Xiaoqing You

Thermal Decomposition of Pentacene Oxyradicals

The energetics and kinetics of the thermal decomposition of pentacene oxyradicals were studied using a combination of ab initio electronic structure theory and energy-transfer master equation modeling. The rate coefficients of pentacene oxyradical decomposition were computed for the range of 1500-2500 K and 0.01-10 atm and found to be both temperature and pressure dependent. The computational results reveal that oxyradicals with oxygen attached to the inner rings are kinetically more stable than those with oxygen attached to the outer rings. The latter decompose to produce CO at rates comparable to those of phenoxy radical, while CO is unlikely to be produced from oxyradicals with oxygen bonded to the inner rings.

Role of Carbon-Addition and Hydrogen-Migration Reactions in Soot Surface Growth

Using density functional theory and master equation modeling, we have studied the kinetics of small unsaturated aliphatic molecules reacting with polycyclic aromatic hydrocarbon (PAH) molecules having a diradical character. We have found that these reactions follow the mechanism of carbon addition and hydrogen migration (CAHM) on both spin-triplet and open-shell singlet potential energy surfaces at a rate that is about ten times those of the hydrogen-abstraction-carbon-addition (HACA) reactions at 1500 K in the fuel-rich postflame region. The results also show that the most active reaction sites are in the center of the zigzag edges of the PAHs. Furthermore, the reaction products are more likely to form straight rather than branched aliphatic side chains in the case of reacting with diacetylene. The computed rate constants are also found to be independent of pressure at conditions of interest in soot formation, and the activation barriers of the CAHM reactions are linearly correlated with the diradical characters.

Benchmark Calculations for Bond Dissociation Enthalpies of Unsaturated Methyl Esters and the Bond Dissociation Enthalpies of Methyl Linolenate

It is important to determine an appropriate computational method for obtaining accurate thermochemical properties of large biodiesel molecules such as methyl linolenate. In this study, we use Kohn-Sham density functional theory (DFT) and coupled cluster theory to calculate bond dissociation enthalpies (BDEs) of seven fragment molecules of methyl linolenate, in particular, propene, methyl formate, cis-3-hexene, 1,4-pentadiene, 1-pentene, butane, and methyl butanoate. The results are compared to BDEs obtained from experiments and to Oyeyemi et al.s multireference averaged coupled pair functional (MRACPF2) calculations. We found that with extrapolation to the complete basis set (CBS) limit, the BDEs derived from coupled cluster calculations with single, double, and triple excitations (CCSDT) and from CCSDT with a perturbative treatment of connected quadruple excitations, CCSDT(2)Q/CBS, are closer to the available experimental values than those obtained by MRACPF2 for propene and methyl formate. The CCSDT/CBS calculations were chosen as the reference for validating the DFT methods. Among the density functionals, we found that M08-HX has the best performance with a mean unsigned deviation (MUD) from CCSDT/CBS of only 1.0 kcal/mol, whereas the much more expensive MRACPF2 has an MUD of 1.1 kcal/mol. We then used the most successfully validated density functionals to calculate the BDEs of methyl linolenate and compared the results with the MRACPF2 BDEs. The present study identifies several Kohn-Sham exchange-correlation functionals that should be useful for modeling ester combustion, especially the M08-HX, M06-2X, M05-2X, M08-SO, and MPWB1K global-hybrid meta functionals, the M11 and MN12-SX range-separated-hybrid meta functionals, the ωB97 range-separated hybrid gradient approximation functional, and the SOGGA11-X global-hybrid gradient approximation functional.

Propagation and Extinction of Mixtures of Air with n-Dodecane, JP-7, and JP-8 Jet Fuels

Laminar flame speeds and extinction strain rates of fuels related to high-speed airbreathing propulsion were determined experimentally and numerically. Among practical jet fuels, JP-7 and JP-8 were investigated at fuel temperatures close to their boiling point. Mixtures of air with n-dodecane were investigated also because of the relevance of the latter to the development of jet fuels surrogates. The experimental data of n-dodecane were compared against numerical simulations obtained using a recently developed kinetic mechanism. The agreements between experimental and predicted values were good for both propagation and extinction, suggesting that the mechanism satisfactorily describes the heat release. It was found that the laminar flame speeds and extinction limits of n-dodecane is comparable to those obtained for both jet fuels, which underlines the significance of n-dodecane in the development of jet fuels surrogates. The effect of thermal cracking on flame propagation was studied also at fuel temperatures well above its boiling point, in view of its relevance to SCRAMJETs.

Dimerization of polycyclic aromatic hydrocarbons in soot nucleation.

A possible pathway of soot nucleation, in which localized π electrons play an important role in binding the polycyclic aromatic hydrocarbon (PAH) molecules having multiradical characteristics to form stable polymer molecules through covalent bonds, is studied using density functional and semiempirical methods. Results show that the number of covalent bonds formed in the dimerization of two identical PAHs is determined by the radical character, and the sites to form bonds are related to the aromaticity of individual six-membered ring structure. It is further shown that the binding energy of dimerization increases linearly with the diradical character in the range relevant to soot nucleation.

Patterns of local aromaticity in graphene oxyradicals

Two families of polyaromatic hydrocarbon (PAH) oxyradicals are investigated using density functional theory (DFT) and the semi-empirical PM6 method. These families result from the edge oxidation of substrates that involve only zigzag edges and those that include both zigzag and armchair edges. Oxyradical stability is shown to correlate with local aromatic character of six-atom rings characterized by the harmonic oscillator measure of aromaticity (HOMA) and with the distribution of HOMA values in molecules. It is demonstrated, that oxidation at the edge has a non-local effect on the structure of PAHs and leads to distinguishable types of HOMA patterns that are common for both families of PAHs.

Role of Spin-Triplet Polycyclic Aromatic Hydrocarbons in Soot Surface Growth.

Using density functional theory, a possible pathway of soot surface growth is studied in the low-temperature, postflame region in which spin-triplet polycyclic aromatic hydrocarbon (PAH) molecules with a small singlet-triplet energy gap react with unsaturated aliphatics such as acetylene via the carbon-addition-hydrogen-migration (CAHM) reaction. Results show that a PAH-core-aliphatic-shell structure is formed and the mass growth rate of this triplet soot surface growth reaction is one order of magnitude larger than that of the surface hydrogen-abstraction-carbon-addition (HACA) reaction at temperatures below 1500 K.

Theoretical Analysis of the Effect of C═C Double Bonds on the Low-Temperature Reactivity of Alkenylperoxy Radicals.

Biodiesel contains a large proportion of unsaturated fatty acid methyl esters. Its combustion characteristics, especially its ignition behavior at low temperatures, have been greatly affected by these C═C double bonds. In this work, we performed a theoretical analysis of the effect of C═C double bonds on the low-temperature reactivity of alkenylperoxy radicals, the key intermediates from the low-temperature combustion of biodiesel. To understand how double bonds affect the fate of peroxy radicals, we selected three representative peroxy radicals from heptane, heptene, and heptadiene having zero, one, and two double C═C bonds, respectively, for study. The potential energy surfaces were explored at the CBS-QB3 level, and the reaction rate constants were computed using canonical/variational transition state theories. We have found that the double bond is responsible for the very different bond dissociation energies of the various types of C-H bonds, which in turn affect significantly the reaction kinetics of alkenylperoxy radicals.

Experimental Investigation on the Evaporation and Combustion Processes of Ammonium-Dinitramide-Based Liquid Propellant

In this work, the evaporation and combustion processes of liquid propellant ammonium dinitramide–methanol aqueous solution were investigated by a thermogravimetric analyzer and a Curie-point pyrolysis unit, respectively, and the gas-phase products were measured by a Fourier transform infrared spectroscopy. The results showed that methanol and water evaporated first, followed by the decomposition of ammonium dinitramide at heating rates of 5–15°C/min. When a catalyst was added, ammonium dinitramide decomposed much faster and released heat that let methanol evaporate immediately as the propellant solution was in contact with the catalyst particles. To explore how the decomposition of ammonium dinitramide is coupled with the oxidation of methanol, the propellant solution was heated by a Curie-point pyrolysis unit at a rate of 3000°C/s to temperatures ranging from 160 to 670°C. Large amounts of nitrous oxide and ammonia were detected at all temperatures, suggesting that most of the decomposition of ammonium d...

Flame Structure of Ammonium-Dinitramide-Based Liquid Propellant in a Small Thruster

In this paper, the flame structure of a liquid propellant ammonium dinitramide–methanol aqueous solution in a small model thruster is investigated. The thruster is composed of an injector and a com...