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Structural Rearrangement of Energetic Materials Under an External Electric Field: A Case Study of Nitromethane Yingzhe Liu, Yiding Ma, Tao Yu, Weipeng Lai, Wangjun Guo, Zhongxue Ge, and Zhinan Ma J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11097 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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The Journal of Physical Chemistry

Structural Rearrangement of Energetic Materials under an External Electric Field: A Case Study of Nitromethane Yingzhe Liu,1,* Yiding Ma,1 Tao Yu,1 Weipeng Lai,1 Wangjun Guo,1 Zhongxue Ge,1 Zhinan Ma2,3* 1

State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute, Xi’an 710065, P. R. China 2

3

School of Science, North University of China, Taiyuan 030051, P. R. China

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, P. R. China

* Corresponding author. E-mail: [email protected] (Y. Liu) and [email protected] (Z. Ma) ACS Paragon Plus Environment

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ABSTRACT: As a significant stimulus, the external electric field (EEF) can change the decomposition mechanism and energy release of energetic materials (EMs). Hence, understanding the response of EMs to an EEF is greatly meaningful for their safe usage. Herein, the structural arrangement, a crucial factor to the impact sensitivity and detonation performance of EMs, under the EEF ranging from 0.0 to 0.5 V/Å was investigated via molecular dynamics simulation. Nitromethane (NM) was taken as a case study due to the simple structure. The simulation results show that there exists a critical EEF strength between 0.2 and 0.3 V/Å, which can induce the transition of NM molecules from relatively disordered distribution to solid-like ordered and compacted arrangement with a large density. In this ordered structure, NM dipoles are aligned in a head-to-tail pattern parallel to the EEF direction because of the favored dipole-dipole interactions and weak C−H···O hydrogen bonds. As the EEF strength is enhanced, the potential energy and cohesive energy density of NM system gradually decreases and increases, respectively, indicative of high thermodynamics stability of ordered arrangement. The results reported here also shed light on the potential of the EEF to induce the nucleation and crystallization to explore new polymorphs of EMs.

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INTRODUCTION The response of energetic materials (EMs) to an accidental stimulus such as impact, shock, friction, and heat that can trigger an unintended and undesired detonation is a continuing concern to the safe usage of EMs in both the civilian and military fields. The electric field is one of important external stimulus, which can be employed as an electrically controlled approach for ignition of EMs.1 It has been indicated the introduction of an external electric field (EEF) into an energetic material can change its sensitivity, decomposition mechanism, and energy release.2 Therefore, understanding the effect of an EEF upon the EMs is crucial for their safety in preparation, processing, transportation, storage and application. In recent years, computational techniques have been widely used as a powerful tool to decipher the decomposition mechanism of EMs and explore the structure-sensitivity relationships. Especially for impact sensitivity, a variety of molecular and crystal parameters, including atomic charge, electrostatic potential, bond order, activation energy, rate constant, free space in crystal, and so on, have been proposed to correlate with it, which allows impact sensitivity to be predicted theoretically.3-12 However, the behaviors of EMs under the external stimuli of EEFs have still remained unclear. Currently, most of computational research on how the EEFs affect the EMs has focused on the structural, energetic, and reactive properties as well as their variations with different strengths and directions of EEFs via single-molecule model.2,13-21 For example, Politzer et al. systematically studied the influences of EEFs upon the “trigger linkage” bonds in EMs, namely, C–NO2, N–N2, O–NO2, and N–NO2, and showed that these bonds can be strengthened along the EEF direction due to the reinforcement of intrinsic molecular polarities.13-15 Ren and coworkers suggested the EEFs can trigger the rupture of N–O bond at the initial stage of decomposition for some EMs and established linear correlations between strength of EEFs and structural parameters including bond length and bond stretching frequency.2,16-17 However, the understanding regarding the role of EEF on intermolecular interactions of EMs is still lacked. It has been reported that the EEFs can induce the molecular rearrangements, e.g. self-assembly of ACS Paragon Plus Environment

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carbon nanomaterials,22 reordering of ionic liquids,23,24 nucleation and crystallization of ice,25-27 and so forth. These studies indicated the ordering effect of EEFs on the molecular arrangement. Molecular arrangement in the crystal plays a key role in the energy and safety of EMs.28,29 Higher compact arrangements can form larger packing density and less free volume in the EM crystal, which is beneficial for detonation performance and mechanical sensitivity, respectively. Taking HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocane), one of the most powerful EMs, as an example, three polymorphs (α, β, and δ) and one hemihydrate (γ) have been identified for the HMX crystal at ambient conditions, and the most compacted form of β-HMX has both the largest velocity of detonation and the lowest impact sensitivity.30 Hence, new avenues have been proposed to improve the stability and sensitivity of EMs by changing their molecular arrangements, such as energetic co-crystallization31,32 and nanoscale confinement.33-36 To our best knowledge, surprisingly, the effect of EEFs on the structural arrangement has not yet been studied in the EM field. Nitromethane (NM, CH3NO2) is one of the simplest nitro compounds that are of great interest to the high explosives, fuels, and propellants. On account of its relatively small size and availability of extensive experimental data, NM has been intensively employed as a prototype system in theoretical studies of EMs to understand the complex reaction mechanism in the processes of combustion and explosion.37-45 Also, NM was probed as a representative of EMs with C–NO2 trigger linkage under the impact of EEFs in the previous work via quantum chemistry calculations.2,13-16,18 Herein, the effect of an EEF upon the molecular arrangement of liquid NM was theoretically studied by nonequilibrium molecular dynamics (MD) simulations. The structural, energetic, and dynamic properties of NM in different EEF strengths were examined, which is envisioned to offer exciting perspectives for tuning the performance of EMs via structural rearrangement.

METHODS Model. The NM molecule has two different conformers, i.e., eclipsed and staggered form, and the energy barrier of transform between them is less than 0.01 kcal/mol in vacuum, indicating that the NO2 ACS Paragon Plus Environment

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group can rotate around the C–N bond almost freely.46 Consequently, only the staggered form of isolated NM molecule was built as the initial structure. NM exists as a liquid under ambient conditions and the density is 1.137 g/cm3 at 293 K.47 The model of liquid NM was composed of 500 molecules that were randomly packed into a cubic simulation cell with a density of about 1.2 g/cm3 via the Monte Carlo fashion to minimize close contacts between atoms, where the COMPASS force filed48 was employed for geometry optimization. Then, the liquid NM was equilibrated in the NPT ensemble at 293 K and 1 bar for 500 ps, which is adequate for system relaxation reflected by steady fluctuation of temperature pressure profiles. The average density of liquid NM during the trajectory of the last 100 ps was calculated as 1.092 g/cm3, which is in good accordance with the experimental one. Finally, the equilibrium structure of liquid NM was used for the simulation with a static EEF applied. Molecular Dynamics Simulation. All the nonequilibrium MD simulations of liquid NM under a static EEF were carried out using the Forcite module of Material Studio 8.0 software.49 The EEF was introduced instantaneously along the +z direction with various strengths ranging from 0.0 to 0.5 V/Å at an interval of 0.1 V/Å. For convenience, these EEF strengths were labeled as E0.0, E0.1, E0.2, E0.3, E0.4, and E0.5, respectively. The isobaric-isothermal ensemble was employed and the periodic boundary conditions were applied in three directions of Cartesian space. The temperature and the pressure were maintained at 298 K and 1 bar with the help of Berendsen thermostat and barostat, respectively. The decay constant for both thermostat and barostat was set to be 0.1 ps. The van der Waals interactions were truncated by a spherical cutoff of 12.5 Å radius, and the electrostatic forces were evaluated by means of Ewald summation approach with a accuracy of 0.001 kcal/mol. The equation of motion was integrated by a time step of 1 fs, and each simulation was performed for 1 ns. The trajectory data were collected every 500 time steps. Partial analysis and visualization of simulation trajectories were conducted via VMD package.50

RESULTS Molecular rearrangements. To analyze the effect of EEFs on the NM rearrangements, the total ACS Paragon Plus Environment

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dipole moment along the +z direction, µz, was monitored to show the degree of molecular alignment with the EEFs. µz was calculated as the vectorial sum of the dipole moments of all the NM molecules,

µz = ∑ µz ,i

(1)

where µz,i is the z-component of the individual dipole moment of each NM molecule. As shown in Figure 1a, µz fluctuate steadily around ca. 0 D in the absence of EEF, indicative of an isotropic fluid for NM. When the EEF was introduced, µz increases gradually and gets the maximum alignment with the EEF after about 100 ps for each system, suggestive of molecular reorientation induced by the EEF. Additionally, the maximum degree of dipole alignment rises with the increase of the EEF strength. For the system E0.3, E0.4, and E0.5, the difference among the maximum alignments is small.

Figure 1. Time evolution of (a) z-component of the total dipole moment and (b) mass density of NM at various EEF strengths

Mass density is also an important factor that reflects the compactness degree of molecular rearrangements. It can be seen from Figure 1b that the NM system is compacted under the EEF, leading to the raising of density. The stronger the EEF strength is, and the larger the equilibrium density is, which is in accordance with µz. For the system E0.2 and E0.3, there is a big gap between their equilibrium densities, indicative of a big structural change. As displayed in Figure 2, the NM molecules are arranged into a highly ordered pattern at the EEF strength of 0.3 V/Å, i.e. the NM dipoles are arrayed head-to-tail in a linear alignment parallel to the EEF direction due to the favored dipole-dipole ACS Paragon Plus Environment

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interactions. Conversely, the NM molecules show a relatively disordered arrangement when the EEF strength is less than 0.3 V/Å, especially for the system E0.0 (see Figure 2a).

(a) E0.0

(b) E0.3

Figure 2. Snapshots of equilibrium structure for the system (a) E0.0 and (b) E0.3

Furthermore, it is evident from Figure 1 that molecular rearrangement under the EEF is a fast process and can be finished after about 100 ps for each system. Therefore, the time scale of 1000 ps is adequate for system relaxation. Here, the initial 200 ps of the simulations were considered as equilibration and the subsequent 800 ps of the trajectories were used for the following analysis. Structural features. To understand structural arrangement of NM molecules induced by the EEF, the NM orientation is quantified by the orientation angle θ, which is defined as the angle formed between the NM dipole and the EEF direction, that is, z axis. The probability distributions gathered in Figure 3 demonstrate that a preferred orientation of the NM dipole exists in all the systems. Even in the absence of the EEF, there is also a broad peak around the orientation angle of 90 º, although the corresponding probability at the peak is very small than the other systems. As the EEF is strengthen gradually, the angle of preferred orientation decreases and the probability increases progressively. The system E0.3, E0.4, and E0.5 have almost the same preferred orientation, i.e. the NM dipole is nearly parallel to the EEF direction, indicative of similar structure arrangements at the large EEF strength (see Figure 2b). On the basis of orientation angle, the order parameter, S, can be determined by the follow formula

S=

1 3cos2 θ − 1 2

(2)

The order parameter can quantify the ordering degree of structural arrangement. The S values of 0 and 1


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mean a fully random arrangement and a perfect alignment parallel to the z axis, respectively. It is expected in Figure 3b that S approximately equal 0 in the absence of the EEF, which signifies that the NM molecules are fully randomly distributed. AS the EEF is applied, the ordering degree of the system structure arises. When the EEF strength is equal or greater than 0.3 V/Å, S is gradually approaching to 1, reflecting a nearly perfect alignment of NM molecules parallel to the EEF direction.

Figure 3. (a) Distribution profiles of the orientation angle that is defined as the angle formed between the NM dipole and z axis. (b) Order parameter of simulation system as a function of the EEF. (c) Radial distribution functions between intermolecular C atom and N atom. (d) Number of week hydrogen bonds among NM molecules in various EEF strengths.

Radial distribution functions (RDFs) between intermolecular C atom and N atom were calculated in order to further analyze the structural features of NM molecules in the presence of the EEF. As shown in Figure 3c, the position and shape of the first peak is almost the same for each system independent of the EEF strength, indicating that the EEF does not change the N···C interaction distance in the first solvation shell. The RDF profiles of the system E0.0, E0.1, and E0.2 are very similar, i.e. the RDF gradually fluctuates weakly and the local density is approaching to the bulk density with the increase of the N···C distance. For the system E0.3, E0.4, and E0.5, in contrast, all the RDF profiles comprise a series of ACS Paragon Plus Environment

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similarly consecutive peaks, which suggests a periodically ordered structure. Hence, it is deducible that there exists a critical EEF strength between 0.2 and 0.3 V/Å that can transform the system structure from relative disordered arrangements to fully ordered alignments. Moreover, the spatial distributions of the NM molecules were collected and visually displayed via the occupancy analysis. Primarily, the 3D space of simulation system was fully divided into cubic grids with the size of 1.0 Å along each axis direction. For each grid, the occupancy was then set to either 1 or 0 depending on whether the grid contained NM atoms. At last, the fractional occupancy was averaged over the simulation trajectory. Figure 4 shows the NM spatial distribution that is the regions encompassed by the surface with the same occupancy. For the system E0.0, obviously, the NM molecules are uniformly distributed in the first solvation shell. On the contrary, the preferred distribution in the specific region is observed in the presence of the EEF. Besides, the second solvation shell can be seen clearly as a ring, which elucidates the role the EEF plays in the orientation and ordering of NM molecules.

(a) E0.0

(b) E0.3

Figure 4. Spatial distribution of the NM molecules for the system (a) E0.0, occupancy=0.54 and (b) E0.3, occupancy=0.68

It was reported that there exist weak hydrogen bonds of C−H···O in liquid NM.51 To illuminate the EEF effect on this hydrogen-bond network, the number of hydrogen bonds was calculated at various EEF strength based on the geometry criterion: the distance C···O < 3.5 Å and the angle C−H···O > 150º. As delineated in Figure 3d, the number of C−H···O hydrogen bond increases as a function of the EEF strength due to the structural rearrangement of NM molecules. A jump in the hydrogen-bond number ACS Paragon Plus Environment

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can be found between E0.2 and E0.3, which reflects a significant structural change caused by the critical EEF strength. Energetic properties. To estimate the thermodynamics stability of the NM arrangements in the presence of the EEF, the average potential energy per NM molecule of each system was calculated and shown in Figure 5. Evidently, the potential energy has negative linear relationship to the EEF strength. As the EEF strength increases, the potential energy decreases gradually, suggesting that the EEF can improve the thermodynamics stability of NM system.

Figure 5. Average potential per NM molecule and cohesive energy density of each system in various EEF strengths.

The interactions among the NM molecules were characterized by the cohesive energy density (CED), which can be calculated via the following formula: CED =

1 1 Ecoh = ( Eint ra − Etotal V V

)

(3)

where Ecoh is the cohesive energy that is the difference between the intermolecular energy (Eintra) and the total energy of the system (Etotal), and V is the volume of the system. As delineated in Figure 5, an approximate s-shape curve can be found for the CED as a function of the EEF strength. There is a steep increase from E0.2 to E0.3, indicative of a significant structure change of NM arrangement. The CED was further decomposed into the van der Waals and the electrostatic components. The results show that the electrostatic forces mainly dominate the intermolecular interactions, which constitutes about 67% ACS Paragon Plus Environment

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contribution to the CED for each system. Dynamic properties. The diffusion behavior of NM molecules under the influence of the EEF was examined though the relationship between mean square displacement (MSD) and simulation time.52 Figure 6 gives the time evolution of the MSD of NM molecule. Clearly, the MSD curve declines step by step with the increase of the EEF strength, suggesting that the EEF restricts the diffusion of NM molecules. The MSD profiles are nearly the same between system E0.4 and E0.5, indicating that the NM diffusion cannot be slower even the EEF strength is further enhanced.

Figure 6. Mean square displacement of NM molecule as a function of simulation time. The dash curve represents a straight line with the slope of 1.

In addition, there exists a critical EEF strength between 0.2 and 0.3 V/Å that distinguishes the MSD profiles into two groups. For the system E0.0, E0.1, and E0.2, the slope of the MSD curve after about 20 ps is close to 1, indicative of a normal diffusion of liquid NM. While the EEF strength is equal or greater than 0.3 V/Å, the MSD gets an approximately steady line, in special for the system E0.4 and E0.5, which demonstrates a solid-like arrangement with very slow diffusion.

DISCUSSION The simulation results show that the EEF can induce an ordering effect on the structure of liquid NM, leading to a high density due to the compact arrangements of NM molecules. When the EEF strength of ACS Paragon Plus Environment

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0.5 V/Å, the maximum strength investigated here, is applied, the highest density of 1.352 g/cm3 can be obtained at room temperature, which is close to the NM crystal density of 1.401 g/cm3 at 228 K.53 Undoubtedly, a significant increase of density can greatly improve the denotation performance of EMs. Besides, the increase of density accompanies the decrease of available free space, which is generally conducive to reduce the impact sensitivity.54 The NM dipoles in the presence of the EEF are arranged into a special pattern, i.e. the head to tail alignment, especially for the system E0.3, E0.4, and E0.5. This structural motif was proved to have high bond dissociation energy of C−NO2 linkage, and thus has a higher thermal stability than random arrangement of liquid NM.35 Previous quantum chemistry calculations also presented that the C−NO2 can be strengthened due to the increase of activation energy barrier and bond-stretching vibration frequency under a weak EEF.15,18 For the system E0.4 and E0.5, the diffusion behavior of NM molecules is very slow, suggestive of a solid-like arrangement. Thus, the EEF can be considered as a potential approach to induce the nucleation and crystallization to explore new polymorphs of EMs, which has been employed successfully for paracetamol.55 When the EEF is removed, the simulation trajectory shows that the ordered structure of NM molecules cannot be maintained and quickly changes to the random arrangements (see Figure 7), indicating that the regulation of EEF on the NM arrangement is reversible.

Figure 7. Time evolution of z-component of the total dipole moment during the EFF-induced reverse transition. ACS Paragon Plus Environment

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Furthermore, the size effect of simulation on the NM structural rearrangement was considered by constructing a large system that is composed of 2000 NM molecules. After 2 ns simulation under an EEF with the same simulation protocol, the main properties of this system were calculated. The results showed that no significant difference on the structural change was observed between the two simulation systems, i.e. there also exists a critical structure transition between 0.2 and 0.3 V/Å for the large system. It does not, however, mean that the critical EEF strength reported herein would be consistent with the practical situation. In the practical experiment, the breakdown of materials will occur when the EEF strength exceeds their dielectric strengths. Although there is no available dielectric strength for NM, the dielectric strengths of other compounds can be found as a comparison, such as 0.81× 104 V/Å of methylamine.56 By contrast, the EEF strengths explored in the present work is very weak, i.e. no more than 0.5 V/Å. In this range of EEF strength, quantum calculation showed that the main change on NM geometry is conformational, e.g. rotation of the methyl groups.13

CONCLUSIONS The molecular-scale understanding on the response of NM to an EEF including the structural, energetic, and dynamic properties was studied by MD simulation. The ordering effect of the EEF on the NM molecules was observed dependent on the EEF strength and a significant structural change from relatively disordered arrangement to solid-like ordered alignment occurred between 0.2 and 0.3 V/Å. The ordered arrangement leads to a large density of NM, which can improve the impact sensitivity and detonation performance. The head-to-tail alignment of NM dipoles parallel to the EEF direction in the ordered structure is driven by the favored dipole-dipole interactions and weak C−H···O hydrogen bonds, which also enhances the thermodynamics stability. When the EEF is removed, the structural transition is reversed. Therefore, the EEF may be regarded as an avenue to tune the performances of EMs based on the structural rearrangement. ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS The authors greatly appreciate the financial support from the National Natural Science Foundation of China (Nos. 21403162, 21503160, 21503195 and 21702195), Natural Science Foundation of Shanxi Province (2015021044), and Open Research Fund of Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University in China. The authors are also grateful to the anonymous reviewers for their insightful comments to improve the contents of this study.

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