RESUMO
The primary objectives of this study are to identify the initiation steps of perchloric acid (HClO4) decomposition and to validate and provide insights into the reaction pathways of O2 formation. To this end, we have performed quantum chemical calculations using the Gaussian 09 program package to identify new reaction pathways and species formed during decomposition. The thermodynamic quantities of the species, such as Gibbs free energy and enthalpy, are calculated using a double-hybrid density functional theory method, B2PLYP, with Jensen's basis set, aug-pc2. For heavy atoms, such as chlorine, the basis set is augmented by adding 2 d functions with a stride factor of 2.5. To incorporate the solvation effect, the conductor-like polarizable continuum model, which is an implicit solvation model, is used. Numerical simulations using a control-volume analysis of an experiment are also performed using the proposed mechanism. In these simulations, rates of the reactions are calculated using transition state theory, incorporating diffusion effects on the rate constants. In order to consider the nonideal behavior of a concentrated HClO4 solution, activity coefficients are used to calculate the effective concentration of acid in solution. The activity coefficient of HClO4 plays a critical role in the calculation of the induction period involved in the HClO4 decomposition. A comparison of numerically predicted O2 evolution and duration of the induction period with experimental data shows that the numerical simulation using the proposed mechanism predicts both the three-stage decomposition characteristics and the induction period observed during HClO4 decomposition, thus validating the proposed mechanism.
RESUMO
This work provides new insights for the liquid-phase decomposition of bis(triaminoguanidinium) azotetrazolate (TAGzT). The liquid-phase decomposition process was investigated using a combined experimental and computational approach. Sub-milligram samples of TAGzT were heated at rates of about 2000 K s-1 to a set temperature (230 to 260 °C) where liquid-phase decomposition occurred under isothermal conditions. Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS) were used to acquire transmittance spectra and mass spectra of the evolved gas-phase species from the rapid thermolysis, respectively. FTIR spectroscopy was also used to acquire the transmittance spectra of the condensate and residue formed from the decomposition. N2, NH3, HCN, N2H4, triaminoguanidine and 3-azido-1,2,4-triazol-4-ide anion were identified as products of liquid-phase decomposition. Quantum chemical calculations were used for confirming the identity of the species observed in experiments and for identifying elementary chemical reactions that formed these species. Based on the calculated free energy barriers of these elementary reactions, important reaction pathways were identified for the formation of each of the product species.
RESUMO
A detailed mechanism has been developed for thermal decomposition of hydroxylammonium nitrate (HAN) solutions, based on quantum mechanical calculations using the SMD-ωB97X-D method. The mechanism describes multiple kinetic processes, including nitration and nitrosation of hydroxylamine, HNO dimerization, and HONO-regeneration pathways involving H-abstraction reactions. Rate constants of elementary reactions were estimated using transition state theory with consideration of species' diffusion effect. Kinetic modeling was performed to predict species' evolutions in 0.1 m HAN in the temperature range of 463-523 K, and results show reasonable agreement with the experimental data from flow reactor studies. For more concentrated solutions, strong autocatalytic behaviors were predicted with the late emergence of NO2 and HONO, whose regeneration was previously considered as the major autocatalytic pathway. Sensitivity analysis results suggest an acid-catalyzed nitration-nitrosation pathway, based on which the autocatalysis should be caused by the rise of solution acidity. A linear correlation can be observed in the previously reported apparent Arrhenius parameters, which may be reconciled via a kinetic compensation effect.
RESUMO
The dimerization of HNO and subsequent yield of N2O in aqueous solution are studied based on the theoretical calculations and kinetic simulations. The initial dimerization reactions were computed at various levels of theory, and large divergence was observed in the predictions of the gas-phase free energies. The T1 diagnostics at CCSD(T)/aug-cc-pVTZ suggests multireference characteristics of the HNO dimers and the transition states. The solution-phase free energies were obtained using the wB97XD method and the SMD solvation model. The pKa values of the (HNO)2 tautomers and their first protonated and deprotonated products were estimated using the cluster-continuum approach. The theoretical results confirmed the original conclusion that the favored cis-pathway is comprised of several rapid proton transfer steps leading to either cis-HONNOH or cis-HONNO¯ before decomposition. Several new water-catalyzed and H3O+/water catalyzed reactions are presented to explain the fast kinetics observed in the experiments. To validate the proposed mechanism, kinetic simulations with the consideration of diffusion-limited kinetics were implemented on several related systems, based on which the previously reported global rate constant was explained as the kinetics of the initial dimerization step and the global kinetics in very dilute HNO solutions.
RESUMO
The determination of the condensed-phase preignition chemistry of hypergolic bipropellant pairs has been considerably challenging due to toxicity and reactivity of the used materials. In this work, we describe a new experimental technique for characterizing preignition behavior of hypergolic materials. Small quantities of the bipropellant pair are brought in contact under isothermal conditions, and spectral transmission measurements on the evolved products are performed using Fourier transform infrared spectroscopy. The gaseous products are subsequently quantified to yield species concentration profiles at various temperatures, hence facilitating the elucidation of the detailed condensed-phase chemical kinetics responsible for the ignition in the gas phase.
