Molecular Dynamics Simulations and Product Vibrational Spectral Analysis for the Reactions of NO₂ with 1-Ethyl-3-methylimidazolium Dicyanamide (EMIM⁺DCA^–), 1-Butyl-3-methylimidazolium Dicyanamide (BMIM⁺DCA^–), and 1-Allyl-3-methylimidazolium Dicyanamide (AMIM⁺DCA^–)

Abstract

Direct dynamics trajectory simulations were carried out for the NO 2 oxidation of 1-ethyl-3-methylimidazolium dicyanamide (EMIM + DCA − ), which were aimed at probing the nature of the primary and secondary reactions in the system. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for NO 2 with EMIM + DCA − , as well as with its analogues 1-butyl-3-methylimidazolium dicyanamide (BMIM + DCA − ) and 1-allyl-3-methylimidazolium dicyanamide (AMIM + DCA − ). Reactions of the dialkylimidazolium− dicyanamide (DCA) ionic liquids (ILs) are all initiated by proton transfer and/or alkyl abstraction between 1,3-dialkylimidazolium cations and DCA − anion, of which two exoergic pathways are particularly relevant to their oxidation activities. One pathway is the transfer of a H β -proton from the ethyl, butyl, or allyl group of the dialkylimidazolium cation to DCA − that results in the concomitant elimination of the corresponding alkyl as a neutral alkene, and the other pathway is the alkyl abstraction by DCA − via a second order nucleophilic substitution (SN 2 ) mechanism. The intra-ion-pair reaction products, including [dialkylimidazolium + − H C2 + ], alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA − , react with NO 2 and favor the formation of nitrite (−ONO) complexes over nitro (−NO 2 ) complexes, albeit the two complex structures have similar formation energies. The exoergic intra-ion-pair reactions in the dialkylimidazolium−DCA ILs account for their significantly higher oxidation activities over the previously reported 1-methyl-4-amino-1,2,4-triazolium dicyanamide [Liu, J.;et al. J. Phys. Chem. B 2019, 123, 2956−2970] and for the relatively higher reactivity of BMIM + DCA − vs AMIM + DCA − as BMIM + has a higher reaction path degeneracy for intra-ion-pair H β -proton transfer and its H β -transfer is more energetically favorable. To validate and directly compare our computational results with spectral measurements in the ILs, infrared and Raman spectra of BMIM + DCA − and AMIM + DCA − and their products with NO 2 were calculated using an ionic liquid solvation model. The simulated spectra reproduced all of the vibrational frequencies detected in the reactions of BMIM + DCA − and AMIM + DCA − IL droplets with NO 2 (as reported by Brotton et al. [

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

“…NWChem is employed for characterizing the reactants, products, intermediates, and transition states. Geometry optimizations and frequency calculations are carried out with B3LYP/6-31G* level of theory, , which has been reported to have a good balance between computation cost and accuracy to model cyanoborohydride-based HIL oxidations . The intermediates are confirmed by the number of frequencies (3 N – 6, N = 11, the number of atoms in the system).…”

“…This is largely due to the challenges involved in probing the highly energetic, reactive, and transient species in the initial stage of the ignition process in situ . Computational studies have proposed that a proton transfer from nitric acid to the anion is the trigger of the hypergolic reaction, − whose overall barriers have been claimed to decrease with additional oxidizer; however, the reported overall reaction barriers are usually too high in energy, e.g., tens of kcal/mol above the reactants, thus seeming to contradict the experimental resultautomatic, fast ignition upon the contact between HIL and oxidizer at room temperature. In fact, ab initio molecular dynamics (AIMD) simulations of the bimolecular collision between the anion of the HIL and the oxidizer have been carried out at elevated energies of a few thousand Kelvin and/or tens of kcal/mol of collision energy in order to collect a meaningful number of reactive trajectories within a reasonable amount of simulation time. − While AIMD is an invaluable tool to find novel, energetically accessible geometries, it alone is simply inadequate to explore these low-reactivity systems at experimental conditions.…”

Molecular Dynamics Simulations, Reaction Pathway and Mechanism Dissection, and Kinetics Modeling of the Nitric Acid Oxidation of Dicyanamide and Dicyanoborohydride Anions

“…The propagation step size is small enough for SCF convergence as well as to keep the system energy-constant within 10 –4 Hartree. The B3LYP/6-31+G­(d) level of theory was chosen for trajectory simulations as it provided a good description of DCA – reactions in our previous work. ,, Representative reaction pathways identified at the B3LYP level were verified using a range-separated functional ωB97XD/6-31+G­(d). Details of trajectory simulations are available in the Supporting Information.…”

“…One direction in the design and synthesis of new hypergolic fuels is to have ignition delay (ID) time (defined as the time between the initial fuel/oxidizer contact and the start of combustion) of less than 5 ms . Studies have shown that anions are directly associated with hypergolicity and play a decisive role during the induction stage of ignition, while cations (such as 1,3-dialkyl-imidazolium and triazolium) tune ILs’ physicochemical properties and play only a secondary role in influencing the ID time. , Dicyanamide anion N­(CN) 2 – (designated as DCA – ) and dicyanoborohydride anion BH 2 (CN) 2 – (designated as DCBH – ) are well known for their hypergolicity in propellant formulations. ,, DCA – -containing hypergolic ILs were first reported in 2008 . The DCA – anion was chosen because of its high N-content and low viscosity, but the resulting ILs exhibit moderate improvement in ID times.…”

“…One challenge for experimentalists is in the difficulty to capture in situ all of the generated transient species and to distinguish primary and secondary products. As demonstrated in our recent work, ,, theoretical modeling provides an alternative to experiments for testing and ascertaining the possibilities of various IL ignition mechanisms and the respective roles of different anions. The work presented in this paper sought to investigate and compare the preignition of DCA – versus DCBH – , including their oxidative capacities, reaction mechanisms, and product channels with HNO 3 by using direct dynamics trajectory simulations, − reaction coordinate and potential energy computations, and kinetics modeling.…”