Ab initiocalculation of anion proton affinity and ionization potential for energetic ionic liquids

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

The composition of the products and the mechanistic routes for the reaction of the hypergolic ionic liquid (HIL) 1-ethyl-3-methylimidazolium cyanoborohydride ([EMIM][CBH]) and nitric acid (HNO₃) at various concentrations from 10% to 70% were explored using a contactless single droplet merging within an ultrasonic levitation setup in an inert atmosphere of argon to reveal the initial steps that cause hypergolicity. The reactions were initiated through controlled droplet-merging manipulation triggered by a frequency chirp pulse amplitude modulation. Utilizing the high-speed optical and infrared cameras surrounding the levitation process chamber, intriguing visual images were unveiled: (i) extensive gas release and (ii) temperature rises of up to 435 K in the merged droplets. The gas development was validated qualitatively and quantitatively with Fourier Transform Infrared Spectroscopy (FTIR) indicating the major gas-phase products to be hydrogen cyanide (HCN) and nitrous oxide (N₂O). The merged droplet was also probed by pulsed Raman spectroscopy which deciphered features for key functional groups of the reaction products and intermediates (-BH, -BH₂, -BH₃, -NCO); reaction kinetics revealed that the reaction was initiated by the interaction of the [CBH]^- anion of the HIL with the oxidizer (HNO₃) through proton transfer. Computations indicate the formation of a van-der-Waals complex between the [CBH]^- anion and HNO₃ initially, followed by proton transfer from the acid to the anion and subsequent extensive isomerization; these rearrangements were found to be essential for the formation of HCN and N₂O. The exoergicity observed during the merging process provides a molar enthalpy change up to 10 kJ mol^-1 to the system, which could be sufficient for a significant fraction of the reactants of about 11% to overcome the reaction barriers in the individual steps of the computationally determined minimum energy pathways.

Theoretical Investigation of the Reactivity of Sodium Dicyanamide with Nitric Acid

There is a need to replace current hydrazine fuels with safer propellants, and dicyanamide (DCA^-)-based systems have emerged as promising alternatives because they autoignite when mixed with some oxidizers. Previous studies of the hypergolic reaction mechanism have focused on the reaction between DCA^- and the oxidizer HNO₃; here, we compare the calculated pathway of DCA^- + HNO₃ with the reaction coordinate of the ion pair sodium dicyanamide with nitric acid, Na[DCA] + HNO₃. Enthalpies and free energies are calculated in the gas phase and in solution using a quantum mechanical continuum solvation model, SMD-GIL. The barriers to the Na[DCA] + HNO₃ reaction are dramatically lowered relative to those of the reaction with the bare anion, and an exothermic exit channel to produce NaNO₃ and the reactive intermediate HDCA appears. These results suggest that Na[DCA] may accelerate the ignition reaction.

Effect of Boron Clusters on the Ignition Reaction of HNO3 and Dicynanamide-Based Ionic Liquids

Many ionic liquids containing the dicynamide anion (DCA^-, formula N(CN)₂^-) exhibit hypergolic ignition when exposed to the common oxidizer nitric acid. However, the ignition delay is often about 10 times longer than the desired 5 ms for rocket applications, so that improvements are desired. Experiments in the past decade have suggested both a mechanism for the early reaction steps and also that additives such as decaborane can reduce the ignition delay. The mechanisms for reactions of nitric acid with both DCA^- and protonated DCAH are considered here, using accurate wave function methods. Complexation of DCA^- or DCAH with borane clusters B₁₀H₁₄ or B₉H₁₄^- is found to modify these mechanisms slightly by changing the nature of some of the intermediate saddle points and by small reductions in the reaction barriers.

Enhanced stability of the model mini-protein in amino acid ionic liquids and their aqueous solutions

Using molecular dynamics simulations, the structure of model mini-protein was thoroughly characterized in the imidazolium-based amino acid ionic liquids and their aqueous solutions. Complete substitution of water by organic cations and anions further results in hindered conformational flexibility of the mini-protein. This observation suggests that amino acid-based ionic liquids are able to defend proteins from thermally induced denaturation. We show by means of radial distributions that the mini-protein is efficiently solvated by both solvents due to a good mutual miscibility. Amino acid-based anions prevail in the first coordination sphere of positively charged sites of the mini-protein whereas water molecules prevail in the first coordination sphere of negatively charged sites of the mini-protein.

Multiscale modeling of the trihexyltetradecylphosphonium chloride ionic liquid

Hierarchical trihexyltetradecylphosphonium cationic and chloride anionic models.