Dynamics Simulations and Statistical Modeling of Thermal Decomposition of 1-Ethyl-3-methylimidazolium Dicyanamide and 1-Ethyl-2,3-dimethylimidazolium Dicyanamide

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.

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

Direct dynamics simulations of HNO₃ with dicyanamide anion DCA^- (i.e., N(CN)₂^-) and dicyanoborohydride anion DCBH^- (i.e., BH₂(CN)₂^-) were performed at the B3LYP/6-31+G(d) level of theory in an attempt to elucidate the primary and secondary reactions in the two reaction systems. Guided by trajectory results, reaction coordinates and potential energy diagrams were mapped out for the oxidation of DCA^- and DCBH^- by one and two HNO₃ molecules, respectively, in the gas-phase and in the condensed-phase ionic liquids using the B3LYP/6-311++G(d,p) method. The oxidation of DCA^- by HNO₃ is initiated by proton transfer. The most important pathway leads to the formation of O₂N-NHC(O)NCN^-, and the latter reacts with a second HNO₃ to produce O₂N-NHC(O)NC(O)NH-NO₂^-(DNB^-). The oxidation of DCBH^- by HNO₃ may follow a similar mechanism as that of DCA^-, producing two analogue products: O₂N-NHC(O)BH₂CN^- and O₂N-NHC(O)BH₂C(O)NH-NO₂^-. Moreover, two new, unique reaction pathways were discovered for DCBH^- because of its boron-hydride group: (1) isomerization of DCBH^- to CNBH₂CN^- and CNBH₂NC^- and (2) H₂ elimination in which the proton in HNO₃ combines with a hydride-H in DCBH^-. The Rice-Ramsperger-Kassel-Marcus (RRKM) theory was utilized to calculate reaction kinetics and product branching ratios. The RRKM results indicate that the formation of DNB^- is exclusively important in the oxidation of DCA^-, whereas the same type of reaction is a minor channel in the oxidation of DCBH^-. In the latter case, H₂ elimination becomes dominating. The RRKM modeling also indicates that the oxidation rate constant of DCBH^- is higher than that of DCA^- by an order of magnitude. This rationalizes the enhanced preignition performance of DCBH^- over DCA^- with HNO₃.

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-)

Direct dynamics trajectory simulations were carried out for the NO₂ 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 NO₂ 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 (SN₂) mechanism. The intra-ion-pair reaction products, including [dialkylimidazolium⁺ - H_C2⁺], alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA^-, react with NO₂ and favor the formation of nitrite (-ONO) complexes over nitro (-NO₂) 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.; 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 NO₂ 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 NO₂ (as reported by Brotton et al. [ J. Phys. Chem. A 2018, 122, 7351-7377] and Lucas et al. [ J. Phys. Chem. A 2019, 123, 400-416]).

Thermal Decomposition and Hypergolic Reaction of a Dicyanoborohydride Ionic Liquid

In this study, in situ infrared spectroscopy techniques and thermogravimetric analysis coupled with mass spectrometry (TGA-MS) are employed to characterize the reactivity of the ionic liquid, 1-butyl-3-methylimidazolium dicyanoborohydride (BMIM⁺DCBH^-), in comparison to the well-characterized 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-) ionic liquid. TGA measurements determined the enthalpy of vaporization (ΔH_vap) to be 112.7 ± 12.3 kJ/mol at 298 K. A rapid scan Fourier transform infrared spectrometer was used to obtain vibrational information useful in tracking the appearance and disappearance of species in the hypergolic reactions of BMIM⁺DCBH^- and BMIM⁺DCA^- with white fuming nitric acid (WFNA) and in the thermal decomposition of these energetic ionic liquids. Attenuated total reflectance measurements recorded the infrared spectra of the reactant sample (BMIM⁺DCBH^-) and the liquid reaction products after reacting with WFNA. Computational chemistry efforts, aided by the experimental results, were used to propose key reaction pathways leading to the hypergolic ignition of BMIM⁺DCBH^- + WFNA. Experimental results indicate that the hypergolic reaction of BMIM⁺DCBH^- with WFNA generates both common and unique intermediates as compared to previous BMIM⁺DCA^- + WFNA investigations: nitrous oxide was generated during both hypergolic reactions indicating that it may play a crucial role in the hypergolic ignition process, NO₂ was generated in significantly higher concentrations for BMIM⁺DCBH^- than for BMIM⁺DCA^-, CO₂ was only generated for BMIM⁺DCA^-, and HCN was only generated during thermal decomposition and hypergolic ignition of BMIM⁺DCBH^-.

Computational Study of the Reaction of 1-Methyl-4-amino-1,2,4-triazolium Dicyanamide with NO2: From Reaction Dynamics to Potential Surfaces, Kinetics and Spectroscopy

Direct dynamics trajectories were calculated at the B3LYP/6-31G(d) level of theory in an attempt to understand the reaction of 1-methyl-4-amino-1,2,4-triazolium dicyanamide (MAT⁺DCA^-) with NO₂. The trajectories revealed an extensive intra-ion-pair proton transfer in MAT⁺DCA^-. The reaction pathways of the ensuing HDCA (i.e., HNCNCN) and [MAT⁺ - H_C5⁺] (i.e., deprotonated at C5-H of MAT⁺) molecules as well as DCA^- with NO₂ were identified. The reaction of NO₂ with HDCA and DCA^- produces HNC(-ONO)NCN and NCNC(-ONO)N^- or NCNCN-NO₂^-, respectively, whereas that with [MAT⁺ - H_C5⁺] results in the formation of 5-O-MAT (i.e., 4-amino-2-methyl-2,4-dihydro-3 H-1,2,4-triazo-3-one) + NO and [MAT⁺ - H₂⁺] + HNO₂. Using trajectories for guidance, structures of intermediates, transition states and products, and the corresponding reaction potential surfaces were elucidated at B3LYP/6-311++ G(d,p). Rice-Ramsperger-Kassel-Marcus (RRKM) theory was utilized to calculate the reaction rates and statistical product branching ratios. A comparison of direct dynamics simulations with RRKM modeling results indicate that the reactions of NO₂ with HDCA and DCA^- are nonstatistical. To validate our computational results, infrared and Raman spectra of MAT⁺DCA^- and its reaction products with NO₂ were calculated using an ionic liquid solvation model. The calculated spectra reproduced the vibrational frequencies detected in an earlier spectroscopic study of MAT⁺DCA^- droplets with NO₂ [ Brotton , S. J. ; J. Phys. Chem. Lett. 2017 , 8 , 6053 ].

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

Thermal decomposition (TD) products of the ionic liquids (ILs) [CnC1Im][BF4] and [CnC1Im][PF6] ([CnC1Im]+ = 1-alkyl-3-methylimidazolium, [BF4]- = tetrafluoroborate, and [PF6]- = hexafluorophosphate) were prepared, ex situ, by bulk heating experiments in a bespoke setup. The respective products, CnC1(C3N2H2)BF3 and CnC1(C3N2H2)PF5 (1-alkyl-3-methylimidazolium-2-trifluoroborate and 1-alkyl-3-methylimidazolium-2-pentafluorophosphate), were then vaporized and analyzed by direct insertion mass spectrometry (DIMS) in order to identify their characteristic MS signals. During IL DIMS experiments we were subsequently able, in situ, to identify and monitor signals due to both IL vaporization and IL thermal decomposition. These decomposition products have not been observed in situ during previous analytical vaporization studies of similar ILs. The ex situ preparation of TD products is therefore perfectly complimentary to in situ thermal stability measurements. Experimental parameters such as sample surface area to volume ratios are consequently very important for ILs that show competitive vaporization and thermal decomposition. We have explained these experimental factors in terms of Langmuir evaporation and Knudsen effusion-like conditions, allowing us to draw together observations from previous studies to make sense of the literature on IL thermal stability. Hence, the design of experimental setups are crucial and previously overlooked experimental factors.

Lamellar structures in fluorinated phosphonium ionic liquids: the roles of fluorination and chain length

We report a new lamellar superstructure and non-Newtonian shear thinning behavior in fluorinated phosphonium dicyanamide ILs.