Unsupervised Reaction Pathways Search for the Oxidation of Hypergolic Ionic Liquids: 1-Ethyl-3-methylimidazolium Cyanoborohydride (EMIM⁺/CBH^–) as a Case Study

Abstract

Hypergolic ionic liquids have come under increased study for having several desirable properties as a fuel source. One particular ionic liquid, 1-ethyl-3-methylimidazolium/cyanoborohydride (EMIM + /CBH − ), and oxidant, nitric acid (HNO 3 ), has been reported to be hypergolic experimentally, but its mechanism is not well-understood at a mechanistic level. In this computational study, the reaction is first probed with ab initio molecular dynamics simulations to confirm that anion−oxidant interactions likely are the first step in the mechanism. Second, the potential energy surface of the anion−oxidant system is studied with an in-depth search over possible isomerizations, and a network of possible intermediates are found. The critical point search is unsupervised and thus has the potential of identifying structures that deviate from chemical intuition. Molecular graphs are employed for analyzing 3000+ intermediates found, and nudged elastic band calculations are employed to identify transition states between them. Finally, the reactivity of the system is discussed through examination of minimal energy paths connecting the reactant to various common products from hypergolic ionic liquid oxidation. Eight products are reported for this system: NO, N 2 O, NO 2 , HNO, HONO, HNO 2 , HCN, and H 2 O. All reaction paths leading to these exothermic products have overall reaction barriers of 6−7 kcal/mol.

Unraveling the initial steps of the ignition chemistry of the hypergolic ionic liquid 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) with nitric acid (HNO3) exploiting chirped pulse triggered droplet merging

“…Fig. 15 and 16 compiles those intermediates, van-der-Waals complexes, products, and reaction pathways 53 fulfilling the aforementioned restrictions; interesting findings are revealed.…”

“…This manuscript would be incomplete without an assessment of our experimental findings in light of recent electronic structure calculations of the gas phase reaction of [EMIM][CBH] with HNO 3 . 53 In such study, unsupervised critical point search was carried out at B3LYP/6-31G* level of theory and found thousands of relevant intermediates leading to the products detected here. Due to the large number of pathways, only the minimum energy paths connecting reactants to various products were ultimately used to justify the reaction mechanism.…”

“…This is merged with the very recent computational results on the oxidation of [EMIM][CBH] by HNO 3 . 53 To our best knowledge, there has been no prior computational studies on the abovementioned system. The only computational study that involves following [EMIM][CBH] also mixes DCA and AMIM as the ionic liquids, which are oxidized by HNO 3 , and the focus of the study was to understand their viscosities, densities, decomposition temperatures, and specific impulses.…”

Hypergolic ionic liquids: to be or not to be?

“…The formation of these closed shell tri-and tetra-atomic products (HCN, N 2 O, NO 2 , HONO) is energetically favored as the mixing energy (Table S6 †) between the liquid droplets can exceed the barrier to reaction of nitric acid (HNO 3 ) and the [CBH] anion. 20,27 Overall, since [EMIM][CBH] does not react with molecular oxygen, we have to conclude that reaction intermediates such as doublet radicals identied previously 20 accessed during the reaction of the [CBH] anion with nitric acid (HNO 3 ) are likely reactants with molecular oxygen.…”