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.

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 not well understood. Capitalizing on electrospray ionization guided-ion beam tandem mass spectrometry and collision-induced dissociation measurements, and augmented by extensive molecular dynamics simulations, this work characterized the structures and reaction dynamics of the species present in the electrosprays of HAN under different conditions, which mimic those possibly occurring in low earth orbit and outer space. While being ionic in nature, the HAN monomer, however, adopts a stable covalent structure HONH2·HNO3 in the gas phase. Spontaneous proton transfer between the HONH2 and HNO3 moieties within the HAN monomer can be induced in the presence of a NO3-, a water ligand or a second HAN monomer within 3-5 Å or a H+ within 8 Å, regardless of their collision impact parameters. These facts imply that HAN proton transfer is trigged by a charge and/or a dipole of the collision partner without the need of chemical interaction or physical contact. Moreover, the addition of NO3- to HAN leads to the formation of a stable -O3N·HONH3+·NO3- anion in negative electrosprays. In contrast, when a proton approaches the HONH2·HNO3 structure, dissociative reactions occur that lead to the H2O, NO2 and HONH2 fragments (and their cations) but not intact HAN species in positive electrosprays.

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.

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, Irn- (n = 1-5), depicts a part of the mechanism, where two hydrogen atoms are removed, likely as H2, and Irn(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)-.

Elucidating the differences in oxidation of high-performance α- and ß- diisobutylene biofuels via Synchrotron photoionization mass spectrometry.

Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel's composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy's (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (ß-DIB). Due to a difference in olefinic bond location, the α- and ß- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1-11.0 eV) available at the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels' ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C4 and C7 fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C4 compounds over C7, while the reverse was true for ß-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design.

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.

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, kobs, 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.

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 Hß-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+ - HC2+], 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 Hß-proton transfer and its Hß-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 ß-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 × 10-20T2.596 exp(-154.2/T), k9 = 2.04 × 10-18T2.154 exp(104.1/T), k10 = 1.13 × 10-20T2.866 exp(-416.3/T), and k11 = 2.41 × 10-23T3.650 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 (ΔHvap) 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+ - HC5+] (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+ - HC5+] 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.

Flow-Tube Investigations of Hypergolic Reactions of a Dicyanamide Ionic Liquid Via Tunable Vacuum Ultraviolet Aerosol Mass Spectrometry.

The unusually high heats of vaporization of room-temperature ionic liquids (RTILs) complicate the utilization of thermal evaporation to study ionic liquid reactivity. Although effusion of RTILs into a reaction flow-tube or mass spectrometer is possible, competition between vaporization and thermal decomposition of the RTIL can greatly increase the complexity of the observed reaction products. In order to investigate the reaction kinetics of a hypergolic RTIL, 1-butyl-3-methylimidazolium dicyanamide (BMIM+DCA-) was aerosolized and reacted with gaseous nitric acid, and the products were monitored via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Reaction product formation at m/z 42, 43, 44, 67, 85, 126, and higher masses was observed as a function of HNO3 exposure. The identities of the product species were assigned to the masses on the basis of their ionization energies. The observed exposure profile of the m/z 67 signal suggests that the excess gaseous HNO3 initiates rapid reactions near the surface of the RTIL aerosol. Nonreactive molecular dynamics simulations support this observation, suggesting that diffusion within the particle may be a limiting step. The mechanism is consistent with previous reports that nitric acid forms protonated dicyanamide species in the first step of the reaction.

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.