Young's modulus from terahertz spectroscopy and first-principles calculations as an effective indicator of impact sensitivity for molecular perovskite energetic materials.

CONTEXT: Impact sensitivity (IS) is a key indicator affecting the safety of energetic materials. Both lattice stability and elastic response are closely related to the IS of energetic materials. However, the microscopic mechanisms by which these two factors regulate sensitivity in molecular perovskite energetic materials remain unclear. In this study, molecular perovskite energetic materials (H2dabco)[M(ClO4)3] (DAPs, dabco = 1,4-diazabicyclo[2.2.2]octane, M = Na+, K+, NH4+, and NH2NH3+ for DAP-1, DAP-2, DAP-4, and DAP-7, respectively) were investigated. By combining terahertz time-domain spectroscopy (THz-TDS), first-principles calculations, Hirshfeld surface (HS) analysis, and quantum theory of atoms in molecules (QTAIM), we systematically explored the correlations among Young's modulus (reflecting elastic response), hydrogen bond (indicating lattice stability), and IS. The results indicate that the variation in hydrogen bond content does not follow the trends typically observed in conventional energetic materials. Assessing sensitivity based on hydrogen bond strength also has limitations. Moreover, Young's modulus derived from low-frequency terahertz vibrations exhibits a negative correlation with experimental IS. First-principles calculations reveal that the relevant vibrational modes primarily arise from lattice vibrations, whose force constants directly determine the elastic response characteristics of the materials. Although local hydrogen bond also exert an influence on the IS, it remains difficult to evaluate the sensitivity based on any single characteristic of the hydrogen bond. Therefore, the regulation of IS in DAPs may be influenced by low-frequency lattice vibrations, which modulate mechanical energy transfer and dissipation processes. Lower force constants and Young's modulus correspond to more deformable lattice potential energy surfaces and greater mechanical energy dissipation efficiency, thereby more effectively suppressing the reaction initiation processes induced by energy localization. This study proposes a new method and approach for evaluating the IS of energetic materials using terahertz spectroscopy. Furthermore, we propose new perspectives and criteria for molecular design based on lattice dynamical properties.

METHODS: First-principles calculations of DAPs were performed using the CP2K software package. The PBE functional was selected, along with the pob-TZVP-rev2 basis set and DFT-D3 dispersion corrections, to perform structural optimization, vibrational analysis, and stress-strain curve calculations. The HS analysis and QTAIM were utilized to quantitatively analyze the hydrogen bond content and strength of each material. Vibrational mode analysis was performed under the harmonic approximation, and Multiwfn program was employed to broaden the calculated vibrational frequencies using the Lorentzian function, in order to obtain terahertz vibrational spectra. Additionally, vibrational vectors were visualized using VMD software. Finally, Young's modulus was determined according to Hooke's law.