Exploding Nitromethane in Silico, in Real TimeEudes Eterno Fileti,*,† Vitaly V. Chaban,*,‡ and Oleg V. Prezhdo§

†Instituto de Ciencia e Tecnologia, Universidade Federal de Sao Paulo, 12231-280, Sao Jose dos Campos, Sao Paulo, Brazil‡MEMPHYS - Center for Biomembrane Physics, Odense M, 5230, Kingdom of Denmark§Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

*S Supporting Information

ABSTRACT: Nitromethane (NM) is widely applied inchemical technology as a solvent for extraction, cleaning, andchemical synthesis. NM was considered safe for a long time,until a railroad tanker car exploded in 1958. We investigate thedetonation kinetics and explosion reaction mechanisms in avariety of systems consisting of NM, molecular oxygen, andwater vapor. Reactive molecular dynamics allows us tosimulate reactions in time-domain, as they occur in real life.High polarity of the NM molecule is shown to play a key role,driving the first exothermic step of the reaction. Rapidtemperature and pressure growth stimulate the subsequent reaction steps. Oxygen is important for faster oxidation, whereas itsoptimal concentration is in agreement with the proposed reaction mechanism. Addition of water (50 mol %) inhibits detonation;however, water does not prevent detonation entirely. The reported results provide important insights for improving applicationsof NM and preserving the safety of industrial processes.

SECTION: Liquids; Chemical and Dynamical Processes in Solution

Nitromethane (NM; CH3NO2) was developed in 1872 fora variety of chemical uses.1 The versatility, functionality,

and efficiency of NM make it a great choice as a chemicaladditive for improving performance of various industrialprocesses.2 An expedient source of nitrogen, NM is a highlyfunctional intermediate in organic synthesis, used widely inmanufacturing of pharmaceuticals,3 as component of deter-gents, and as a chemical stabilizer in pesticides, explosives,fibers, and coatings.1,2

NM was not considered volatile in its pure form until the1950s, when numerous accidents occurred as it was beingtransported.4 However, its use as fuel and an explosive5−8

became quite common, mainly due to its low cost and safety.NM is utilized as a hobby fuel in model airplanes and racingcars, offering greater power and better performance. Explosiveproperties and combustion behavior of fuels based on NM havebeen studied at different levels, both theoretically andexperimentally.9−25 The processes involved in the shockinitiation of detonation in pure NM have been studied byplate impact experiments under a steady one-dimensional strainfor shock pressures ranging from 8.5 to 12 GPa.23 Theexperimental results confirm the main steps of the classicalhomogeneous model,26 but also show that the build-up processis more complex. When used as one of the additive componentsin propellants and explosives, liquid NM evaporates and mixeswith air to form flammable gaseous nitromethane/airmixtures.14,22 The influence of the relative concentration ofNM on the rate of increase of combustion pressure is relativelycomplicated.14 Dilution of a liquid explosive, such as nitro-

benzene, with an inert, nonexplosive liquid, such as methanol,benzene, or water, can decrease the detonation velocity and,ultimately, destroy the ability of this mixture to detonate atambient conditions. Expected from the energetic point of view,this situation has been investigated with nitrobenzene mixturesin recent years.18 It has been concluded that the elusivebehavior of the critical mixture cannot be completely describedby simplified models of one-dimensional detonation theory.18

Computational and numerical modeling have also beensporadically used.15−17,21 For instance, the decompositionmechanism of hot liquid NM has been investigated usingdensity functional theory and reactive force field (ReaxFF)molecular dynamics (MD) simulations.16,27 Both studieselucidate molecular aspects of pure, liquid NM, revealingdetailed chemical reactions that occur during detonation.Insights regarding the reaction mechanisms governing theinitiation and detonation processes have been provided.In this work, we investigate the explosion process for a

variety of systems consisting of pure NM and NM mixed withmolecular oxygen and water vapor. Nonequilibrium reactiveMD simulation allows us to observe reactions in real time, as itoccurs in experiment. Vigorous heat emission during a shortperiod of time stimulates subsequent reaction steps during theexplosion. An interesting methodology for the observation ofchemical reactions in real-time was recently introduced by Joshi

Received: September 2, 2014Accepted: September 19, 2014Published: September 19, 2014

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and co-workers.28 We do not use parallel replica dynamics inthe present work. Rather, we record system evolution betweenthe initial and final states, along a concentration gradient. Thismethodology enables us to analyze the reactive mechanismsthat drive such mixtures to explosion. We provide a generaldescription of interactions of the mixture components,revealing the differences in the detonation processes at eachcomposition.The MD simulations were performed using the ReaxFF

methodology, as detailed in the Supporting Information. Thenitromethane molecule is relatively small and simple, allowingus to perform efficient computations. The list of the simulatedsystems is provided in Table 1. Ten to 50 oxygen moleculeswere added to each system (Figure 1) to represent atmosphere.Oxidants are expected to play an important role inthermodynamics and kinetics of the explosion reaction.Unlike methane, NM is liquid at ambient temperature and

pressure with density of 1137 kg m−3,29 and enthalpy ofvaporization of 38.25 kJ mol−1.30 Van Duin and co-workershave investigated this liquid at ambient conditions, and at highpressures and temperatures. The reactive model providesdensity of 1111 kg m−3. The difference of only 2% from theexperimental value16 suggests a high level of accuracy in thesimulated physical chemical properties.Figure 2 shows temperature evolution in the pure NM

systems at different densities. Initial heating up to 1000 Ktriggers the first reaction step (unimolecular decomposition).Afterward, temperature grows drastically during the first 20 ps,irrespective of system density. Equilibrium in the lowest-densitysystem (d = 317 kg m−3) is achieved during 40 ps. Higher

Table 1. Simulated Systems: Molecular Compositions and Representative Parameters

# # CH3NO2 # O2 # atoms density, kg m−3 time-step, fs duration, ps T, K ensemble

1 25 0 175 317 0.10 100 1000+ NVE2 40 0 280 507 0.10 100 1000+ NVE3 60 0 420 706 0.10 100 1000+ NVE4 70 0 490 887 0.10 100 1000+ NVE5 25 10 195 380 0.10 100 1000+ NVE6 25 20 215 450 0.10 100 1000+ NVE7 25 25 225 480 0.10 100 1000+ NVE8 25 50 275 650 0.10 100 1000+ NVE9 70 0 490 887 0.05 150 5000 NVT10 25 25 225 380 0.05 150 5000 NVT11 200 0 1400 1210 0.25 675 300 NPT12 50 50H2O 500 882 0.1 100 1000+ NVE

Figure 1. Molecular snapshots of the simulated liquid nitromethane at 300 K: (a) starting configuration; (b) final configuration with the density of1210 kg m−3; (c) final configuration (hard spheres representation). Oxygen atoms are red, nitrogen atoms are blue, carbon atoms are gray, hydrogenatoms are black.

Figure 2. Temperature evolution in systems 1−8 computed in theconstant total energy ensemble. (Top) systems of various density (seelegends, in kg m−3) containing only nitromethane. (Bottom) systemscontaining 25 nitromethane molecules and various quantities ofoxygen molecules. The oxygen molecules are added to represent anatmosphere, which plays a significant role in the explosion process viaoxidation.

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density of the explosive brings higher temperature due to alarger number of chemical bonds per unit volume. Temperatureexceeds 4000 K after the first 100 ps and continues to increaseslowly. Interestingly, the difference between systems 2, 3, and 4is smaller than their difference from system 1. If liberated heatis not dissipated efficiently, gaseous pressure in the reactionvessel increases by 500−2000 MPa (Figure 3).

Our result correlates reasonably with the recent data, whereNM was simulated at high compression at 4000 K. Accordingto van Duin and co-workers, application of 5000 MPa to NMresults in a highly dense gas (d = 1111 kg m−3).16 The density(Figure S1) and structure of liquid NM at 300 K are insatisfactory agreement with the experimental data and previoussimulation results. NM is 20% denser than water at the sameconditions, making it a more energetic material. The densityincrease comes mainly from the larger dipole moment of theNM molecule and, therefore, stronger electrostatic attractionenergy.Atmospheric oxidants are expected to participate vigorously

in the NM detonation, since metastable nitrogen and carboncontaining particles need to transform into stable forms viaoxidation. In particular, we envision two major reactionpathways:

+ = + +4CH NO 7O 4CO 4NO 6H O 4: 7 ratio3 2 2 2 2 2(I)

+ = + +4CH NO O 2CO 2N 6H O 4: 1 ratio3 2 2 2 2 2(II)

Formation of molecular nitrogen appears more probable,because N2 is thermodynamically more stable at hightemperatures. Formation of nitrogen dioxide is another

possibility. These two competing reactions generate differentamounts of CO2. More importantly, they provide differentnumbers of newly synthesized gas molecules, which affect totalpressure. That is, the particular pathway must depend onapplied pressure and, therefore, on system density (Table 1).Another important observation is that different amounts ofoxygen molecules are involved in the formation of NO2 and N2.Indeed, oxygen-poor systems follow mechanism II, assupported clearly by the lower temperature and pressure inthese systems (Figures 2 and 3). Interestingly, the oxygen-richest system, n(NM):n(O2) = 1:1, also exhibits a slowerreaction rate due to formation of a variety of oxygen containingmetastable particles. Temperature of the NM/O2 mixturegrows rapidly during the first tens of picoseconds and continuesa slower increase beyond 20 ps. Oxygen enrichment leads todensity increase (Table 1) and, consequently, an increase inexplosion pressure. The generated pressure is roughly propor-tional to oxygen content (Figure 3).Figures 4 and 5 characterize the kinetics of formation of the

main reaction products, water, and carbon dioxide. Figure S2

depicts a real-time decrease of the NM content. The NMdecomposition rate clearly depends on system density, but it isnot sensitive to O2 concentration. Oxygen does not participatein the first stage of the explosion reaction. Molecular oxygen isa relatively weak oxidant. Oxidation at 1000 K is a slow processas compared to the time scale of the investigated detonationreactions. Interestingly, in the presence of oxygen, newlysynthesized CO2 and H2O molecules are more stable than inthe case of the oxygen-free detonation. The quantity of oxygenatoms obtained from decomposition of NM is enough to giverise to CO2 only, but hydrogen atoms cannot be oxidized (togenerate water vapor). A possible pathway in this case is toform some compounds with nitrogen at high temperature andpressure, e.g., ammonia. However, the content of ammonia in

Figure 3. Pressure generated due to explosion in the simulatedsystems: (top) systems of various density (see legends, in kg m−3)containing only nitromethane; (bottom) systems containing 25nitromethane molecules and various quantities of oxygen molecules.Compare with Figure 2.

Figure 4. Kinetics of CO2 formation during explosion. The CO2content is depicted as a percentage of the final CO2 content. If theinstantaneous content is greater than the final content, the valueexceeds 100%. In the O2 containing systems, CO2 starts forming laterthan in the systems without oxygen.

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the reaction byproducts is negligible. If an excess of oxygenwere supplied, the reaction would end with all involvedelements in their highest oxidation states: NO2, CO2, H2O.Note that molecular oxygen is an efficient oxidant only at hightemperatures, such as in the present case.Figure 6 compares reaction products in the oxygen-free

systems of various density and oxygen containing systems.

More water molecules than CO2 molecules are created in theO2 deficient systems, while the trend inverts in the oxygenabundant systems. Modest amounts of N2 are present in bothkinds of systems. These three products are represented most,whereas a variety of other particles are formed in tracequantities (NO2, NH3, NHC, HNO, etc.). If oxygen is lacking,the amount of such byproducts exceeds 50%. An abundance oftrace admixtures is also explained by a very high temperature(ca. 4000 K). The kinetic energy per degree of freedom exceedsthe bond energies of many potentially stable products of NMdecomposition, such as nitrogen oxides. In addition, thepresence of both CO2 and H2O in both kinds of systemssuggests that the formation of water vapor is much moreenergetically favorable than the formation of molecularhydrogen or even the more reduced form of hydrogen, NH3.Note that the initial oxidation state of hydrogen in this reactionis −1 (carbon hydride).NM is a popular solvent involved in a variety of industrial

applications. Its safety, therefore, presents an importantproblem. To gain insight into how the explosivity of NM-containing systems can be controlled, we simulated anequimolar NM−water mixture, system 12. The heat-initiatedexplosion was performed using the same computationalprocedure as for systems 1−8 (Figure 7). Water and NM

exhibit a perfect mutual miscibility. A large amount of waterdoes not block the explosion, but somewhat decreases theresulting temperature. Water induces strong hydrogen bondingwith NM during the first stage, which was interpreted by ouralgorithm as the formation of a separate metastable molecule.Decrease in the water content during the first picosecondsillustrates this effect (Figure 7). The resulting compound,CH3−N(O)(OH)2, decomposes promptly providing water andcarbon dioxide.To recapitulate, we have simulated a heat-initiated real-time

explosion of NM in the absence and presence of molecularoxygen and water. While oxygen favors the detonation reaction,

Figure 5. Formation of water vapor during explosion. The content ofCO2 molecules is depicted as a percent of the final CO2 content. If theinstantaneous content is greater than the final content, the valueexceeds 100%. Water molecules are highly unstable at the simulatedtemperatures, and their quantity fluctuates greatly.

Figure 6. Final composition of the explosion products computed inthe simulated systems 9−10, Table 1: (top) in the system containingonly CH3NO2 molecules (887 kg m−3 density); (bottom) in thesystem containing equimolar mixture of CH3NO2 and O2.

Figure 7. (Top) Temperature increase upon explosion of water−nitromethane equimolar mixture. (Bottom) Formation of water andcarbon dioxide products during the first 100 ps of the reaction.

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an excess of oxygen slows the reaction down. Such behavior isin agreement with the proposed reaction pathway. The gaseousproducts of the exothermic reaction generate significantpressures depending on the initial density of the reactants.Water impact on the detonation kinetics is explored for the firsttime, in particular, in regards to safety of NM as a popularsolvent. We show that even 50 mol % addition of water doesnot block detonation in the case of heat-driven initiation.However, such a dilution of NM can be an important safetymeasure at room temperature and ambient pressure.

■ ASSOCIATED CONTENT*S Supporting InformationDescription of the ReaxFF molecular dynamics methodology,and Figures S1 and S2 providing details on system equilibrationand explosion can be found in Supporting Information. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: vvchaban@gmail.com or chaban@sdu.dk.*E-mail: fileti@gmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSE.E.F. thanks the Brazilian agencies FAPESP and CNPq forsupport. MEMPHYS is the Danish National Center ofExcellence for Biomembrane Physics. The Center is supportedby the Danish National Research Foundation. O.V.P. acknowl-edges Grant CHE-1300118 from the U.S. National ScienceFoundation. V.V.C. cordially thanks Prof. Adri van Duin forimportant discussions regarding reactive molecular dynamicsmethodology and force field development. Dr. Fedor Goumans(Scientific Computing & Modeling, Amsterdam) has providedinvaluable assistance in visualization of the simulation results.

■ REFERENCES(1) Klapotke, T. M. Chemistry of High-Energy Materials, 2nd ed.; deGruyter: New York, 2012.(2) Markofsky, S. B. Nitro Compounds, Aliphatic. In Ullmann’sEncyclopedia of Industrial Chemistry, 7th ed.; Wiley-VCH: Weinheim,Germany, 2011.(3) Jouyban, A.; Kenndler, E. Capillary Electrophoresis with OrganicSolvents in Pharmaceutical Analysis: A Systematic Guide through theBackground. Curr. Anal. Chem. 2014, 10, 248−266.(4) Makovky, A.; Lenji, L. Nitromethane - Physical Properties,Thermodynamics, Kinetics of Decomposition, and Utilization as Fuel.Chem. Rev. 1958, 58, 627−644.(5) Sharia, O.; Tsyshevsky, R.; Kuklja, M. M. Surface-AcceleratedDecomposition of δ-HMX. J. Phys. Chem. Lett. 2013, 4, 730−734.(6) Katz, G.; Zybin, S.; Goddard, W. A.; Zeiri, Y.; Kosloff, R. DirectMD Simulations of Terahertz Absorption and 2D SpectroscopyApplied to Explosive Crystals. J. Phys. Chem. Lett. 2014, 5, 772−776.(7) Kuklja, M. M.; Rashkeev, S. N. Self-Accelerated Mechanochem-istry in Nitroarenes. J. Phys. Chem. Lett. 2010, 1, 363−367.(8) Landerville, A. C.; Conroy, M. W.; Oleynik, I. I.; White, C. T.Prediction of Isothermal Equation of State of an Explosive NitrateEster by van der Waals Density Functional Theory. J. Phys. Chem. Lett.2010, 1, 346−348.(9) Brown, K. E.; McGrane, S. D.; Bolme, C. A.; Moore, D. S.Ultrafast Chemical Reactions in Shocked Nitromethane Probed withDynamic Ellipsometry and Transient Absorption Spectroscopy. J. Phys.Chem. A 2014, 118, 2559−2567.

(10) Rivera-Rivera, L. A.; Sewell, T. D.; Thompson, D. L. Post-ShockRelaxation in Crystalline Nitromethane. J. Chem. Phys. 2013, 138,084512.(11) Bhattacharya, A.; Guo, Y.; Bernstein, E. R. A Comparison of theDecomposition of Electronically Excited Nitro-Containing Moleculeswith Energetic Moieties C−NO2, N−NO2, and O−NO2. J. Chem.Phys. 2012, 136, 024321.(12) Shekhar, H. Explosive Characteristics and Shaped ChargeApplications of Nitromethane (NM): A Review. Cent. Eur. J. Energ.Mater. 2012, 9, 87−97.(13) Koldunov, S. A.; Anan’in, A. V.; Garanin, V. A.; Torunov, S. I.Detonation Properties of Diluted Liquid Explosives: A Mixture ofNitromethane with Nitrobenzene. Combust., Explos., Shock Waves2012, 48, 106−111.(14) Zhang, Q.; Li, W.; Lin, D. C.; He, N.; Duan, Y. Influence ofNitromethane Concentration on Ignition Energy and ExplosionParameters in Gaseous Nitromethane/Air Mixtures. J. Hazard.Mater. 2011, 185, 756−62.(15) Liu, L.; Liu, Y.; Zybin, S. V.; Sun, H.; Goddard, W. A. ReaxFF-lg:Correction of the ReaxFF Reactive Force Field for London Dispersion,with Applications to the Equations of State for Energetic Materials. J.Phys. Chem. A 2011, 115, 11016−11022.(16) Rom, N.; Zybin, S. V.; van Duin, A. C. T.; Goddard, W. A.;Zeiri, Y.; Katz, G.; Kosloff, R. Density-Dependent Liquid Nitro-methane Decomposition: Molecular Dynamics Simulations Based onReaxFF. J. Phys. Chem. A 2011, 115, 10181−10202.(17) Menikoff, R.; Shaw, M. Modeling Detonation Waves inNitromethane. Combust. Flame 2011, 158, 2549−2558.(18) Koldunov, S. A.; Anan’in, A. V.; Garanin, V. A.; Sosikov, V. A.;Torunov, S. I. Detonation Characteristics of Diluted Liquid Explosives:Mixtures of Nitromethane with Methanol. Combust., Explos., ShockWaves 2010, 46, 64−69.(19) Bazin, M.; Ptasin ska, S.; Bass, A. D.; Sanche, L. ElectronInduced Dissociation in Condensed-Phase Nitromethane I: Desorp-tion of Ionic Fragments. Phys. Chem. Chem. Phys. 2009, 11, 1610−1618.(20) Hervouet, A.; Desbiens, N.; Bourasseau, E.; Maillet, J.-B.Microscopic Approaches to Liquid Nitromethane Detonation Proper-ties. J. Phys. Chem. B 2008, 112, 5070−5078.(21) Megyes, T.; Balint, S.; Grosz, T.; Radnai, T.; Bako, I.; Almasy, L.Structure of Liquid Nitromethane: Comparison of Simulation andDiffraction Studies. J. Chem. Phys. 2007, 126, 164507.(22) Sturtzer, M. O.; Lamoureux, N.; Matignon, C.; Desbordes, D.;Presles, H. N. On the Origin of the Double Cellular Structure of theDetonation in Gaseous Nitromethane and Its Mixtures with Oxygen.Shock Waves 2005, 14, 45−51.(23) Leal-Crouzet, B. Shock Initiation of Detonation in Nitro-methane. Combust. Flame 2000, 122, 463−473.(24) Courtecuisse, S.; Cansell, F.; Fabre, D.; Petitet, J. P.Comparative Raman Spectroscopy of Nitromethane-H3, Nitrome-thane-D3, and Nitroethane up to 20 GPa. J. Chem. Phys. 1998, 108,7350−7355.(25) Campbell, A. W.; Malin, M. E.; Holland, T. E. TemperatureEffects in the Liquid Explosive, Nitromethane. J. Appl. Phys. 1956, 27,963−963.(26) Presles, H. N.; Vidal, P. Detonation Generation and Propagationin Homogeneous Liquid Explosives. J. Phys. IV 1995, 05, C4-143−C4-152.(27) Manaa, M. R.; Reed, E. J.; Fried, L. E.; Galli, G.; Gygi, F. EarlyChemistry in Hot and Dense Nitromethane: Molecular DynamicsSimulations. J. Chem. Phys. 2004, 120, 10146−53.(28) Joshi, K. L.; Raman, S.; van Duin, A. C. T. Connectivity-BasedParallel Replica Dynamics for Chemically Reactive Systems: FromFemtoseconds to Microseconds. J. Phys. Chem. Lett. 2013, 4, 3792−3797.(29) Haynes, W. M. CRC Handbook of Chemistry and Physics, 94thed.; CRC Press: New York, 2013.

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dx.doi.org/10.1021/jz501848e | J. Phys. Chem. Lett. 2014, 5, 3415−34203419

(30) Jones, W. M.; Giauque, W. F. The Entropy of Nitromethane.Heat Capacity of Solid and Liquid. Vapor Pressure, Heats of Fusionand Vaporization. J. Am. Chem. Soc. 1947, 69, 983−987.

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