Computational Elucidation of Mechanisms of Alkaline Hydrolysis of

Article pubs.acs.org/JPCA

Computational Elucidation of Mechanisms of Alkaline Hydrolysis of Nitrocellulose: Dimer and Trimer Models with Comparison to the Corresponding Monomer Manoj K. Shukla* and Frances Hill US Army Engineer Research and Development Center, Environmental Laboratory, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180-6199, United States ABSTRACT: Density functional theory (DFT) investigation has been undertaken to explore alkaline hydrolysis mechanisms for nitrocellulose in the gas phase and in bulk water solution by considering the dimer and trimer forms of 2,3,6trinitro-β-D-glucopyranose in the 4C1 chair conformation and by comparing the computed results with the monomer. Ground and transition state geometries were optimized using the B3LYP functional and the 6-311G(d,p) basis set both in the gas phase and in the bulk water solution. The nature of respective potential energy surfaces was ascertained through harmonic vibrational frequency analysis. Intrinsic reaction coordinate calculations were performed to ensure that computed transition state connects to the respective reactants and products. Single-point energy calculations were also performed using the recently developed M06-2X functional and the ccpVTZ basis set using the B3LYP/6-311G(d,p) optimized geometries. Effect of the bulk water solution was modeled using the polarizable continuum model (PCM) approach. It has been suggested that the dimeric form of 2,3,6-trinitro-β-D-glucopyranose can be considered as the smallest model to study the nitrocellulose system regarding the alkaline hydrolysis reaction. It was predicted that the peeling-off reaction will start after the denitration of various sites, which will follow a C3 → C6 → C2 denitration route. Further, it was determined that the peeling-off reaction will be more preferred than the ring cleavage through the ring CO bond.

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nitrocellulose.3,5,6,8−13 It has been suggested that irradiation techniques can be excellent at the laboratory scale, but for fieldscale, it may not be cost-effective.3,13 Of the two hydrolysis approaches, alkaline hydrolysis has been found to be more effective than acidic hydrolysis for the destruction of nitrocellulose.5,12 To determine the alkaline hydrolysis reaction profiles of nitrocellulose, we have recently performed detailed theoretical investigations at the density functional theory (DFT) level exploring the possibilities of potential reaction pathways.14,15 In these investigations, the trinitro form of β-D-glucopyranose (2,3,6-trinitro-β-D-glucopyranose) in the 4C1 chair conformation as monomer of nitrocellulose and the α-anomeric analogue were considered. For these investigations, it was assumed that alkaline hydrolysis will follow a multistep process under the SN2 framework leading to denitration of a nitrate group through addition−elimination reactions at the C2, C3, and C6 sites of the monomer. On the basis of these computational investigations,14,15 we found that the initial reaction in the addition−elimination mechanism involves the formation of a reactant complex between the monomer and the OH− ion and that the reaction progresses through the transition state in

INTRODUCTION Nitrocellulose, which is the nitrated form of cellulose and also called as guncotton, is an energetic material and has been used for military as well as civilian applications for a long time.1−5 It is prepared by the nitration of cotton with nitric acid under acidic conditions, and the quality of cotton is a key factor in determining the degree of nitration.2 In the environment, particulates of nitrocellulose are stable and very resistant to bacterial decomposition.5,6 The higher environmental stability and industrial scale production of nitrocellulose can lead to contamination in a variety of environments including military ranges. Recent sampling by the Department of Defense (DoD) on live-fire training ranges suggest the persistence of propellant residues on soil surfaces.7 These residues occur in the form of discrete fibrous particles where 2,4-dinitrotoluene (2,4-DNT) is embedded within the nitrocellulose. On the basis of laboratory experiments, it has been found that nitrocellulose is difficult to elute from soil samples collected on operational ranges. Further, it is recalcitrant toward hydrolysis under environmentally relevant conditions.7 Therefore, a proper and effective technology is needed to decontaminate the affected environment. Since cellulose itself is nontoxic, it would be logical to convert nitrocellulose to cellulose through denitration reactions. Several techniques, for example, acidic and alkaline hydrolysis, thermal decomposition, microbial decomposition, and irradiation techniques, have been used to decompose This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Received: May 3, 2012 Revised: June 22, 2012

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dx.doi.org/10.1021/jp3042878 | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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The effect of bulk water solution was modeled using the polarizable continuum model (PCM) approach18 as implemented in the Gaussian09 suite of programs.19 Harmonic vibrational frequency analysis was performed to ascertain the nature of local minima and transition states. Transition states were also validated through examination of normal modes associated with the respective imaginary frequencies. Such visualization can provide information on whether reaction paths connect to the reactants and the expected products. Intrinsic reaction coordinate (IRC) calculations were also performed for selected cases for the forward and reverse reactions connecting the transition state with the respective reactants and products. Desired reactant and product structures were obtained, and these calculations validated our computed transition state structures. Total energies of computed species were corrected for zero-point vibrational energy (ZPE). Further, given the size of the system, the electron-correlated MP2 method with reasonably good basis set is not practical. However, the recently developed M06-2X meta-hybrid functional20 offers a good compromise between accuracy and affordability for large-sized systems. Therefore, in the present investigation, single-point energy calculations were performed at the M06-2X/cc-pVTZ level using the cc-pVTZ basis set utilizing the B3LYP/6311G(d,p) optimized geometries both in the gas phase and in the bulk water solution. The computed single-point M06-2X energies were corrected for the ZPE obtained at the B3LYP/6311G(d,p) level. For the computation of enthalpy and Gibbs free energy at the single point M06-2X level, the total energy obtained at the M06-2X level was used, while other parameters were obtained at the B3LYP/6-311G(d,p) level. Activation energy barriers were computed as the energy difference between the transition state and the corresponding reactant complex. All calculations were performed using the Gaussian03 and Gaussian09 suites of programs.19,21

stepwise processes leading to denitration. It was also revealed that the hydrogen bonds between an OH− ion and other sites of a nitrocellulose monomer play an important role in the stabilization of transition state structures. It was predicted that an addition−elimination reaction in the nitrocellulose monomer will start at the C3 site and will progress in the C2 → C6 direction.14 Further, comparison of data suggested that the αanomeric form will be more reactive than the β-anomeric form with regard to the alkaline hydrolysis reaction, although, the alkaline hydrolysis routes may be different.14,15 It is known that β-elimination plays an important role in the depolymerization of cellulose molecules.16,17 In this reaction, each glucose unit is released from cellulose due to the βelimination at the C4 carbon atom. This phenomenon is called a depolymerization or peeling-off reaction. However, if βelimination occurs at positions other than the C4 site, the reaction does not continue because the hexose ring does not detach from the cellulose molecule and therefore terminates the depolymerization. Our earlier investigations suggested that denitration of various sites of a nitrocellulose monomer requires significantly less activation energy than the cleavage of the hexose CO ring bond.14,15 Further, theoretical computation on alkaline hydrolysis reaction of a nitrocellulose monomer (NCM) provided information on the sequence of the stepwise denitration, but it did not provide information about the peeling-off reaction and whether such reaction will be preferred over the denitration of various sites of the nitrocellulose. In other words, whether the monomer unit (2,3,6-trinitro-β-D-glucopyranose) will be detached before denitration or whether such cleavage of glycosidic bond will proceed after the denitration of C2, C3, and C6 sites. To answer these questions, a detailed computational analysis of addition−elimination reactions within the SN2 framework for the nitrocellulose polymer is needed. However, it is not possible to consider a large system such as an extensive nitrocellulose polymer to study alkaline hydrolysis reactions using a high-level quantum chemical method. Fortunately, dimer (NCD) and trimer (NCT) forms of 2,3,6-trinitro-β-Dglucopyranose can be reliably investigated using the DFT approach. Experimentally, it has been suggested that the degradation route of cellulose involves the breaking of the monomeric ring (β-D-glucopyranose) from the polymeric chain as the first step that subsequently follows the cleavage of the pyranose ring.16,17 To shed light on the possibility of various alkaline hydrolysis mechanistic routes of nitrocellulose and to determine if the trinitro derivative of the β-D-glucopyranose monomer will be the first to be sliced from the polymer chain, we have considered the dimer and trimer forms of 2,3,6trinitro-β-D-glucopyranose. The activation barrier associated with denitration of various sites within the SN2 framework and that of the cleavage of the monomeric ring in the alkaline environment have been computed. Relevant results have been compared with the corresponding data for a nitrocellulose monomer. It was revealed that the dimer can be considered as the smallest model for the nitrocellulose system able to provide insight into both denitration and ring cleavage reactions for the investigation of alkaline hydrolysis.

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RESULTS AND DISCUSSION Molecular geometries and atomic numbering schemes for NCM, NCD, and NCT are shown in Figure 1. The molecular geometries of reactant complexes obtained between the hydroxyl ion (OH−) and dimer (NCD) and trimer (NCT) of 2,3,6-trinitro-β-D-glucopyranose, transition states, and product intermediates (monodenitrated forms of NCD and NCT) at the B3LYP/6-311G(d,p) level are shown in Figures 2 and 3. It should be noted that, in the alkaline hydrolysis reaction, the OH− ion can attack the relevant site in the addition− elimination reaction from the same side (angular attack) or from the opposite side (direct attack) of the nitro group under consideration for replacement by the hydroxyl ion.14 We have predicted that, in the case of the nitrocellulose monomer, both in the gas phase and in the bulk water solution, the angular attack requires significantly larger activation energy than direct attack suggesting the later reaction as the most probable for the alkaline hydrolysis of nitrocellulose.14 Therefore, in the present investigation, only direct attack by an OH− ion has been considered. Further, the alkaline hydrolysis reaction under consideration is a two-step process where reaction from the reactant complex proceeds through the transition state leading to the formation of monodenitrated species and that of the nitrate anion (NO3−). Additionally, we will use nomenclature like NCx-nTSI in the present manuscript, where x can be M, D, or T, n refers the electrophilic site of denitration, TS refers to transition state, and I represents the intermediate. Thus, NCD2TSI represents the intermediate product of the nitrocellulose

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COMPUTATIONAL DETAILS Geometries of the reactant complexes, product intermediates, and transition states were optimized at the density functional theory level using the B3LYP functional and the 6-311G(d,p) basis set both in the gas phase and in the bulk water solution. B



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dimer obtained by denitration of the C2 site through the NCD2TS− transition state. Similarly, NCx+nOH− represent the reactant complex where hydroxyl ion will initiate reaction at the nth site. Energetics and thermodynamic properties (enthalpies and Gibbs free energy) discussed in the current article, unless otherwise described, were obtained at the M06-2X/cc-pVTZ// B3LYP/6-311G(d,p) level. For comparison, activation energies for denitration of different sites of NCM were also computed at the similar theoretical level both in the gas phase and in the bulk water solution. Denitration of NCM. Detailed analyses of alkaline hydrolysis reaction pathways for nitrocellulose monomer (trinitro form of β-D-glucopyranose) were performed in our earlier work.14 Here, we will only very briefly describe the activation energy needed for denitration of C2, C3, and C6 sites at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level in the bulk water solution and will show that obtained results are similar to those obtained in the previous investigation at the MP2/cc-pVTZ//B3LYP/6-311G(d,p) level.14 As discussed earlier, denitration in the alkaline environment will start with the formation of a reactant complex between NCM and OH− where orientation of the hydroxyl ion will promote addition− elimination reaction at the C2, C3, or C6 sites. For denitration reaction starting at the C2 site, about 32.20 kcal/mol energy (ΔG = 33.11 kcal/mol) will be needed to overcome the transition state reaction barrier and about 51.25 kcal/mol of energy (ΔG = 62.90 kcal/mol) will be released due to

Figure 1. Atomic numbering schemes of nitrocellulose monomer (NCM), dimer (NCD), and trimer (NCT).

Figure 2. Alkaline hydrolysis reaction pathways for denitration of the C2, C3, and C6 sites of NCD in the gas phase and in the bulk water solution. Bond distances are in Å, and values in parentheses correspond to those in the bulk water solution. ΔG values (top, gas phase; bottom (italics), in water) are in kcal/mol and obtained at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level. C



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Figure 3. Alkaline hydrolysis reaction pathways for denitration of the C2, C3, and C6 sites of NCT in the gas phase and in the bulk water solution. Bond distances are in Å, and values in parentheses correspond to those in the bulk water solution. ΔG values (top, gas phase; bottom (italics), in water) are in kcal/mol and obtained at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level.

pVTZ//B3LYP/6-311G(d,p) level prediction regarding alkaline hydrolysis of NCM agrees very well with those obtained at the MP2/cc-pVTZ//B3LYP/6-311G(d,p) level and can be used reliably for the comparative study for dimer and trimer in the present investigation. Addition−Elimination Reaction at Various Sites of NCD. The denitration of NCD through the addition− elimination reaction within the SN2 framework can start at C2, C3, or C6 sites, and only the first step of the reaction (monodenitration) has been considered. Assuming that the C2 site will be the first to be denitrated, in the gas phase, such reaction will start with the formation of reactant complex NCD +2OH− between the NCD and the OH− ion, where a hydroxyl ion will be in a suitable position to facilitate the reaction at the C2 site. In the NCD+2OH− complex, an hydroxyl ion attracts a proton from the hydroxyl group at the C1 site and forms a water molecule leaving NCD in the anionic form and is hydrogen bonded with oxygen at C1, hydrogen, and −ONO2 group at the C2 site (Figure 2). The reaction will proceed with the formation of NCD-2TS− transition state, and about 40.96 kcal/mol of activation energy in the gas phase will be needed to cross the transition barrier. In the transition state NCD-2TS−, the OH− ion is hydrogen bonded with hydrogen atoms at the C2 and C4 sites and is about 2.3 Å away from the C2 site. The presence of hydrogen bonds provides stability to the transition state. Further, the leaving NO3− ion is about 1.725 Å away from the C2 site (Figure 2). The formation of the monodenitrated intermediate product (NCD-2TSI) via the transition state

denitration (substitution) of the C2 site in the bulk water solution (Table 1). Thus, denitration reaction will be exothermic by about 19 kcal/mol of energy (about 30 kacl/ mol of Gibbs free energy). However, at the MP2/cc-pVTZ// B3LYP/6-311G(d,p) level, the same reaction was found to be exothermic by about 13 kcal/mol of energy (about 23 kcal/mol of Gibbs free energy).14 For denitration starting at the C3 site at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level, the reaction was predicted to be exothermic by about 21 kcal/ mol (ΔG = 32 kcal/mol), and the same reaction at the MP2/ cc-pVTZ//B3LYP/6-311G(d,p) level was predicted to be exothermic by about 15 kcal/mol (about 26 kcal/mol of Gibbs free energy). Similarly, the C6 site denitration reaction at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level was predicted to be exothermic by about 13 kcal/mol (about 23 kcal/ mol of Gibbs free energy), and the same reaction was found to be exothermic by about 8 kcal/mol of energy (about 19 kcal/ mol of Gibbs free energy) at the MP2/cc-pVTZ//B3LYP/6311G(d,p) level. Further, on the basis of activation energy data shown in Table 1 at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level, it is clear that both the sites C3 and C6 will have equal likelihood of denitration through the scission of the C-ONO2 bond in the alkaline hydrolysis reaction, though such reaction at the C3 site should be slightly more preferable. Similar prediction was also made at the MP2/cc-pVTZ//B3LYP/6311G(d,p) level, although the preference toward the C3 site was slightly more pronounced.14 Thus, the M06-2X/ccD



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Table 1. Computed Total Energy (ΔE, kcal/mol), Enthalpy (ΔH, kcal/mol), and Gibbs Free Energy (ΔG, kcal/mol) of Formation of Transition States and Product Intermediates during the First Step in the Alkaline Hydrolysis of Nitrocellulose Monomer (NCM), Nitrocellulose Dimer (NCD), and Nitrocellulose Trimer (NCT) with Initial Reaction Starting at the C2, C3, and C6 Sites and That of Peeling-Off Reaction through Cleavage of Glycosidic Bond in the Dimer and Trimer in the Gas Phase and in the Bulk Water Solution at the B3LYP/6-311G(d,p) and M062X/cc-pVTZ//B3LYP/6-311G(d,p) Levels gas phase B3LYP

water solution M06-2X/B3LYP

B3LYP

M06-2X/B3LYP

reaction

ΔE

ΔH

ΔG

ΔE

ΔH

ΔG

ΔE

ΔH

ΔG

ΔE

ΔH

ΔG

NCM+2OH → NCM-2TS NCM-2TS → NCM-2TSI + NO3 NCM+3OH → NCM-3TS NCM-3TS → NCM-3TSI + NO3 NCM+6OH → NCM-6TS NCM-6TS → NCM-6TSI + NO3 NCD+2OH → NCD-2TS NCD-2TS → NCD-2TSI + NO3 NCD+3OH → NCD-3TS NCD-3TS → NCD-3TSI + NO3 NCD+6OH → NCD-6TS NCD-6TS → NCD-6TSI + NO3 NCD+4OH → NCD-4TS NCT+2OH → NCT-2TS NCT-2TS → NCT-2TSI + NO3 NCT+3OH → NCT-3TS NCT-3TS → NCT-3TSI + NO3 NCT+6OH → NCT-6TS NCT-6TS → NCT-6TSI + NO3 NCT-3TSI+2OH → NCT-3TSI2TS NCT-3TSI2TS → NCT-3TSI2TSI + NO3 NCT-3TSI+6OH → NCT-3TSI6TS NCT-3TSI6TS → NCT-3TSI6TSI + NO3 NCT+4OH → NCT-4TS

25.36 −16.93 22.29 −15.62 22.57 −6.94 42.12 −26.83 7.74 −20.37 20.42 −22.12 38.54 34.83 −9.82 7.63 −18.20 22.66 −20.39 30.84 −21.96

24.49 −16.62 21.69 −15.56 21.73 −6.92 41.82 −27.05 7.43 −20.55 19.73 −22.06 38.06 34.25 −9.65 7.32 −18.28 22.15 −20.37 30.08 −21.70

27.20 −29.84 23.63 −28.12 23.96 −18.71 43.06 −39.22 8.23 −33.78 22.42 −35.22 38.92 35.09 −21.58 8.12 −32.61 24.01 −33.10 32.35 −35.03

25.91 −21.13 25.98 −20.49 26.23 −11.94 40.96 −30.24 8.06 −19.52 20.69 −24.82 44.95 40.77 −15.98 8.04 −17.40 23.23 −24.04 36.84 −27.79

25.03 −20.82 25.38 −20.42 25.39 −11.91 40.66 −30.46 7.74 −19.69 20.00 −24.77 44.47 40.19 −15.81 7.73 −17.48 22.71 −24.01 36.08 −27.53

27.74 −34.04 27.32 −32.98 27.62 −23.71 41.90 −42.64 8.54 −32.93 22.70 −37.92 45.33 41.03 −27.75 8.57 −31.81 24.57 −36.74 38.35 −40.86

29.01 −45.44 24.41 −44.16 24.77 −36.70 42.33 −55.54 11.51 −52.73 17.99 −53.03 48.44 33.47 −43.66 11.48 −52.37 16.38 −52.88 26.83 −47.88

28.53 −45.44 23.89 −44.12 24.07 −36.79 42.26 −55.84 11.14 −53.06 17.21 −53.00 47.69 32.81 −43.54 11.15 −52.65 15.91 −53.10 26.12 −47.58

29.93 −57.09 25.55 −56.25 25.61 −48.25 42.66 −66.84 11.97 −65.56 20.21 −66.91 49.38 34.68 −56.63 11.68 −65.46 17.59 −64.39 28.38 −61.02

32.20 −51.25 28.34 −49.19 28.79 −41.63 42.99 −58.95 13.40 −53.79 17.11 −53.71 56.96 38.43 −48.97 13.29 −53.26 15.58 −53.95 31.46 −53.56

31.72 −51.24 27.82 −49.16 28.09 −41.72 42.92 −59.35 13.04 −54.12 16.33 −53.69 56.21 37.78 −48.85 12.96 −53.54 15.11 −54.17 30.75 −53.26

33.11 −62.90 29.48 −61.28 29.63 −53.18 43.32 −70.35 13.87 −66.62 19.33 −67.60 57.90 39.65 −61.94 13.49 −66.35 16.79 −65.46 33.01 −66.70

16.54 −14.68

16.32 −14.70

16.35 −27.23

19.96 −19.10

19.74 −19.12

19.77 −31.65

10.48 −47.62

10.12 −47.73

10.88 −60.07

13.64 −51.35

13.29 −51.46

14.04 −63.79

38.24

37.78

38.28

45.06

44.59

45.09

48.26

47.59

48.78

56.55

55.88

57.07

by about 4 kcal/mol (Table 1). The NCD-6OH− reactant complex has a complicated structure where an OH− ion is involved in hydrogen bonding with both monomeric units of the NCD. Particularly, it forms hydrogen bonds with hydrogen atoms at the C4, C6, C1′, and C5′ sites. However, in the NCD6TS− transition state, OH− forms a hydrogen bond with hydrogen at the C4 site and is about 2.217, and the leaving NO3− ion is about 1.77 Å away from the C6 site (Figure 2). Optimized geometrical parameters in the bulk water solution of reactant complexes, transition states, and monodenitrated intermediate products depicted in Figure 2 and computed energetics presented in Table 1 show that, although geometries are not substantially changed, but as expected, energetics are significantly modified compared to those in the gas phase. In the bulk water solution, also, the denitration reaction will start with the formation of a reaction complex between the NCD and OH− where the location of the hydroxyl ion will be instrumental for the addition−elimination reaction at the C2, C3, and C6 sites. Thus, denitration of the C2 site will be initiated with the formation of the NCD+2OH− reactant complex and reaction will progress through NCD-2TS− transition state yielding the monodenitrated species (NCD2TSI) and the leaving NO3− ion. About 42.99 kcal/mol (ΔG = 43.32 kcal/mol) of energy will be needed to cross the transition state reaction barrier, while about 58.95 kcal/mol (ΔG = 70.35 kcal/mol) of energy will be released consequent to the NCD2TSI formation. Thus, in the bulk water solution, this reaction

consequent to the addition−elimination reaction at the C2 site will lead to the release of about 30.24 kcal/mol of energy. Thus, in the gas phase, monodenitration at the C2 site will be endothermic by about 11 kcal/mol (Table 1). In the case of denitration at the C3 site, the reaction will be initiated with the formation of reactant complex NCD+3OH− where the orientation of the hydroxyl ion will facilitate an addition−elimination reaction at the C3 site (Figure 2). In this complex, OH− forms multiple hydrogen bonds with hydrogen atoms at the C1, C3, and C5 sites. The reaction will progress through the NCD-3TS− transition state, which is stabilized by the presence of hydrogen bonds between OH− and hydrogen atoms at C1 and C5 sites. Moreover, the OH− ion is about 2.379, and the leaving NO3− ion is about 1.714 Å away from the C3 site. The monodenitrated intermediate product NCD-3TSI will be formed through the NCD-3TS− transition state where about 8.06 kcal/mol of energy in the gas phase will be needed to overcome the transition state reaction barrier and about 19.52 kcal/mol of energy will be released. Thus, denitration of the C3 site will be exothermic by about 11 kcal/mol (Table 1). Similarly, the denitration of NCD at the C6 site in the gas phase will start with the formation of NCD+6OH− reactant complex and will progress through the NCD-6TS− transition state, which has a barrier height of about 20.69 kcal/mol (Table 1). About 24.82 kcal/mol of energy will be released consequent to the formation of monodenitrated NCD-6TSI product. Thus, the denitration of NCD at the C6 site will also be exothermic E



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utilized for the formation of NCT-3TS− transition state but about 17.40 kcal/mol of energy will be released due to the denitration of the C3 site forming an intermediate complex NCT-3TSI and that of the leaving nitrate ion. Thus, denitration of the C3 site will be exothermic by about 9 kcal/mol (Table 1). The NCT-3TS− transition state is stabilized by the presence of hydrogen bonds between OH− ion and hydrogen atoms at the C1 and C5 site. Further, the OH− ion is about 2.376 and the leaving NO3− ion is about 1.714 Å away from the C3 site (Figure 3). However, the denitration of the C6 site of NCT will be initiated with formation of NCT+6OH− reactant complex where OH− forms complex hydrogen bonds with hydrogen atoms at the C4 and C6 sites and that at the C1′ site. The orientation of OH− in the complex is suitable for addition− elimination reaction at the C6 site, and the presence of several hydrogen bonds provides stability for the complex. Reaction will proceed through the NCT-6TS− transition state, which is stabilized by the hydrogen bond between OH− and hydrogen at the C4 site. Further, OH− is about 2.212 and the leaving NO3− ion is about 1.773 Å away from the C6 site. About 23.23 kcal/ mol of energy will be needed to overcome the transition state barrier, and about 24.04 kcal/mol of energy will be released consequent to the monodenitration at the C6 site. Thus, in the gas phase, the addition−elimination reaction leading to the denitration of the C6 site will be exothermic only by about 1 kcal/mol of energy (Table 1). Selected geometrical parameters shown in Figure 3 and data shown in Table 1 clearly show that, although geometrical parameters in the bulk water solution are similar to those of the corresponding gas phase, as expected, energetics are significantly changed. Specifically, about 38.43 kcal/mol of energy (ΔG = 39.65 kcal/mol) will be needed to cross the NCT-2TS− transition state reaction barrier in the water solution, but about 48.97 kcal/mol (ΔG = 61.94 kcal/mol) of energy will be released consequent to the formation of the monodenitrated NCT-2TSI intermediate product. In case of the denitration reaction starting at the C3 site of NCT in the bulk water solution, about 13.29 kcal/mol (ΔG = 13.49 kcal/mol) of energy will be needed to overcome the NCT-3TS− transition state energy barrier, but about 53.26 kcal/mol (ΔG = 66.35 kcal/mol) of energy will be released due to the formation of intermediate reaction products NCT-3TSI and NO3−. Similarly, for the reaction starting at the C6 site, about 15.58 kcal/ mol (ΔG = 16.79 kcal/mol) of energy will be needed to cross the NCT-6TS− transition state reaction barrier, and about 53.95 kcal/mol (ΔG = 65.46 kcal/mol) of energy will be released consequent to the formation of reaction intermediate products NCT-6TSI and NO3− in the bulk water solution. It is evident that denitration of C6 site in the bulk water solution will be an exothermic process where about 38 kcal/mol of energy (about 49 kcal/mol of Gibbs free energy) will be released (Table 1). Thus, it is clear that the C3 site will be the first to be denitrated among all available sites (C2, C3 at C6). However, information about the next site to be denitrated for the dimer and trimer forms is less clear; although for the monomer, the C3 → C2 → C6 route was predicted to be the most preferred for the complete denitration,14 but it is not clear if the same route can be extrapolated for the dimer and trimer forms, which can be applicable for the denitration of nitrocellulose. To shed light on these questions and to predict the next denitration step, we also investigated the denitration of C2 and C6 sites of the NCT-3TSI, the monodenitrated intermediate obtained by

(denitration of C2 site) will be exothermic by about 16 kcal/ mol (about 27 kcal/mol of Gibbs free energy). However, the same reaction in the gas phase was found to be endothermic by about 11 kcal/mol (Table 1). For denitration of the C3 site, about 13.40 kcal/mol (ΔG = 13.87 kcal/mol) of energy will be needed to overcome the energy barrier due to the NCD-3TS− transition state, and about 53.79 kcal/mol (ΔG = 66.62 kcal/ mol) of energy will be released due to the monodenitrated product (NCD-3TSI) formation (Table 1). Thus, this reaction will be exothermic by about 40 kcal/mol (about 53 kcal/mol of Gibbs free energy), but the same reaction in the gas phase was exothermic by only about 11 kcal/mol of energy. The denitration of the C6 site will require about 17.11 kcal/mol of energy (ΔG = 19.33 kcal/mol) to overcome NCD-6TS− transition barrier and about 53.71 kcal/mol (ΔG = 67.60 kcal/ mol) of energy will be released due to the formation of NCD6TSI monodenitrated species. This reaction will also be exothermic by about 37 kcal/mol of energy (about 48 kcal/ mol of Gibbs free energy). However, the corresponding reaction in the gas phase was exothermic by only about 4 kcal/mol of energy (Table 1). Thus, on the basis of the activation energy data shown in the Table 1, it is evident that, for NCD, the C3 site will be the first and that the C2 site will be the last to be denitrated. In other words, the addition− elimination reaction in alkaline hydrolysis will initiate at the C3 site, and it may be followed by the same reaction at the C6 and C2 sites of the NCD. Addition−Elimination Reaction at Various Sites of NCT. The addition−elimination reactions at various sites of NCT leading to denitration were also studied in the gas phase and in bulk water solution. Geometries and selected parameters of reactant complexes, transition states and intermediate monodenitrated products are shown in Figure 3, and computed energies are shown in Table 1. As discussed earlier, denitration through an addition−elimination reaction can start at any of the C2, C3, and C6 sites both in the gas phase and in bulk water solution. Considering denitration reactions in the gas phase and assuming denitration at the C2 site, the addition−elimination in alkaline hydrolysis will be initiated through the formation of NCT+2OH− reactant complex between NCT and the incoming hydroxyl ion. In this complex, OH− ion attracts a proton from the hydroxyl group at the C1 site and forms a water molecule leaving NCT in the anionic form. Reaction will progress through the NCT-2TS− transition state forming a monodenitrated intermediate product (NCT-2TSI) and the nitrate ion. In the NCT-2TS− transition state, OH− forms hydrogen bonds with hydrogen at C4 and hydroxyl group at the C1 site and is about 2.203 Å away from the C2 site. The leaving NO3− ion is about 1.822 Å away from the C2 site. About 40.77 kcal/mol energy will be needed to cross the barrier due to the transition state and about 15.98 kcal/mol energy will be released due to the formation of the monodenitrated product. Thus, the denitration of the C2 site in the gas phase will be enothermic by about 25 kcal/mol. If the C3 site of the NCT is the first to be denitrated, reaction will proceed with the formation of NCT+3OH− reactant complex between NCT and OH− ion where the location of OH− ion in the complex will facilitate the denitration of the C3 site. In the NCT+OH− complex, OH− forms complex hydrogen bonds with hydrogen atoms at the C1, C3, and C5 sites, and these hydrogen bonds provide stability for the complex (Table 1). The reaction will progress at the expense of about 8.04 kcal/mol of energy, which will be F



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Figure 4. Subsequent alkaline hydrolysis reaction pathways for NCT after denitration of the C3 site. Bond distances are in Å, and values in parentheses correspond to those in the bulk water solution. ΔG values (top, gas phase; bottom (italics), in water) are in kcal/mol and obtained at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level.

Figure 5. Alkaline hydrolysis reaction pathways for cleavage of glycosidic bond in NCD and NCT in the gas phase and in the bulk water solution. Bond distances are in Å, and values in parentheses correspond to those in the bulk water solution. ΔG values (top, gas phase; bottom (italics), in water) are in kcal/mol and obtained at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level.

revealed that, though in the gas phase the water molecule forms a hydrogen bond (2.615 Å) with hydrogen at the C5′ site, such interaction is almost nonexistent in the bulk water where such distance is revealed to be 4.061 Å (Figure 4). Reaction will proceed through transition state NCT-3TSI2TS−. In the transition state, OH− is hydrogen bonded with hydrogen at the C4 site and hydroxyl group at the C1 site. Further, it is about 2.135 Å away from the C2 site, while the NO3− ion is about 1.893 Å away from the same site. Generally, significant geometrical change is not revealed for the transition state in the bulk water solution. In the gas phase, about 36.84 kcal/mol of energy will be needed to overcome the energy barrier

the denitration of the C3 site of NCT. The structures of reactant complexes, intermediates, and products both in the gas phase and in the bulk water solution are shown in the Figure 4, while the computed energetics are shown in Table 1. If the subsequent denitration reaction proceeds through the C2 site, it will be initiated with the formation of the reactant intermediate complex NCT-3TSI+2OH− formed between NCT-3TSI and OH − in suitable orientation for the addition−elimination reaction at the C2 site. In this complex, OH− grasps a proton from the hydroxyl group at the C1 site forming a water molecule and is involved in complex hydrogen bonding at various sites as shown in Figure 4. It has been G



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associated with the NCT-3TSI2TS− transition state, and only 27.79 kcal/mol of energy will be released consequent to product formation (NCT-3TSI2TSI and NO3−) from the transition state. In the bulk water solution, about 31.46 kcal/ mol of energy (ΔG = 33.01 kcal/mol) will be needed to cross the transition state barrier, but about 53.56 kcal/mol of energy (ΔG = 66.70 kcal/mol) will be released due to the product formation from the transition state. Thus, in the bulk water solution, the reaction will become exothermic by about 22 kcal/ mol of energy (about 34 kcal/mol of Gibbs free energy). However, if the denitration reaction progresses through the C6 site, it will proceed with the formation of reactant intermediate complex NCT-3TSI+6OH− formed between NCT-3TSI and OH− in suitable orientation to promote such reaction as depicted in Figure 4. In this reactant complex, OH− forms complex hydrogen bonds at various sites as shown in Figure 4. Reaction will progress through the NCT-3TSI6TS− transition state where, in the gas phase, about 19.96 kcal/mol of energy will be required to overcome the height of barrier due to transition state and about 19.10 kcal/mol of energy will be released due to the formation of products through the transition state. The corresponding energies in the bulk water solution would be 13.64 (ΔG = 14.04 kcal/mol) and 51.35 kcal/mol of energies (ΔG = 63.79 kcal/mol), respectively. Thus, the denitration of the C6 site in the bulk water solution will be exothermic by about 38 kcal/mol of energy (about 50 kcal/mol of Gibbs free energy). On the basis of these discussions, it is clear that, once the denitration reaction is started at the C3 site, further denitration will proceed with the denitration of the C6 site and therefore, denitration of the CNT will follow the C3 → C6 → C2 denitration route. Peeling-Off Reaction. The peeling-off reactions in the NCD and NCT were also investigated at the B3LYP/6311G(d,p) level both in the gas phase and in the bulk water solution. In these reactions, the OH− anion will attack at the C4 site leading to the cleavage of the C4−O bond (β(1 → 4) glycosidic bond). Such reaction in the case of NCD will start with the formation of reaction complex with OH− as depicted in Figure 5. In this complex (NCD+4OH−), the orientation of OH− is suitable for addition−elimination reaction at the C4 site leading to the peeling-off reaction due to the cleavage of the glycosidic bond. Further, the structure of the reactant complex NCD+4OH− is stabilized by the presence of hydrogen bonds between the OH− and hydrogen atoms at the C1, C3, and C5 sites. Reaction will progress through the transition state NCD4TS− leading to the cleavage of the glycosidic bond. In the transition state, the glycosidic bond (C4O bond) is elongated to 1.834 Å, and the OH−···C4 distance (heavy−heavy atoms) is about 2.082 Å in the gas phase (Figure 5). Such reactions were also investigated in the bulk water solution, and selected geometrical parameters are also shown in Figure 5. It is clear that geometries do not change appreciably in the bulk water solution, though some change among the depicted bond distances is evident. Noticeably, the glycosidic bond distance is increased by about 0.07 Å in the water solution (Figure 5). Further, in the bulk water solution, about 56.96 kcal/mol (ΔG = 57.90 kcal/mol) of energy will be needed to cross the NCT-4TS− transition state reaction barrier leading to the breaking of the glycosidic bond. Similarly, in the case of NCT, the peeling-off reaction will start with the formation of a reactant complex (NCT+4OH−) between NCT and the OH− ion where OH− forms multiple

hydrogen bonds with hydrogen atoms at the C1, C3, and C5 sites and is in suitable orientation for addition−elimination reaction at the C4 site. Reaction will proceed through the NCT-4TS− transition state as depicted in Figure 5. In this transition state, the C4O bond (glycosidic bond) distance is increased to 1.830 Å, the OH−···C4 distance (heavy−heavy atoms) is about 2.085 Å, and the OH−···C4′O (O···O distance) is about 2.451 Å. Geometries were also optimized in the bulk water solution, and selected parameters are also shown in Figure 5. About 56.55 kcal/mol (ΔG = 57.07 kcal/mol) in the bulk water solution will be needed to overcome the NCT-4TS− transition state reaction barrier. Further, from the comparison of the geometrical parameters and that of the activation barrier for both the dimer and trimer (NCD and NCT), it is evident that extending the chain length is not expected to have a significant effect on the energetics and geometrical parameters associated with the peeling-off reaction of the nitrocellulose. Mechanism for Alkaline Hydrolysis Reaction. Comparison of activation Gibbs free energy data for addition− elimination reaction at the C2, C3, and C6 sites leading to denitration reaction and the Gibbs free energy release data for intermediate product formations in the bulk water solution for NCM, NCD, and NCT at the M06-2X/cc-pVTZ//B3LYP/ 6-311G(d,p) level are depicted in Figure 6. It is evident from data shown in Table 1 and Figure 6 that, for all considered species (NCM, NCD, and NCT), the lowest amount of activation energy (as well as activation Gibbs free energy) needed to overcome the transition state reaction barrier will be for addition−elimination reaction leading to denitration of the C3 site. The denitration of the C6 site will require a slightly larger amount of activation energy compared to that needed for the same reaction at the C3 site for NCM, but this difference is much larger for dimer and trimer. The C2 site will require the largest amount of energy for denitration. Important information revealed in the present investigation is that, for the denitration of the C3 and C6 sites, the activation energy is decreased from monomer to dimer and trimer, but for denitration of the C2 site, the required activation energy to overcome the relevant transition state reaction barrier is significantly increased in going from the NCM to NCD and NCT (Figure 6). Interestingly, the activation energies are not predicted to have a significant effect in going from NCD to NCT. Further, from the previous section, we noticed that once denitration reaction started at the C3 site reaction will progress in the C6 direction leading to the C3 → C6 → C2 denitration route for the complete hydrolysis reaction. Further, it is clear from the data shown in Table 1 that the peeling-off reaction through breaking of the C4−O bond (β(1 → 4) glycosidic bond) during alkaline hydrolysis of nitrocellulose in bulk water solution would require about 57 kcal/ mol of energy. However, maximum amount of activation energy was found to be associated with the denitration of the C2 site and amounted about 43 kcal/mol for NCD and about 38 kcal/mol for NCT. Thus, it is clear that denitration would be preferred over the peeling-off reaction for the alkaline hydrolysis of the nitrocellulose. Further, on the basis of our earlier investigation,14 it was revealed that cleavage of the CO ring bond for the nitrocellulose monomer would require about 76 kcal/mol of activation energy in the bulk water solution computed at the B3LYP/6-311G(d,p) level. However, the activation energy needed for the peeling-off reaction for NCD and NCT at the same theoretical level in the bulk water solution was found to be around 48 kcal/mol (Table 1). Thus, H



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place before the denitration. Alkaline hydrolysis reaction will start with the formation of reactant complex between the nitrocellulose species and that of the OH− ion. Reaction will progress through the relevant transition state leading to the formation of denitrated species. The structures of reactant complexes and transition states will be stabilized by the presence of hydrogen bonds between hydroxyl ion and various sites of nitrocellulose. The activation energy needed for denitration of the C3 and C6 sites was found to be decreased, while that of the C2 site was increased in going from the monomer to dimer or trimer. Significant change in the activation energy was not predicted in going from the dimer to trimer, which indicates the suitability of these species in modeling the alkaline hydrolysis reaction mechanisms of nitrocellulose. Further, the third monomeric unit of NCT was also not found to be involved in interaction with the OH− ion in the reactant complexes and corresponding transition states formation. On the basis of the current investigation, we conclude that the C3 site will be the first and the C2 site will be the last to be denitrated, and thus, the denitration reaction will follow the C3 → C6 → C2 denitration route in alkaline hydrolysis of nitrocellulose. The peeling-off reaction will start after the complete denitration of the terminal monomer of nitrocellulose and before the ring cleavage leading to decomposition reactions. Further, the dimer form can be considered as the smallest unit for modeling alkaline hydrolysis reaction of nitrocellulose.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. The tests described and the resulting data presented herein, unless otherwise noted, were obtained from research conducted under the Environmental Quality Technology Program of the United States Army Corps of Engineers and the Environmental Security Technology Certification Program of the Department of Defense by the USAERDC. Permission was granted by the Chief of Engineers to publish this information. The findings of this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. The authors thank Dr. Andrea Scott of USACE and Dr. Leonid Gorb of SpecPro, Inc., for their editorial comments.

Figure 6. Comparison of Gibbs free energy of formation of transition state and intermediate product leading to the denitration of NCM, NCD, and NCT at the (a) C2, (b) C3, and (c) C6 sites obtained at the M06-2X/cc-pVTZ//B3LYP/6-311G(d,p) level in the bulk water solution. Energies are shown with respect to respective reactant complexes, and x represents M, D, or T.

this discussion suggests that the peeling-off reaction would be preferred over the ring cleavage reaction. Thus, on the basis of the current computational investigation, it appears that, during the alkaline hydrolysis of nitrocellulose, the denitration of various sites will dominate over the peeling-off reaction and that the ring cleavage reaction will require a significantly greater amount of the activation energy than the peeling-off reaction.

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REFERENCES

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CONCLUSIONS Current theoretical investigation of alkaline hydrolysis reaction mechanisms of dimer and trimer of nitrocellulose and its comparison with the monomer in the gas phase and in the bulk water solution predicted that peeling-off reactions will not take I



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Computational Elucidation of Mechanisms of Alkaline Hydrolysis of

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