1-Ethyl-3-methylimidazolium dinitramide (EMImDN): a comprehensive study of synthesis, characterization, thermal behavior and hypergolicity

Abstract

1-Ethyl-3-methylimidazolium dinitramide (EMImDN), an energetic ionic liquid, was synthesized and characterized by IR, ¹H NMR, ¹³C NMR, ¹⁵N NMR, and ¹⁷O NMR techniques. The thermal properties of EMImDN were investigated through DSC and TG analyses. The TG curve revealed three consecutive exothermic decomposition events with peak temperatures at 223 °C, 277 °C and 308 °C, accompanied by an 80% mass loss between 180 °C and 280 °C. The thermal decomposition kinetics were analyzed using the Kissinger method and Flynn–Wall–Ozawa isoconversional method. The activation energies for the three decomposition stages were calculated to be 41.1, 35.9 and 43.2 kcal mol⁻¹ respectively by the Kissinger method and 39.0, 36.8 and 42.6 kcal mol⁻¹, respectively, by the FWO method. Pyrolysis GC-MS analysis identified the decomposition products, and the decomposition mechanism was predicted to involve dealkylation of the imidazole ring via bimolecular nucleophilic substitution (S_N2). The proposed decomposition mechanism was further supported by density functional theory (DFT) calculations at the B3LYP/6-311+G(d,p) level. Additionally, the molar enthalpy of formation of EMImDN (+54.5 kcal mol⁻¹), determined through combustion calorimetry, underscores the energetic nature of this ionic liquid. Notably, a 10 wt% solution of NH₃BH₃ in EMImDN exhibited hypergolicity with red fuming nitric acid. Preliminary investigations into the interactions of EMImDN with HNO₃ and NH₃BH₃ provide insights into the initial stages of this hypergolic reaction.