Comparative Study of Experiments and Calculations on the

Feb 10, 2016 - 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is the most powerful explosive. However, the application of this com...
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Comparative Study of Experiments and Calculations on the Polymorphisms of 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12hexaazaisowurtzitane (CL-20) Precipitated by Solvent/Antisolvent Method Xianfeng Wei, Jinjiang Xu, Hongzhen Li, Xinping Long, and Chaoyang Zhang* Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-327, Mianyang, Sichuan 621900, China S Supporting Information *

ABSTRACT: 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is the most powerful explosive. However, the application of this compound is limited by its high sensitivity and serious polymorphic transformations. Thus, elucidating the mechanism of crystallization and polymorphic transformation of CL-20 is crucial. This work presents a comparative study of experiments and calculations to clarify the mechanism of CL-20 precipitation using an solvent/antisolvent method. Calculations show that the β-formed CL-20 conformations are always the most energetically favored. These conformations have generally the highest content in solutions, and the intermolecular conformational transformations in solutions have low energy barriers. In addition, it is predicted that the β-CL-20 crystal possesses the lowest lattice energy among all polymorphs. The calculated results are successfully applied to explain the experimental observations, as β-CL-20 crystal is initially precipitated from most of the highly supersaturated solutions and then converted into ε-CL-20 crystal. This precipitation is kinetically controlled by the dominance of β-CL-20 molecules in a metastable phase and rapid crystallization. The final conversion into ε-CL-20 crystal is attributed to its low energy barrier for polymorphic transformation and stability, that is, the conversion is dynamically dominated. Furthermore, calculated coherent energy densities (CEDs) of various CL-20 polymorphs, including hydrates with different hydration degrees, agree well with the thermal stabilities, as the higher CED corresponds to the higher thermal stability. Therefore, the complex crystallization of CL-20 is elucidated by combining experimental observations with theoretical calculations and simulations. CL-20 has three polymorphs, namely, β-, γ-, and ε-forms, and a hydrate α-form at ambient conditions.1 Meanwhile, a ξ-form of CL-20 crystal exists at 1.44 GPa.16 These forms have distinct powers and safety, which are the two most important properties of explosives. The ε-form presents the highest packing density of approximately 2.04 g/cm3 and the best stability against thermal and impact stimuli. Thus, the crystal engineering of CL-20s mainly aims to achieve an ε-formed polymorph, avoiding transformation to other less stable ones.17−21 Crystallization conditions, such as solvents, addition method, temperature, and agitation rate, can distinctively affect the polymorphs of CL-20 and their transformations. For example, polymorphs of CL-20 transform more easily in solutions than in solids. Transformations in solutions start at low temperatures, even at the room temperature.18 By contrast, transformations in solids of β → γ and ε → γ are initiated above 400 K.17,18 In addition, experimental results show that β-formed polymorph is initially precipitated from many solutions and then transformed into a more stable ε-form.22 We examined a series of CL-20 crystallization using solvent/antisolvent

1. INTRODUCTION 2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) is currently the most powerful explosive.1−6 The energy of this compound is superior to that of its nitroamino cousins, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tertranitro-1,3,5,7- tetrazocine (HMX). However, the application of CL-20 is limited because of its high sensitivity to accidental initiation and cost. The high sensitivity is strongly related with its low chemical stability and easy transformation to more unstable phases. Several methods, such as improvement of crystal qualities,7 cocrystallization with other components,8−11 and increasing coating efficiency in plasticbonded explosives, have been performed to enhance the safety of CL-20 and expand its applicability. Explosive crystals, with minimal imperfections, ideal crystal shapes, sizes, and distributions, and a single polymorph with the highest packing density and stability, have enhanced safety and increased applicability.7 Thus, controlling the crystallization of CL-20 to its most stable form is the key to realize the applications of this compound. Also, the crystal shape control is crucial to safety, in particular for spherical crystals. It has been verified that the spherical CL-20 crystals are favorable to lowered impact sensitivity in contrast to other shaped ones, and even less sensitive than HMX and RDX crystals.12−15 © 2016 American Chemical Society

Received: January 12, 2016 Revised: February 10, 2016 Published: February 10, 2016 5042

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The Journal of Physical Chemistry C precipitation and found that the β-form appears first, which is then transformed into the ε-form when ethyl acetate (EA) solution is added and saturated with CL-20 using numerous antisolvents, except in water.22 Swift et al. and Ghosh et al. also found the coexistence of different polymorphs of CL-20 and their transformations in various solutions.23,24 Polymorphic forms could be related to the polarity of applied solvents. The β- and/or ε-forms usually appear in low-polarity and nonpolar solvents at low temperatures, whereas the γ (α)-form appears in high-polarity solvents at high temperatures.22,25−27 In contrast to experiments, calculations related to CL-20 crystallization are insufficient, resulting in obscurity in the mechanism of polymorphic transformation at the molecular level. Experiment results show the following characteristics of CL-20: high solubility in carbonyl-containing solutions, such as acetone (ACE) and EA; low solubility in alcohol, ether, and nitroalkane; and insolubility in hydrocarbon, halogenated hydrocarbon, and water, among others. Moreover, solubility of CL-20 is slightly dependent on temperature. Thus, no recrystallization of CL-20 is formed by decreasing temperature. The solvent/antisolvent method, rather than solvent evaporation by heating, is used in most cases for CL-20 recrystallization because temperature can accelerate CL-20 decay. Thus, it is interesting to investigate the dynamic behaviors of CL-20 molecules in a solution. Nevertheless, to our knowledge, no report on the dynamic behavior of these molecules exists. This study aims to investigate the dynamic behavior, static structures, and energy of CL-20 molecules in solutions. The dynamic behavior of CL-20 molecules in solutions can be reasonably considered as the basis to determine their crystallization. For instance, among the four crystal forms of CL-20, α- and γ-forms have similar molecular conformations regardless of chirality but are distinct from those in the β- and ε-forms.28 Thus, the corresponding molecular conformations in these forms are called α (γ), β, and ε-formed ones in this paper. Distinct CL-20 molecular conformations are the root factors for stacking various polymorphs apart from the hydrated α-form, further indicating that molecular conformational transformations are the possible bases for polymorphic transformations. The CL-20 molecule is composed of a rigid cage and six revertible NO2 groups.1 The rigid cage is assumed to change slightly in the solution, contrary to the considerable change in the NO2 groups. Figure 1 illustrates that CL-20 molecular conformations result from the rotations of the NO2 groups. Therefore, we investigated the orientations or molecular conformations of the NO2 groups during simulation of the CL-20 molecules in solutions. Molecular dynamic (MD) simulations that demonstrate well micro details were performed to examine the dynamic behavior of CL-20 molecules in solutions. Moreover, the interaction energy (IE) between a CL20 molecule and solvent molecules surrounding it in solution was provided to discuss the relationship between IE and solvent features, such as polarity. We designed a series of experiments to examine the polymorphic transformation of CL-20 in detail using solvent/ antisolvent precipitation methods, which have been previously reported in a few studies.16,22,29 These experiments and molecular calculations may be helpful in establishing a detailed mechanism of CL-20 crystallization. The results may guide technical optimization in processing CL-20 crystals.

Figure 1. α (γ), β, and ε-formed molecular conformations of CL-20 and their transformations. Gray, white, red, and blue represent C, H, O, and N atoms, respectively. NO2 groups in colored ellipses are considered. Orange and yellow represent the axial and exquatorial orientations of NO2 groups, respectively.

2. METHODOLOGIES 2.1. Experiments. Chemicals. CL-20 (99% purity, ε-form) was provided by Beijing Institute of Technology. All reagents and solvents applied, namely, heptane (HE), cyclohexane (CH), carbon tetrachloride (TCM), toluene (TOL), EA, dichloromethane (DCM), dichloroethane (DCE), ACE, and water, were received without further purification unless otherwise specified. Solvent/Antisolvent Crystallization of CL-20. Solubility of CL-20 in various solvents is weakly dependent on temperature, and high temperature can accelerate CL-20 decay.3,16,17 Thus, solvent/antisolvent method was applied to recrystallize CL-20 and examine the polymorphic transformation in a solution. CL20 powder (5 g) was completely dissolved with 12.5 g of EA (or ACE) in a 150 mL three-necked volume flask. An antisolvent (1:4 volume ratio of solvent to antisolvent), namely, DCM, TCM, BEN, DCE, HE, TOL, or CH, was normally added to the solution, or the solution was reversely added to an antisolvent with agitation at 160 rev/min. Moreover, precipitation temperatures were conditionally varied. The effect of addition method and crystallization temperature on the polymorphic behavior of CL-20 was then investigated. Transformation processes among the four polymorphs were also examined using a polarized microscope. Powder X-ray Diffraction (PXRD) Examination. All polymorphic transformations were examined in our institute using a Bruker D8 Advance with Cu Kα radiation (λ = 1.54180 Å) and a Vantec-1 detector at 40kV/40 Ma, without any monochromator. The 2θ range was 5°−50° with an increment of 0.02°/0.1 s. PXRD was employed to quantify the relative amounts of the precipitated CL-20 crystals. The polymorphic content or the polymorphic fraction (X) of CL-20 was evaluated by a full pattern fitting method based on Rietveld refinement and without a standard sample.30,31 In contrast to the traditional methods, this method can efficiently deal with the superposition problem by reducing the disadvantages caused by extinction and preferred orientation and thus can obtain more accurate intensities and quantitative results. The fitting was carried out using the Topas package. If a serious issue of preferred orientation exists, the PO Spherical Harmonics model and initial parameter of 8 can be adopted to correct the preferred orientation.32 5043

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The Journal of Physical Chemistry C 2.2. Calculations. In Figure s1, we defined the axial (a) or equatorial (e) orientations of each NO2 group in the α-formed conformation. The NO2 groups in other conformations were similarly confirmed. Although CL-20 molecules have eight stable molecular conformations,33 we only focused on the α (γ), β, and ε-formed conformations involved in the crystal forms that exist at ambient conditions (Figure 1). We used these orientations to confirm whether a conformation is of interest in the following MD simulations. In practice, we found that these conformations possess bigger populations than other conformations, and numerous unstable conformations exist dynamically in solution. Static and dynamic calculations were also conducted. We used static calculations to determine the geometries and energies of CL-20 conformations in gaseous state and solvents, as well as the effects of the solvent on these conformations and their intertransformation. By contrast, we investigated the effects of temperature and solvent on the distributions of these conformations in the dynamic simulations. The geometries of α (γ)-, β-, and ε-formed CL-20 conformations in gaseous state and solvents, as well as all solvent molecules, were fully optimized in the static calculations at the B3LYP/cc-pVDZ level of density functional theory. The B3LYP method includes generalized gradient approximation, exact exchange using the Becke three-parameter exchange functional,34−36 and nonlocal correlation functional of Lee, Yang, and Parr.37 This method can provide the structural and energetic information on nitramine cousins of CL-20, such as HMX and RDX, in conjunction with a moderate basis set, such as cc-pVDZ.38−41 Solvent effects were evaluated using the IEF-PCM method in the self-consistent reaction field (SCRF) QM/MM theory.42 In this technique, PCM calculations were performed using the integral equation formalism model,43−45 and universal force field (UFF) radii were drawn from the data derived from the UFF.46 Corresponding to experiments, nine solvents with a large range of dielectric constants (ε) within 1.92−78.39, namely, HE, CH, TCM, TOL, EA, DCM, DCE, ACE, and water, were chosen for the study. The dipole moments (D) of these solvents were also calculated at the B3LYP/cc-pVDZ level because the polarization of the solvents is possibly important to these conformations. The abbreviations, ε, and calculated D of the solvents are listed in Table s1. We collected four experimental and predicted parameters, namely, ε, molecular volume, density, and solvent radius, which are required in SCRF calculations for an important applied solvent EA because no default parameters are available for this solvent in Gaussian 03 programs.47,48 Soft scan method49 was applied to explore the barriers of intertransformations among the interesting conformations. Scanning increment was set to 5°. All static calculations were performed using Gaussian 03 package.47 Two extreme cases were considered to simulate the CL-20 molecules in the solutions. One resembled a case in which CL20 was diluted infinitely, that is, two neighboring CL-20 molecules were too far from each other to cause interaction (a low concentration of CL-20 in solution). The other resembled a case similar to that with CL-20 molecules on the crystal surfaces. The distance between two neighboring CL-20 molecules in this case is close to that in the lattice (a high concentration of CL-20 in solution). Remaining cases of CL-20 molecules in solutions were between these two extreme cases. For case 1, we selected eight solvents with various dielectric constants and dipoles, as well as two temperatures (298 and

350 K), for simulation. MD simulations were performed at 1 atm with COMPASS force field based on first-principle calculations and experimental data,50 resembling the conditions of CL-20 crystallization.22−27,29,51 The reliability of COMPASS to the interesting systems was validated by comparing the predicted and experimental densities (Table s2), and the related MD simulation details are shown in S3. For the second case, we simulated the dynamic behavior of CL-20 molecules around crystal/solute interfaces. We built 2 × 3 × 6 supercells of β- and ε-formed CL-20 as the crystal bulk as shown in Figure 2a. Afterward, we elongated the c-axes of the

Figure 2. Plots showing (a) the crystal bulk CL-20 with surfaces and (b) the crystal CL-20 in EA. L1, L2, and L3 denote the layers in the crystal according to the distance to the surface.

two supercells to 200 Å to build (001) faces and added 100 AE molecules near the surfaces for simulation, as illustrated in Figure 2b. In this case, two neighboring CL-20 molecules were close to each other. Moreover, we partitioned the molecular layers into layers L1, L2, and L3, according to the distances between the molecular layers and solvent. NPT simulations of the two systems were performed at 1 atm, 298 K, and 350 K for 10 ns. The final equilibrated structures were used as the initial ones for the subsequent NVT simulations at 298 and 350 K for 10 ns each with an increment of 1 fs. Frames were recorded per 1 ps. The last 1000 frames were used to analyze the occurrences of molecular conformations in the models. Andersen52,53 and Berendsen54 methods were employed in all MD simulations for thermostat and barostat, respectively. The atom-based and Ewald summation methods were used for van der Waals and electrostatic interaction calculations, respectively. All MD simulations were performed with Accelrys’ Material Studio code.55 MD trajectories were analyzed using some Perl scripts written by us. Moreover, we verified that data were from dynamically equilibrated structures and of statistic meanings and that the equilibrated CL-20 molecular structures in solution were independent of the initial ones before we analyzed the MD simulation results (see S4).

3. RESULTS AND DISCUSSION 3.1. Polymorphs and Their Transformations at Room Temperature. CL-20 polymorphs and their transformations caused by EA/various antisolvent precipitations at room temperature are listed in Table 1. The ε-formed CL-20 crystals were finally formed when precipitated by EA/antisolvent method, except in water. β-CL-20 crystal was initially observed during the reverse addition of saturated CL-20/EA to the HE, CH, DCM, TCM, DCE, and TOL. This form subsequently transformed into a more stable ε-form. This behavior was 5044

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found in the first several minutes of CL-20 precipitation by EA/ DCE (Figure 3). With continuous reaction, the slenderness ratio decreased and diamond shapes appeared. This crystal shape transformation suggests polymorphic transformation (i.e., β → ε), as β- and ε-formed CL-20 feature needle and diamond shapes, respectively. These crystal shape transformations are also demonstrated in Figures s8 and s9. The PXRD pattern evolution in Figure 4, as well as in Figures s3− s7, also shows the β → ε transformation of CL-20, compared with the standard patterns of β- and ε-formed CL-20. The normal addition of an antisolvent to the saturated CL20/EA solution yielded results distinct from those of the reverse addition. That is, β- and ε-forms, or only the ε-form, can be observed first. Rates of addition of HE and TCM influenced the recrystallization mechanism. The fast addition of HE at 15 mL/min resulted in the initial precipitation of both βand ε-forms, whereas slow addition (1 mL/min) only produced ε-forms. The slow addition of TCM led to the initial formation of β- and ε-forms, whereas the fast addition first produced βform only, which subsequently transformed into the ε-form, similar to the case of the reverse addition. The initial crystal which formed in CH, DCM, DCE, and TOL was independent of the rates of normal addition. However, the initial crystal forms were not similar to one another: β and ε formed in CH, β in DCM and DCE, and ε in TOL. Reverse addition was performed at an extremely fast rate. Table 1 shows that faster addition of saturated CL-20/EA solution to the seven antisolvents resulted in a higher tendency of initial formation of β-CL-20. Detailed evolutions vary from one another although CL-20 recrystallized by various methods are finally of ε-form, which is the most stable polymorph at room temperature. Figure 5

Table 1. Crystal Forms and Polymorphic Transformations of CL-20 Recrystallized from EA/Anti-Solvent Precipitation at Room Temperaturea antisolvents

A

B

C

HE CH DCM TCM DCE TOL water

ε (β+ε)→ε β→ε (β+ε)→ε β→ε ε α

(β+ε)→ε (β+ε)→ε β→ε β→ε β→ε ε α

β→ε β→ε β→ε β→ε β→ε β→ε α

a

A and B represent the normal addition of an antisolvent to the saturated CL-20/EA solution at 1 and 15 mL/min, respectively. C represents the instant reverse addition of the saturated CL-20/EA solution to an antisolvent.

verified from the crystal shape and PXRD pattern examinations (Figures 3 and 4, and S5). Fine needle-shaped crystals were

Figure 3. Microscopic image of the evolution of CL-20 precipitated from the reverse EA/DCE addition at room temperature.

Figure 5. Evolution of CL-20 transformed from β- to ε-forms in various antisolvents at room temperature.

illustrates the difference in polymorphic transformation rate when the reverse addition was performed. This difference distinguishes the effects of antisolvents on the β → ε transformation of CL-20. Figure 5 demonstrates that the transformation in TOL and HE was almost completely finished within 10 min, whereas those in DCE and DCM took more than 70 min. Transformations in TCM and CH were mediated in contrast to TOL, HE, DCE, and DCM. The transformation rates were consistent with the initial precipitation forms of CL20 under normal slow addition, which are shown in Table 1.

Figure 4. Powder X-ray diffraction (PXRD) pattern of the evolution of CL-20 precipitated from reverse EA/DCE addition at room temperature.

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The Journal of Physical Chemistry C The ε-, ε- and β-, and β-forms correspond to decreasing transformations. ε-CL-20 is the first crystal formed in lowsupersaturated HE and TOL at normal slow addition. This phenomenon suggests that it is more energetically and kinetically favored to form this most stable form of ε-CL-20 crystal relative to other antisolvents. The formation of the most stable crystal consumes the largest time for nucleation (Figure 5). This phenomenon also implies the faster transformation of β → ε during reverse addition in highly supersaturated solution. This induction can be applied to elucidate the remaining cases. 3.2. Polymorphs and Their Transformations at Increased Temperature. Polymorphic transformations of CL-20 in solutions are sensitive to temperature. In this study, we selected the case of reverse EA/DCE addition at 82 °C for comparison. In addition, transformation was slowest among all antisolvents at this temperature (Figure 5), which is beneficial for acquiring more accurate transformation mechanism because of the sufficient time for examination. Figures 6 and 7 verify the

With the PXRD verification in Figure 7, the evolution of the content fraction of the three polymorphs is shown in Figure 8.

Figure 8. Evolution of polymorphic transformations of CL-20 precipitated from reverse EA/DCE addition at 82 °C.

Compared with the transformation at room temperature, the elevated temperature of 82 °C resulted in a new phase of γ-CL20, besides β-CL-20 and ε-CL-20. The results listed in Table 1 show that the γ-form does not always appear in solutions at room temperature. This phenomenon shows that high temperature is necessary to obtain γ-CL-20 in the solution. Hence, γ-CL-20 is more stable than ε-CL-20 in the solution at high temperature. Figure 10 shows that γ-CL-20 is more stable than ε-CL-20 in EA/DCE when temperature is higher than approximately 43 °C. The evident transformation of ε- to γform from pure ε-CL-20 reportedly occurred at temperatures much higher than 100 °C. This result indicates that solvent inclusions in CL-20 will reduce the transformation temperature, decreasing its safety. 3.3. Polymorphs and Transformations Induced by Water. Water usually appears in the preparation and application environments of CL-20. Thus, the effect of water on the polymorphic transformations of CL-20 should be determined. As illustrated in Figure 9, α-CL-20, which is hydrated CL-20, was formed when we added water to EA or ACE solution saturated with CL-20. The hydrated CL-20 was stable at room temperature, and no polymorphic transformation was found within several days. This product was remarkably different from β-CL-20, which was readily converted into ε-CL-20 in a solution. As the amount of hydrated water increases, α-CL-20 becomes more stable, or the conversion becomes more difficult. 3.4. Molecular Properties. Polymorphs of CL-20 are directly related to its molecular conformations. Thus, establishing its molecular properties in a solution is important. Properties in the gaseous state will also be discussed for comparison. We initially analyzed the dipole moments of three interesting CL-20 conformations in various solvents. We performed SCRF calculations and found the increasing order of dipole moments of optimized β-, α/γ-, and ε-CL-20 molecules in any solution or isolated gaseous state. We also examined the solvation effect on CL-20 molecular conformations. Figure s10 illustrates that all solvents with different polarities (for example, dipolar moment of 0−2.74 D shown in

Figure 6. Microscopic image of the evolution of CL-20 precipitated from the reverse EA/DCE addition at 82 °C.

Figure 7. PXRD pattern of the evolution of CL-20 precipitated from reverse EA/DCE addition at 82 °C.

polymorphic transformations of CL-20 in the solution of EA/ DCE at 82 °C, in terms of crystal shapes and PXRD patterns, respectively. CL-20 crystals with mixed shapes of needle, diamond, and flake (characteristic shapes of β-, ε-, and γ-CL-20, respectively) appeared before 10 min. Most crystals appeared in diamond-shaped crystals, whereas needle-shaped crystals disappeared at 10 min. The flake-shaped crystals then increased. 5046

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Figure 10. Transformational energy barriers (ΔEs) of three interesting CL-20 conformations in different solvents and gaseous state.

Figure 9. PXRD patterns of α-CL-20 precipitated from EA/W and ACE/W at room temperature.

Table s5 illustrates that all dihedrals change from a negative value to a positive one, suggesting that NO2 groups can change readily between a- and e-orientations. Moreover, the solvent reduced the spans of these dihedrals. The spans of the dihedrals in the gaseous state were larger than those in the solutions, suggesting that the solvent molecules can constrain molecular vibrations. The most shrinking effect occurred when the CL-20 molecule was dipped in water, which had the largest dielectric constant. By contrast, slight shrinking occurred in solvents with low dielectric constants, such as DCM and CH. This phenomenon indicates that polar solvent molecules strongly interact with CL-20 molecules through negatively charged NO2 groups. Moreover, the effects of solvent and temperature on the molecular conformations were re-evaluated. The signs of D1 to D4 corresponded to the orientations of the NO2 groups, that is, the positive and negative signs denote the e and a-oriented NO2 groups, respectively. Orientations of the NO2 groups under dynamic conditions were analyzed accordingly and are summarized in Table s6. The e- and a-oriented NO2 groups appeared in the gas state and almost all solvents at 298 and 350 K. The coexistence of e and a-oriented NO2 groups in solvents and gaseous state suggests the easy transition among all CL-20 molecular conformations. This result is consistent with the statically calculated results of low transition barriers. Hence, this coexistence should be a basis for the easy polymorphic transitions of CL-20 in solvents observed experimentally.17−22 Furthermore, we summarize the occurrences of the interesting α, β, and ε-formed conformations and their summation in gaseous state and solutions in Figure 11. The interesting conformations consisted 50% of the total occurrences regardless of whether in gaseous state or in solutions. This characteristic suggests that CL-20 molecular conformations in solid remarkably differ from those in gas and solutions. Figure 11 shows other important characteristics. β-Formed molecular conformation exhibited the highest content in most solvents (five out of nine interesting cases at 298 and 350 K), contrary to the smallest conformation shown by the β- or aform. These MD results are consistent with energy analyses in section 3.2.1. β-Formed conformation is most energetically favored and therefore had the highest content, in particular, in low polarity solvents. Meanwhile, the effects of temperature and

Table s1, or dielectric constants of 0−78) displayed solvation effects on the conformations. Strong solvation tendency corresponding to high polarities was also observed. The εformed conformation always exhibited several kilojoules per mole of total energy higher than α and β-forms. Moreover, the total energy of the α-form was slightly higher or almost similar to that of the β-form in the gaseous state and solvent. The calculated REs of gaseous conformations (Figure s11), including ε-form (6.99 kJ/mol), α/γ-forms (4.73 kJ/mol−1), and β-form (0.0 kJ/mol), agree well with the previous report by Kholod et al.33 This phenomenon implies that β-formed CL-20 molecule is the most energetically favored conformation. Moreover, we calculated the IE between CL-20 and solvent molecules to show solvation by MD at 298 and 350 K. Figure s12 demonstrates that IE can be distinctively influenced by solvents. The solvent with high polarity (dielectric constant) can apparently lead to a large IE, which is in agreement with the SCRF calculated results. This phenomenon may be attributed to the six NO2 groups surrounding a cage to form a CL-20 molecule. These groups are negatively charged and can cause a large electrostatic interaction with polar solvent molecules. This behavior can partly explain the insolubility of CL-20 in hydrocarbon or halogenated hydrocarbon. CL-20 can also be insoluble in water because of the large cohesive energy density of water, despite a big IE between their molecules. We examined the transformation energy barriers (ΔEs) with molecular conformational transformations as the bases for polymorphic transformations. Figure 10 shows that ΔEs are small (