Initial Hypergolic Reaction Pathways Between Ammonia Borane and Nitric Acid in the Condensed Phase: Quantum Chemical Study

Abstract

Amine–borane compounds are promising low-toxicity alternatives to conventional hydrazine-based hypergolic fuels. However, rational molecular design remains impeded because the underlying chemical reaction mechanism leading to hypergolic ignition is poorly understood. This study presents the first molecular-level elucidation of the initial condensed-phase hypergolic reaction pathways between ammonia borane (NH3BH3) and nitric acid (HNO3) using quantum chemical calculations and detailed chemical kinetic simulations. This kinetic modeling reveals a rapid, spontaneous, and exothermic reaction mechanism driven by in situ generated N2O5. The initial exothermicity associated with the reaction between NH3BH3 and HNO3 accelerates the self-decomposition of HNO3 into N2O5. The resulting N2O5 promotes low-barrier exothermic B–O bond formation, raising the system temperature to over 900 K within 1 ms, thereby instantly supplying the heat required for hypergolic ignition. Additionally, reaction pathways were elucidated in which HOBO, a key preignition intermediate identified in prior studies, is spontaneously produced via the decomposition of NH3BH(ONO2)2 at elevated temperatures. Furthermore, it was revealed that the hydridic nature of the boron-bound hydrogen atoms in NH3BH3 triggers the hypergolic reaction of this system, providing insights into the rational design of next-generation amine–borane hypergolic fuels.