Intermolecular interactions and thermodynamic properties of LLM-105

Abstract

<sec>2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) is a typical high-energy and low-sensitivity energetic material (EM), which has excellent detonation performance and thermal stability. In the quasi-harmonic approximation, the dispersion corrected density functional theory is used to study the intermolecular interactions and thermodynamic properties of energetic LLM-105 crystal. By introducing the zero-point energy and temperature effect corrections, PBE-D3 dispersion correction scheme can significantly improve the calculation accuracy of structural parameters at an experimental temperature (294 K). The temperature dependent lattice parameters and thermal expansion coefficients exhibit strong anisotropy, especially the thermal expansivity in <i>b</i>-axis orientation (intermolecular layers) is much higher than that in the <i>ac</i> plane (intramolecular layers). Through Hirshfeld surface and fingerprint analysis, it is found that the intermolecular interactions of LLM-105 are mainly O···H hydrogen bonding interactions. The change of intermolecular interactions will result in the rotation of nitro group, which can contribute to forming new hydrogen-bonding interaction pattern. Mulliken population analysis shows that the bond order of C−NO₂ bond is more sensitive to the change of temperature, so this bond may be a trigger bond for the high-temperature decomposition reaction of LLM-105.</sec><sec>The fundamental thermodynamic properties of EMs can not only provide key parameters for mesoscopic or macroscopic thermodynamic simulations, but also gain theoretical insights into the temperature effects of EMs. Specific heat capacity reflects the amount of heat to be supplied to heating the matter and it is important to make the risk assessment of EMs during storage or when exposed to external thermal stimuli. Herein, the basic thermodynamic parameters, such as heat capacity, entropy, bulk modulus and elastic constants under different conditions are predicted. Among them, the calculated heat capacity and entropy describe the nonlinear behaviors within a temperature range of 0−500 K, and the calculated isobaric heat capacity <i>C</i>_<i>p</i>(<i>T</i>) is in good agreement with the available experimental measurements. The elasticity of material describes the macroscopic response of crystal to external force, and the bulk modulus <i>B</i>₀ of molecular crystal can be determined through the equation of state, which is an important parameter for evaluating material stiffness. The bulk modulus under adiabatic condition is in reasonable agreement with experimental value, and the evolution of bulk modulus with temperature reflects the softening behavior of LLM-105 at temperature. Furthermore, the complete set of second-order elastic constants (SOECs) of LLM-105 is calculated and 13 independent SOECs (<i>C</i>₁₁, <i>C</i>₁₂, <i>C</i>₁₃, <i>C</i>₁₅, <i>C</i>₂₂, <i>C</i>₂₃, <i>C</i>₂₅, <i>C</i>₃₃, <i>C</i>₃₅, <i>C</i>₄₄, <i>C</i>₄₆, <i>C</i>₅₅, <i>C</i>₆₆) are predicted. With the increasing temperature, all elastic constants gradually decrease due to the weakening of intermolecular interactions of LLM-105. Overall, these results will fundamentally provide a deep understanding of temperature effects and serve as a reference for the experimental measurement of the thermodynamic parameters of EMs.</sec>