Reduction Pathways of 2,4,6-Trinitrotoluene: An Electrochemical

Article pubs.acs.org/JPCC

Reduction Pathways of 2,4,6-Trinitrotoluene: An Electrochemical and Theoretical Study Chun Kiang Chua,† Martin Pumera,*,† and Lubomír Rulíšek*,‡ †

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ Institute of Organic Chemistry and Biochemistry, Gilead Sciences & IOCB Research Center, Academy of Sciences of the Czech Republic, Flemingovo náměstí 2, 166 10 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: The reduction pathways of trinitrotoluene are studied using electrochemical and computational methods. The electrochemical reduction of three nitro groups in 2,4,6trinitrotoluene (TNT) is characterized by three major reduction peaks in cyclic voltammograms at the peak potentials of −0.310, −0.463, and −0.629 V vs a normal hydrogen electrode (NHE). The second and third peaks coincide with the two peaks observed for the 2-amino-4,6dinitrotoluene (at the potentials of −0.475 and −0.627 V vs NHE), whereas the two peaks in the 4-amino-2,6-dinitrotoluene voltammograms appear at −0.537 and −0.623 V and deviate more significantly from the corresponding two peaks in 2,4,6-trinitrotoluene. It suggests that the first NO2 group reduced in the overall process is the one in ortho position with respect to the CH3 group. Analogously, the 2,6-diamino-4-nitrotoluene exhibits a reduction peak at −0.629 V, almost identical to the third and second reduction peaks of 2,4,6-trinitrotoluene and 2-amino-4,6dinitrotoluene, respectively. Since the other isomer, 2,4-diamino-6-nitrotoluene, exhibits a reduction peak at −0.712 V, we conclude that the second reduction occurs also in the ortho position with respect to the methyl group. Most of these observations are corroborated by quantum chemical calculations, which yielded reduction potentials in a good agreement with the experimental values (in relative scale). Thus, studying in detail all of the possible protonation and redox states in the reduction of the first nitro group and the key steps in the reduction of the second and third nitro groups, we have obtained a comprehensive and detailed picture of the mechanism of the full 18e−/18H+ reduction of TNT. Last but not least, the calculations have shown that the thermodynamic stabilities of (isomeric) neutral radical species (X + e− + H+)presumably the regioselectivitydetermining steps in the 6e−/6H+ reductions of the individual NO2 groupsare within 2 kJ·mol−1 (i.e., comparable to RT). Therefore, the course of the reduction can be governed by the effect of the surroundings, such as the enzymatic environment, and a different regioselectivity can be observed under biological conditions.

1. INTRODUCTION Environmental remediation and safety are at the forefront of general public interest to sustain safe living conditions. It is wellknown that the most used component of high-performance military explosives, 2,4,6-trinitrotoluene (TNT), is reduced in the soil and groundwater to the corresponding amino compounds,1−3 which are toxic and carcinogenic and may contaminate the drinking water resource and expose the population to long-term doses of such compounds, leading to chronic poisoning.4−7 Several reduction mechanisms of TNT have been proposed.8,9 However, to the best of our knowledge, the detailed structural and energetical characterization of all intermediates on the reduction pathway of TNT has not been carried out. This is of enormous interest in that it may ultimately bring us closer to a conclusive answer concerning this problem. The molecule of TNT has three reducible nitro groups. The mechanism of the reduction of one nitro group in an aqueous (protic) environment can be described as shown in Scheme 1. © 2012 American Chemical Society

The reduction of one nitro group to the amino group is a process requiring six electrons and six protons in total. It can be resolved in three steps, which implies the existence of two intermediates. In the first step, the reduction of one nitro group proceeds via the 2e−/2H+ electron and proton transfers to yield (upon the elimination of the water molecule) the nitroso intermediate.10 Since the nitroso group is more easily reducible than the nitro group,10 it is immediately further reduced by the 2e−/2H+ process into a hydroxylamine intermediate.10 Finally, the hydroxylamine intermediate is reduced by the two electrons into the aromatic amine.8 Again, this process requires two protons and involves then elimination of the water molecule. As can be seen, the process of reducing one nitro group is quite similar to the reduction of nitrobenzene which has been studied Received: February 16, 2011 Revised: December 13, 2011 Published: January 9, 2012 4243

dx.doi.org/10.1021/jp209631x | J. Phys. Chem. C 2012, 116, 4243−4251

The Journal of Physical Chemistry C

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Scheme 1. General Mechanism for the Electrochemical Reduction of the Nitro Group Bound to the Aryl Moiety in an Aqueous Media, Exemplified Here by TNT10

Scheme 2. Possible Reduction Pathways of the Full Reduction of TNT into the Triaminotoluene

were found to correlate with their reduction rate constants under various conditions14,15 and also with toxicity.16 On the other hand, the quantum chemical calculations provide complementary information and the mapping between the structures of the intermediates and their energies or free energies. Despite the fact that an accurate calculation of the reduction potentials is a highly challenging task,17−20 which is also true also for NACs,21 we believe that the agreement between the calculated and experimental data may yield an unambiguous picture about TNT reduction.

and described in even more detail (solvent dependence, alternative pathways) several decades ago, and we may point a reader to the excellent work of Smith and Bard.11 They have shown that in the nonaqueous solutions, two reduction waves can be detected, at the peak potentials of −0.44 and −1.26 V vs Ag|Ag+ electrode which implies, in the context of our work, that the first elementary step in the overall process is the 1e− step that is followed (or concomitant with) by the proton transfer step. Indeed, the existence of a shortlived intermediate corresponding to the nitrobenzene radical anion has been detected by electron spin resonance (ESR) in alkaline solutions (pH > 10).12 The same radical has been found to be stable in the nonaqueous solutions.13 The full reduction of 2,4,6-TNT involves three nitro groups. Scheme 2 depicts the possible reduction pathways for the full reduction of TNT into triaminotoluene. As can be seen, 2,4,6TNT can be reduced to either 2-amino-4,6-dinitrotoluene (2-A4,6-DNT) or 4-amino-2,6-dinitrotoluene (4-A-2,6-DNT). Consequently, the asymmetric molecule of 2-A-4,6-DNT can be reduced to either 2,4-diamino-6-nitrotoluene (2,4-DA-6NT) or 2,6-diamino-4-nitrotoluene (2,6-DA-4-NT); on the other hand, only one reduction product is possible for 4-A-2,6DNT, namely, 2,4-DA-6-NT. Finally, these diamino products are reduced to triaminotoluene. The aim of this study is the elucidation of the above reduction pathways by combining electrochemical methods and quantum chemical calculations. The advantage of the electrochemical approach is that the electrons are used as reducing agents, and the reducing power of the electrons can be regulated by varying the electrochemical potential and the Fermi level of electrons in the electrode. It is therefore possible by fine-tuning of the electrochemical potential to identify the minute differences in the possible reduction pathways and find the most energetically favorable pathway of TNT. Moreover, the reduction potentials of the nitroarene compounds (NACs)

2. EXPERIMENTAL METHODS 2,4,6-Trinitrotoluene, 4-amino-2,6-dinitrotoluene, and 2-amino4,6-dinitrotoluene were obtained from Sigma-Aldrich, USA, as the 1000 ppm/mL (4.40 mM for 2,4,6-TNT, 5.07 mM for 4-A-2,6DNT, and 2-A-4,6-DNT) analytical standard. 2,4-Diamino-6nitrotoluene and 2,6-diamino-4-nitrotoluene were obtained from AccuStandard, CT, USA, as the 100 ppm/mL (598 μM) analytical standard. The phosphate buffer components (mono-, di-, and tribasic phosphate salts) were obtained from Sigma Aldrich and were of the highest commercially available purity. All of the voltammetric experiments were performed using an Autolab 302 electrochemical analyzer (Ecochemie, Utrecht, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems v. 4.9 software (Ecochemie). The electrochemical experiments were carried out in a 5 mL voltammetric cell at room temperature (25 °C) using a three-electrode configuration. A platinum electrode served as an auxiliary electrode and a Ag/AgCl electrode as a reference electrode; a glassy carbon electrode (diameter: 3 mm) was utilized as a working electrode. All potentials stated were converted from reference to Ag/AgCl to the reference to normal hydrogen electrode (NHE) by adding a factor of 0.197 V. Prior to measurement, the glassy carbon electrode was polished with 0.05 μm alumina on a 4244



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polishing cloth. All of the electrodes were obtained from CHInstruments, TX, USA.

3. COMPUTATIONAL DETAILS The quantum chemical calculations were performed using the Turbomole 6.2 program.22 The geometry optimizations were carried out at the DFT level, using the Perdew−Burke− Ernzerhof (PBE) functional.23 The DFT/PBE calculations were expedited by expanding the Coulomb integrals in an auxiliary basis set, the resolution-of-identity (RI-J) approximation.24,25 For the geometry optimization, the def2-SVP basis set was employed on all of the atoms.26,27 Single-point energies were then calculated using the B3LYP28 and PBE23 functionals. For these calculations, the def2-TZVP and basis sets were employed on all of the atoms.26 The solvation effects were taken into account by using the COSMO−RS (conductor-like screening model−real solvent) method29,30 using Turbomole 6.2 for the COSMO calculation 31 with the ε r = ∞ (ideal screening) and the COSMOtherm32 program for the subsequent COSMO−RS calculation. The recommended protocol involving the Becke− Perdew (B−P) functional28a,33 for the in vacuo and the εr = ∞ calculations and the def-TZVP basis set was used. The Gibbs free energy was then calculated as the sum of the following contributions G = Eel + Gsolv + E ZPE − RT ln(qtransqrotqvib)

Figure 1. Cyclic voltammograms of TNT and its amino derivatives. Conditions: 50 mM phosphate buffer, pH 8.0; a scan rate of 0.1 V s−1. Concentration of the nitroaromatic compound: 88.1 μM for TNT; 101 μM for 2-A-4,6-DNT and 4-A-2,6-DNT; 119 μM for 2,6-DA-4NT and 2,4-DA-6-NT. All potentials are vs NHE.

(1)

where Eel is the in vacuo energy of the system (at the RI-PBE/ def2-TZVP//RI-PBE/def2-SVP level); Gsolv is the solvation free energy (calculated using the RI-BP/def-TZVP(COSMO-RS, ε = 1, ε = ∞) method as described above); EZPE is the zeropoint energy; and −RT ln(qtransqrotqvib) accounts for the entropic terms. The thermal correction to the enthalpy is obtained from a frequency calculation using the same method and software as for the geometry optimization at the RI-PBE/def2-SVP level, 298 K, and 1 atm using the ideal-gas approximation.34 The reduction potentials were then calculated according to the equation

potential values in the following text are stated as vs NHE. Throughout the manuscript we use the peak potentials (Ep), as the standard reduction potential cannot be directly established from Ep.39 To identify whether the first reduction step of TNT leads to the 2-A-4,6-DNT or to the 4-A-2,6-DNT, we performed a voltammetric investigation on these two compounds. The reduction of 2-A-4,6-DNT resulted in two peaks, thus reflecting the reduction of the two nitro groups, at the peak potentials of −0.475 and −0.627 V (Figure 1, B, black line), whereas the reduction of the 4-A-2,6-DNT results into two overlapping peaks, at the potentials of −0.537 and −0.623 V (Figure 1, B, red line). The data indicate that the reduction of the second nitro group of TNT, which corresponds to the reduction of the first nitro group of amino-dinitrotoluene, occurs at an almost identical potential as that of 2-A-4,6-DNT (−0.463 and −0.475 V, respectively). This is contrary to the electrochemical reduction of 4-A-2,6-DNT, which shows the first peak at the potential of −0.537 V and substantially differs from TNT and 2-A-4,6-DNT. A comparison of the reduction potentials of these three compounds while bearing in mind the possible reduction pathways in Scheme 2 led us to the conclusion that the first reduced nitro group of TNT is in the ortho position with respect to the methyl group and that 2-A-4,6DNT is the major product of the first reduction step. It can also be mentioned that there is a kinetic effect of the two nitro groups in the ortho position (vs the single nitro group in the para position), which should further favor the reduction of this o-NO2 group. The reduction of the second nitro group of TNT can take place in either the ortho or para position vs the methyl group of TNT, leading to 2,6-DA-4-NT or 2,4-DA-6-NT isomers, respectively. In a similar fashion as with previous experiments, we have compared the electrochemical behavior of 2-A-4,6DNT and two potential products, 2,6-DA-4-NT or 2,4-DA-6NT. The 2,6-DA-4-NT exhibits a reduction peak at −0.629 V (Figure 1, C, black line), whereas the 2,4-DA-6-NT reduction

E 0 [V] = 27.21(Gox [au] − Gred [au]) − E 0abs(NHE) [V]

(2)

where Gox and Gred are the free energies calculated according to eq 1 and the E0abs(NHE) is the absolute potential of the NHE. In the literature, the values for the E0abs(NHE) ranged between 4.24 and 4.5 V,35−37 perhaps with the most recent value of E0abs(NHE) = 4.281 V advocated by Isse and Gennaro38 which we use in our work. Moreover, the correction of (1.9Δn) kcal·mol−1 (corresponding to the difference between the concentration of the ideal gas at 298 K and 1 atm and its 1 mol·L−1 concentration) has been applied for the reactions (processes) in which the number of moles (Δn) changed (in the calculations of pKa values).

4. RESULTS AND DISCUSSION 4.1. Cyclic Voltammetry. The reduction pathways of TNT were studied by means of cyclic voltammetry. Figure 1 depicts the cyclic voltammograms of TNT and its derivatives2-A-4,6-DNT (DNT, dinitrotoluene) and 4-A-2,6-DNT. The nitro groups of TNT are electrochemically reduced, resulting in three major reduction peaks at the potentials of −0.310, −0.463, and −0.629 V vs the normal hydrogen electrodeNHE (Figure 1, A). All 4245



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peak appears at −0.712 V (Figure 1, C, red line). As the TNT exhibits a reduction peak for its third group at the potential −0.629 V and 2-A-4,6-DNT exhibits a reduction of the second reduction peak at −0.627 V, we conclude that the reduction pathway of TNT for the second nitro group is also in the ortho position with respect to the methyl group. The final reduction then proceeds to the third group, i.e., to triaminotoluene in the para-position. To ascertain that the first reduction process of TNT took place at the ortho position toward the formation of 2-A-4,6-DNT, we performed an exhaustive controlled potential electrolysis (CPE) of TNT at the potential of the first reduction peak (−0.301 V vs NHE). The number of transferred electrons per molecule of TNT was 4.5 (total experiment time, 2200 s). The cyclic voltammogram of the resulting product shows a main reduction peak at the potential of −0.488 V, which closely corresponds to the first reduction peak of 2-A4,6-DNT at −0.475 V (Figure 2, A). In a similar fashion, we

potential critically depends on the accuracy of the calculated electron affinities (EA)the attachment of an electron to the neutral (oxidized) species of the redox couple and the accuracy in the calculated solvation energies of both the reduced and oxidized species. To calibrate our data and select the appropriate computational method, we first addressed the accuracy of the computed electron affinities. Since we could not find any value of EA for the studied system (TNT) in the literature, we used the experimental values for similar systems (nitrobenzene, 1,3dinitrobenzene, 1,4-dinitrobenzene, 2,6-DNT, and 4-nitrobenzaldehyde) and compared them to the calculated data. The experimental EAs between the five chemically similar species differ by 1 eV, and we consider them as suitable test systems for the performance of the quantum chemical methods. The results are summarized in Table 1. Several conclusions can be drawn from the data presented in Table 1. First, the performance of both the PBE and B3LYP functionals is acceptable with both functionals slightly overestimating the EA of nitrobenzene derivatives. It can be noticed that for all of the disubstituted benzenes the difference between the calculated and experimental EAs is almost constant, 0.30−0.33 eV for the PBE and 0.16−0.22 eV for the B3LYP functional (using the def2-TZVP basis set), which is quite encouraging. For nitrobenzene, the agreement with the experimental data is almost quantitative (Δexp/calc = 0.05 eV for PBE and 0.02 eV for B3LYP). We hypothesize that this deviation from the otherwise systematic shifts for the disubstituted nitrobenzenes can be attributed to the underestimation of the EA for the simplest modelthe NO2 molecule: EAexp = 2.27 eV46 and EAcalc (this work) = 1.80 eV for the PBE/def2-TZVP and 1.98 eV for the B3LYP/def2-TZVP and is almost quantitatively reproduced only by highly expensive CCSD(T)/aug-cc-pVTZ calculations that yield the value of EAcalc = 2.21 eV. Therefore, there is a tendency in both of the DFT functionals to converge to higher values of EA, in comparison with the experimental data, ongoing from the simple NO2 through a monosubstituted benzene to more complex disubstituted species as will also be demonstrated in the comparison of the experimental and calculated data for the TNT derivatives. The systematic overestimation of the EA by the PBE and B3LYP functionals also correlates with the fact that calculated potentials are slightly less negative (∼0.2 V) than the reported experimental peak potentials. Second, there is almost a constant difference between the two basis sets, the smaller def2-TZVP basis set and the larger def2QZVP basis set. For both functionals, the former values are smaller by an almost constant value (0.07−0.09 eV) and closer to the experimental values, which may be partially fortuitous and points to a possible cancellation of errors between the functional used and the finite basis set. In our benchmark calculations, the agreement between the calculated and experimental electron affinities was much worse for the standard wave function methods (data not shown), such as the Hartree−Fock and even MP2 (second-order Møller−Plesset). Therefore, we decided not to use the composite procedures such as G3(MP2) or its RAD variants for open-shell species (which are normally the recommended highlevel methods of calculation of EA and reduction potentials)47 for our systems and rather used the RI-PBE method as the method of choice in our study. Finally, the calculated and calibrated EAs provide us also with an approximate estimate of the error bar in the calculated values of the reduction potentials, which can be, in an absolute scale, higher than ∼0.2 V. Taking into account an error in the calculated solvation energies, we expect the accuracy in the calculated reduction potentials of 0.2−0.4 V. It can be pointed out that the

Figure 2. Cyclic voltammograms of (A) TNT before and after exhaustive controlled potential electrolysis with its amino derivatives and (B) 2-A-4,6-DNT before and after electrolysis with its diamino derivatives. Conditions: 50 mM phosphate buffer, pH 8.0; a scan rate of 0.1 V s−1. Concentration of the nitroaromatic compound: 88.1 μM for TNT; 101 μM for 2-A-4,6-DNT and 4-A-2,6-DNT; 119 μM for 2,6-DA-4-NT and 2,4-DA-6-NT. All potentials are vs NHE.

performed an exhaustive CPE of 2-A-4,6-DNT at a potential of −0.478 V, which corresponds to the first reduction peak of 2-A4,6-DNT. The number of transferred electrons per molecule of 2-A-4,6-DNT was 5.3 (total experiment time, 2200 s). The cyclic voltammogram of the resulting product exhibits a main reduction peak at −0.627 V, which closely corresponds to the reduction peak from 2,6-DA-4-NT at −0.629 V (Figure 2, B). Further details related to the electronic structure of the studied compounds that can provide the answer to the remaining question of why the reduction pathway proceeds according to the proposed mechanism are best addressed by quantum chemical calculations. 4.2. Quantum Chemical Calculations: Methodological Issues. Equations 1 and 2 imply that the calculated reduction 4246



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Table 1. Calculated Adiabatic Electron Affinities for the Nitrobenzene, 1,3-Dinitrobenzene, 1,4-Dinitrobenzene, 2,6-DNT, and 4-Nitrobenzaldehydea EAcalc

a b

system

EAexp

nitrobenzene 1,3-dinitrobenzene 1,4-dinitrobenzene 4-nitrobenzaldehyde 2,6-DNT

± ± ± ±

1.00 1.66 2.00 1.69 ∼1.5 f

b

0.01 0.1c 0.1d 0.09e

RI-PBE/TZg

B3LYP/TZ

RI-PBE/QZg

B3LYP/QZ

1.05 1.97 2.30 1.97 1.81

1.02 1.83 2.22 1.85 1.67

1.14 2.05 2.38 2.05 1.89

1.11 1.90 2.29 1.92 1.74

All of the values are in eV. The calculated values include the ZPVE correction. All of the geometries were optimized at the RI-PBE/def2-SVP level. Ref 40. cRef 41. dRef 42. eRef 43. fFrom refs 44 and 45. gTZ and QZ stand for the def2-TZVP and def2-QZVP basis sets.

experimental uncertainty in the EAs is quite often ∼0.1 eV. Therefore, the same (or higher) error bar would probably be obtained using experimental EAs and experimental solvation energies (which are not available for most of the studied reduced speciesanions) as the parameters of the thermodynamic cycle used to calculate the reduction potentials. However, we are more optimistic about the relative errors in the calculated reduction potentials for TNT and its various derivativesintermediates on the reduction pathway. This optimism is partially based on the systematic errors encountered for a given set of systems presented in Table 1 and the expected systematic errors in the solvation energies that may cancel in the ΔE0 values (i.e., differences between various compounds studied). 4.3. Quantum Chemical Calculations: Reduction Potentials. On the basis of the results of the benchmark calculations discussed in the previous section and the comparison of the calculated data with the experimental results, we decided to use RI-PBE/def2-TZVP//RI-PBE/def2-SVP model chemistry to calculate the reduction potentials of the TNT and its reduction intermediates. Compared to the B3LYP, it deviates in absolute values slightly more from the experimental EAs, but the window of the systematic shift is narrower (albeit marginally). Also, owing to the RI approximation, the method is by an order of magnitude faster, which can make a difference in the calculations of the reduction potentials of larger systems than TNT. For the selected intermediates, we also report the pKa values calculated according to the equation

The results of the calculations of reduction potentials and acidity constants are summarized in Table 2. We studied all of the possible intermediates involved in the reduction of the first nitro group, whereas we only calculated the reduction potentials and stabilities of the key intermediates(+1e−/1H+) neutral radicals50on the reduction pathway of the aminoDNT and the reduction potentials of diamino-NT, thus assuming that the same reaction mechanism applies for the reduction of the second and third nitro group. It can also be mentioned that for many intermediates several isomers may exist (various protonation sites in the TNT and its partially reduced derivatives). We investigated all of the possibilities but report only the data for the most stable intermediate for the particular protonation and redox state. The structures of the most stable intermediates are depicted in Figure 3. Starting from TNT, we arrive at the first key intermediate, which is a neutral radical (TNT + e− + H+). This intermediate is considered to have a decisive role in governing the regioselectivity of the overall 6e−/6H+ process of the reduction of the first nitro group,50 as it is quite evident that the reduction will continue on the first group being attacked. The calculations indicate, in full agreement with electrochemical data reported in this paper, that the ortho-position is slightly energetically preferred (∼2 kJ·mol−1). The calculated pKa values for the protonation of the TNT anion radical is ∼4.5, which suggests that this process is not spontaneous and requires a small amount of energy at a neutral pH. The calculated reduction potentials for the second one-electron reduction are −0.099 and −0.055 V, both above the calculated value for the first reduction (−0.108 V vs experimental Ep(exp) = −0.310 V), and the process is therefore feasible under the applied external potential.51 Moreover, the calculated pKa value of −5 and −6 strongly supports the (anticipated) notion that the second reduction precedes (or, more precisely, does not come after; since the issue whether these processes might be proton-coupled electron transfer processes is beyond the scope of this work the reader is referred to recent excellent reviews52−54 on the subject dealing with this nontrivial phenomenon) the second protonation. It is nevertheless interesting that the stability of the ortho-/paraisomers is reversed in the TNT-H− intermediate (2.5 kJ·mol−1). However, we do not consider this step as regioselectivity-determining (vide supra). Finally, the protonation of the TNT-H− anion leads to a spontaneous dissociation of the water molecule and the nitroso intermediates. For the nitroso intermediates, we have found a strong thermodynamic preference of the ortho-isomer (13.5 kJ·mol−1). The reduction potential has been calculated at ∼0.34 V, much higher than the −0.108 V for the initial reduction step, thus demonstrating that the process is feasible. The pKa value for the NO−DNT-H• neutral radical is ∼13, and therefore the first anion radical is readily protonated. The second reduction of the nitroso

pK a = − ΔGdiss /(RT ln 10) where ΔGdiss is the free energy difference for the dissociation of the protonated form (acid) into the deprotonated form (base) and a proton. Also, the free energy differences between the relevant ortho- and para-isomers using the same method and protocol as for the calculation of the reduction potentials are reported. It must be emphasized that we consider the pKa values only as indicative (or qualitative) with an estimated error bar of at least 2 pKa units. On an absolute scale, they were anchored to the pKa of acetic acid (4.756),48 which yielded the proton solvation energy for the given computational protocol ΔG0aq(H+) = −1085.5 kJ·mol−1. Admittedly, this value is by 19 kJ·mol−1 lower (less negative) than the value of ΔG0aq(H+) = −1104.5 kJ·mol−1, recommended by Tissandier et al.,49 but this shift does not influence the relative values. To further verify the accuracy of pKa and estimate the error bars of pKa calculations, we have calculated pKa for aniline, which is chemically similar to some of the studied intermediates, and obtained a pKa value (or its conjugate acid) of 6.68, which deviates by 1.79 from the experimental value (4.87).48 We consider it as a relevant and quick test of the accuracy of (and error estimate in) our pKa predictions. 4247



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Table 2. Calculated Reduction Potentials (E0, in V), pKa Values, and Free Energy Differences between the ortho- and paraIsomers (ΔGo/p, in kJ·mol−1) for the Intermediates on the Reduction Pathway of TNT to 2,4,6-Triaminotoluenea,b E0(calc) ΔGo/p Ep(exp) [V] pKa(calc) [kJ·mol‑1] [V]

system TNT TNT-H• (TNT + e− + H+) TNT-H− (TNT + 2e− + H+)

−0.108 ortho (o-) −0.099 4.6e

−1.9 f

para (p-) −0.055 4.2 o-

0.0 2.5

pTNT-H2+• (TNT + e− o+ 2H+) p(o-)d 2-NO-4,6-DNTc 4-NO-2,6-DNTc (p-)d 2-NO-4,6-DNT-H• (o-) 4-NO-4,6-DNT-H• (p-) 2-NO-4,6-DNT-H− (o-) 4-NO-4,6-DNT-H− (p-) 2-NO-4,6-DNT-H2+• (o-) 4-NO-4,6-DNT-H2+• (p-) 2-NHOH-4,6-DNT (o-) 4-NHOH-2,6-DNT (p-)

−5.0 −6.1 0.351 0.327 0.104 12.5 0.022 13.0

−5.0 −6.1 −0.351 −0.188

E0(calc) [V] pKa(calc)

system

−0.310

2-NHOH-4,6-DNT-H+ 4-NHOH-2,6-DNT-H+ 2-NH-4,6-DNT+ 4-NH-2,6-DNT+ 2-NH-4,6-DNT• 4-NH-2,6-DNT• 2-NH-4,6-DNT− 4-NH-2,6-DNT− 2-NH2-4,6-DNT+• 4-NH2-2,6-DNT+• 2-A-4,6-DNT 4-A-2,6-DNT 2-A-4,6-DNT-H•

0.0 −7.9 0.0 −13.5 0.0 −12.9 0.0 −20.8 0.0 −10.3 0.0 −19.9 0.0

2-A-4,6-DNT-H• 4-A-2,6-DNT-H• 2,6-DA-4-NT 2,4-DA-6-NT

(o-) (p-) (o-) (p-) (o-) (p-) (o-) (p-) (o-) (p-)

−(dissoc.) −(dissoc.) 2.158 2.085 0.671 0.608

6.9 8.0 −0.265 −0.225

H in 6 pos. H in 4 pos.

ΔGo/p Ep(exp) [V] [kJ·mol‑1] −13.2 0.0 −10.1 0.0 −17.1 0.0 −23.2 0.0 −11.2 0.0 −20.4 0.0 2.4

−0.475 −0.537

0.0

−0.369 −0.460

19.2 −15.1 0.0

−0.629 −0.722

a

Experimental peak potentials (Ep) refer to the NHE. bThe calculated data were obtained using the RI-PBE/def2-TZVP//RI-PBE/def2-SVP computational protocol and COSMO-RS (at the BP/def-TZVP level) for the solvation energies. cNitroso-DNT is the first stable intermediate on the 6e−/6H+ reduction pathway (TNT amino-DNT). dThe (o-), (p-) symbols are left in the table for the sake of convenience, although they are redundant in the lower part of the table. eThe pKa values refer to the deprotonation (dissociation) process. fΔGo/p is the free energy difference between the given pair of ortho- and para-isomers with the energy of the para-isomer in the given pair set to zero (in one case, the amino-DNT-H radical, we have compared three possible structures)

Figure 3. Molecular geometries of the selected intermediates on the 18e−/18H+ reduction pathway of TNT: (a) the parent TNT molecule; (b) the neutral radical TNT-H• (TNT + e− + H+); (c) 2-NO-4,6-DNT with a dissociating water molecule; (d) the neutral radical 2-NO-4,6-DNT-H•; (e) 2NO-4,6-DNT-H2+•; (f) the hydroxylamine intermediate 2-NHOH-4,6-DNT; (g) 2-NHOH-4,6-DNT-H+ with a dissociating water molecule; (h) the 2-NH-4,6-DNT• radical; (i) 2-A-4,6-DNT; (j) 2-A-4,6-DNT-H• with H in the 6 position; (k) 2-A-4,6-DNT-H• with H in the 4 position.

0.104 and 0.022 V for the ortho- and para-isomers, respectively, well above −0.108 V. The second protonation is characterized by

intermediate (the fourth electron in the overall process) is associated with the redox potential, which has been calculated at 4248



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the computed values of pKa = −5 (and −6), suggesting again that the second proton does not come before the second electron. Finally, it can be mentioned that all of the ortho-structures along the nitroso to hydroxylamine reduction pathway are energetically much more stable than the para-isomers. The last intermediate in the reduction of the first nitro group contains a hydroxylamine group. In this case, the calculated reduction potentials (−0.351 and −0.188 V for the ortho- and para-isomer, respectively) are below the value of −0.108 V (i.e., more negative), and it therefore appears that the external potential is not sufficient to promote the fifth reduction step. However, and this is contrary to the nitro- and nitroso-intermediates, the proton attack on the hydroxylamine intermediate leads to the spontaneous, barrierless dissociation of the water molecule (the pKa value cannot be calculated since there is no equilibrium between the protonated and deprotonated forms). Therefore, we put forward the hypothesis that the protonation precedes a reduction in this (overall) fifth elementary step. Once the water molecule is eliminated, the NHDNT+ readily accepts an electron (E0 = 2.158, 2.085 V; as can be expected for cationic species), and NH-DNT• is formed and reduced by the last electron (E0 = 0.671, 0.608 V) to NH-DNT‑. The calculated pKa value for the NH2-DNT+ is close to the value of 7 and does not allow us to elucidate unambiguously whether the protonation or reduction occurs first in the last (sixth) step of the 6e−/6H+ reduction of the first nitro group. Of the two products, 2-A-4,6-DNT (the ortho-isomer) is significantly (20 kJ·mol−1) more stable than 4-A-2,6-DNT (the para-isomer), which is perfectly in line with the computational and electrochemical data above. The calculated reduction potential for the 2-A-4,6-DNT is again consistent with the experimental data if we account for the 0.17 V shift (−0.265 vs −0.475 V), whereas the agreement for the 4-A-2,6-DNT is less satisfactory (−0.225 vs −0.537 V), at the edge of the estimated error bar.

As mentioned above, we assume that the 12 elementary steps in the reduction of the second and third nitro groups are analogical, and therefore detailed calculations of all the structures along the remaining 12e−/12H+ reduction pathway have not been carried out. There are still, however, a few points of interest worth addressing. First, we calculated the stability of the 2-A-4,6-DNTH• and 4-A-2,6-DNT-H• neutral radicals, i.e., the initial steps in the reduction of the second nitro group. Whereas the 4-A-2,6DNT-H• isomer (with the hydrogen atom on one of the orthopositions) is considerably less stable (19 kJ·mol−1) than the two possible 2-A-4,6-DNT-H• isomers, as can be expected from the thermodynamic stabilities of the two A-DNT isomers discussed above, the calculations predict that the first hydrogen should be preferably attached in the para- rather than the second orthoposition (with respect to the methyl group). The preference is rather small (2 kJ·mol−1), of the order of RT, but we do not have a plausible explanation of the computed phenomena at the moment. Nevertheless, it can be seen from the calculated values that the thermodynamic stability of the 2,6-DA-4-NT is considerably higher (15 kJ·mol−1) than that of the 2,4-DA-6-NT, which is in agreement with the experimental observation of the preferred reduction of the o-NO2 over the p-NO2 group. Finally, we calculated the redox potentials for the diamino derivatives, namely, 2,6-DA-4-NT and 2,4-DA-6-NT, and found them consistent with the experimental data. We believe that the theoretical calculation and the experimental electrochemical data allow us to arrive at an unambiguous, consistent reaction pathway of the 18e−/18H+ reduction of TNT to triaminonitrotoluene. This pathway is depicted in Figure 4. Finally, it can be mentioned that by accomplishing the aims of our study we believe that we have added comprehensive arguments to our understanding of the TNT reduction pathways. For example, when discussing the enzymatic degradation

Figure 4. Overall mechanism of the TNT reduction consistent with the electrochemical data and quantum chemical calculations. 4249



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(MINDEF/NTU, Singapore) is gratefully acknowledged. We thank Prof. R. D. Webster (NTU) for valuable comments.

of TNT, one has to consider two factors: the inherent properties of TNT (studied in this work) and the complicated catalytic machinery of the enzyme that may alter the TNT tendency to be reduced in the ortho- or para-position. The calculations clearly show that the differences in the regioselectivity of the NO2 reduction are minute (∼2 kJ·mol−1) and can be changed by small variations in the enzymatic environment. In this respect, it is not surprising that it is very difficult to predict the regioselectivity by theoretical calculations50 since the well-calibrated and carefully chosen level of theoretical calculations must be used and the solvation and entropy effects included.

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5. CONCLUSIONS In this work, we have attempted to provide a detailed mechanism of the complete electrochemical reduction of TNT into triaminotoluene. This complicated process requires 18 electrons and 18 protons and involves the elimination of six water molecules. By combining electrochemical data (acquired for the TNT, 2-A-4,6-DNT, 4-A-2,6-DNT, 2,4-DA-6-NT, and 2,6-DA-4-NT compounds) with quantum chemical calculations, we arrived at a consensual picture of the mechanism that includes all of the details of all the elementary 1e− and 1H+ steps. The most important finding is that both the experiments and calculations suggest that in the first 6e−/6H+ step the nitro group in the ortho-position (with respect to the methyl group) is reduced. It is likely followed by the reduction of the second ortho-group, although the calculations predicted that the addition of the electron and proton to position 4 of the 2-ADNT intermediate is slightly preferred (∼2 kJ·mol−1). This is, however, compensated by the much higher thermodynamic stability of the “di-ortho” isomer. Comparing the experimental peak potentials with the theoretical calculations, we believe that the relative accuracy of the computed reduction potentials can be estimated as ∼0.1 V, which is probably about the error bar of current computational methods (or even by using the experimental electron affinities and solvation energies). It provides a certain level of confidence for applying the computational protocol for more complex systems (such as the various redox states in metalloenzymes), where the accurate predictions of reduction potentials may serve as a discriminative tool for studying the possible reaction catalytic pathways. Finally, we believe that we have added an important piece of theoretical and experimental evidence to the discussion of TNT degradation, which is an important environmental issue.

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ASSOCIATED CONTENT

S Supporting Information *

The equilibrium geometries (xyz) and electronic energies of all of the molecules studied in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +420-220-183-263. Fax: +420-220-183-578. +65-67911961. E-mail: [email protected]; [email protected].

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ACKNOWLEDGMENTS The financial support from Projects Z40550506 and LC512 (MSMT CR, Czech Republic) and Grant JPP10/07 4250



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Reduction Pathways of 2,4,6-Trinitrotoluene: An Electrochemical

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