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A wide-range experimental and theoretical investigation of ammonia gas-phase oxidation is performed, and a predictive, detailed kinetic model is developed.

Despite extensive kinetics/theoretical studies, information on the detailed mechanism (primary products, branching ratios (BRs)) for many important combustion reactions of O( 3 P) with unsaturated hydrocarbons is still lacking. We report synergic experimental/theoretical studies on the mechanism of the O( 3 P) + C 3 H 6 (propene) reaction by combining crossed-molecular-beam experiments with mass spectrometric detection at 9.3 kcal/mol collision energy (E c ) with high-level ab initio electronic structure calculations of underlying triplet/ singlet potential energy surfaces (PESs) and statistical (RRKM/Master Equation) computations of BRs including intersystem crossing (ISC). The reactive interaction of O( 3 P) with propene is found to mainly break apart the three-carbon atom chain, producing the radical products methyl + vinoxy (32%), ethyl + formyl (9%), and molecular products ethylidene/ethylene + formaldehyde (44%). Two isomers, CH 3 CHCHO (7%) and CH 3 COCH 2 (5%), are also observed from H atom elimination, reflecting O atom attack to both terminal and central C atoms of propene. Some methylketene (3%) is also formed following H 2 elimination. As some of these products can only be formed via ISC from triplet to singlet PESs, from BRs an extent of ISC of about 20% is inferred. This value is significantly lower than recently observed in O( 3 P) + ethylene (∼50%) and O( 3 P) + allene (∼90%) at similar E c , posing the question of how important it is to consider nonadiabatic effects for these and similar combustion reactions. Comparison of the derived BRs with those from recent kinetics studies at 300 K and statistical predictions provides information on the variation of BRs with E c . ISC is estimated to decrease from 60% to 20% with increasing E c . The present results lead to a detailed understanding of the complex reaction mechanism of O + propene and should facilitate the development of improved models of hydrocarbon combustion.

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The production of OH and HO(2) in Cl-initiated oxidation of cyclohexane has been measured using pulsed-laser photolytic initiation and continuous-laser absorption detection. The experimental data are modeled by master equation calculations that employ new G2(MP2)-like ab initio characterizations of important stationary points on the cyclo-C(6)H(11)O(2) surface. These ab initio calculations are a substantial expansion on previously published characterizations, including explicit consideration of conformational changes (chair-boat, axial-equatorial) and torsional potentials. The rate constants for the decomposition and ring-opening of cyclohexyl radical are also computed with ab initio based transition state theory calculations. Comparison of kinetic simulations based on the master equation results with the present experimental data and with literature determinations of branching fractions suggests adjustment of several transition state energies below their ab initio values. Simulations with the adjusted values agree well with the body of experimental data. The results once again emphasize the importance of both direct and indirect components of the kinetics for the production of both HO(2) and OH in radical + O(2) reactions.

The low- and high-temperature oxidation mechanisms of n-heptane have been extensively studied in recent and past literature because of its importance as a primary reference fuel. Recent advanced analytical methods allowed for the identification of several intermediate oxygenated species at very low-temperature conditions in jet-stirred reactors. On these bases, new classes of successive reactions involving hydroperoxide species, already discussed for propane and n-butane oxidation, were included in the low-temperature oxidation mechanism of n-heptane. These new reactions allowed for the improvement of the overall mechanism, not only obtaining a satisfactorily agreement with reaction products, such as organic acids, diones, and ketones, but also in terms of system reactivity. Moreover, deeper attention was also paid to the formation of ketohydroperoxides, rarely experimentally measured. Because of n-heptane importance as a primary reference fuel, the overall POLIMI kinetic mechanism is validated in a wide range of conditions, in both the high- and low-temperature regimes. Moreover, the reliability of the updated oxidation mechanism is further proven in a couple of more complex applications, such as the autoignition of nheptane droplets in microgravity conditions and the oxidation of lean n-heptane/air mixtures in homogeneous charge compression ignition (HCCI) engines

Trichlorosilane is the most used precursor to deposit silicon for photovoltaic applications. Despite of this, its gas phase and surface kinetics have not yet been completely understood. In the present work, it is reported a systematic investigation aimed at determining what is the dominant gas phase chemistry active during the chemical vapor deposition of Si from trichlorosilane. The gas phase mechanism was developed calculating the rate constant of each reaction using conventional transition state theory in the rigid rotor-harmonic oscillator approximation. Torsional vibrations were described using a hindered rotor model. Structures and vibrational frequencies of reactants and transition states were determined at the B3LYP/6-31+G(d,p) level, while potential energy surfaces and activation energies were computed at the CCSD(T) level using aug-cc-pVDZ and aug-cc-pVTZ basis sets extrapolating to the complete basis set limit. As gas phase and surface reactivities are mutually interlinked, simulations were performed using a microkinetic surface mechanism. It was found that the gas phase reactivity follows two different routes. The disilane mechanism, in which the formation of disilanes as reaction intermediates favors the conversion between the most stable monosilane species, and the radical pathway, initiated by the decomposition of Si2HCl5 and followed by a series of fast propagation reactions. Though both mechanisms are active during deposition, the simulations revealed that above a certain temperature and conversion threshold the radical mechanism provides a faster route for the conversion of SiHCl3 into SiCl4, a reaction that favors the overall Si deposition process as it is associated with the consumption of HCl, a fast etchant of Si. Also, this study shows that the formation of disilanes as reactant intermediates promotes significantly the gas phase reactivity, as they contribute both to the initiation of radical chain mechanisms and provide a catalytic route for the conversion between the most stable monosilanes.