Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O2. This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NOX, which typically rapidly terminates autoxidation in urban areas, the studied C6–C10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for engine efficiency and urban air quality.

A modification of the energy transfer model recently proposed by two of us (ref 4) is tested in this work by an extensive comparison with the simulation results for O 3 scattering from a perfluorinated self-assembled monolayer (F-SAM) as well as with previous NO + FSAM and Ar + F-SAM scattering results. The model fits very well the trajectory data over a ∼10 3 -fold of incident energies. The percentage of energy transferred to the surface, predicted by the model at high incident energies, decreases with the number of degrees of freedom of the projectile because they compete with the surface degrees of freedom as possible destinations of the incident energy. The distributions of the scattered ozone molecules over translational and rotational states show a low-energy component characterized by a Maxwell−Boltzmann (MB) distribution at the surface temperature that survives at the highest collision energies. The dependence of the fraction of the MB component on the incident energy is an exponential decay function and the rate of decay is similar for the rotational and translational distributions. A nonnegligible number of the O 3 + F-SAM trajectories that penetrate the surface at high energies have very long residence times (longer than the simulation time), which enables thermal accommodation of the rotational and translational degrees of freedom. A new method to categorize the O 3 + F-SAM trajectories, based on the residence time, shows that, at very low incident energies (<10 kcal/mol), thermal accommodation can be achieved in a single collision event.

A full-dimensional analytical potential energy surface (PES) for the OH + NH3 → H2O + NH2 gas-phase reaction was developed based exclusively on high-level ab initio calculations. This reaction presents a very complicated shape with wells along the reaction path. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction path, and points on the reaction swath) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the coupled-cluster single double triple (CCSD(T)) = FULL/aug-cc-pVTZ//CCSD(T) = FC/cc-pVTZ single point level, which represents a severe test of the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K. The forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. However, the detailed analysis of the experimental equilibrium constants (from which the reverse rate constants are obtained) permits us to conclude that the experimental reverse rate constants must be re-evaluated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs), which were not included in the fitting procedure. The KIEs reproduce the values obtained from ab initio calculations in the common temperature range, although unfortunately no experimental information is available for comparison.

The need for renewable and cleaner sources of energy has made biofuels an interesting alternative to fossil fuels, especially in the case of butanol isomers, with its favorable blend properties and low hygroscopicity. Although C alcohols are prospective fuels, some key reactions governing their pyrolysis and combustion have not been adequately studied, leading to incomplete kinetic models. Enols are important intermediates in the combustion of C alcohols, as well as in atmospheric processes. Butanol reactions kinetics is poorly understood. Specifically, the unimolecular tautomerism of propen-2-ol ↔ acetone, which is included in butanol combustion kinetic models, is assigned rate parameters based on the tautomerism vinyl alcohol ↔ acetaldehyde as an analogy. In an attempt to update current kinetic models for tert- and 2-butanol, a theoretical kinetic study of the titled reaction was carried out by means of CCSD(T,FULL)/aug-cc-pVTZ//CCSD(T)/6-31+G(d,p) ab initio calculations, with multistructural torsional anharmonicity and variational transition state theory considerations in a wide temperature and pressure range (200-3000 K; 0.1-10 kPa). Results differ from vinyl alcohol ↔ acetaldehyde analogue reaction, which shows lower rate constant values. It was observed that decreasing pressure leads to a decrease in rate constants, describing the expected falloff behavior. Tunneling turned out to be important, especially at low temperatures. Accordingly, pyrolysis simulations in a batch reactor for tert- and 2-butanol with computed rate constants showed important differences in comparison with previous results, such as larger acetone yield and quicker propen-2-ol consumption.

We present for the first time an analytical potential energy surface (PES) for the reaction of hydrogen abstraction from ammonia by a chlorine atom. It has a very complicated shape with various maxima and minima. The functional form used in the development of the PES considered the stretching and bending nuclear motions, and the parameters in the calibration process were fitted to reproduce exclusively highlevel ab initio electronic structure calculations obtained at the CCSD(T) ¼ FULL/aug-ccpVTZ//CCSD(T) ¼ FC/cc-pVTZ single point level. Thus, the surface is completely symmetric with respect to the permutation of the three ammonia hydrogen atoms, and no experimental information is used in the process. The ab initio information used in the fit includes a wide spectrum of properties (equilibrium geometries, relative energies, and vibrational frequencies) of the reactants, products, saddle point, intermediate complexes in the entry and exit channels, points on the reaction path, and points on the reaction swath. By comparison with the reference results, we show that the resulting PES reproduces not only the ab initio data used in the fitting procedure but also other thermochemical and kinetics results computed at the same ab initio level, which were not used in the fit-equilibrium constants, rate constants, and kinetic isotope effects. This represents a severe test for the new surface. As a first application, we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K, on this analytical PES. The forward rate constants reproduce the sparse experimental measurements, while the reverse ones reproduce the change of activation energy with temperature reported in another theoretical study, although unfortunately there are no experimental