Quantum Chemical Insights into the Initial Reaction Pathways Governing the Self-Ignition of Hypergolic Ionic Liquids
Miori Nogamida1, Hirotoshi Mori1
1Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
Abstract:
Hypergolic ionic liquids (HILs), characterized by low volatility and improved safety, have attracted increasing attention as next-generation green propellants. However, the ignition mechanism of HILs with hydrogen peroxide (H2O2) remains poorly understood at the molecular level. In this study, we investigate the initial ignition processes of representative HIL anions reacting with H2O2 by combining automated reaction path exploration, kinetic analysis, energy density analysis (EDA), and natural bond orbital (NBO) analysis. The reactivity of each anion is systematically evaluated based on activation free energies. The results show that [BH4]- and [BH3CN]- exhibit hypergolic behavior, whereas [BH2(CN)2]- and [N-(CN)2]- do not, in qualitative agreement with experimental trends. Among the systems studied, [BH4]- exhibits the lowest activation barrier (20.6 kcal/mol) and the largest heat release (-105.4 kcal/mol), while cyano substitution progressively reduces reactivity. Further analysis reveals that ignition behavior is primarily governed by the activation barrier of the initial reaction, which is controlled by the efficiency of electron donation from the anion bonding orbitals to the antibonding orbital of H2O2. These findings establish an activation-controlled framework for understanding self-ignition in HIL/H2O2 systems and provide molecular-level design principles for next-generation green hypergolic propellants.
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