimated to be 3.1 kcal mol-1 lower than addition of a TCS radical to a neutral TCS dimer. Similar to the formation of TCS dimers, a TCS trimer may form via addition of a dimer phenoxy radical to each of the six nonsubstituted carbons in a third TCS molecule (Table 2), creating 36 TCS trimer conformers. However, only six TCS trimer peaks were observed (Figure 2). On the basis of the calculated dimer conformer energies shown in Table 2, the phenoxy radicals from conformers 1 and 2 are more likely candidates to form trimers with a third TCS molecule, because the energies of those two dimers are the lowest of the six dimer conformers. The higher the energy of a dimer conformer, the higher steric hindrance it has, making it less likely for a third TCS molecule to approach the dimer phenoxy radical. Furthermore, according to the optimized geometries, TCS dimer conformer 2 has more open space around the reactive phenoxy radical than dimer conformer 1, making it easier for another TCS molecule to attach to the former (Figure S2-5, Supporting Information). It is reasonable to assume that peaks 7-12 were TCS trimer conformers formed from attachment of TCS dimer conformer 2 to each of the six carbons in a TCS molecule (Table 2). When considering the optimized spatial arrangement of a TCS molecule (Table 2), our computer simulations showed that nonsubstituted carbon at the 2-position had the most open space of the nonsubstituted carbons. Hence, attachment of dimer conformer 2 phenoxy radical to the nonsubstituted carbon at the 2-position of a third TCS molecule is likely to produce the trimer product (Figure S2-5, Supporting Information) with the lowest energy. This trimer conformer would most likely be peak 10, which has the highest peak intensity among the six trimer peaks (Figure 2). On the basis of the close energies of the TCS dimers, the other TCS trimers are likely to have similar energies to the one calculated. TCS-Fe(III)-Montmorillonite Reaction Pathway. On the basis of the experimental and computational modeling results, the reaction pathways for TCS abiotic transformation oxidation by Fe(III)-montmorillonite are proposed in Figure 3. Detection of 2,4-dichlorophenol suggests cleavage of the diaryl ether bond of TCS after formation of TCS radicals via Fe(III) oxidation. Hydrolytic cleavage of the TCS diaryl ether bond was unlikely because this reaction requires acidic 672

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solution with the presence of a strong nucleophile, which was not present in our experiment. Diaryl ether bond cleavage under ambient conditions requires formation of radical as a first step, followed by homolytic or heterolytic transferring of the unpaired electron from the aryl ring (34). More 2,4dichlorophenol was detected in the presence of Fe(III)-montmorillonite compared to Na-montmorillonite (Figure S2-2, Supporting Information). At day 20, close to 60% and 10% of the TCS molecules initially added to the Fe(III)- and Na-montmorillonite, respectively, were transformed to 2,4dichlorophenol. At initial concentration of 0.01 mmol TCS g-1 mineral, 2,4-dichlorophenol may couple with another 2,4-dichlorophenol to form 2,4-dichlorophenol dimer (Figure 4). After 30 days the production of 2,4-dichlorophenol reached a plateau in the presence of Fe(III)-montmorillonite but decreased significantly in the presence of Na-montmorillonite. Formation of the 2,4-dichlorophenol dimer was not observed for reactions with initial concentration of 0.3 mmol TCS g-1 mineral up to 30 days (Figures 2a and 4). At 100 days, a significant amount of 2,4-dichlorophenol dimer was produced only in the reactions with Fe(III)-montmorillonite (Figure 4). It is possible that at higher TCS concentration, much of the 2,4-dichlorophenol radical initially reacted with an excess amount of TCS to form product at peak 1 instead of forming 2,4-dichlorophenol dimer. Only when the quantity of TCS becomes much less at day 100 does 2,4-dichlorophenol begin to form. At the higher initial TCS concentration, the total peak areas of 3-chloro-2-(2,3-dichlorophenoxy)-6-(2,4dichlorophenoxy)phenol and 3-chloro-2-(3-chlorophenoxy)6-(2,4-dichlorophenoxy)phenol (products B and C, Figure 3) increased with increasing reaction time, while at the lower initial TCS concentration it remained unchanged with time until 30 days, followed by a sharp decrease at day 100 (Figure 4). The TCS phenoxy radicals, produced by Fe(III) oxidation of TCS, react with other TCS molecules, forming four TCS dimer conformers (Figure 3). At higher initial TCS concentration, the total peak area of the four dimer conformers increased with reaction time and reached a plateau between 30 and 100 days, while at the lower initial TCS concentrations it increased with time up to 30 days, followed by a decrease (Figure 4). Six TCS trimer conformers are formed via addition of a TCS dimer phenoxy radical, produced by Fe(III) oxidation

FIGURE 4. Change of peak areas for TCS transformation products with time in the presence of Fe(III)- and Na-montmorillonite at initial concentrations of 0.3 and 0.01 mmol TCS g-1 mineral.

of TCS dimers, to a TCS molecule (Figure 3). The total peak area of the six trimer conformers increased with time up to 30 days, followed by a sharp decrease at 100 days except for the reaction with Na-montmorillonite at the higher initial TCS concentration (Figure 4). Figure 4 shows that significantly higher amounts of TCS transformation products were formed when TCS was reacted with Fe(III)-montmorillonite compared to Na-montmorillonite. The formation of small quantities of similar TCS transformation products in the reactions with Na-montmorillonite indicates that the structural Fe(III) in the Na-montnorillonite might also be capable of oxidizing TCS, however, to a

much less extent compared to the easily accessible interlayer Fe(III) in the Fe(III)-montmorillonite. It is important to point outthatalthoughat100days,bothNa-andFe(III)-montmorillonite with lower initial TCS concentrations transformed similar amounts of TCS (Figure 1), Figure 4 illustrates orders of magnitude lower peak areas for the products in the Na-montmorillonite treatments compared with the Fe(III)montmorillonite treatments. It is unknown at this point whether in the Na-montmorillonite treatments a portion of TCS transformation product(s) were not extractable by the acetone extraction method or not identifiable by the GC/MS or LC/MS analysis methods. VOL. 44, NO. 2, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Acknowledgments We wish to thank Dr. Jeong-wook Kwon for his assistance with the LC/MS analysis.

Supporting Information Available Detailed information on chemicals used; Fe(III)-montmorillonite preparation; sample extraction; instrumental analysis; and coordinates of TCS, TCS dimer conformers, and the lowest energy TCS trimer. This material is available free of charge via the Internet at http://pubs.acs.org.

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