Journal of Molecular Modeling

, 20:2091

A DFT study of permanganate oxidation of toluene and its ortho-nitroderivatives

Open Access
Original Paper
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Abstract

Calculations of alternative oxidation pathways of toluene and its ortho-substituted nitro derivatives by permanganate anion have been performed. The competition between methyl group and ring oxidation has been addressed. Acceptable results have been obtained using IEFPCM/B3LYP/6-31+G(d,p) calculations with zero-point (ZPC) and thermal corrections, as validated by comparison with the experimental data. It has been shown that ring oxidation reactions proceed via relatively early transition states that become quite unsymmetrical for reactions involving ortho-nitrosubstituted derivatives. Transition states for the hydrogen atom abstraction reactions, on the other hand, are late. All favored reactions are characterized by the Gibbs free energy of activation, ΔG, of about 25 kcal mol−1. Methyl group oxidations are exothermic by about 20 kcal mol−1 while ring oxidations are around thermoneutrality.

Figure

Oxidation of toluene and its ortho-nitroderivatives

Keywords

Permanganate B3LYP DFT HOMA Nitroaromatic pollutants Toluene 

Introduction

Anthropogenic influence on the natural environment results in the presence of a wide range of aromatic pollutants in soil, sediments, as well as surface- and groundwaters since aromatic compounds are widely used by industries but also they are a component of gasoline and oils [1, 2]. These compounds are of high toxicity, stability and ability of bioaccumulation and depending on the component of the ecosystem in which they are present, they may undergo a transition through the various abiotic or biological processes. In addition, products of such degradation reactions may also pose a significant environmental hazard [1, 2]. In recent years mechanisms of these processes have been intensively studied in search of the best methods for removal of aromatic compounds from the environment.

It has been shown than oxidative degradation of many of these contaminants, both biotic and abiotic, may proceed via two competitive pathways: aromatic ring oxidation and methyl group oxidation [3, 4]. In environmental field studies compound specific isotope analysis (CSIA) is increasingly used for quantitative estimates of ongoing degradation processes. In the case of polynitroaromatic pollutants, such as mono-, dinitrotoluenes, typical analysis of carbon and hydrogen isotope fractionation combined with reaction progress is difficult to establish as outlined in a companion paper [5]. Thus a more fundamental understanding of possible oxidation pathways is essential not only for the selection of an appropriate treatment but also for improvement of CSIA-based accessing of degradation processes of nitroaromatic compounds [6]. As permanganate, the most popular oxidant for the in situ chemical oxidation, is capable of oxidizing both aromatic ring and aliphatic chains [5, 7] we have used it as a model oxidant for our studies of oxidative degradation of common aromatic pollutants.

The mechanism of toluene oxidation by permanganate has been the subject of detailed experimental [8, 9, 10, 11, 12] and theoretical studies [13]. However degradation of nitroaromatic compounds by permanganate was not studied to an extent that would allow one to assess the relative shares of oxidation at the alkyl vs. aryl moieties. Herein we present detailed theoretical study of the rate-determining step of permanganate oxidation of three aromatic pollutants; toluene and its two nitro derivatives, 2-nitrotoluene and 2,6-dinitrotoluene, which were chosen due to their environmental importance.

Methodology

Two DFT functionals M05-2X [14, 15] and B3LYP [16, 17, 18] expressed in 6-31+G(d,p) [19, 20, 21, 22, 23] basis set with aqueous solution modeled by IEFPCM continuum solvent model [24] utilizing the UFF [25] atom radii have been used. These levels of theory have been chosen based on our previous studies [26, 27]. Energy calculations for the selected stationary points have been carried out using the same functionals in combination with a significantly larger basis set, aug-cc-pVTZ [28]. Except for the hydrogen abstraction from the methyl group where unrestricted open shell method [29] was applied, singlet state using default restricted closed shell method was used. All quantum-mechanical calculations were performed using Gaussian package G09 rev. A.02 [30] with default convergence criteria. Vibrational analysis was performed not only to confirm that obtained optimized geometries indeed correspond to stationary points (either local minimum or first order saddle point) on the potential energy surfaces but also to evaluate contributions of vibrational motions to thermochemistry calculations. Merz-Singh-Kollman population analysis [31, 32] has been performed for all obtained stationary points. Transition states of modeled reactions have been located using Berny algorithm [33, 34]. All reaction pathways have been investigated using intrinsic reaction coordinate (IRC) [35] protocol in which end points have been subsequently optimized to either reactants or products. Calculations of reaction pathways probabilities, were based on Eyring-Polanyi equation [36, 37, 38]. The influence of the tunneling was tested using Wigner correction [39]. Aromaticity indexes have been calculated for all structures using reformulated harmonic oscillator model of aromaticity (HOMA) [40, 41]. Bond orders were calculated using Pauling equation [42].

Results and discussion

Toluene

Environmental studies of the oxidation of toluene nitroderivatives by permanganate anion show that hydrogen atom abstraction from the methyl group competes with the ring oxidation. The first, rate-determining step in the case of hydrogen abstraction is formation of the benzyl radical while in the case of the ring oxidation it is formation of the adduct. We have considered a simple model for these processes, i.e., reactions of permanganate anion with toluene using previously employed theory level [26, 27]. Three different regio-selective attacks of permanganate anion on the aromatic ring are possible here as presented in Fig. 1. Results collected in Table 1 show that Gibbs free energy of activation (ΔG) for aromatic ring oxidation is smaller than the one for the hydrogen atom abstraction from methyl group. The percentage contributions of alternative pathways of toluene oxidation, %F, (calculated from ratios of Gibbs free energies of activation) do not, however, agree with the experimental results [5], which indicate that the predominant pathway of this reaction is hydrogen atom abstraction from the methyl group. These results question the applicability of the theory level used previously [26, 27] to the present systems. We have, therefore, started our studies by identifying a theory level that properly describes the competition between the pathways a – d presented in Fig. 1.
Fig. 1

Possible reactions of toluene and with permanganate anion at positions: a Cm, b C1-C2, c C2-C3, d C3-C4

Table 1

Activation Gibbs free energies, exothermicity and contribution of alternative pathways (%F) of oxidation of toluene by permanganate anion at several different theory levels (all with IEFPCM)

Attack

ΔG [kcal mol−1]

ΔGR [kcal mol−1]

% F

M05-2X/6-31+G(d,p)

 Cm

25.0

−71.9

0.0 (0.0)

 C1-C2a

21.0

−53.9

10.4 (10.4)

 C2-C3

19.7

−54.2

86.1 (86.1)

 C3-C4

21.6

−52.4

3.5 (3.5)

M05-2X/6-31+G(d,p) with ZPC and thermal corrections

 Cm

22.2

−66.6

25.6

 C1-C2

22.6

−48.4

12.8

 C2-C3

21.8

−48.1

48.2

 C3-C4

22.6

−47.0

13.3

M05-2X/aug-cc-pVTZ//M05-2X/6-31+G(d,p) with ZPC and thermal corrections

 Cm

18.3

−70.2

99.8

 C1-C2

22.5

−50.2

0.1

 C2-C3

22.7

−50.3

0.1

 C3-C4

26.1

−49.2

0.0

B3LYP/6-31+G(d,p)

 Cm

25.9

−25.9

13.1 (16.5)

 C1-C2

26.1

−5.8

10.2 (9.8)

 C2-C3

25.2

−6.8

43.6 (41.9)

 C3-C4

25.4

−6.4

33.1 (31.8)

B3LYP/6-31+G(d,p) with ZPC and thermal corrections

 Cm

27.2

−20.2

99.7

 C1-C2

32.2

2.4

0.1

 C2-C3

30.8

0.9

0.2

 C3-C4

33.3

3.4

0.0

B3LYP/aug-cc-pVTZ//B3LYP/6-31+G(d,p) with ZPC and thermal corrections

 Cm

24.4

−21.0

99.9

 C1-C2

33.3

4.0

0.0

 C2-C3

32.0

2.6

0.1

 C3-C4

34.6

5.0

0.0

Experimental5

 Cm

  

100

 Ring oxidation

  

0

aFor atom numbering see Fig. 2

Since the percentage contribution of the competing pathways results from the energetic barriers, we have extended IEFPCM/M05-2X/6-31+G(d,p) by including ZPC and thermal corrections and by calculating energies using larger basis set. As can be seen from the results listed in Table 1, only after including all of these correction does one obtain the agreement between experiment and theory. When, however, B3LYP functional has been used instead of M05-2X, even the results obtained with smaller basis set became acceptable when ZPC and thermal corrections were included (see the last three entries in Table 1). Furthermore, inclusion of tunneling correction (values reported in parenthesis in the last column) did not affect the results significantly. Therefore B3LYP/6-31+G(d,p) with ZPC and thermal corrections has been used in the present studies.

Optimized structures of the transition states corresponding to alternative pathways of toluene oxidation by permanganate together with atom numbering used are shown in Fig. 2. In all ring oxidation cases, we have observed formation of the C-O-Mn-O-C ring, which is almost perpendicular to the aromatic ring surface. In the case of methyl group oxidation, the atoms H-O-Mn-O-C form a similar pseudo-cyclic structure. No bridging structures of transition states corresponding to C1-C3, C1-C4 attacks or combining ring carbon with methyl group carbon attack have been observed; all these initial structures converged to one of those presented in Fig. 2.
Fig. 2

Optimized structures of transition states of modeled oxidation reactions of toluene with permanganate anion at positions (from upper, left): Cm, C1-C2, C2-C3, and C3-C4

Geometric results are presented in Table 2. In all reactions involving ring oxidation, similar products differing only in the position of attack, are obtained. Corresponding changes of bond distances are also almost identical in all three cases. The same is true for valence angles despite the fact that initial values differ significantly. These reactions proceed analogically to benzene oxidation [27] however transition state structures are not symmetric due to steric hindrance caused by the methyl group. In the transition state of toluene oxidation at the C1-C2 bond these distances are different and larger than in other cases; C1-O1 bond length equals to 1.98 Å and C2-O2 is 1.96 Å, corresponding to bond orders of 0.22 and 0.23, respectively. Elongation of these bonds is a consequence of the steric hindrance exerted by the methyl group. In reactions in which the attack occurs at C2-C3 and C3-C4 the corresponding values are 1.94 Å and 1.95 Å (bond order of about 0.24) and 1.97 Å (bond orders of 0.22), respectively. Interestingly, the above bond orders for toluene oxidation do not correlate with the barriers as one would expect a slightly earlier transition state for the reaction with lowest barrier. Dihedral angle Φ, defined as C-C-O-Mn, varies for all considered reactions indicating that permanganate anion rotates over the aromatic ring. Interestingly, in the case of both C2-C3 and C3-C4 oxidation, MnO4- rotates in one direction, stops at the transition state (dihedral angles are almost 0), and then rotates back but in the case of C1-C2 attack rotation is in one direction only which again may be ascribed to the presence of steric hindrance.
Table 2

Selected geometric parameters (d – distances in Å, α,Φ – angles in °) and HOMA aromaticity indexes of modeled oxidation processes of toluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

Toluene Cm attack

Toluene C1-C2 attack

 dCm-H

1.098

1.521

2.448

dC1-C2

1.403

1.451

1.552

 dH-O1

2.583

1.103

0.969

dC1-O1

4.430

1.978

1.450

 dCm-O2

4.245

2.726

1.414

dC2-O2

4.564

1.957

1.435

 dO1-Mn

1.600

1.703

1.819

dO1-Mn

1.600

1.658

1.804

 dO2-Mn

1.600

1.629

1.822

dO2-Mn

1.600

1.661

1.806

 αO1-H-Cm

169.5

179.3

116.8

αO1-C1-C2

109.0

103.9

106.7

 αH-Cm-O2

35.5

54.6

80.2

αC1-C2-O2

86.7

106.6

107.9

 αCm-O2-Mn

94.3

103.8

126.4

αC2-O2-Mn

100.4

115.5

115.0

 αO2-Mn-O1

109.4

95.5

97.9

αO2-Mn-O1

109.5

97.4

87.8

 αMn-O1-H

117.4

105.4

112.1

αMn-O1-C1

94.8

116.1

115.3

 ΦC1-Cm-O2-Mn

−89.1

3.3

−82.3

ΦC1-Cm-O2-Mn

29.5

−5.6

−23.7

 HOMA

0.958

0.909

0.962

HOMA

0.959

0.459

−1.890

Toluene C2-C3 attack

Toluene C3-C4 attack

 dC2-C3

1.398

1.444

1.537

dC3-C4

1.398

1.444

1.538

 dC2-O1

4.589

1.942

1.443

dC3-O1

4.730

1.969

1.437

 dC3-O2

4.936

1.968

1.435

dC4-O2

4.586

1.945

1.443

 dO1-Mn

1.600

1.662

1.807

dO1-Mn

1.600

1.661

1.808

 dO2-Mn

1.600

1.660

1.810

dO2-Mn

1.600

1.662

1.807

 αO1-C2-C3

102.4

105.8

107.4

αO1-C3-C4

78.1

105.0

107.3

 αC2-C3-O2

92.1

105.4

107.1

αC3-C4-O2

115.0

106.2

107.9

 αC3-O2-Mn

97.3

115.3

114.3

αC4-O2-Mn

110.0

115.8

113.9

 αO2-Mn-O1

109.5

97.4

87.9

αO2-Mn-O1

109.4

97.4

80.0

 αMn-O1-C2

106.4

116.1

113.7

αMn-O1-C3

123.4

115.5

114.8

 ΦC1-Cm-O2-Mn

34.1

1.4

25.9

ΦC1-Cm-O2-Mn

−28.9

0.1

−25.2

 HOMA

0.958

0.485

−1.722

HOMA

0.958

0.490

−1.701

We have carried out calculations of HOMA indices to compare how addition at different positions of the ring influences the aromaticity. Indices collected in Tables 2 and 3 indicate that dearomatization during all toluene ring oxidation reactions increases as the attack occurs closer to the methyl group. Interestingly, this trend is opposite to the one that could be expected from the C-O bond lengths in the corresponding transition state structures; the shortest being observed for C2-C3 (average of 1.55 Å) and the longest for C1-C2 attack (average of 1.97 Å). This result of the steric hindrance exerted by the neighboring methyl group illustrates how subtle the balance is between different factors influencing reactivity in the opposite directions.
Table 3

Bond lengths in aromatic rings (in Å) for HOMA analysis of modeled oxidation processes of toluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

Toluene Cm attack

Toluene C1-C2 attack

 C1-C2

1.405

1.417

1.403

C1-C2

1.403

1.451

1.552

 C2-C3

1.398

1.393

1.398

C2-C3

1.399

1.438

1.508

 C3-C4

1.399

1.402

1.399

C3-C4

1.398

1.369

1.344

 C4-C5

1.398

1.402

1.398

C4-C5

1.399

1.434

1.466

 C5-C6

1.399

1.393

1.399

C5-C6

1.398

1.368

1.345

 C1-C6

1.404

1.417

1.403

C1-C6

1.405

1.445

1.515

 HOMA

0.958

0.909

0.962

HOMA

0.959

0.459

−1.890

 EN

0.040

0.066

0.037

EN

0.039

0.224

1.157

 GEO

0.002

0.025

0.001

GEO

0.002

0.317

1.733

Toluene C2-C3 attack

Toluene C3-C4 attack

 C1-C2

1.404

1.449

1.519

C1-C2

1.404

1.373

1.347

 C2-C3

1.398

1.444

1.537

C2-C3

1.399

1.438

1.509

 C3-C4

1.399

1.437

1.508

C3-C4

1.398

1.444

1.538

 C4-C5

1.399

1.369

1.345

C4-C5

1.399

1.440

1.509

 C5-C6

1.399

1.434

1.467

C5-C6

1.398

1.368

1.345

 C1-C6

1.404

1.372

1.349

C1-C6

1.405

1.442

1.475

 HOMA

0.958

0.485

−1.722

HOMA

0.958

0.490

−1.701

 EN

0.040

0.224

1.128

EN

0.040

0.224

1.117

 GEO

0.002

0.291

1.594

GEO

0.002

0.286

1.584

The above differences regarding the reaction advancement in the transition state gathered from electronic and geometric data were further investigated by performing Merz-Singh-Kollman population analysis (see Table 4). The analysis revealed that initial charges in the reactant complex with orientation for the attack at the C1-C2 bond are 0.38 a.u. for C1 and −0.32 a.u. at C2. In the transition state the charge on C1 remains unchanged while that on C2 becomes positive (0.25 a.u.). In the other two reactions these charges systematically and simultaneously increase. In all cases attacked carbons become positively charged in product followed by increasing negative charge on both attacking oxygen atoms and reduction of positive charge on manganese atom.
Table 4

Charge distribution based on Merz-Singh-Kollman population analysis of selected atoms in modeled oxidation processes of toluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

Toluene Cm

Toluene C1-C2 attack

 H

0.202

0.435

0.396

C1

0.383

0.381

0.782

 Cm

−0.628

−0.782

0.196

C2

−0.324

0.246

0.638

 O1

−0.543

−0.708

−0.867

O1

−0.546

−0.567

−0.721

 O2

−0.539

−0.632

−0.677

O2

−0.546

−0.572

−0.764

 Mn

1.163

1.219

1.244

Mn

1.192

1.172

1.152

 O3

−0.542

−0.619

−0.672

O3

−0.553

−0.646

−0.659

 O4

−0.544

−0.619

−0.662

O4

−0.551

−0.668

−0.669

Toluene C2-C3 attack

Toluene C3-C4 attack

 C2

−0.320

−0.067

0.401

C3

−0.105

0.213

0.347

 C3

−0.107

0.220

0.327

C4

−0.177

−0.054

0.67

 O1

−0.543

−0.485

−0.635

O1

−0.555

−0.544

−0.675

 O2

−0.546

−0.519

−0.671

O2

−0.555

−0.524

−0.680

 Mn

1.183

1.110

1.150

Mn

1.226

1.176

1.108

 O3

−0.543

−0.636

−0.660

O3

−0.560

−0.653

−0.650

 O4

−0.552

−0.663

−0.671

O4

−0.559

−0.675

−0.661

In the reaction of hydrogen atom abstraction one of the oxygen atoms attacks the hydrogen atom of methyl group while another oxygen atoms moves in the direction of the methyl carbon (and in fact, in the subsequent step the Cm-O bond is formed). The pseudo-cyclic H-O-Mn-O-C structure is nearly perpendicular to the aromatic ring. The geometry of this part of the transition state structure is similar to the one obtained with higher basis set [13] although breaking the toluene C-H bond at IEFPCM/B3LYP/6-31+G(d,p) has the length of about 1.52 Å (bond order of 0.30) while it is 1.67 Å in the case of B3LYP/6-311++G(d,p) while forming O-H bond is about 1.1 Å (bond order of 0.68) in the case of IEFPCM/B3LYP/6-31+G(d,p) and 1.05 Å in the case of higher basis set. A small difference is also observed in the forming C-O bond; 2.73 Å in the case of smaller basis set (bond order of 0.02) and 2.70 Å in the case of higher. Evolution of the dihedral angle throughout oxidation of toluene is quite interesting. These values change from −89.1° for reactants through 3.3° at the transition state to −82.3° for products.

Analysis of HOMA indices for the methyl group oxidation is also interesting; temporary lowering of aromaticity is observed in the transition state. Analysis of HOMA factors indicate that in this case bond elongation term (EN) is responsible for the change, as opposite to the ring oxidation reactions where it was caused by the bond alternation term (GEO). Population analysis reveals that in the case of methyl group oxidation environment has lower influence on the charge distribution then in the case of aromatic ring oxidation. The attacked carbon atoms become more negatively charged in the transition state (change of 0.15 a.u.) but in the products they are positively charged. Positive charge located initially on the abstracted hydrogen atom (about 0.20 a.u.) increases in the transition state (about 0.43 a.u.) and decreases in products (about 0.40 a.u.). These results do not support earlier suggestions of the hydride transfer in the toluene oxidation by permanganate [10].

2-Nitro- and 2,6-dinitrotoluene

We have selected these two compounds as models because of the extreme differences in relative contributions of alternative oxidation pathways observed for them experimentally. While in the case of symmetrically substituted dinitroderivative alternative pathways are similar to those found for toluene the situation is more complicated in the case of monosubstitution since all six possible ring oxidation processes lead to different products; schematic representation of all possible pathways is given in Fig. 3. In Tables 5, 6, 7, 8, 9, 10 and 11 corresponding energetic parameters and resulting percentage contributions of each alternative reaction in the overall conversion of 2-nitrotoluene and 2,6-nitrotoluene are collected.
Fig. 3

Modeled oxidation reactions of 2-nitrotoluene (R1 = NO2, R2 = H) and 2,6-dinitrotoluene (R1, R2 = NO2) with permanganate at positions: a Cm, b C1-C2, c C2-C3, d C3-C4, e C4-C5, f C5-C6, g C1-C6

Table 5

Activation Gibbs free energies, exothermicity, contribution of alternative pathways and comparison of methyl group vs. ring oxidation with experimental data in oxidation reactions of 2-nitrotoluene and 2,6-dinitrotoluene by permanganate anion

Attack

ΔG [kcal mol−1]

ΔGR [kcal mol−1]

% F

% FDFT/% Fexp5

2-nitrotoluene

 Cm

25.8

−19.1

93.1

93/87

 C1-C2

29.9

−5.0

0.1

7/13

 C2-C3

28.5

−7.7

1.0

 C3-C4

27.9

−2.7

3.2

 C4-C5

28.9

0.2

0.6

 C5-C6

28.2

−0.7

1.8

 C1-C6

29.4

−0.4

0.2

2,6-dinitrotoluene

 Cm

24.2

−20.3

67.6

68/58

 C1-C2

28.0

−8.3

0.1

32/42

 C2-C3

26.1

−11.0

2.5

 C3-C4

24.7

−2.2

29.8

Table 6

Selected geometric parameters (d – distances in Å, α,Φ – angles in °) of modeled oxidation processes of 2,6-dinitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2,6-dinitrotoluene Cm attack

2,6-dinitrotoluene C1-C2 attack

 dCm-H

1.096

1.593

2.863

dC1-C2

1.409

1.475

1.582

 dm-O1

2.434

1.077

0.966

dC1-O1

4.056

1.711

1.418

 dCm-O2

3.828

2.897

1.407

dC2-O2

4.325

2.400

1.369

 dO1-Mn

1.601

1.701

1.814

dO1-Mn

1.599

1.679

1.817

 dO2-Mn

1.599

1.604

1.822

dO2-Mn

1.599

1.609

1.844

 αO1-H-Cm

169.4

176.5

116.9

αO1-C1-C2

76.7

107.3

105.5

 αH-Cm-O2

44.1

55.1

60.8

αC1-C2-O2

119.1

100.1

109.5

 αCm-O2-Mn

98.3

100.4

141.7

αC2-O2-Mn

99.7

103.2

115.9

 αO2-Mn-O1

109.2

99.3

101.2

αO2-Mn-O1

109.4

100.9

86.1

 αMn-O1-H

112.1

108.8

111.5

αMn-O1-C1

134.3

127.2

141.8

 ΦC1-Cm-O2-Mn

83.9

−22.9

−21.3

ΦC1-C2-O2-Mn

−12.4

9.3

17.6

2,6-dinitrotoluene C2-C3 attack

2,6-dinitrotoluene C3-C4 attack

 dC2-C3

1.396

1.457

1.562

dC3-C4

1.391

1.444

1.536

 dC2-O1

3.857

2.314

1.363

dC3-O1

3.358

1.725

1.423

 dC3-O2

3.315

1.678

1.427

dC4-O2

4.085

2.229

1.433

 dO1-Mn

1.599

1.614

1.841

dO1-Mn

1.600

1.689

1.817

 dO2-Mn

1.603

1.692

1.807

dO2-Mn

1.599

1.623

1.815

 αO1-C2-C3

61.2

99.3

110.2

αO1-C3-C4

124.3

109.2

107.6

 αC2-C3-O2

129.6

110.8

107.9

αC3-C4-O2

75.7

101.5

108.6

 αC3-O2-Mn

104.6

124.5

115.5

αC4-O2-Mn

111.6

106.8

113.7

 αO2-Mn-O1

108.9

99.9

86.8

αO2-Mn-O1

109.3

99.4

87.7

 αMn-O1-C2

116.9

105.4

116.4

αMn-O1-C3

117.9

122.6

114.8

 ΦC2-C3-O2-Mn

57.7

2.9

−18.5

ΦC3-C4-O2-Mn

11.8

−7.4

−24.5

Table 7

Charge distribution based on Merz-Singh-Kollman population analysis of selected atoms in modeled oxidation processes of 2,6-dinitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2,6-dinitrotoluene Cm attack

2,6-dinitrotoluene C1-C2 attack

 H

0.254

0.403

0.373

C1

0.045

0.136

0.289

 Cm

−0.695

−0.745

−0.048

C2

0.104

0.330

0.941

 O1

−0.532

−0.650

−0.820

O1

−0.539

−0.413

−0.618

 O2

−0.532

−0.541

−0.587

O2

−0.539

−0.473

−0.711

 Mn

1.148

1.242

1.201

Mn

1.186

1.087

1.103

 O3

−0.543

−0.533

−0.647

O3

−0.552

−0.520

−0.585

 O4

−0.536

−0.538

−0.640

O4

−0.551

−0.523

−0.589

2,6-dinitrotoluene C2-C3 attack

2,6-dinitrotoluene C3-C4 attack

 C2

0.070

0.071

0.293

C3

−0.302

−0.012

0.136

 C3

−0.259

0.221

0.658

C4

0.023

−0.139

0.399

 O1

−0.525

−0.477

−0.640

O1

−0.518

−0.450

−0.570

 O2

−0.537

−0.479

−0.694

O2

−0.533

−0.462

−0.607

 Mn

1.161

1.171

1.155

Mn

1.151

1.107

1.070

 O3

−0.542

−0.538

−0.593

O3

−0.542

−0.542

−0.597

 O4

−0.535

−0.546

−0.606

O4

−0.543

−0.555

−0.613

Table 8

Bond lengths in aromatic rings (in Å) for HOMA analysis of modeled oxidation processes of 2,6-dinitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2,6-dinitrotoluene Cm attack

2,6-dinitrotoluene C1-C2 attack

 C1-C2

1.410

1.444

1.412

C1-C2

1.409

1.475

1.582

 C2-C3

1.396

1.394

1.396

C2-C3

1.396

1.405

1.508

 C3-C4

1.391

1.391

1.391

C3-C4

1.391

1.379

1.344

 C4-C5

1.391

1.391

1.391

C4-C5

1.391

1.407

1.450

 C5-C6

1.396

1.395

1.396

C5-C6

1.396

1.378

1.349

 C1-C6

1.408

1.444

1.412

C1-C6

1.409

1.484

1.537

 HOMA

0.956

0.726

0.944

HOMA

0.956

0.243

−2.502

 EN

0.029

0.123

0.035

EN

0.029

0.286

1.398

 GEO

0.015

0.151

0.021

GEO

0.015

0.470

2.104

2,6-dinitrotoluene C2-C3 attack

2,6-dinitrotoluene C3-C4 attack

Parameter

R

TS

P

Parameter

R

TS

P

 C1-C2

1.408

1.435

1.527

C1-C2

1.409

1.386

1.353

 C2-C3

1.396

1.457

1.562

C2-C3

1.396

1.459

1.517

 C3-C4

1.392

1.459

1.500

C3-C4

1.391

1.444

1.536

 C4-C5

1.391

1.353

1.339

C4-C5

1.391

1.395

1.501

 C5-C6

1.397

1.433

1.463

C5-C6

1.396

1.389

1.342

 C1-C6

1.409

1.392

1.354

C1-C6

1.409

1.437

1.478

 HOMA

0.957

0.344

−2.063

HOMA

0.956

0.543

−1.695

 EN

0.030

0.289

1.245

EN

0.029

0.237

1.140

 GEO

0.013

0.367

1.819

GEO

0.015

0.220

1.556

Table 9

Selected geometric parameters (d – distances in Å, α,Φ – angles in °) of modeled oxidation processes of 2-nitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2-nitrotoluene Cm attack

    

 dCm-H

1.095

1.538

2.504

    

 dH-O1

2.559

1.097

0.967

    

 dCm-O2

4.066

2.862

1.412

    

 dO1-Mn

1.600

1.700

1.816

    

 dO2-Mn

1.599

1.615

1.824

    

 αO1-H-Cm

166.0

178.5

116.0

    

 αH-Cm-O2

46.6

54.4

81.6

    

 αCm-O2-Mn

95.8

100.6

127.7

    

 αO2-Mn-O1

109.4

97.9

99.3

    

 αMn-O1-H

117.1

107.9

115.3

    

 ΦC1-Cm-O2-O1

−85.0

−94.2

−81.2

    

2-nitrotoluene C1-C2 attack

2-nitrotoluene C2-C3 attack

 dC1-C2

1.412

1.469

1.571

dC2-C3

1.401

1.455

1.549

 dC1-O1

4.229

1.729

1.431

dC2-O1

3.597

2.234

1.368

 dC2-O2

3.957

2.268

1.345

dC3-O2

4.376

1.718

1.427

 dO1-Mn

1.599

1.685

1.808

dO1-Mn

1.599

1.627

1.844

 dO2-Mn

1.599

1.623

1.838

dO2-Mn

1.599

1.688

1.810

 αO1-C1-C2

73.2

108.3

106.3

αO1-C2-C3

118.2

100.5

110.2

 αC1-C2-O2

122.4

100.9

109.6

αC2-C3-O2

80.1

110.1

107.4

 αC2-O2-Mn

104.2

106.9

116.3

αC3-O2-Mn

108.3

122.9

115.1

 αO2-Mn-O1

109.4

98..9

86.4

αO2-Mn-O1

109.4

98.9

86.8

 αMn-O1-C1

115.4

124.6

116.1

αMn-O1-C2

121.8

107.6

115.3

 ΦC1-C2-O2-O1

−8.7

4.8

15.3

ΦC2-C3-O2-O1

−8.3

1.6

21.4

2-nitrotoluene C3-C4 attack

2-Nitrotoluene C4-C5 attack

 dC3-C4

1.389

1.440

1.532

dC4-C5

1.399

1.447

1.534

 dC3-O1

3.931

1.846

1.436

dC4-O1

4.397

2.099

1.425

 dC4-O2

4.478

2.055

1.427

dC5-O2

4.347

1.788

1.439

 dO1-Mn

1.599

1.671

1.813

dO1-Mn

1.599

1.641

1.819

 dO2-Mn

1.599

1.645

1.817

dO2-Mn

1.600

1.68

1.813

 αO1-C3-C4

121.0

106.9

107.1

αO1-C4-C5

109.9

103.3

107.9

 αC3-C4-O2

121.7

104.6

106.9

αC4-C5-O2

86.2

107.9

107.4

 αC4-O2-Mn

76.2

111.7

113.7

αC5-O2-Mn

119.5

120.1

113.4

 αO2-Mn-O1

115.7

98.0

87.7

αO2-Mn-O1

109.5

98.1

87.9

 αMn-O1-C3

109.4

118.8

113.2

αMn-O1-C1

105.1

110.3

113.8

 ΦC3-C4-O2-O1

−1.5

−0.3

28.0

ΦC4-C5-O2-O1

16.5

−4.9

27.2

2-nitrotoluene C5-C6 attack

2-nitrotoluene C1-C6 attack

 dC5-C6

1.395

1.443

1.534

dC1-C6

1.405

1.459

1.562

 dC5-O1

3.501

1.811

1.432

dC1-O1

3.774

1.833

1.437

 dC6-O2

4.128

2.099

1.435

dC6-O2

4.375

2.087

1.426

 dO1-Mn

1.601

1.676

1.811

dO1-Mn

1.599

1.671

1.810

 dO2-Mn

1.600

1.640

1.816

dO2-Mn

1.600

1.642

1.813

 αO1-C5-C6

132.9

107.8

106.9

αO1-C1-C6

124.1

105.4

106.3

 αC5-C6-O2

58.6

103.2

108.0

αC1-C6-O2

73.4

105.1

107.1

 αC6-O2-Mn

116.9

110.7

113.4

αC6-O2-Mn

115.0

110.6

114.5

 αO2-Mn-O1

108.9

98.2

87.8

αO2-Mn-O1

109.4

98.1

87.4

 αMn-O1-C2

160.1

119.6

114.4

αMn-O1-C1

115.0

120.7

115.1

 ΦC5-C6-O2-O1

41.6

6.7

26.5

ΦC1-C6-O2-O1

−21.2

−0.2

−27.5

Table 10

Charge distribution based on Merz-Singh-Kollman population analysis of selected atoms in modeled oxidation processes of 2,6-dinitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2-nitrotoluene Cm attack

 

 H

0.214

0.395

0.474

    

 Cm

−0.585

−0.824

0.294

    

 O1

−0.538

−0.648

−0.915

    

 O2

−0.543

−0.574

−0.713

    

 Mn

1.175

1.212

1.285

    

 O3

−0.546

−0.567

−0.671

    

 O4

−0.548

−0.570

−0.674

    

2-nitrotoluene C1-C2 attack

2-nitrotoluene C2-C3 attack

 C1

0.191

0.382

0.472

C2

0.055

0.136

0.638

 C2

0.126

0.451

1.042

C3

−0.239

0.141

0.400

 O1

−0.534

−0.524

−0.716

O1

−0.551

−0.495

−0.631

 O2

−0.528

−0.529

−0.772

O2

−0.556

−0.485

−0.576

 Mn

1.160

1.163

1.165

Mn

1.240

1.133

1.110

 O3

−0.550

−0.582

−0.618

O3

−0.568

−0.570

−0.597

 O4

−0.541

−0.590

−0.622

O4

−0.561

−0.585

−0.612

2-nitrotoluene C3-C4 attack

2-nitrotoluene C4-C5 attack

 C3

−0.271

0.063

0.408

C4

−0.142

−0.144

0.159

 C4

−0.090

−0.063

0.187

C5

−0.065

0.319

0.634

 O1

−0.516

−0.484

−0.620

O1

−0.540

−0.490

−0.626

 O2

−0.531

−0.498

−0.640

O2

−0.540

−0.512

−0.662

 Mn

1.129

1.137

1.130

Mn

1.176

1.121

1.074

 O3

−0.534

−0.605

−0.638

O3

−0.545

−0.590

−0.616

 O4

−0.540

−0.624

−0.642

O4

−0.549

−0.610

−0.628

2-nitrotoluene C5-C6 attack

2-nitrotoluene C1-C6 attack

 C5

−0.127

0.342

0.335

C1

0.234

0.440

0.780

 C6

−0.193

−0.305

0.414

C6

−0.200

0.058

0.420

 O1

−0.545

−0.508

−0.671

O1

−0.520

−0.537

−0.702

 O2

−0.544

−0.474

−0.619

O2

−0.536

−0.528

−0.713

 Mn

1.188

1.093

1.114

Mn

1.154

1.162

1.169

 O3

−0.548

−0.579

−0.629

O3

−0.543

−0.604

−0.644

 O4

−0.547

−0.601

−0.638

O4

−0.547

−0.626

−0.653

Table 11

Bond lengths in aromatic rings (in Å) for HOMA analysis of modeled oxidation processes of 2-nitrotoluene with permanganate ions (R – reactants, TS – transition states, P – products) at IEFPCM/B3LYP/6-31+G(d,p) theory level

Parameter

R

TS

P

Parameter

R

TS

P

2-nitrotoluene Cm attack

 

 C1-C2

1.412

1.432

1.406

    

 C2-C3

1.401

1.406

1.401

    

 C3-C4

1.389

1.385

1.390

    

 C4-C5

1.399

1.407

1.399

    

 C5-C6

1.395

1.383

1.394

    

 C1-C6

1.405

1.426

1.403

    

 HOMA

0.948

0.824

0.962

    

 EN

0.038

0.088

0.030

    

 GEO

0.014

0.088

0.008

    

2-nitrotoluene C1-C2 attack

2-nitrotoluene C2-C3 attack

 C1-C2

1.412

1.469

1.571

C1-C2

1.412

1.438

1.525

 C2-C3

1.401

1.422

1.509

C2-C3

1.401

1.455

1.549

 C3-C4

1.389

1.369

1.343

C3-C4

1.389

1.457

1.506

 C4-C5

1.399

1.431

1.464

C4-C5

1.399

1.361

1.343

 C5-C6

1.395

1.360

1.342

C5-C6

1.395

1.430

1.464

 C1-C6

1.405

1.473

1.519

C1-C6

1.405

1.379

1.348

 HOMA

0.948

0.230

−2.230

HOMA

0.948

0.385

−1.921

 EN

0.038

0.275

1.263

EN

0.038

0.264

1.186

 GEO

0.014

0.495

1.967

GEO

0.014

0.351

1.735

2-nitrotoluene C3-C4 attack

2-nitrotoluene C4-C5 attack

 C1-C2

1.412

1.395

1.370

C1-C2

1.412

1.449

1.481

 C2-C3

1.401

1.449

1.513

C2-C3

1.401

1.382

1.344

 C3-C4

1.389

1.440

1.532

C3-C4

1.389

1.412

1.503

 C4-C5

1.399

1.421

1.502

C4-C5

1.399

1.447

1.534

 C5-C6

1.395

1.376

1.346

C5-C6

1.395

1.451

1.508

 C1-C6

1.405

1.429

1.466

C1-C6

1.405

1.369

1.348

 HOMA

0.948

0.597

−1.471

HOMA

0.948

0.478

−1.625

 EN

0.038

0.237

1.151

EN

0.038

0.237

1.089

 GEO

0.014

0.166

1.320

GEO

0.014

0.284

1.537

2-nitrotoluene C5-C6 attack

2-nitrotoluene C1-C6 attack

 C1-C2

1.412

1.395

1.359

C1-C2

1.412

1.467

1.527

 C2-C3

1.401

1.436

1.467

C2-C3

1.401

1.382

1.354

 C3-C4

1.389

1.359

1.340

C3-C4

1.389

1.411

1.448

 C4-C5

1.399

1.449

1.504

C4-C5

1.399

1.379

1.347

 C5-C6

1.395

1.443

1.534

C5-C6

1.395

1.416

1.503

 C1-C6

1.405

1.429

1.527

C1-C6

1.405

1.459

1.562

 HOMA

0.948

0.501

−1.726

HOMA

0.948

0.454

−1.975

 EN

0.038

0.240

1.163

EN

0.038

0.248

1.221

 GEO

0.014

0.259

1.564

GEO

0.014

0.298

1.754

As can be seen ring attack probability increases with the increase of nitro groups attached to the aromatic ring. In the case of 2-nitrotoluene the obtained activation Gibbs free energy of methyl group oxidation suggests that reaction proceeds almost exclusively through the methyl group oxidation (93 %). In the case of 2,6-dinitrotoluene oxidation the it is only around 68 % with the remaining 32 % proceeding mostly by the attack at the C3-C4 bond. This is probably caused by the electron-withdrawing properties of this substituent, which may have negative influence on stabilization of the transition state of methyl group oxidation.

In the case of ring oxidation of nitroaromatics geometries of transition states differ significantly from those obtained for benzene [27] and toluene. Opposite to both C-O bonds (of about 1.95 Å) being nearly equally advanced in the transition state, in the case of toluene nitroderivatives these bonds are quite different; one of them oscillates around 1.7 Å corresponding to bond being nearly half formed (bond order of about 0.45), while the C-O distance remains quite large, around 2.2 Å indicating that the formation of this bond hardly started (bond order of about 0.1). This asymmetry is smaller in the case of 2-nitrotoluene and diminishes slightly with the distance from the nitrosubstituent, with C-O forming bond lengths being about 1.8 and 2.1 Å. This is paralleled by significantly stronger dearomatization occurring in the transition states of ring oxidation of 2-nitro- and 2,6-dinitrotoluene for the attack involving C1-C2 and C2-C3 bonds. Charge distribution on attacking oxygen atoms follows the same pattern in all reactions of initial slight increase from about −0.55 a.u. to −0.48 a.u. on the transition from the reactants to the transition state and final decrease in the products to average of −0.65 a.u. With the sole exception of the unusually small partial charge on the C6 atom (−0.31 a.u.) in the reaction proceeding with the attack on the C5-C6 bond, atomic charges on the attacked carbon atoms, on the other hand, generally increase systematically from the reactant complex to the transition state to product although absolute changes between reactions are very diverse.

As illustrated in Fig. 4 geometries of the transition states of the methyl group oxidation of the considered nitroderivatives are significantly different. In the case of 2,6-dinitrotoluene the structure is almost symmetric and very similar to the one observed in the corresponding toluene oxidation. In the case of 2-nitrotoluene, however, the permanganate anion is rotated about 90 degrees relative to the Cm-C1 bond. Changes of the dihedral angle Φ throughout the 2,6-dinitrotoluene oxidation molecule are similar to those observed in the case of toluene. In the case of 2-nitrotolune, however, these changes are negligible; the dihedral angle changes from −85° in reactants complex to −94° in the transition state to −81° in the product.
Fig. 4

Transition state structures of methyl group oxidation in modeled oxidation reactions of 2-nitrotoluene and 2,6-dinitrotoluene with permanganate

The length of the breaking C-H bond in 2,6-dinitrotoluene transition state is 1.59 Å, which corresponds to the bond order of 0.24, and is longer than in the case of mono-nitrosubstituted derivative where the corresponding values to 1.54 Å and 0.29, respectively. Analogously, the forming O-H bond in the doubly substituted derivative transition state is 1.08 Å (bond order of 0.73) and is noticeably shorter than in the case of 2-nitrotoluene where the corresponding values are 1.10 Å and 0.69. These results indicate that in both reactions the transition states are late. The overall trend obtained in our studies shows, in agreement with expectations, the increasingly later transition state in the order: toluene, 2-nitrotoluene, 2,6-nitrotoluene. This sequence agrees with the calculated ring dearomatization in the transition state, which increases from toluene to 2-nitrotoluene and 2,6-nitrotoluene with the corresponding HOMA indices equal to 0.91, 0.82 and 0.73, respectively. Partial atomic charges of reacting C · · · H · · · O atoms, on the other hand, do not reveal any significant differences; all changes follow the same pattern although the absolute values differ.

Conclusions

We have performed calculations of alternative oxidation pathways of toluene and its ortho-substituted nitroderivatives by permanganate anion. Based on the obtained structures of reactants and transition states kinetic isotope effects for each carbon and nitrogen position and subsequently averaged elemental isotopic fractionation have been calculated. These values, compared with experimentally determined ones [5], validated the used theory level. This combination of theoretical and experimental analysis greatly enhances our understanding of oxidative degradation processes of common environmentally important aromatic pollutants.

Our studies show that the preference of the attack position of permanganate anion in oxidation reactions with selected aromatic compounds changes with positions and number of substituents in aromatic ring. On the example of the well studied [8, 9, 10, 11, 12] case of toluene oxidation we have shown that the correct preference of methyl group oxidation is predicted when Gibbs free energies from IEFPCM/B3LYP/6-31+G(d,p) calculations, including ZPC and thermal corrections, are used. Furthermore, applying continuum solvent model results in slightly earlier transition states than in the corresponding reaction modeled in gas phase [13]. Obtained charge distribution does not support hydride transfer in toluene oxidation by permanganate. For nitrosubstituted derivatives competitive ring oxidation has been predicted in agreement with the experiment.

From the chemical point of view, ring oxidation reactions proceed via relatively early transition states that become quite unsymmetrical for reactions involving ortho-nitrosubstituted derivatives. Transition states for the hydrogen atom abstraction reactions, on the other hand, are late, with C-H bond breaking advanced in about 70 %. All favored reactions are characterized by the Gibbs free energy of activation of about 25 kcal mol−1. Methyl group oxidations are exothermic by about 20 kcal mol−1 while ring oxidations are around thermoneutrality.

Notes

Acknowledgments

This work is supported by the grant PSRP-025/2010 from the Polish-Swiss Research Program. Computing time at the ACK Cyfronet AGH (Krakow, Poland) is gratefully acknowledged.

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Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

Authors and Affiliations

  1. 1.Institute of Applied Radiation Chemistry, Faculty of ChemistryLodz University of TechnologyLodzPoland
  2. 2.EawagSwiss Federal Institute of Aquatic Science and TechnologyDübendorfSwitzerland

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