Directly Insight Into the Inter- and Intramolecular Interactions of CL-20

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Directly Insight Into the Inter- and Intramolecular Interactions of CL-20/TNT Energetic Cocrystal Through the Theoretical Simulations of THz Spectroscopy Lu Shi, Xiaohui Duan, Li-Guo Zhu, Xun Liu, and Chonghua Pei J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10782 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 8, 2016

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The Journal of Physical Chemistry

Directly Insight Into the Inter- and Intramolecular Interactions of CL-20/TNT Energetic Cocrystal Through the Theoretical Simulations of THz Spectroscopy Lu Shi 1, Xiao-Hui Duan *1, Li-Guo Zhu *2, Xun Liu 1, and Chong-Hua Pei 1

1. State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, P. R. China

2. Institute of Fluid Physic, China Academy of Engineering Physics, Mianyang 621900, Sichuan, P. R. China.

ACS Paragon Plus Environment

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ABSTRACT Compared with cocrystal co-formers, explosive cocrystal has distinctive packing arrangements and complex intermolecular interactions. Identifying the spectral signatures of explosive cocrystal and understanding the molecular low-frequency modes by means of the spectrum in terahertz range are of great worth to the explicit mechanism of cocrystal formation. In this work, based on the joint with molecular dynamics (MD) simulations and solid-state density functional theory (DFT) calculations, we have investigated the terahertz (THz) absorption spectra of CL-20/TNT cocrystal and its different directions as well as cocrystal co-formers and determined the systematic and all-sided assignments of corresponding THz vibration modes. The THz spectral comparison of the cocrystal with different directions and the cocrystal co-formers indicates that CL-20/TNT cocrystal has five fresh low-frequency absorption features as unique and discernible peaks for identification, in which 0.25, 0.73 and 0.87 THz are attributed to intensive crystalline vibrations; 0.87THz is also caused by C-H … O hydrogen-bonding bending vibrations; 1.60 and 1.85THz features originate from C-H…O hydrogen-bond stretching vibrations. Additionally, THz spectrum of (001) direction of CL-20/TNT cocrystal verifies that the molecular conformation of CL-20 is the same as that in β-polymorph, other than the initial conformation of raw material ε-CL-20.


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1. Introduction Energetic cocrystal, a new crystal engineering strategy, alleviate the safety-power contradiction of existing explosives, has been aroused much interest in the field of energetic materials.1 This new crystal engineering is widely recognized as an opportunity to modify explosive structures and physical-chemical properties by integrating two or more energetic molecules into one crystal lattice.2-8 Energetic cocrystal creates a distinct solid-state arrangement at the molecular level, which is implemented by the combination of intermolecular non-covalent interactions, such as hydrogen bonds, π-π stacking, van der Waals forces, instead of common chemical bond.9-12 So far, the characterization and detection for these weak non-covalent interactions in energetic cocrystal rely on traditional solid-state materials analytical tools, for example, X-ray powder diffraction (XRPD),13-14 Fourier transformation infrared spectroscopy (FT-IR)15 and Raman vibrational spectroscopy16. These currently used analytical techniques devote to intramolecular interactions, namely strong polar covalent bond stretching or angle bending. Yet there are no lattice vibrations and the majority of intermolecular interactions involved. In addition, single-crystal X-ray diffraction (SXRD) is also a favorable tool to the analysis of cocrystal structure if not strict demand to the preparation of single crystals.12,

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All of these

characterization methods often lack direct, effective and abundant information to identify the intricate intermolecular interactions. Therefore, it is worthwhile searching for an


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efficient and promising alternative as a complemental means to quantificationally monitor the formation of the energetic cocrystal.

Based on several decades of advances due to approaching the “terahertz gap” in electromagnetic spectrum, Terahertz Time-Domain spectroscopy (THz-TDS) has been employed as an attractive tool to medical imaging, security checks or nondestructive testing because of their high penetrability and low phonon energy.16,

18-21

THz

spectroscopy, the molecular vibration spectrum of the region from 0.1 to10 THz (from 3 to 333 cm-1, with 1 THz = 33.33 cm-1) between microwaves and infrared regimes in the electromagnetic radiation, has been founded as a valuable and supplementary analytical method to provide direct insights into the vibration spectra of energetic crystalline materials in THz range.22 These frequencies correspond to the energy level transition of external librations and internal torsions of explosive molecules, which yield “fingerprint” featured information of skeletal vibration of macromolecule, low-frequency crystal-lattice vibrations and weak intermolecular interactions including hydrogen bonding, van der Waals forces.19, 23-24 Hence, as the theory goes, THz absorption spectrum aims to provide a potential way for understanding and analyzing the cocrystal formation mechanism.

CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, often referred to as CL-20, or HNIW) 25 is one of the most famous high energy density compound for practical applications at present. Among the four reported crystalline forms, denoted as α, β, γ and ε,


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ε-CL-20 is the densest and most thermodynamically stable polymorph under ambient conditions, which is used typically.26 However, CL-20 has not achieved widespread engineering applications because of relative high sensitivity to heat, friction, and shock.27 CL-20 and TNT cocrystal with 1:1 molar ratio has been prepared by Bolton et al. and co-workers,7 which combine the stability and economy of TNT with power of CL-20 to create a low-sensitive and high-energetic cocrystal. The crystal structures of CL-20/TNT cocrystal and the cocrystal co-formers, ε-CL-20 and TNT, are shown in Figure 1. The crystal structure of CL-20/TNT cocrystal and the corresponding molecular conformations are shown in Figure 1(a). We can see that the cocrystal exhibits alternatively layered-layered mode of molecular stacking between CL-20 and TNT molecular chain with a huge C-H···O hydrogen bonds network to its next neighbors. In CL-20/TNT cocrystal, the conformation of CL-20 molecules is the same as that in β- CL-20 due to NO2 space orientation but differ from the initial conformation in raw material ε-CL-20. Besides, CL-20/TNT cocrystal shows obviously different intermolecular interactions along different crystalline directions, such as noncovalent interatomic hydrogen bonds contact between CL-20 nitro group and the 3-position hydrogen of TNT, while nitro group of same TNT molecule and piperazine rings of another CL-20 molecule forming a repeating zigzagging chain in the (010) direction. Also, the interactions lie between CL-20 nitro groups and the electron-poor ring of TNT along (120) direction, and the interactions in (001) direction are those between adjacent CL-20 molecules.7 In consideration of these


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distinctive intermolecular non-covalent interactions, in this work, we choose CL-20/TNT cocrystal as a prototype to investigate the response of THz spectrum on the weak nonbonding interactions, hoping to develop the utilization of the THz spectrum in the field of the energetic cocrystals.

Figure 1. Unit cells of CL-20/TNT cocrystal (a), ε-CL-20 crystal (b) and TNT crystal (c), and the corresponding molecular conformations. C, H, O, and N elements are represented in gray, white, red and blue, respectively.


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Recently, several groups have reported the experimental THz spectra of CL-20 28-30 and TNT 28, 31-38 crystalline powders, exhibiting their unique absorption features in THz range. However, it is very difficult to obtain the experimental THz spectra of the perfect single crystals of energetic cocrystals due to the limit of water interference, sample microstructure

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and a slow even hard preparation of low-defect or perfect single

crystals.39 Furthermore, systematic assignment of the vibrational modes is remain fraught with challenges because the high overlap of absorption peaks make individual feature distinguished difficultly.40 By contrast, theory simulations of THz spectra can effectively compensate these disadvantages, avoiding interference of some unessential factors and acquiring THz features of the perfect single crystal and assigning their vibration modes. So far, we only found the THz spectra of γ-CL-20 28-29 and TNT 38 molecules obtained by the density functional theory (DFT) method. Due to neglecting the vibration and intermolecular interactions, these isolated-molecular spectra should have very distinct difference from corresponding crystals. Therefore, in this study, THz spectrum of CL-20/TNT cocrystal has been studied by the crystal-lattice dynamic theoretical simulations. The characteristic THz spectrum of CL-20/TNT cocrystal and the corresponding vibrational modes are determined by combining with the THz spectrum studies of cocrystal co-formers (ε-CL-20 and TNT) and different orientations of CL-20/TNT cocrystal. Consequently, the response of THz spectrum is acquired on the


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different intermolecular interactions, which are the main driven forces for the formation of CL-20/TNT cocrystal.

2. Theoretical Methods

2.1 Theoretical

When THz radiation propagates through matter, polarity molecules can absorb energy under the action of oscillating electric field, thus induce forces on the molecular dipoles. Since the system accessing to the thermal equilibrium system, linear response theory can be effective to obtain the THz absorption spectroscopy. Based on linear response, the absorption spectra depend on the time autocorrelation function of electric dipole moment. The relationship between the absorption line shape I( ) and the freely propagating dipole-dipole time autocorrelation function (DDACF)





M (t )  M ( 0 ) was shown in

following relation:

I( ) 

h 



2(1  exp( h  ))  





M (t )  M (0)e itdt

With   kT  ; this is included to account approximately for frequency dependence of 1

the quantum occupation numbers.41-42

 Where M (t ) denotes the dipole moment operator of the sample and the brackets is the average over an appropriate equilibrium ensemble, which is proportional to frequency.


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Linear response together with MD simulation has developed to become the primary method to theoretically calculate molecular vibration spectrum. The spectrum is then obtained by evaluating the power dissipation as a function of frequency.

2.2 Modeling Computational Methods

In this paper, all the simulations were performed on Materials Studio 3.0 (MS) program by Accelrys Software, Inc. The calculations utilized COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field.43 This force field was exemplified accurately and simultaneously to the prediction of structural, conformational, vibrational, and thermos-physical properties for a broad range of molecules in isolation and in condensed phases. It has been successfully applied for ε-CL-20 44-45 and TNT46.

The unit cells of ε-CL-20 and TNT were built according to the crystal parameters derived from CCDC (Cambridge Crystallographic Data Centre), and then extended to the three-dimensionally periodic 3a×3b×4c (5184 atoms, comprised 144 ε-CL-20 molecules) and 3a×3b×3c (4536 atoms, comprised 216 TNT molecules) supercells, respectively. Analogously, based on the crystal parameters obtained from X-ray crystallographic experimental data,7 the unit cell of CL-20/TNT and a 3D periodic supercell (5472 atoms, comprised 96 ε-CL-20 molecules and 96 TNT molecules) in 3a×2b×2c arrangement were built. All three supercells were equilibrated for 200 ps using isothermal-isobaric


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(NPT) ensemble corresponding to atmospheric pressure and T=298K; in which “Berendsen” was chosen as the thermostat and barostat; Ewald and Atom based summation were respectively used for electrostatic interactions and van der Waals forces, and all atom-atom interactions were summed to a 15.5 Å direct cutoff distance for the intermolecular potentials. The equations of motion were integrated with a step of 0.1 fs. Approximately after 100 ps, the fluctuation of energy and temperature has been less than 5%, meaning the systems reached equilibrium. After equilibrium, the differences of the density and crystal parameters between the original and equilibrated supercells are also less than 5%. Based on the equilibrium structures from NPT simulations, the NVE (isochoric/isoergic) trajectories of 100 ps length with the step size of 0.1 fs were calculated to accomplish the THz absorption spectroscopy. Atomic positions were recorded every 4.0 fs, which ensured that this sampling frequency captured the entire vibrational frequency range of CL-20/TNT cocrystal including C-H stretches. The DDACF was calculated separately for each trajectory, after which the ensemble-averaged DDACF was Fourier transformed by using the discrete cosine transform.47 Here, it is worth mentioning that only the first 20.0 ps of the DDACF were used to analyze in the Fourier transform since the precision of the DDACF became lower for times comparable to the total simulation time41. Initially, the DDACF are relatively large due to the robust positive correlation between   M (t ) and M (0) , which is very beneficial for the accurate prediction of THz spectrum.

Subsequently, DDACF will delay toward zero along the runtime. In order to validate


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reliability of MD simulation, solid-state density function theory (DFT) calculations of the unit cells of co-formers and cocrystal were performed using DMol3 program. We employed periodic boundary condition (PBC) with generalized gradient approximation (GGA)-PBE density function incorporating DNP basis set (double numerical with d and p polarization).48 “Fine” quality was chosen corresponding to grid sizes of 0.04 Å-1 and energy convergence criteria of 1×10-6Ha.

3. Results and Discussion

3.1 THz spectroscopy of cocrystal co-formers: ε-CL-20 and TNT

The calculated absorption spectra of ε-CL-20 and TNT crystals in THz frequency range at ambient temperature and pressure are shown in Figure 2. Comparison of calculated and experimental THz spectra and the vibrational assignment were listed in Table 1. From Table 1, we can find that the simulation results exhibit an excellent agreement with the experimental findings, verifying the rationality and reliability of two simulation methods. Besides, the THz absorption peaks obtained using MD simulations are also consistent with solid-state DFT calculations.

As respect to ε-CL-20, it is characterized by three pronounced maxima at 1.22, 1.41and 2.02 THz, respectively attributed to crystal lattice vibration (i.e., longitudinal phonon mode) in lower frequency region and NO2 torsion accompanied ring rotation, coinciding


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well with experimental results below 5.0 THz. Additionally, the two less prominent absorption peaks at 1.71 and 2.28 THz, attaching on the vicinity of 2.02 THz, may have the same vibration assignment as that of 2.02 THz, namely NO2 rotational and ring normal modes. A slightly remarkable absorption feature at 3.85 THz corresponds to the experimental data, 3.72 THz. It may be attributed to NO2 out of plane wagging mode. Compared to the experimental results, theoretical THz spectra present slight red shifts to low frequencies, probably arising from the differences of the crystal quality, morphology, and purity between the powders and single crystals.


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Figure 2. The calculated THz absorption spectra of explosive compounds (a) ε-CL-20 and (b) TNT from the DDACF analysis. Insert in (b) exhibits the enlarged view of THz absorption feature below 3.0 THz. The THz absorption peaks for TNT are shown in Figure 2(b) at 0-8 THz, and this frequency range is accessible experimentally. The enlarged view of 0-3 THz spectra has shown in the inset in gray exhibits unambiguous THz absorption peaks. Very weak absorptions below 3.0 THz are presented at 0.77, 1.28 and 2.16 THz, and the experimentally observed spectra below 3THz regime approximately exist at 1.6 and 2.2 THz. All of them could be assigned to low-frequency lattice modes. The THz spectral features shown in the regime excessed 3.0 THz of Figure 2 (b) are nearly identical to the results obtained from experimental measurement. Namely, the strong absorptions at 3.78, 4.65, 5.64 THz, respectively attributed to C-C ring torsion, CH3 rotations ring torsion as


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well as NO2 and CH3 wagging out of plane mode, are in agreement with 3.69, 4.71, 5.59 THz peaks given by Leahy-Hoppa M.R.et al37) (see Table 1).

Table 1. Comparison of calculated THz vibrational frequencies by MD simulations with solid-state DFT calculations and experimental absorption peaks of ε-CL-20 and TNT. Vibrational assignments are shown in the last column. Vibrational Frequencies (THz)

Calculated frequencies

Vibrational assignment

(THz) ε-CL-20

Exp. 128

Exp. 230

1.32

MD

DFT

1.22

1.18 Lattice vibrations

1.43

1.46

1.41

1.44

1.75

1.71

1.72

2.00

2.02

2.06

2.37

2.28

2.35

3.72

3.85

3.86

1.17

0.77

1.21

1.62

1.67

1.28

1.59

2.2

2.02

2.16

2.10

3.69

3.78

3.67

4.11

4.11

2.08

TNT

Exp. 131

Symmetric NO2 rotations Ring rotations modes NO2 wagging modes

Exp. 237

Lattice modes

C-C ring torsions


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4.71

5.59

4.53

4.70

CH3 rotations

4.65

4.63

Ring torsions

4.90

4.90

5.44

5.55

NO2 and CH3 wagging

5.64

5.61

out of plane modes

3.2 THz spectroscopy of CL-20/TNT cocrystal

The contradistinctive THz spectra of CL-20/TNT cocrystal, ε-CL-20 and TNT from 0 to 8 THz are shown in Figure 3. The distinct THz absorption peaks and the corresponding vibration modes description have been listed in Table 2. From the comparison in Table 2, we can see that the MD simulation results are in agreement with solid-state DFT calculations, which further illustrates the reliability of MD simulations for the prediction of THz spectroscopy. In Figure 3, THz spectrum of CL-20/TNT cocrystal exhibits five distinctive features that could be used for identification. Mainly, there are three absorption features at 0.25, 0.38 and 0.87 THz, maybe ascribed to crystalline lattice vibrations and C-H … O hydrogen-bonding bending vibration accompanied with the strengthen of intermolecular interactions at 0.87 THz. The other two THz responses respectively at 1.60 and 1.85 THz are attributed to the C-H…O hydrogen-bonding stretching modes. These can be selected as a reference to distinguish the CL-20/TNT cocrystal. Besides, the vanishment of 1.22 and 1.41 THz absorption peaks in ε-CL-20 is another sign to monitor the formation of CL-20/TNT cocrystal. Even so, the comparison shows that the THz absorption peaks of


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CL-20/TNT cocrystal overlap partially with cocrystal co-formers, but on the other hand, CL-20/TNT cocrystal has more spectral features, especially focus on low-frequency 0-2.5 THz range, which may result from the coupling of CL-20 and TNT molecules. Namely, it could be related that the integration of two different molecules in the same solid-state asymmetric unit gives rise to the decrease of space group symmetry and the variation in molecular packing, resulting in more IR-active normal modes of vibration in terms of intermolecular interactions.

Figure 3. Calculated THz absorption spectra for CL-20/TNT cocrystal, ε-CL-20 and TNT: Top: CL-20/TNT cocrystal; Middle: TNT crystal; Bottom: ε-CL-20 crystal.


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Table 2. Calculated THz Vibrational Frequencies of CL-20/TNT cocrystal by MD simulation and solid-state DFT calculation, vibration assignments and intensities of THz absorption peaks. Calculated frequencies(THz) MD

Intensities

Vibrational assignments

DFT

0.25

m

0.38

s

0.73

0.65

s

0.87

0.91

s

Crystalline lattice vibration

0.92 1.60

1.59

s

Benzene ring torsion

1.62 1.85

1.83

C-N caged-Ring torsion in CL-20

s

NO2 rotation modes in CL-20 Asymmetric C-CH3 out of plane wagging

1.84

A pair NO2 wagging in TNT 2.20

2.20

m

Benzene ring torsion NO2 swing modes C-CH3 wagging

3.79

3.79

w

4.15

4.15

w

NO2 rotation modes in CL-20 and TNT CH3 and a pair NO2 wagging in TNT Asymmetric three pair NO2 wagging out of plane in CL-20

5.45

5.22

w

A pair NO2 wagging in CL-20 C-H out of plane swing and asymmetric a


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pair NO2 wagging in TNT

w, weak; m, medium; s, strong; 3.3 Intramolecular Vibrational Modes in the THz Domain

As demonstrated by the THz spectrum of CL-20/TNT cocrystal, almost each of those THz absorption frequencies corresponds significantly to given vibration mode in prominent regions of the spectrum, i.e., 1.6-2.2 THz and 3.7-5.5 THz ranges. What is the dynamical reason for these modes? To this end, the visual dynamical animations of six low-frequency intramolecular modes peaking from 0 to 8 THz regime are found, seen from the Figure 4. The low-frequency modes with the maxima at 0.73, 0.87 THz are the crystalline lattice vibrations (see Figure 4a). The ring torsions of C-N caged-ring in CL-20 and benzene ring in TNT, together with C-CH3 out of plane wagging and NO2 rotation modes are shown at 1.60, 1.85 and 2.2 THz (see Figure 4b, c). The modes at 3.79 and 4.15 THz (see Figure 4d, e) can be described as the partial NO2 rotation and CH3 wagging in CL-20 and TNT, obviously weaker than lower-frequency THz vibrations. Seen from Figure 4f, the THz modes merely display individual NO2 wagging and C-H out of plane swing of TNT. Overall, the intensities of intramolecular vibrations increase and intermolecular relative vibrations weaken along with higher frequencies. For THz vibrational assignments, it can be shown that the low-frequency resonance observed in spectrum can be assigned to crystalline lattice vibration below 1.0 THz and ring torsion at


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around 2.0 THz, intramolecular NO2 and CH3 rotations together with wagging modes at 3.79, 4.15, 5.45 THz.

Figure 4. Important THz modes of CL-20/TNT cocrystal. All displayed intramolecular THz modes obtained from a part of CL-20/TNT cocrystal; others should be identical to this fragment because symmetry exists in crystal. The vibration frequencies at (a) 0.73, 0.87 THz; (b) 1.60, 1.85 THz; (c) 2.2 THz; (d) 3.79 THz; (e) 4.15 THz; (f) 5.45 THz. 3.4 THz Spectroscopy of CL-20/TNT Cocrystal in Different Orientations


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Crystal anisotropy along with different crystal faces gives separate and legible crystal structure that is crucial for understanding the intermolecular interactions. The structures of CL-20/TNT supercell cut along the (001), (120) and (010) crystal faces are shown in Figure 5. Three different directions present three distinctive molecular arrangements and intermolecular interactions, such as, the C-H…O hydrogen bond along (010) direction, π-π stacking following (120) direction and CL-20-CL-20 intermolecular electrostatic interactions in (010) direction, respectively. The different responses of THz spectrum on these crystal faces could be devoted to the investigation of these weak non-covalent interactions. Based on above-mentioned comparison of MD simulations with solid-state DFT calculations and experiential results, it is quite clear that MD simulations can be used to successfully predict the THz absorption features. Taking the computation efficiency into account, only MD simulations are adopted in this section.


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Figure 5. Representation of CL-20/TNT cocrystal along the three different directions: a (010) face, b (120) face, and c (001) face. Figure 6 displays the THz absorption spectra of CL-20/TNT cocrystal along fixed crystalline orientations (001), (120), (010) and the interaction modes in three orientations, respectively. The red line of Figure 6 (a) presents the THz spectrum of (120) direction in CL-20/TNT cocrystal. It is characterized by a prominent absorption peak centered at 0.07THz. As described above, π-π stacking is predominate in this direction, corresponding to the interactions of electron-rich nitro groups of CL-20 with the electron-poor ring of TNT, or the CL-20 bond with nitrogen of TNT nitro group in an N=O…NO2 arrangement, seen from Figure 6 (b). Thus, we speculate that this sharp peak may originate from the π-π stacking. The corresponding energy of this THz frequency is agreement with the energy level transition of π-π stacking. The THz spectrum of (010) direction depicts as blue line in Figure 6 (a). We observe six low-frequency THz absorption peaks below 3.0 THz including 0.16, 0.31, 0.81, 1.09, 1.78, 1.95 THz, in which 0.81, 1.78 and 1.95 THz peaks, nearly agreeing with the 0.87, 1.60 and 1.85 THz features of CL-20/TNT cocrystal, respectively originate from C-H…O hydrogen-bond bending and stretching vibrations. Besides, it is obviously that the THz spectrum of (010) direction is consistent with that of CL-20/TNT cocrystal, only except for slight red shifts of hydrogen-bond network modes. This agreement further confirms that the pronounced intermolecular interactions


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throughout CL-20/TNT cocrystal are C-H … O hydrogen-bond interactions network propagating in CL-20/TNT cocrystal.

Figure 6. (a) THz absorption spectra of CL-20/TNT cocrystal with different orientations. The green line shows the spectra of (001) direction. The red line is THz line shape regarding to (001) direction. The blue line displays spectra of in (010) direction; (b) the schematic interactions modes of the different orientations in CL-20/TNT Cocrystal:


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nitro-hydrogen and nitro-nitro interactions between CL-20 molecules in (001) direction; nitro-aromatic and nitro-nitro interactions along (120) direction; C-H…O hydrogen bonds in (010) direction. Turning to the (001) direction, nitro-hydrogen and nitro-nitro interactions are dominant between CL-20-CL-20 molecules. The THz line shape of this direction is shown in the green line in Figure 6 (a). Since the CL-20 conformation in CL-20/TNT cocrystal is the same as that in β-CL-20 (see Figure 1), it might be speculated that CL-20-CL-20 intermolecular interactions in (001) crystal face are more similar to that of β-form, rather than that in ε-form. This hypothesis has been proved by the agreement of THz absorption spectrum of β-CL-20 with that of (001) crystal face, seen from Figure 7.

Figure 7. The contrastive THz absorption spectra of ε-CL-20 crystal (top), (001) crystal face (medium), and β-CL-20 crystal (bottom).


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4. Conclusion In this study, the THz absorption spectra and the corresponding vibrational assignments of CL-20/TNT cocrystal and the cocrystal conformers, ε-CL-20 and TNT crystals, have been investigated by the theoretical simulations. The comparison of MD simulation results with the solid-states DFT and experimental findings confirms the reliability of MD methods to predict the THz spectrum. Theoretical simulations indicate that there are five novel distinctive THz peaks for CL-20/TNT cocrystal, located respectively at 0.25, 0.38, 0.87, 1.60 and 1.85 THz. Hereinto, 0.25, 0.38, 0.87THz absorption peaks originate from the lattice vibrations; meanwhile, 0.87THz is also caused by C-H…O hydrogen-bonding bending vibrations; 1.60 and 1.85THz features is assigned to the C-H…O hydrogen-bond stretching vibrations. In addition, the THz spectrum of (120) crystal face has been used to conclude that 0.07THz peak is a symbol of CL-20-TNT π-π stacking interaction. Also, CL-20-CL-20 interactions and C-H…O hydrogen bonding in CL-20/TNT cocrystal have been further verified by means of the THz spectral features of (001) and (010) crystal faces. The THz spectra comparison of ε-and β-CL-20 crystals as well as (001) crystal face indicates that the CL-20 molecule conformation in CL-20/TNT cocrystal is the same as that in β-CL-20 other than in ε-CL-20. All these inspections embody the superiority of THz technology in the characterization of energetic cocrystals, especially the weak intermolecular interactions.


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AUTHOR INFORMATION Corresponding Author *Xiaohui Duan. E-mail: [email protected]

*Liguo Zhu. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are very grateful for the financial help from the National Natural Science Foundation of China (No. 11572270), the Postgraduate Innovation Fund Project by Southwest University of Science and Technology (No. 15ycx008), the Fund Project for National Defense Basic Research (No. 13ZG6101) and the Innovation Team Construction Program of Southwest University of Science and Technology (No. 14tdfk06).

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Directly Insight Into the Inter- and Intramolecular Interactions of CL-20

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