Chemosphere 134 (2015) 31–38

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Alkaline hydrolysis of hexahydro-1,3,5-trinitro-1,3,5-triazine: M06-2X investigation Liudmyla K. Sviatenko a,b, Leonid Gorb c, Frances C. Hill d, Danuta Leszczynska e, Sergiy I. Okovytyy b, Jerzy Leszczynski a,⇑ a

Interdisciplinary Center for Nanotoxicity, Department of Chemistry and Biochemistry, Jackson State University, Jackson, MS 39217, USA Department of Organic Chemistry, Oles Honchar Dnipropetrovsk National University, Dnipropetrovsk 49000, Ukraine HX5, LLC, Vicksburg, MS 39180, USA d US Army ERDC, Vicksburg, MS 39180, USA e Interdisciplinary Nanotoxicity Center, Department of Civil and Environmental Engineering, Jackson State University, Jackson, MS 39217, USA b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 RDX hydrolysis reaction was modeled

along with simulation of UV–VIS spectra.  The initial step of hydrolysis is deprotonation of RDX and nitrite elimination.  Reaction proceeds toward cycle cleavage.  Stable products are 4-nitro-2,4diazabutanal, nitrite, formaldehyde, nitrous oxide.  Alkaline hydrolysis of RDX is highly exothermic process.

a r t i c l e

i n f o

Article history: Received 24 January 2015 Received in revised form 28 March 2015 Accepted 31 March 2015 Available online 22 April 2015 Handling Editor: I. Cousins Keywords: Hexahydro-1,3,5-trinitro-1,3,5-triazine 4-Nitro-2,4-diazabutanal Mechanism UV–VIS spectra

a b s t r a c t Alkaline hydrolysis mechanism of possible environmental contaminant RDX (hexahydro-1,3,5-trinitro1,3,5-triazine) was investigated computationally at the PCM(Pauling)/M06-2X/6-311++G(d,p) level of theory. Results obtained show that the initial deprotonation of RDX by hydroxide leads to nitrite elimination and formation of a denitrated cyclohexene intermediate. Further nucleophilic attack by hydroxide onto cyclic C@N double bond results in ring opening. It was shown that the presence of hydroxide is crucial for this stage of the reaction. The dominant decomposition pathway leading to a ring-opened intermediate was found to be formation of 4-nitro-2,4-diazabutanal. Hydrolytic transformation of its byproduct (methylene nitramine) leads to end products such as formaldehyde and nitrous oxide. Computational results are in a good agreement with experimental data on hydrolysis of RDX, suggesting that 4-nitro-2,4-diazabutanal, nitrite, formaldehyde, and nitrous oxide are main products for early stages of RDX decomposition under alkaline conditions. Ó 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Interdisciplinary Center for Nanotoxicity, Department of Chemistry and Biochemistry, Jackson State University, 1325 J.R. Lynch Street, P.O. Box 17910, Jackson, MS 39217-0510, USA. Tel.: +1 601 979 3482; fax: +1 601 979 7823. E-mail address: [email protected] (J. Leszczynski). http://dx.doi.org/10.1016/j.chemosphere.2015.03.064 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) is an energetic material used in both military and commercial applications. Release of RDX to the environment can occur during

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manufacturing, transportation, storage, and disposal (ATSDR, 2012; USEPA, 2014). The EPA has classified RDX as a possible human carcinogen, weight-of-evidence carcinogenic classification of C (USEPA, 2014). It biodegrades very slowly under aerobic conditions (Sikka et al., 1980), and can migrate to groundwater easily since its water solubility is 0.04 g L1 or 2  104 M (Lynch et al., 2001). RDX is stable to hydrolysis in an aqueous solution at a pH range normally found in natural waters (Sikka et al., 1980), however under alkaline conditions, hydrolysis can occur. For example, alkaline hydrolysis may play an active role in the in situ natural attenuation of RDX in coastal seawaters (Hoffsommer and Rosen, 1973; Monteil-Rivera et al., 2008). Various technologies, such as treatment with iron metal (Wanaratna et al., 2006), chemical reduction using buffered sodium hydrosulfite (Luo et al., 2012), phytoremediation (Lamichhane et al., 2012; Panz and Miksch, 2012), photolysis (Hawari et al., 2002), adsorption by activated carbon (Wujcik et al., 1992), in situ bioremediation (Waisner et al., 2002), hydrolysis under different conditions (Hoffsommer et al., 1977; Croce and Okamoto, 1979; Heilmann et al., 1996; Balakrishnan et al., 2003; Hwang et al., 2004, 2006; Davis et al., 2007; Larson et al., 2008; Gent et al., 2010), and electrochemical decomposition (Gent et al., 2010) have been investigated as possible methods to treat RDX-contaminated water and soil for safe removal of nitramine from the environment. Development of those technologies focuses on finding an innovative, effective, low-cost method of transforming RDX to nontoxic end products. In most cases the observed end products include nitrite, nitrate, nitrous oxide, nitrogen gas, ammonia, formaldehyde, formic acid, and carbonate (Heilmann et al., 1996; Hawari et al., 2002; Balakrishnan et al., 2003; Hwang et al., 2006; Wanaratna et al., 2006). In particular, it was found that alkaline hydrolysis is one of the most effective treatment approaches for remediating RDX-contaminated water, soil, and sediment (Heilmann et al., 1996; Davis et al., 2007; Gent et al., 2010). It was extensively studied at different temperatures and pH levels and was effectively demonstrated on active military training ranges through the application of hydrated lime (Larson et al., 2008). Kinetics of the alkaline hydrolysis of RDX were studied in aqueous solutions (Hoffsommer et al., 1977; Heilmann et al., 1996; Balakrishnan et al., 2003; Hwang et al., 2006), in aqueous acetone (Epstein and Winkler, 1952), methanol (Jones, 1954), and soil slurries (Brooks et al., 2003). The reaction was shown to be a second-order reaction with respect to RDX and OH concentrations (Hoffsommer et al., 1977; Heilmann et al., 1996; Balakrishnan et al., 2003; Hwang et al., 2006). RDX hydrolyzes at pH > 10 to form end products including nitrite, nitrous oxide, ammonia, formate, and formaldehyde (Hoffsommer et al., 1977; Croce and Okamoto, 1979; Heilmann et al., 1996; Balakrishnan et al., 2003). The end products produced depended on pH, temperature, and reaction time during the experiment. Summarizing the results from these publications, in order to reach those end-products, multi-step chemical transformations are necessary. Despite the amount of experimentally known information, a step-by-step chemical mechanism for the alkaline hydrolysis of RDX has not been determined. Recently, we proposed a computational approach to analyze multi-step chemical reactions. The applied protocol includes a generation of multistep Gibbs free energy reaction profile for the transformations of the reagents to end-products followed by evaluation of rate constants, construction of the corresponding kinetic equations and subsequent solution of the resulting equations. This procedure is semi-empirical since it is based on available experimental data, and allows the experimentally-determined data to be significantly extended by addition of computationally predicted data. This procedure was successfully applied to the prediction of kinetics of alkaline hydrolysis of such energetic materials as

trinitrotoluene, dinitrotoluene and dinitroanisole (Sviatenko et al., 2014). The procedure consists of two major stages. The first involves the scan of possible pathways to obtain Gibbs free energy reaction profiles that include all possible pathways of alkaline hydrolysis. This stage requires the application of an accurate quantum-chemical approximation (M06-2X was used here) (Sousa et al., 2007; Zhao and Truhlar, 2008) and so called continuum model to model influence of bulk water (SMD(Pauling) approach was used here) (Marenich et al., 2009). The final step of the first stage is identification of the most probable pathway from the steps obtained, considering both kinetic and thermodynamic controls. The second step uses the same approach as step one with inclusion of the influence of specific hydration to create a more accurate model (Sviatenko et al., 2014). Only possible chemical reactions from step one for which kinetic equations and chemical kinetics were obtained are used in step two. The current study considers the first stage of the proposed multi-step chemical reactions evaluation. The following experimental data were taken into account: (1) The disappearance of RDX during hydrolysis in water (pH 10) is accompanied by the accumulation of the key ring cleavage intermediate 4-nitro-2,4-diazabutanal (IV) (Scheme 1) (Balakrishnan et al., 2003); (2) early intermediates pentahydro-3,5-dinitro1,3,5-triazacyclohex-1-ene (I) and 4,6-dinitro-2,4,6-triaza-hexanal (II) were identified as a denitrated cyclohexene intermediate, and as a hydrolyzed product of I, respectively (Hoffsommer et al., 1977; Balakrishnan et al., 2003); (3) early intermediate 5-hydroxy-4-nitro-2,4-diaza-pentanal (III) was tentatively identified as a hydrolyzed product of II (Balakrishnan et al., 2003). Based on these data, a degradation mechanism includes the initial denitration of RDX via the bimolecular E2 mechanism in which a proton is removed by hydroxide ion from the methylene group and, simultaneously, a nitrite ion is removed from the adjacent ring nitrogen resulting in the C@N double bond formation (Scheme 1). Denitrated cyclohexene intermediate (I) undergoes further nucleophilic attack, which produces a ring-opened intermediate (II). Several pathways for its decomposition, connected to cleavage of different CAN bonds, led through various intermediates to end products (nitrite, nitrous oxide, ammonia, formaldehyde, and formate). It should be noted that an alternative mechanism of RDX biodegradation includes enzyme catalyzed sequential transfer of two single electrons to RDX, which cause denitration to form intermediate V (Scheme 1). The latter is unstable in water and hydrolyzes to yield hypothetical intermediate VI, which spontaneously decomposes producing 4-nitro-2,4-diazabutanal and formamide (Bhushan et al., 2003).

2. Materials and methods All calculations were performed using the Gaussian 09 suite of programs (Frisch et al., 2009). The relevant stationary points (intermediates, transition states, and products) in aqueous solution were fully optimized at the PCM(Pauling)/M06-2X/6-311++G(d,p) level (Cossi et al., 1996; Sousa et al., 2007; Zhao and Truhlar, 2008). The functional M06-2X has been successfully used in our studies of alkaline hydrolysis of nitroaromatics (Hill et al., 2012; Sviatenko et al., 2014). It was also shown that Pauling radii should be used for appropriate reproduction of solvation effects (Hill et al., 2012). Stationary points were further characterized as either local minima having all real frequencies, or as transition states possessing only one imaginary frequency, by calculation of the analytic harmonic vibrational frequencies at the same theory level as geometry optimization. The Cartesian coordinates for optimized local minima and transition states are listed in Supplementary material. UV–VIS spectra of initial compound, intermediates and

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Scheme 1. Proposed RDX degradation pathway for alkaline hydrolysis (Balakrishnan et al., 2003) and RDX biodegradation pathway (Bhushan et al., 2003).

AAA

EEA

twist

0.00

3.89

4.12

Fig. 1. Stable conformers of RDX with calculated relative Gibbs free energies (in kcal mol1).

products of alkaline hydrolysis of RDX were also calculated at the PCM(Pauling)/M06-2X/6-311++G(d,p) level. The simulated UV– VIS absorption spectra were visualized using GaussView version 5.0 (Dennington et al., 2009). Gibbs free energies of all considered species were calculated using standard expression:

DG ¼ DH  T DS at T ¼ 298:3 K: 3. Results and discussion 3.1. Initial step of alkaline hydrolysis of RDX It has been already established that RDX may exist in gas state in several conformations: AAA, AAE, EEA, boat, and twist (Karpowicz and Brill, 1984; Rice and Chabalowski, 1997; Molt et al., 2011; Al-Saidi et al., 2012). All these structures were the starting points for the minimization of the energy of the conformers in aqueous solution at PCM(Pauling)/M06-2X/6-311++G(d,p) level of the theory. As the result of minimization the gas phase boat

conformation has been relaxed into the twist one and AAE conformer has converged yielding the AAA species. The relative Gibbs free energies for the conformations that are stable in aqueous solution are displayed in Fig. 1 along with corresponding notation. According to our calculations the most stable structure in aqueous solution is AAA. This species was previously predicted to be the most probable conformer in gas phase, acetonitrile, dimethyl sulfoxide, and acetone solutions, and b-solid RDX (Karpowicz and Brill, 1984; Rice and Chabalowski, 1997). In addition, it was reported that AAE and AAA conformers would contribute the most to the RDX RR spectra in the gas phase and in acetonitrile solution (Al-Saidi et al., 2012). The calculated values of Gibbs free energies suggest that the estimated equilibrium constants for AAA ? EEA and AAA ? twist transformations are in the range of 103 to 104. This indicates that the presence of populations of EEA and twist conformers is negligible. Therefore, the most stable AAA conformer was selected for study of the mechanism of alkaline hydrolysis of the nitramine. As shown previously (Hoffsommer et al., 1977; Balakrishnan et al., 2003) the hydrolysis of RDX starts from elimination by E2

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mechanism which consists of simultaneous detachment of proton and nitrite from nitramine under the influence of hydroxide resulted in double bond C@N formation. The computational results obtained in this study show that the initial denitration of RDX occurs in two sequential steps (Fig. 2). The first step represents nucleophilic attack of hydroxide onto methylene hydrogen with the formation of unstable negatively charged intermediate INT1. The second step includes an easy removal of nitrite ion and transformation to very stable unsaturated INT2. This process is accompanied by releasing of large amount of energy, that provides evidence in favor of its irreversibility. Exothermicity of the nitrous acid elimination from cyclic nitramines is experimentally and theoretically supported (Bishop et al., 2000; Hawari et al., 2002; Zhang et al., 2011). Existence of INT2 in decomposition pathway of RDX was tentatively identified in several experimental studies (Hoffsommer et al., 1977; Hawari et al., 2002; Balakrishnan et al., 2003). 3.2. Pathways for cycle opening Further transformation of INT2 proceeds with six-membered cycle opening which could be initiated by nucleophilic attacks of water molecule as well as hydroxide ion onto carbon atom of double C@N bond. Calculations show that attaching a water molecule to C@N bond requires three-times more energy as compared to hydroxide binding (Fig. 3a and b). A large value for Gibbs free activation energy (more that 50 kcal mol1), indicates that water attachment to RDX within this pathway is rather unlikely. This is also suggested by experimental data, which confirm that hydrolytic decomposition of RDX occurs under photolytic, enzymatic, alkaline conditions (Hawari et al., 2002; Waisner et al., 2002; Balakrishnan et al., 2003; Gent et al., 2010). Hydroxide attack onto C2 atom leads to unstable intermediate INT3 with negative charge localized at nitrogen atom N1. There are three possible pathways for further transformation of INT3 which are characterized by small activation barriers in a range of 1.11–3.53 kcal mol1 (Fig. 3c and d). However, these pathways lead to intermediates with different stabilities. Proton transfer between the oxygen and nitrogen atoms occurs with involvement of a water molecule as a catalyst and results in formation of INT5 with negative charge localized at oxygen atom. However, the direct C2AN3 bond breaking requires about 2 kcal mol1 less activation energy, and leads to a more stable cycle-opened intermediate INT6, as compared with previous pathway. Both intermediates further transform into INT7. While the process of C2AN3 bond breaking is very easy for INT5 (activation barrier is 0.17 kcal mol1), the proton transfer from oxygen at C2 to nitrogen N1 requires 27.52 kcal mol1 activation energy (Fig. 3c). We suggest that an intermediate INT3 could acquire a proton from water molecule and transform to uncharged intermediate INT3H, which is only slightly more stable than INT3 (Fig. 3d).

Further C2AN3 bond cleavage with activation barrier of 11.78 kcal mol1 could lead to ring-opened intermediate INT7H through unstable INT8. Based on energetics, INT7H is approximately three-times less stable than INT7. Therefore, it is more probably that INT3 transforms into more stable INT7. Hydroxide may also participate in direct nucleophilic attack of the C4 atom and lead to cycle cleavage and formation of INT4 through SN2 mechanism. However, activation energy for this reaction is 32.68 kcal mol1. This is two-times more than free energy barrier for hydroxide attack onto C2 atom. We concluded that pathway leading to INT4 is unfavorable because of this high activation barrier. Based on the results obtained, one can deduce that the most favorable channel for cycle cleavage, which characterizes the smallest activation energies, is INT2 ? INT3 ? INT5(INT6) ? INT7 (Fig. 3). It should be mentioned that experimentally observed INT7H (4,6-dinitro-2,4,6-triaza-hexanal) (Hawari et al., 2002; Balakrishnan et al., 2003) may be formed from INT7 during neutralization of reaction mixture sample before analysis. Possible reaction pathways for INT7 will be investigated in the next section. 3.3. Ring-opened intermediate transformation Ring-opened intermediate INT7 has several bonds that potentially can be broken. Hydroxide attack onto C3 atom leads to C3AN2 and C3AN4 bonds cleavage, while attack onto C5 atom causes breaking of C5AN4 and C5AN6 bonds. The probability of such processes is determined by activation barriers and stability of formed intermediates. As can be seen from Fig. 4, C3AN2 and C5AN6 bonds ruptures require very high (more than 50 kcal mol1) activation energies and, therefore, are unfavorable. Search to find the transition state for hydroxide attack onto C5 atom with C5AN4 bond rupture leads to conclusion that the C5AN4 bond breaks without participation of hydroxide. Although the calculated reaction barrier is not high, the formed intermediates INT13 and INT14 are less stable than INT7. Hydroxide attack onto C3 atom, leading to C3AN4 bond cleavage and formation of two stable intermediates INT11 and INT12 through SN2 mechanism, has high activation barrier of 39.22 kcal mol1, that is twice more than the barrier encountered for C5AN4 bond rupture. Therefore this pathway is also deemed unfavorable. In our attempt to determine the most favorable pathway for INT7 decomposition, we examined the further transformation of INT14 and found that hydroxide attached with a small activation barrier of 7.73 kcal mol1 and formed very stable intermediate INT16 (Fig. 5). Based on this evidence we concluded that under alkaline conditions INT7 decays mainly into INT13 and INT16. Negligible amounts of INT11 and INT12 can also be produced. Calculated results support experimental finding that 4-nitro-2,4diazabutanal is a major intermediate of cycle-opened intermediate decomposition (Hawari et al., 2002; Balakrishnan et al., 2003). Unfavorable formation of experimentally observed INT15 (5-

Fig. 2. Computer generated initial pathway of RDX alkaline hydrolysis, corresponding Gibbs free energy diagram.

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Fig. 3. Computer generated pathways of INT2 alkaline hydrolysis, corresponding Gibbs free energy diagram.

Fig. 4. Computer generated pathways of INT7 alkaline hydrolysis, corresponding Gibbs free energy diagram.

hydroxy-4-nitro-2,4-diaza-pentanal) (Hawari et al., 2002; Balakrishnan et al., 2003) during hydroxide attack at C5 atom encourages us to investigate the reaction of INT7H with hydroxide leading to INT15. The activation barrier for this pathway is 38.34 kcal mol1, too high to be feasible (Fig. S1, Supplementary material).

3.4. End products Transformation of INT16 to the end products, which are nitrous oxide and formaldehyde, was found to occur in three steps including two proton transfers between oxygen and nitrogen atoms and CAN bond rupture (Fig. 5). Proton transfer involving a water molecule

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Fig. 5. Computer generated pathways of INT14 alkaline hydrolysis, corresponding Gibbs free energy diagram.

G, kcal/mol 20 INT1

10 0

RDX + OH

-10 -20 -30 -40 -50

INT3 INT2+NO 2

-60

INT5

-70

INT6

-80

INT13+INT14

-90

INT7 INT17+HCOH

-100 INT16

-110 -120

NHNO2

N2O

-130

Reaction coordinate Fig. 6. Relative Gibbs free-energy profile (kcal mol1) for the most favorable pathway of the alkaline hydrolysis of RDX calculated at the PCM(Pauling)/M06-2X/6311++G(d,p) level.

required more than 20 kcal mol1 activation energy, while the CAN bond broke easily. As a whole, decomposition of INT16 to the end products is exothermic. Experimentally reported product distribution for alkali hydrolysis of RDX (0.972 ± 0.056 mol NO–2, 0.951 ± 0.032 mol 4-nitro-2,4-diazabutanal, 0.939 ± 0.037 mol HCOH, 0.770 ± 0.130 mol N2O per 1 mol RDX) (Balakrishnan et al., 2003) corresponds to computationally obtained ones. Simulated UV–VIS spectra for RDX, stable intermediates and end products show an absence of absorption in the visible region that support reported experimental data about colorless reaction mixture during the whole process of alkaline hydrolysis of RDX (Fig. S2, Supplementary material) (Hoffsommer et al., 1977; Hwang et al., 2004). Summarizing, Fig. 6 presents the general profile of the change of the Gibbs free energy along the reaction coordinate that starts from the reactants (RDX and hydroxide) and ends at the products. Initial hydroxide attack resulted in proton and nitrite elimination from

RDX and double C@N bond formation in the cycle. Hydroxide attachment to the carbon atom at double bond initiated formation of ring-opened intermediate, which mainly decomposed into 4-nitro-2,4-diazabutanal, nitrous oxide and formaldehyde. Based on computationally obtained results we concluded that RDX alkali hydrolysis is a highly exothermic multistep process. However, immediate release of the heat energy is prevented by presence of few relatively high activation barriers. We also predicted the lack of single rate determining step for this transformation. This means that complex simulation of kinetics will need to include all the steps that are kinetically and thermodynamically feasible. 4. Conclusions The first quantum-chemical investigation of alkaline hydrolysis of RDX was performed at PCM(Pauling)/M06-2X/6-311++G(d,p) level. The obtained results demonstrate that the studied reaction

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is a multistep exothermic process in which the presence of hydroxide is crucial for this process due to conclusion that hydroxide initiates a decomposition of nitramine through deprotonation and elimination of nitrite. Further hydroxide attachment to double C@N cyclic bond promotes ring cleavage. Major pathway in decomposition of ring-opened intermediate leads to formation of 4-nitro2,4-diazabutanal and methylene nitramine. The last one further transforms into formaldehyde and nitrous oxide. Computationally predicted dominated products and their distribution agree well with available experimental data. 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. Results in this study were funded and obtained from research conducted under the Environmental Quality Technology Program of the United States Army Corps of Engineers 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. We thank Engineer Research and Development Center (ERDC) for financial support (Grant No. is W912HZ-13-P-0037). The computation time was provided by the Extreme Science and Engineering Discovery Environment (XSEDE) by National Science Foundation Grant No. OCI-1053575 and XSEDE award allocation Number DMR110088 and by the Mississippi Center for Supercomputer Research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2015.03.064. References Al-Saidi, W.A., Asher, S.A., Norman, P., 2012. Resonance Raman spectra of TNT and RDX using vibronic theory, excited-state gradient, and complex polarizability approximations. J. Phys. Chem. A 116, 7862–7872. Agency for Toxic Substances and Disease Registry (ATSDR), 2012. Toxicological Profile for RDX. U.S. Department of Health and Human Services, Atlanta, GA, 229 p. Balakrishnan, V.K., Halasz, A., Hawari, J., 2003. Alkaline hydrolysis of the cyclic nitramine explosives RDX, HMX, and CL-20: new insights into degradation pathways obtained by the observation of novel intermediates. Environ. Sci. Technol. 37, 1838–1843. Bhushan, B., Trott, S., Spain, J.C., Halasz, A., Paquet, L., Hawari, J., 2003. Biotransformation of hexahydro-1,3,5-trinitro-1,3,5-Triazine (RDX) by a rabbit liver cytochrome P450: insight into the mechanism of RDX biodegradation by Rhodococcus sp. strain DN22. Appl. Environ. Microbiol. 69, 1347–1351. Bishop, R.L., Harradine, D.M., Flesner, R.L., Larson, S.A., 2000. Safe operating temperatures for pressurized alkaline hydrolysis of HMX-based explosives. Ind. Eng. Chem. Res. 39, 1215–1220. Brooks, M.C., Davis, J.L., Larson, S.L., 2003. Topical Lime Treatment for Containment of Source Zone Energetics Contamination. Final report ERDC/ELTR-03-19. Environmental Laboratory U.S. Army Engineer Research and Development Center. Vicksburg, MS, 100 p. Cossi, M., Barone, V., Cammi, R., Tomasi, J., 1996. Ab initio study of solvated molecules: a new implementation of the polarizable continuum model. Chem. Phys. Lett. 255, 327–335. Croce, M., Okamoto, Y., 1979. Cationic micellar catalysis of the aqueous alkaline hydrolyses of 1,3,5-triaza-1,3,5-trinitrocyclohexane and 1,3,5,7-tetraaza1,3,5,7-tetranitrocyclooctane. Org. Chem. 44, 2100–2103. Davis, J.L., Nestler, C.C., Felt, D.R., Larson, S.L., 2007. Effect of Treatment pH on the End Products of the Alkaline Hydrolysis of TNT and RDX. Final report ERDC/EL TR-07-4. Environmental Laboratory U.S. Army Engineer Research and Development Center. Vicksburg, MS. Dennington, R., Keith, T., Millam, J., 2009. GaussView, Version 5. Semichem Inc., Shawnee Mission, KS. Epstein, S., Winkler, C.A., 1952. Studies of RDX and related compounds VII. Relation between RDX and HMX production in the Bachmann reaction. Can. J. Chem. 30, 734–742.

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