We present a combined experimental and theoretical investigation of formaldehyde ($H_{2}CO$) dissociation to H2and CO at energies just above the threshold for competing H elimination. High-resolution ...state-resolved imaging measurements of the CO velocity distributions reveal two dissociation pathways. The first proceeds through a well-established transition state to produce rotationally excited CO and vibrationally cold H2. The second dissociation pathway yields rotationally cold CO in conjunction with highly vibrationally excited H2. Quasiclassical trajectory calculations performed on a global potential energy surface for$H_{2}CO$suggest that this second channel represents an intramolecular hydrogen abstraction mechanism: One hydrogen atom explores large regions of the potential energy surface before bonding with the second H atom, bypassing the saddle point entirely.
The hydrazine-derived ionic liquid 2-hydroxyethylhydrazinium nitrate (HEHN) is a promising fuel component of green monopropellants. For successful implementation of these propellants, it is necessary ...to understand the kinetics and mechanism of HEHN decomposition, but the available data are scarce. The objective of the present work was to investigate the kinetics of HEHN thermal decomposition using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) as well as to identify the evolved gas products with mass spectrometry (MS) and Fourier-transform infrared (FTIR) spectroscopy. Both TGA and DSC have revealed that the decomposition of high-purity HEHN has two distinct stages. The effective kinetic parameters of both stages were determined using the Ozawa-Wall-Flynn, Kissinger, and model-based methods. The model-based analysis has shown autocatalytic behaviour of the involved reactions and produced apparent activation energies of 113.7 ± 1.7 kJ/mol at the first stage and 123.6 ± 2.5 kJ/mol at the second stage, close to the literature data (124.8 kJ/mol) on the autocatalytic reaction between HEHN and HNO3. The evolved gas analysis has shown that the first stage generates H2O, N2, NH3, NO, N2O, and NO2, while the second stage also generates HNO3 and CO2. The observed existence of two stages in the thermal decomposition of HEHN could be explained by the formation of a condensed by product, which, in turn, decomposes via a highly activated reaction during the second (high-temperature) stage.
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.
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 Hβ-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 Hβ-proton transfer and its Hβ-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 ).
Monopropellant catalytic thrusters have been used for decades in space systems due to their simplicity and reliability. However, the degradation or loss of the catalyst is a major lifetime limiting ...effect, both for hydrazine fueled systems as well as for newer hydrazine-replacement fueled systems. Currently there is no in-situ diagnostic technique to measure the health status of the catalyst in such devices. Herein, we report a gas-phase diagnostic technique for measuring iridium, a common catalyst material for monopropellant thrusters. Laser induced breakdown spectroscopy was employed to quantify iridium in the gas phase by thermally vaporizing various organometallic iridium compounds to known concentrations within a test cell and observing the resulting iridium optical emission lines that had no interfering lines in the region from other possible emitting species (such as nitrogen, oxygen, and carbon). A limit of detection for the iridium was determined to be 6.21 μmol/L (197 ppm by volume), and this, to our knowledge, is the first report on the quantitative analysis of gas-phase iridium by laser induced breakdown spectroscopy.
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•Ir-organometallic compounds were vaporized into the gas phase to known quantities.•LIBS was used to measure the gas-phase iridium LOD as low as 197 ppm (by volume).•LIBS provides for real-time iridium catalyst loss monitoring in thruster plumes.
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.
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.
The N
H
+ NO
reaction plays a key role during the early stages of hypergolic ignition between N
H
and N
O
. Here for the first time, the reaction kinetics of N
H
in excess NO
was studied in 2.0 Torr ...of N
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
/
+47 amu ion signal. For each chosen NO
, the observed HONO trace was fitted to a biexponential kinetics expression, which yielded a value for the pseudo-first-order rate coefficient,
', for the reaction of N
H
with NO
. The slope of the plot of
' versus NO
yielded a value for the observed bimolecular rate coefficient,
, which could be fitted to an Arrhenius expression of (2.36 ± 0.47) × 10
exp((520 ± 350)/
) cm
molecule
s
. The errors are 1σ and include estimated uncertainties in the NO
concentration. The potential energy surface of N
H
+ NO
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 N
H
NO
adducts, while roaming-mediated isomerization reactions yield the N
H
ONO 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
-HONO and
-N
H
below 500 K and the formation of NO + NH
NHO 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.
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-3H-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 .
Direct dynamics trajectory simulations were carried out for the NO
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
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
-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
) mechanism. The intra-ion-pair reaction products, including dialkylimidazolium
- H
, alkylimidazole, alkene, alkyl-DCA, HDCA, and DCA
, react with NO
and favor the formation of nitrite (-ONO) complexes over nitro (-NO
) 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.;
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
-proton transfer and its H
-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
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
(as reported by Brotton et al.
2018, 122, 7351-7377 and Lucas et al.
2019, 123, 400-416).
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