Liquid Structure and Hydrogen Bonding in Aqueous Hydroxylammonium Nitrate

Hydroxylammonium nitrate (HAN) has emerged as a promising component in ionic liquid-based spacecraft propellants. However, the physicochemical and structural properties of aqueous HAN have been largely overlooked. The purpose of this study is to investigate the hydrogen bonding in aqueous HAN and understand its implications on these properties and the proton transfer mechanism as a function of concentration. Classical polarizable molecular dynamics simulations have been employed with the APPLE&P force field to analyze the geometry of individual hydrogen bonds and the overall hydrogen-bonding network in various concentrations of aqueous HAN. Radial distribution functions (RDFs) and spatial distribution functions (SDFs) indicate the structural arrangement of the species and their hydrogen bonds. Projections of water density and the orientation of its electric dipole moment near the ions provide insight into the hydrogen-bonding network. The incorporation of water into the hydrogen-bonding network at high ion concentrations occurs via interstitial accommodation around the ions immediately outside the first solvation shell. While ion pairs are observed at all concentrations considered, the frequency of Ha···On hydrogen bonds increases substantially with the ion concentration. The findings contribute to a better fundamental understanding of HAN and the precursors of reactivity, crucial to the development of "green" spacecraft propellants.

Structural Properties of HEHN- and HAN-Based Ionic Liquid Mixtures: A Polarizable Molecular Dynamics Study

Molecular dynamics simulations of binary mixtures comprising 2-hydroxyethylhydrazinium nitrate (HEHN) and hydroxylammonium nitrate (HAN) were conducted using the polarizable APPLE&P force field to investigate fundamental properties of multimode propulsion (MMP) propellants. Calculated densities as a function of temperature were in good agreement with experiments and similar simulations. The structural properties of neat HEHN and HAN-HEHN provided insights into their inherent, protic nature. Radial distribution functions (RDFs) identified key hydrogen bonding sites located at N-H···O and O-H···O within a first solvation shell of approximately 2 Å. Angular distribution functions further affirmed the relatively strong nature of the hydrogen bonds with nearly linear directionality. The increased hydroxylammonium cation (HA+) mole fraction shows the influence of competitively strong hydrogen bonds on the overall hydrogen bond network. Dominant spatial motifs via three-dimensional distribution functions along with nearly nanosecond-long hydrogen bond lifetimes highlight the local bonding environment that may precede proton transfer reactions.

Formation and fragmentation of 2-hydroxyethylhydrazinium nitrate (HEHN) cluster ions: a combined electrospray ionization mass spectrometry, molecular dynamics and reaction potential surface study

The 2-hydroxyethylhydrazinium nitrate ([HOCH₂CH₂NH₂NH₂]⁺NO₃^-, HEHN) ionic liquid has the potential to power both electric and chemical thrusters and provide a wider range of specific impulse needs. To characterize its capabilities as an electrospray propellant, we report the formation of HEHN cluster ions in positive electrospray ionization (ESI) and their collision-induced dissociation. The experiment was carried out using ESI guided-ion beam mass spectrometry which mimics an electrospray thruster in terms of ion emission, injection into a vacuum and fragmentation in space. Measurements include compositions of primary ions in the electrospray plume and their individual dissociation product ion cross sections and threshold energies. The results were interpreted in light of theoretical modeling. To determine cluster structures that are comprised of [HE + H]⁺ and NO₃^- constituents, classical mechanics simulations were used to create initial guesses; and for clusters that are formed by reactions between ionic constituents, quasi-classical direct dynamics trajectory simulations were used to mimic covalent bond formation and structures. All candidate structures were subject to density functional theory optimization, from which global minimum structures were identified and used for construction of reaction potential energy surface. The comparison between experimental values and calculated dissociation thermodynamics was used to verify the structures for the emitted species [(HEHN)_nHE + H]⁺, [(HEHN)_n(HE)₂ + H]⁺, [(HE)_n+1 + H]⁺ and [(HE)_nC₂H₄OH]⁺ (n = 0-2), of which [(HE)_1-2 + H]⁺ dominates. Due to the protic nature of HEHN, cluster fragmentation can be rationalized by proton transfer-mediated elimination of HNO₃, HE and HE·HNO₃, and the latter two become dominant in larger clusters. [(HE)₂ + H]⁺ and [(HE)_nC₂H₄OH]⁺ contain H-bonded water and consequently are featured by water elimination in fragmentation. These findings help to evaluate ion formation and fragmentation efficiencies and their impacts on electrospray propulsion.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

Structures, Proton Transfer and Dissociation of Hydroxylammonium Nitrate (HAN) Revealed by Electrospray Ionization Tandem Mass Spectrometry and Molecular Dynamics Simulations

Hydroxylammonium nitrate (HAN) is a potential propellant candidate for dual-mode propulsion systems that combine chemical and electrospray thrust capabilities for spacecraft applications. However, the electrospray dynamics of HAN is currently...

Study of the Reaction of Hydroxylamine with Iridium Atomic and Cluster Anions (n = 1-5)

Elucidating the multifaceted processes of molecular activation and subsequent reactions gives a fundamental view into the development of iridium catalysts as they apply to fuels and propellants, for example, for spacecraft thrusters. Hydroxylamine, a component of the well-known hydroxylammonium nitrate (HAN) ionic liquid, is a safer alternative and mimics the chemistry and performance standards of hydrazine. The activation of hydroxylamine by anionic iridium clusters, Ir_n^- (n = 1-5), depicts a part of the mechanism, where two hydrogen atoms are removed, likely as H₂, and Ir_n(NOH)^- clusters remain. The significant photoelectron spectral differences between these products and the bare clusters illustrate the substantial electronic changes imposed by the hydroxylamine fragment on the iridium clusters. In combination with DFT calculations, a preliminary reaction mechanism is proposed, identifying the possible intermediate steps leading to the formation of Ir(NOH)^-.

Experimental and Theoretical Investigations of the Radical-Radical Reaction: N2H3 + NO2

The N₂H₃ + NO₂ reaction plays a key role during the early stages of hypergolic ignition between N₂H₄ and N₂O₄. Here for the first time, the reaction kinetics of N₂H₃ in excess NO₂ was studied in 2.0 Torr of N₂ and in the narrow temperature range 298-348 K in a pulsed photolysis flow-tube reactor coupled to a mass spectrometer. The temporal profile of the product, HONO, was determined by direct detection of the m/z +47 amu ion signal. For each chosen [NO₂], the observed [HONO] trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient, k', for the reaction of N₂H₃ with NO₂. The slope of the plot of k' versus [NO₂] yielded a value for the observed bimolecular rate coefficient, k_obs, which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10^-12 exp((520 ± 350)/T) cm³ molecule^-1 s^-1. The errors are 1σ and include estimated uncertainties in the NO₂ concentration. The potential energy surface of N₂H₃ + NO₂ was investigated by advanced ab initio quantum chemistry theories. It was found that the reaction occurs via a complex reaction mechanism, and all of the reaction channels have transition state energies below that of the entrance asymptote. The radical-radical addition forms the N₂H₃NO₂ adducts, while roaming-mediated isomerization reactions yield the N₂H₃ONO isomers, which undergo rapid dissociation reactions to several sets of distinct products. The RRKM multiwell master equation simulations revealed that the major product channel involves the formation of trans-HONO and trans-N₂H₂ below 500 K and the formation of NO + NH₂NHO above 500 K, which is nearly pressure independent. The pressure-dependent rate coefficients of the product channels were computed over a wide pressure-temperature range, which encompassed the experimental data.

Ionic Liquid Clusters Generated from Electrospray Thrusters: Cold Ion Spectroscopic Signatures of Size-Dependent Acid-Base Interactions

We determine the intramolecular distortions at play in the 2-hydroxyethylhydrazinium nitrate (HEHN) ionic liquid (IL) propellant, which presents the interesting case that the HEH⁺ cation has multiple sites (i.e., hydroxy, primary amine, and secondary ammonium groups) available for H-bonding with the nitrate anion. These interactions are quantified by analyzing the vibrational band patterns displayed by cold cationic clusters, (HEH⁺)_n(NO₃^-)_n-1, n = 2-6, which are obtained using IR photodissociation of the cryogenically cooled, mass-selected ions. The strong interaction involving partial proton transfer of the acidic N-H proton in HEH⁺ cation to the nitrate anion is strongly enhanced in the ternary n = 2 cluster but is suppressed with increasing cluster size. The cluster spectra recover the bands displayed by the bulk liquid by n = 5, thus establishing the minimum domain required to capture this aspect of macroscopic behavior.

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]).

Ab Initio Kinetics of Methylamine Radical Thermal Decomposition and H-Abstraction from Monomethylhydrazine by H-Atom

Methylamine radicals (CH₃NH) and amino radicals (NH₂) are major products in the early pyrolysis/ignition of monomethylhydrazine (CH₃NHNH₂). Ab initio kinetics of thermal decomposition of CH₃NH radicals was analyzed by RRKM master equation simulations. It was found that β-scission of the methyl H-atom from CH₃NH radicals is predominant and fast enough to induce subsequent H-abstraction reactions in CH₃NHNH₂ to trigger ignition. Consequently, the kinetics of H-abstraction reactions from CH₃NHNH₂ by H-atoms was further investigated. It was found that the energy barriers for abstraction of the central amine H-atom, two terminal amine H-atoms, and methyl H-atoms are 4.16, 2.95, 5.98, and 8.50 kcal mol^-1, respectively. In units of cm³ molecule^-1 s^-1, the corresponding rate coefficients were found to be k₈ = 9.63 × 10^-20T^2.596 exp(-154.2/T), k₉ = 2.04 × 10^-18T^2.154 exp(104.1/T), k₁₀ = 1.13 × 10^-20T^2.866 exp(-416.3/T), and k₁₁ = 2.41 × 10^-23T^3.650 exp(-870.5/T), respectively, in the 290-2500 K temperature range. The results reveal that abstraction of the terminal amine H-atom to form trans-CH₃NHNH radicals is the dominant channel among the different abstraction channels. At 298 K, the total theoretical H-abstraction rate coefficient, calculated with no adjustable parameters, is 8.16 × 10^-13 cm³ molecule^-1 s^-1, which is in excellent agreement with Vaghjiani's experimental observation of (7.60 ± 1.14) × 10^-13 cm³ molecule^-1 s^-1 ( J. Phys. Chem. A 1997, 101, 4167-4171, DOI: 10.1021/jp964044z).

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 ].

Ignition Delay Reduction with Sodium Addition to Imidazolium-Based Dicyanamide Ionic Liquid

A range of ionic liquids (ILs) have been synthesized and modeled to better understand the role of the cation in the ignition of hypergolic ionic liquids. Vogelhuber et al. have shown by density functional theory methods that the addition of sodium cations to an ionic liquid promotes ignition with white fuming nitric acid (WFNA) by lowering energy barriers. To validate this prediction, solid sodium dicyanamide (Na⁺DCA^-) was added at various weight percents to 1-butyl-3-methylimidazolium dicyanamide (BMIM⁺DCA^-). The ignition delay was measured for each mixture with WFNA. Overall, it was found that the Na⁺DCA^- lowered the ignition delay by 11 ms at 7 wt %. The calculations done by Vogelhuber et al. appear to be consistent with this observation. The sodium cation may play a role by orienting the anion with the WFNA resulting in the favorable reaction energetics observed.

Spectroscopic Investigation of the Primary Reaction Intermediates in the Oxidation of Levitated Droplets of Energetic Ionic Liquids

The production of the next generation of hypergolic, ionic-liquid-based fuels requires an understanding of the reaction mechanisms between the ionic liquid and oxidizer. We probed reactions between a levitated droplet of 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]), with and without hydrogen-capped boron nanoparticles, and the nitrogen dioxide (NO₂) oxidizer. The apparatus exploits an ultrasonic levitator enclosed within a pressure-compatible process chamber equipped with complementary Raman, ultraviolet-visible, and Fourier-transform infrared (FTIR) spectroscopic probes. Vibrational modes were first assigned to the FTIR and Raman spectra of droplets levitated in argon. Spectra were subsequently collected for pure and boron-doped [MAT][DCA] exposed to nitrogen dioxide. By comparison with electronic structure calculations, some of the newly formed modes suggest that the N atom of the NO₂ molecule bonds to a terminal N on the dicyanamide anion yielding [O₂N-NCNCN]^-. This represents the first spectroscopic evidence of a key reaction intermediate in the oxidation of levitated ionic liquid droplets.

Catalytic Decomposition of Hydroxylammonium Nitrate Ionic Liquid: Enhancement of NO Formation

Hydroxylammonium nitrate (HAN) is a promising candidate to replace highly toxic hydrazine in monopropellant thruster space applications. The reactivity of HAN aerosols on heated copper and iridium targets was investigated using tunable vacuum ultraviolet photoionization time-of-flight aerosol mass spectrometry. The reaction products were identified by their mass-to-charge ratios and their ionization energies. Products include NH₃, H₂O, NO, hydroxylamine (HA), HNO₃, and a small amount of NO₂ at high temperature. No N₂O was detected under these experimental conditions, despite the fact that N₂O is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. Upon introduction of iridium catalyst, a significant enhancement of the NO/HA ratio was observed. This observation indicates that the formation of NO via decomposition of HA is an important pathway in the catalytic decomposition of HAN.

Ab initio kinetics and thermal decomposition mechanism of mononitrobiuret and 1,5-dinitrobiuret

Mononitrobiuret (MNB) and 1,5-dinitrobiuret (DNB) are tetrazole-free, nitrogen-rich, energetic compounds. For the first time, a comprehensive ab initio kinetics study on the thermal decomposition mechanisms of MNB and DNB is reported here. In particular, the intramolecular interactions of amine H-atom with electronegative nitro O-atom and carbonyl O-atom have been analyzed for biuret, MNB, and DNB at the M06-2X/aug-cc-pVTZ level of theory. The results show that the MNB and DNB molecules are stabilized through six-member-ring moieties via intramolecular H-bonding with interatomic distances between 1.8 and 2.0 Å, due to electrostatic as well as polarization and dispersion interactions. Furthermore, it was found that the stable molecules in the solid state have the smallest dipole moment amongst all the conformers in the nitrobiuret series of compounds, thus revealing a simple way for evaluating reactivity of fuel conformers. The potential energy surface for thermal decomposition of MNB was characterized by spin restricted coupled cluster theory at the RCCSD(T)/cc-pV∞ Z//M06-2X/aug-cc-pVTZ level. It was found that the thermal decomposition of MNB is initiated by the elimination of HNCO and HNN(O)OH intermediates. Intramolecular transfer of a H-atom, respectively, from the terminal NH2 group to the adjacent carbonyl O-atom via a six-member-ring transition state eliminates HNCO with an energy barrier of 35 kcal/mol and from the central NH group to the adjacent nitro O-atom eliminates HNN(O)OH with an energy barrier of 34 kcal/mol. Elimination of HNN(O)OH is also the primary process involved in the thermal decomposition of DNB, which processes C2v symmetry. The rate coefficients for the primary decomposition channels for MNB and DNB were quantified as functions of temperature and pressure. In addition, the thermal decomposition of HNN(O)OH was analyzed via Rice-Ramsperger-Kassel-Marcus/multi-well master equation simulations, the results of which reveal the formation of (NO2 + H2O) to be the major decomposition path. Furthermore, we provide fundamental interpretations for the experimental results of Klapötke et al. [Combust. Flame 139, 358-366 (2004)] regarding the thermal stability of MNB and DNB, and their decomposition products. Notably, a fundamental understanding of fuel stability, decomposition mechanism, and key reactions leading to ignition is essential in the design and manipulation of molecular systems for the development of new energetic materials for advanced propulsion applications.

Binding of alkenes and ionic liquids to B-H-functionalized boron nanoparticles: creation of particles with controlled dispersibility and minimal surface oxidation

The interaction of B-H-functionalized boron nanoparticles with alkenes and nitrogen-rich ionic liquids (ILs) is investigated by a combination of X-ray photoelectron spectroscopy, FTIR spectroscopy, dynamic light scattering, thermogravimetric analysis, and helium ion microscopy. Surface B-H bonds are shown to react with terminal alkenes to produce alkyl-functionalized boron particles. The interaction of nitrogen-rich ILs with the particles appears, instead, to be dominated by boron-nitrogen bonding, even for an ILs with terminal alkene functionality. This chemistry provides a convenient approach to producing and capping boron nanoparticles with a protective organic layer, which is shown to protect the particles from oxidation during air exposure. By controlling the capping group, particles with high dispersibility in nonpolar or polar liquids can be produced. For the particles capped with ILs, the effect of particle loading on hypergolic ignition of the ILs is reported.

Reactions of Ions with Ionic Liquid Vapors by Selected-Ion Flow Tube Mass Spectrometry

Room-temperature ionic liquids exert vanishingly small vapor pressures under ambient conditions. Under reduced pressure, certain ionic liquids have demonstrated volatility, and they are thought to vaporize as intact cation-anion ion pairs. However, ion pair vapors are difficult to detect because their concentration is extremely low under these conditions. In this Letter, we report the products of reacting ions such as NO(+), NH4(+), NO3(-), and O2(-) with vaporized aprotic ionic liquids in their intact ion pair form. Ion pair fragmentation to the cation or anion as well as ion exchange and ion addition processes are observed by selected-ion flow tube mass spectrometry. Free energies of the reactions involving 1-ethyl-3-methylimidazolium bis-trifluoromethylsulfonylimide determined by ab initio quantum mechanical calculations indicate that ion exchange or ion addition are energetically more favorable than charge-transfer processes, whereas charge-transfer processes can be important in reactions involving 1-butyl-3-methylimidazolium dicyanamide.

248-nm Laser Photolysis of CHBr3/O-Atom Mixtures: Kinetic Evidence for UV CO(A) Chemiluminescence in the Reaction of Methylidyne Radicals with Atomic Oxygen

The 4th positive and Cameron band emissions from electronically excited CO have been observed for the first time in 248-nm pulsed laser photolysis of a trace amount of CHBr(3) vapor in an excess of O atoms. O atoms were produced by dissociation of N(2)O (or O(2)) in a cw-microwave discharge cavity in 2.0 Torr of He at 298 K. The CO emission intensity in these bands showed a quadratic dependence on the laser fluence employed. Temporal profiles of the CO(A) and other excited-state products that formed in the photoproduced precursor + O-atom reactions were measured by recording their time-resolved chemiluminescence in discrete vibronic bands. The CO 4th positive transition (A(1)Pi, v' = 0 --> X(1)Sigma(+), v' ' = 2) near 165.7 nm was monitored in this work to deduce the pseudo-first-order decay kinetics of the CO(A) chemiluminescence in the presence of various added substrates (CH(4), NO, N(2)O, H(2), and O(2)). From this, the second-order rate coefficient values were determined for reactions of these substrates with the photoproduced precursors. The measured reactivity trends suggest that the prominent precursors responsible for the CO(A) chemiluminescence are the methylidyne radicals, CH(X(2)Pi) and CH(a(4)Sigma(-)), whose production requires the absorption of at least 2 laser photons by the photolysis mixture. The O-atom reactions with brominated precursors (CBr, CHBr, and CBr(2)), which also form in the photolysis, are shown to play a minor role in the production of the CO(A or a) chemiluminescence. However, the CBr(2) + O-atom reaction was identified as a significant source for the 289.9-nm Br(2) chemiluminescence that was also observed in this work. The 282.2-nm OH and the 336.2-nm NH chemiluminescences were also monitored to deduce the kinetics of CH(X(2)Pi) and CH(a(4)Sigma(-)) reactions when excess O(2) and NO were present.