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A DFT Study Toward the Reaction Mechanisms of TNT With Hydroxyl Radicals for Advanced Oxidation Processes Xi He,†,‡,§ Qun Zeng,†,§ Yang Zhou,*,† Qingxuan Zeng,‡ Xianfeng Wei,† and Chaoyang Zhang† †

Institute of Chemical Materials, Chinese Academy of Engineering and Physics, Mianyang 621010 China School of Mechano-electronic Engineering, Beijing Institute of Technology, Beijing 100081, China

‡

S Supporting Information *

ABSTRACT: The degradation pathway of environmental contaminant 2,4,6-trinitrotoluene (TNT) was investigated computationally at the SMD(Pauling)/M06-2X/6-311+G(d,p) level of theory. The dominant decomposition pathway of TNT → 4,6-dinitro-o-cresol → 4,6-dinitro-2-hydroxybenzylalcohol → 4,6-dinitro-2-hydroxybenzaldehyde was provided, and the corresponding predicted products and their distributions are in a good agreement with available experimental data on TNT degradation by Fenton reaction. It was shown that the mechanism of addition−elimination is crucial for this stage of the reaction. The reaction of H atom abstraction is a minor competing pathway. The details on transition states, intermediate radicals, and free energy surfaces for all proposed reactions are given and make up for a lack of experimental knowledge.

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INTRODUCTION Only for military purpose, the whole world can produce tens of thousands of tons of energetic compounds every year. Therefore, serious environmental problems by energetics at current and former conflict zones, manufacturing sites, and military ranges have been an international concern.1 As one of widely used explosives and a class C carcinogen rated by the Environmental Protection Agency (EPA), 2,4,6-trinitrotoluene (TNT) is a serious worldwide pollutant that has toxic effect on all living organisms. EPA regulated TNT contamination in soil to 17.2 ppm and in water to 2 μg·L−1.2,3 However, the documented TNT concentrations have reached the amazing range from 0.08 to 87 000 mg·kg−1 at contaminated sites.4 These serious situations raise extensive studies on TNT degradation. Early techniques like incineration, detonation, and ocean dumping were dangerous and illegal due to accidental explosion and secondary pollution. Recently, by comparing several techniques,5−14 advanced oxidative processes (AOPs)9−11 gradually become one of the most promising methods for removing TNT from contaminated soils, sediments, and water. Considering the generation of hydroxyl radicals, several AOPs, such as Fenton, Fenton-like, Fenton/UV, ElectroFenton, had been extensively studied and used to degrade TNT.9 These works mainly focused on the ways to accelerate and improve the rate of TNT removal. However, to the best of our knowledge, little is known about the detailed degradation pathways for AOPs. Clearly, the corresponding information is very significant to suggest some strategies to improve the effectiveness of AOPs. Virtually, this type of work is a challenge © 2016 American Chemical Society

for experiments, because of complicated reaction systems including several radicals and lively reactants, such as •OH, HO2•, OH−1, H+, and the short-lived and difficultly detected intermediates. In this work, we focus on the reaction mechanism of TNT with hydroxyl radical (•OH) for AOPs at early stages by means of quantum chemistry (QM) methods. Therefore, we would like to summarize the most recent experimental data, and the corresponding results are given in Figure 1. It is worth noting that Figure 1 only gives the detected intermediates that are only related to •OH radicals. On the basis of the identified products in Figure 1, we can roughly distinguish the four possible pathways of TNT degradation by •OH. In detail, Pathway A is a series of Habstraction reactions, which can result in the oxidation of −CH3 into −CHO, then to −COOH group. During this process, it is interesting that the second intermediate of 2,4,6trinitrobenzaldehyde (II) is detected10,15,16 before the first 2,4,6-trinitrobenzylalcohol (I). I is only detected in the recent Fenton degradation of TNT by Ayoub et al.10 As for the third intermediate, 2,4,6-trinitrobenzoic acid (III) was not detected by Fenton process;9 however, it was detected by UV/Fenton,17 ozonation, and photo-ozonation processes.18 In the Pathway B, the intermediates of 2,4,6-trinitro-cyclohexa-2,4-dienol (IV), picric acid (IV′), and 1-methyl-2,4,6-trinitro-cyclohexadiene epoxide (V) were found in the Fenton,10 photo-Fenton,13 and Fenton-like reactions,19 respectively. Obviously, except for Received: April 8, 2016 Revised: May 1, 2016 Published: May 2, 2016 3747

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DNT, DNAN, and RDX by Leszczynski’s group,7,8,20 Therefore, we do not repeat their reliabilities. All stationary points were further characterized as either local minima (intermediates, no imaginary frequency) or saddle points (TSs, one and only one imaginary frequency). Additionally, zero-point vibrational energies, the corrections to enthalpy, entropy, and Gibbs free energy were also determined by calculation of the analytic harmonic vibrational frequencies at the same theory level as geometry optimization. The intrinsic reaction coordinate (IRC)29 path was traced to check the energy profiles connecting each TS to the two associated minima of the proposed mechanism. Gibbs free energies of all considered species were calculated using standard expression: ΔG = ΔH − T·ΔS at T = 298.3 K. All intermediates that had been identified by experiments in Figure 1 are orderly named by Roman numerals, and then we only use Roman numerals to represent them, not the compound name. In the following figures, the corresponding intermediates and TSs for detailed pathways are labeled as IN and TS combining Arabic numerals, respectively. The numbers of 2 and 4 after the decimal point represent the ortho and meta positions of the methyl group in TNT, respectively. The last letter “R” corresponds to the radical.

Figure 1. Possible degradation pathways for TNT reacting with •OH based on the detected intermediates by experiments. I, 2,4,6trinitrobenzylalcohol; II, 2,4,6-trinitrobenzaldehyde; III, 2,4,6-trinitrobenzoic acid; IV, 2,4,6-trinitro-cyclohexa-2,4-dienol; IV′, picric acid; V, 1-methyl-2,4,6-trinitro-cyclohexadiene epoxide; VI, hydroxy-dinitrotoluene isomer; VII, hydroxy-dinitrobenzaldehyde isomer.

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OH, the other reactants also participate in the formation of IV. As for the formation of IV′ and V, like the intermediates of hydroxyl-dinitrotoluene isomer (VI) and hydroxyl-dinitrobenzaldehyde isomer (VII) in the Pathway C, we can know that the first step is the formation of Meisenheimer complexes by the addition of •OH to the aromatic ring. However, the next step, the further adsorption or the direct substitution, is little known. In addition, Hess et al.12 described the direct substitution of nitro groups in TNT and 1,3,5-trinitrobenzene (TNB) by the radical of •OOH under the Fe(III)/H2O2 condition. Additionally, the detailed information on the above pathways, such as transition states (TSs), free energy barriers (ΔG), is still ambiguous. These indeterminacies limit new AOP technologies to promote TNT removal. Hence, the investigation on the reaction mechanism of TNT degradation by • OH radicals is an urgent task. Indeed, QM methods plus experimental data can provide a powerful research approach to delineate the detailed TNT degradation pathways. Leszczynshi et al. have successfully used density function theory (DFT) to study the alkaline hydrolysis of energetic compounds (including TNT, DNAN, RDX, and so on).7,8,20 In addition, several simulations had investigated the mechanisms of toxic aromatic compounds (not explosives) with • OH.21−24 To the best of our knowledge, the theoretical simulations on explosive compounds with •OH are rare so far. Here, we mainly focus on the reaction mechanisms of TNT with •OH depending on the experimental result.9−11 Our DFT investigations will provide a meaningful help for the understanding of the degradation process of TNT with •OH and promote the devolvement of AOP technologies for removal of TNT and other energetic compounds.

RESULTS AND DISCUSSION For the initial steps of TNT reacting with •OH, the possible pathways are proposed by experimental identified intermediates and drawn in Figure 1. There are H-abstraction from the methyl group of TNT (Pathway A) and •OH addition to aromatic rings (Pathways B and C). The difference is that •OH attacks the C1 atom with the CH3 group, for Pathway B, and the C2 or C4 atom with the NO2 group for Pathway C. In addition, Ayoub and co-workers9,10 inferred that I and II would evolve to VII through unidentified steps. To explore its probability, we also design the fourth Pathway D. Then, we will introduce the four pathways in detail, and the corresponding calculated reaction (ΔE) and activation Gibbs free energies (ΔG) are provided in Table S1 of Supporting Information. Pathway A. The detailed reaction mechanism, which goes through the intermediates of I, II, and III, are delineated and given in Figure 2. On the basis of the H atoms at different sites, the reaction from I to II can be divided as two pathways, that is, attacking H linked with −CHOH (A1, dash) and H linked with −OCH2 (A2, dot), respectively. Along the whole path from TNT to III, there are four H-abstraction reactions and one dehydration reaction. Then, the geometries of TSs and the profiles of ΔG for the whole pathways are drawn in Figure 3 and Figure 4, respectively. In Figure 3, the lengths of the C−H breaking and H−O forming bonds at these TSs are 1.212 and 1.318 Å at TS1, 1.177 and 1.404 Å at TS2, 1.134 (O−H) and 1.189 Å at TS2′, and 1.182 and 1.448 Å at TS4, which indicates that the H-abstraction reaction by • OH is a simple asynchronous atom-transfer reaction in which the old C−H or O−H (only for TS2′) bond is broken and a new O−H bond is formed. As for TSs associated with the dehydration reactions, the lengths of two O−H (l1) and C−O (l3) breaking bonds at TS3 are 1.373 and 1.616 Å, and those of C−H (l1) and O−O (l3) breaking bonds at TS3′ are 1.485 and 1.608 Å, respectively. The lengths of O−H (l2) forming a water molecule at TS3 and TS3′ are 1.152 and 1.234 Å, respectively. It indicates that the dehydration reaction is also an asynchronous process that first breaks two bonds and later forms one bond. In detail, the first H-abstraction from the CH3 group of TNT needs to overcome an obstacle of 5.4 kcal·mol−1 and form the

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COMPUTATIONAL METHODS All calculations were performed with the Gaussian 09 suite of programs.25 Relevant stationary points in reaction pathways were fully optimized at the M06-2X/6-311+G(d,p) level.26,27 The solvent effect was described by SMD(pauling) solvent model,28 using the experimental solvent (water). The function M06-2X and SMD(pauling) solvation model had been successfully used to study the alkaline hydrolysis of TNT, 3748

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radical of IN1R. The next combination of two radicals (IN1R and •OH) is a diffusion-controlled process without the energy barrier. Similarly, the last H-abstraction from II to IN4R and the combination of two radicals (IN4R and •OH) also have an energy barrier of 3.3 kcal·mol−1. The lower energy barrier hints that two H-abstraction processes (TNT → IN1R and II → IN4R) can spontaneously take place under the room temperature. Interestingly, there are two paths from I to II. Path A1 (abstracting H-bonded C atom) and Path A2 (abstracting H-bonded O atom) have the energy barriers (ΔG) of 2.2 and 7.3 kcal·mol−1, respectively. The difference (ΔGA2 is more than 3 times of ΔGA1) can result in the obvious different reaction rate. It is in accordance with the findings in a majority of aliphatic compounds; that is, the reaction of robbing H atoms bound to oxygen is usually disfavored compared to those bound to carbon.30 The subsequent dehydration reactions also show the obvious different energy barriers, that is, 44.0 kcal·mol−1 for Path A1 and 62.2 kcal·mol−1 for Path A2, respectively. All these variations indicate that Path A1 will be energetically more favorable. In a word, H-abstraction reaction of TNT by •OH radicals is a spontaneous exothermic reaction. Moreover, from Table S1 we can also know that the solvent effect can significantly decrease the activation energy barrier of H-abstraction reaction. However, the further reaction of the radical IN1R would be induced by other species, not •OH, in a real AOP system. Indeed, the radicals like IN1R often have relatively low activation energy barriers for reaction with O2.30 As a result, an alternative reaction route with O2 from TNT to II had been proposed.31 Generally, the concentration of •OH is much lower than that of dissolved oxygen.32 Hence, the probability of IN1R reacting with O2 will be higher than that with •OH, which results in the direct formation of II avoiding I. This maybe explains why the product I was only detected up to date.10 Pathway B. For the intermediates of IV′ and V, we proposed the reaction mechanism of two •OH radicals sequentially adding into C1 and C2 (or C6) atoms in the aromatic ring (see Figure 5). According to the different

Figure 2. Pathway A for the degradation reaction of TNT only by • OH based on the detected intermediates. From I to II, there are two pathways, i.e., A1 (dash) and A2 (dot).

Figure 3. Geometries of TSs involved in Pathway A. The distances (li, i = 1, 2, 3) are given in angstroms.

Figure 5. Pathway B for the reaction of •OH addition to C atoms in the aromatic ring. After IN5R, there are two pathways, i.e., B1 for the elimination of CH3OH (dash) and B2 for the elimination of H2O (dot).

elimination products, there are also two routes of Path B1 (CH3OH elimination) and Path B2 (H2O elimination), in which there are two transition states of TS5′ and TS5, respectively. In Figure 6, for TS5′, the length of the first breaking bond C−C (l1) is 2.428 Å far more than the common C−C bond, and the second breaking bond C−O (l3) is 1.431 Å less than the forming C−O (l2) bond of 2.034 Å. It clearly indicates that the asynchronous process of CH3OH elimination

Figure 4. Free energy profiles for Pathway A. From I to II, there are two pathways, i.e., A1 (black) and A2 (red). The relative free energy is calculated by considering the balance of reaction equations.

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Figure 6. Geometries of TSs involved in Pathway B. The distances (li, i = 1, 2, 3) are given in angstroms.

is first the departure of CH3 and OH groups from the ring and then the formation of CH3OH. For TS5, the lengths of two breaking bonds, C−O (l1) and O−H (l3), are 1.612 and 1.547 Å, respectively. The length of the forming bond O−H (l2) is 1.034 Å, which is very close to the common O−H bond. Therefore, the asynchronous process of H2O elimination is different from CH3OH elimination and is first the formation of H2O and then the break of H−O bond. Figure 7 draws the evolution of Gibbs free energy barriers (ΔG) for Pathway B. As reported in previous works,24,30 for

Figure 8. A potential energy surface for the elimination of the methyl group at IN5R. The variation of total energy (kcal·mol−1) is given to roughly estimate the free-energy barrier.

simplify the reaction as a broken-bond reaction, that is, the leaving of methyl group. Therefore, we virtually obtained the upper limit value of activation energy for the factual reaction, not the accurate activation energy value. From Figure 8, we can know that the CH3 elimination has a barrier of ∼21 kcal·mol−1. For the broken-bond reaction, this value can be used to estimate the free-energy barrier of this reaction, which is lower than those from the elimination of CH3OH and H2O. Hence, it seems that the substitution pathway (addition−elimination) is more possible. However, the reaction from TNT to IN6 (or IN6′) is exothermic; the released heat (∼101 kcal·mol−1) is enough to sustain the reaction from IN6 to V (or IN6′ to IV′) with the high energy barrier. The pathway from IN6 to V (or IN6′ to IV′) proposed by Figure 6 is also a competing reaction with the minor probability. This maybe is the reason that a small amount of the product V was detected by the experiment.19 Pathway C. As shown in Figure 9, we design two routes (Path C1 and Path C2) to investigate the reaction processes of • OH addition to C2 and C4 atoms in the aromatic ring, respectively. The resulting intermediates IN9.4 and IN9.2 can further transform into the product VII by two possible Habstraction pathways similar to the forenamed Path A1 and Path A2, respectively. Therefore, we take IN9.4 as an example and also give two routes of Path C2−1 (dash) and Path C2−2 (dot) in Figure 9, respectively. In addition, TS structures and activation energy barriers for the reaction pathways of Figure 9 are drawn in Figure 10 and Figure 11, respectively. Similar to Pathway B, we also first choose the addition of two •OH radicals into the ring as the favorable pathway (i.e., Path C1 and Path C2). Then, the elimination reaction of HNO3 through the transition states of TS6 and TS6′ become the only way that must be passed. From Figure 10, we can find that TS6 has two breaking bonds of C−N (l1 = 2.576 Å) and C−O (l3 = 1.684 Å) and one forming bond of N−O (l1 = 1.871 Å), and TS6′ has also two C−N (l1 = 2.485 Å) and C−O (l3 = 1.743 Å) breaking bonds and one N−O (l1 = 1.875 Å) forming bond, which indicates that they also correspond to an asynchronous bondchanging process. Interestingly, again the free-energy profile in Figure 11 shows that the eliminations of HNO3 through TS6 and TS6′ have very high activation energy barriers of ∼91 kcal· mol−1 for both of Path C1 and Path C2. In terms of thermodynamics, the reaction mechanism of addition−addition difficultly occurs at the room temperature. After this, the two processes from VI.2 to IN9.2 and VI.4 to IN9.4 have the very close barrier of ∼1.6 kcal·mol−1 for the first reaction of H

Figure 7. Free-energy profiles for the reaction of •OH addition to the aromatic ring. After IN5R, there are two pathways, i.e., B1 (red) and B2 (black). The relative free energy is calculated by considering the balance of reaction equations.

aromatic compounds, the reaction of •OH addition at the ring generally prevails over H atom abstraction when competition is possible. The reason is that •OH addition to aromatic ring is an exothermic process with a negative reaction barrier,24 like that from TNT to IN5R (see Figure 7). After the first •OH addition, possible evolution products are a saturated dihydroxylated compound by the second •OH addition, or an unsaturated hydroxyl derivative by the direct substitution (that is to say, the elimination of the CH3 radical), and so on. Since the formation of the former product does not need to overcome any energy barriers, we think it should be a more favorable path, and also give its energy profile in Figure 7. However, free energy (ΔG) profiles show that the elimination reactions after the second •OH addition have the very high energetic barriers of 71.4 and 83.8 kcal·mol−1 for Path B2 (black) and Path B1 (red), respectively. Meanwhile, as shown in Table S1, these elimination reactions for all pathways are hardly influenced by the solvent effects. Clearly, it significantly decreases the probability of the path for two •OH radicals addition to the aromatic ring. Hence, we also explore the potential energy surface of latter reaction by scanning the elimination of CH3 at IN5R, and the results are drawn in Figure 8. The reaction is actually the reaction of two radicals into two molecules if considering the participation of •OH. Here, we 3750

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Figure 9. Pathway C for the reaction of •OH addition to C atoms in the aromatic ring. On the basis of the different C atoms, there are two pathways, i.e., C1 (•OH addition to C2 atom) and C2 (•OH addition to C4 atom). From IN9.4 to VII, there are also two H-abstraction pathways like A1 and A2, which are defined as C2−1 (dash) and C2− 2 (dot).

Figure 10. Geometries of TSs involved in Path C1 and Path C2. The distances (li, i = 1, 2, 3) are given in angstroms.

abstraction, which is far less than that (5.4 kcal·mol−1) of TNT → IN1R in Pathway A. Compared with TNT, the only difference is that a NO2 group of VI.2 or VI.4 is substituted by an OH group. Thus, it can be seen that the variation of substituent groups at the aromatic ring would significantly influence the energetic barrier of H-abstraction reactions. For the reactions from IN9.4 to VII, the corresponding TSs (TS7, TS7′, TS8, TS8′, TS9, and TS9′), the evolution of free energy surface (blue square in Figure 11) and TS structures (Figure 10) are very similar to those involved in Pathway A and hence introduced no longer. In fact, the proposed addition− addition mechanism at the first step of Pathway C decreased the probability of this route infinitely. In Pathway B, we provide the other mechanism for the transformation from CH3 to OH, that is, addition−elimination, which shows the lower energetic barrier than that of addition−addition. Here, we also try this addition−elimination mechanism and explore the potential energy surface of addition−elimination reaction by scanning the elimination of NO2 groups at IN7.4R. The results are shown in Figure 12. On the basis of Figure 12, we can know that the broken-bond reaction of the stable NO2 radical has a very low barrier of ∼1.47 kcal·mol−1, which is far less than those from the elimination of HNO3 and H2O. It also testifies that the direct substitution (addition−elimination) path is more favorable.

Figure 11. Free-energy profiles for the reaction of C1 (red) and C2 (black). After IN9.4, there are also two pathways, i.e., C2−1 (black) and C2−2 (blue). The relative free energy is calculated by considering the balance of reaction equations.

Pathway D. Because of the different sites attacked by •OH, the last pathway from II to VII also has two possible paths, that is, Path D1, •OH addition to C2, and Path D2, •OH addition to C4. The details can be seen in Figure 13. According to the above results, we can believe that the •OH addition radicals (IN12.2R and IN12.4R) prefer the mechanism of addition− elimination to achieve the elimination of nitro group. This time, we directly provide the potential energy surface of substitution reaction by scanning the elimination of NO2 groups at IN12.4R (see Figure 14). Clearly, the elimination of the stable NO2 radical from IN12.4R still has a very low barrier of ∼1.25 kcal· mol−1, which is similar to the elimination of NO2 radical from 3751

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transferred to the product VII by an H-abstraction with the low barrier of ∼1.4 kcal·mol−1 and then an H2O elimination with the barrier of ∼43.7 kcal·mol−1. Though the high barrier for the last elimination reaction, the former exothermic reactions release enough heat to make the reaction overcome the obstacle. Accordingly, the product VII would appear as predominant and quickly generated, and other intermediates cannot be easily detected due to the rapid exhaustion. This is in agreement with the fact in experiments.10 Additionally, as one of the competing reactions, hydrogen abstraction would pass over an energy barrier of ∼5.4 kcal·mol−1, and the formed radical IN1R often will react with the oxygen30 and produce the product II detouring I.31 This can explain the reason that the product I was found after II.10 It must be noted that the reaction involving other reactants (such as O2, H+, OH−), the dimerization, the further degradation (ring cleavage), as well as the kinetics information, are not investigated in this work.

Figure 12. A potential energy surface for the elimination of the NO2 group at IN7.4R. The variation of total energy (kcal·mol−1) is given to roughly estimate the free-energy barrier.

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CONCLUSIONS In summary, the TNT degradation reaction with hydroxyl radicals in AOPs has been studied with DFT methods at the SMD(Pauling)/M06-2X/6-311+G(d,p) level. The results show that the proposed reaction pathway is a multistep exothermic process in which the mechanism of addition−elimination is crucial for this process due to the very low energy barrier. As for the reaction of H atom abstraction, it is only a possible competing pathway. The details on transition states, intermediate radicals, and free-energy surfaces for all proposed reactions are given and make up for a lack of experimental knowledge. The determined main pathway can rightly explain the phenomena detected by Ayoub et al. experiment,10 and at the same time, our results prefer an alternative pathway that is not supported by the above experiments.

Figure 13. Pathway D for the transformation from II to VII. On the basis of the different C atoms, there are two pathways; i.e., D1 (•OH addition to C2 atom) is labeled by the dash line, and D2 (•OH addition to C4 atom) is labeled by the dot line.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03596. Tabulated calculated reaction and activation Gibbs free energies for main reactions in four pathways. (PDF)

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Figure 14. A potential energy surface for the elimination of the nitro group at IN12.4R.

AUTHOR INFORMATION

Corresponding Author

*E-Mail: [email protected].

−1

Author Contributions

IN7.4R (1.47 kcal·mol ) in Figure 12. Compared with IN7.4R, the difference is that IN12.4R has a CHO substitute at the C1-atom site. It indicates that the mechanism of addition−elimination is most favorable for the transformation from NO2 to OH group at the aromatic ring. Meanwhile, the substituent group (CH3 or CHO) at the C1 atom has hardly the obvious effect on the activation energy barrier. This is also different with the reaction of H abstraction, which is easily influenced by the changed substitute. Comparison. Through the comparison of the four explored routes, we can obtain the most possible reaction pathway of TNT degradation by hydroxyl radicals in AOPs, in combination with experimental results.9,10 That is, by means of an adsorption without the energy barrier the single •OH first attack a carbon (C2 or C4) atom of benzene ring, then the NO2 group departs from the attacked C atom through a broken-bond reaction with the energy barrier less than 1.5 kcal· mol−1. Next, the formed intermediate IN9.2 or IN9.4 would be

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These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All the authors appreciate very much the financial support from Foundation of CAEP (Nos. 2014-1-075 and 2014B0302040) and National Nature Sciences Foundation of China (No. 11402241).

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REFERENCES

(1) Pichtel, J. Distribution and Fate of Military Explosives and Propellants in Soil: A Review. Appl. Environ. Soil Sci. 2012, 2012, 617236. (2) Alnaizy, R.; Akgerman, A. Oxidative Treatment of High Explosives Contaminated. Wastewater. Water Res. 1999, 33, 2021− 2030.

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DOI: 10.1021/acs.jpca.6b03596 J. Phys. Chem. A 2016, 120, 3747−3753

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DOI: 10.1021/acs.jpca.6b03596 J. Phys. Chem. A 2016, 120, 3747−3753