Directly Insight Into the Inter- and Intramolecular Interactions of CL-20/TNT Energetic Cocrystal through the Theoretical Simulations of THz Spectroscopy.

Compared with cocrystal coformers, an explosive cocrystal has distinctive packing arrangements and complex intermolecular interactions. Identifying the spectral signatures of an explosive cocrystal and understanding the molecular low-frequency modes by means of the spectrum in the terahertz range are of great worth to the explicit mechanism of cocrystal formation. In this work, on the basis of the joint molecular dynamics (MD) simulations and solid-state density functional theory (DFT) calculations, we have investigated the terahertz (THz) absorption spectra of the CL-20/TNT cocrystal and its different directions as well as cocrystal coformers 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 coformers indicates that the 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; 0.87 THz is also caused by C-H···O hydrogen-bonding bending vibrations; 1.60 and 1.85 THz features originate from C-H···O hydrogen-bond stretching vibrations. Additionally, the THz spectrum of the (001) direction of the CL-20/TNT cocrystal verifies that the molecular conformation of the CL-20 is the same as that in the ß-polymorph, other than the initial conformation of raw material ε-CL-20.

Low-temperature dependence on the THz spectrum of CL-20/TNT energetic cocrystal by molecular dynamics simulations.

Based on the unique advantages of terahertz (THz) spectrum on the detection of energetic cocrystals, the low-temperature dependent THz spectra of CL-20/TNT cocrystal were investigated by using molecular dynamics (MD) simulations from 5 to 296 K, as well as three different crystal faces, (001), (120), and (010). When the temperature decreases below 95 K, we have observed two new peaks for CL-20/TNT cocrystal, at 4.58 and 5.99 THz, respectively. Also, the THz peaks below 1.5 THz gradually disappear under cooling from 296 to 5 K, and they should originate from the lattice thermal vibrations. THz absorption peaks of CL-20/TNT cocrystal reveal frequency shifting, linearly dependent on temperature. Four of them are red shift and other two are blue shift of THz vibrational peaks of CL-20/TNT cocrystal with the temperature increase. The frequency shifts can be attributed to the effects of lattice thermal expansion on inter-/intramolecular vibrational modes as well as their coupling. From the temperature-dependent THz spectra of different crystal faces, we further confirm the response of different kinds of intermolecular interactions on the THz spectrum of CL-20/TNT cocrystal. Graphical abstractThe intermolecular interactions and peak positions of THz spectra of CL-20/TNT cocrystal in the range of 0-6 THz versus temperature.

Molecular dynamics simulations of void defects in the energetic material HMX.

A molecular dynamics (MD) simulation was carried out to characterize the dynamic evolution of void defects in crystalline octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX). Different models were constructed with the same concentration of vacancies (10 %) to discuss the size effects of void. Energetic ground state properties were determined by annealing simulations. The void formation energy per molecule removed was found to be 55-63 kcal/mol(-1), and the average binding energy per molecule was between 32 and 34 kcal/mol(-1) according to the change in void size. Voids with larger size had lower formation energy. Local binding energies for molecules directly on the void surface decreased greatly compared to those in defect-free lattice, and then gradually increased until the distance away from the void surface was around 10 Å. Analysis of 1 ns MD simulations revealed that the larger the void size, the easier is void collapse. Mean square displacements (MSDs) showed that HMX molecules that had collapsed into void present liquid structure characteristics. Four unique low-energy conformers were found for HMX molecules in void: two whose conformational geometries corresponded closely to those found in HMX polymorphs and two, additional, lower energy conformers that were not seen in the crystalline phases. The ratio of different conformers changed with the simulated temperature, in that the ratio of α conformer increased with the increase in temperature.