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Confusion over how to account for symmetry numbers when reactants are identical can cause significant errors in isotopic studies. An extraneous factor of 2 in the reaction symmetry number, as proposed in the literature, violates reaction equilibrium and causes huge enrichment errors in isotopic analysis. In actuality, no extra symmetry factor is needed with identical reactants.

Bio-derived isobutanol has been approved as a gasoline additive in the U.S., but our understanding of its combustion chemistry still has significant uncertainties. Detailed quantum calculations could improve model accuracy leading to better estimation of isobutanol’s combustion properties and its environmental impacts. This work examines 47 molecules and 38 reactions involved in the first oxygen addition to isobutanol’s three alkyl radicals located α, β, and γ to the hydroxide. Quantum calculations are mostly done at CCSD(T)-F12/cc-pVTZ-F12//B3LYP/CBSB7, with 1-D hindered rotor corrections obtained at B3LYP/6-31G(d). The resulting potential energy surfaces are the most comprehensive isobutanol peroxy networks published to date. Canonical transition state theory and a 1-D microcanonical master equation are used to derive high-pressure-limit and pressure-dependent rate coefficients, respectively. At all conditions studied, the recombination of α- isobutanol radical with O2 forms HO2 and isobutanal. The recombination of γ-isobutanol radical with O2 forms a stabilized hydroperoxy alkyl radical below 400 K, water and an alkoxy radical at higher temperatures, and HO2 and an alkene above 1200 K. The recombination of β-isobutanol radical with O2 results in a mixture of products between 700-1100 K, forming acetone, formaldehyde and OH at lower temperatures and forming HO2 and alkenes at higher temperatures. The barrier heights, high-pressure-limit rates, and pressure-dependent kinetics generally agree with the results from previous quantum chemistry calculations. Six reaction rates in this work deviate by over three orders of magnitude from kinetics in detailed models of isobutanol combustion, suggesting the rates calculated here can help improve modeling of isobutanol combustion and its environmental fate.

This perspective gives our views on general aspects and future directions of gas‐phase atmospheric chemical kinetic mechanism development, emphasizing on the work needed for the sustainable development of chemically detailed mechanisms that reflect current kinetic, mechanistic, and theoretical knowledge. Current and future mechanism development efforts and research needs are discussed, including software‐aided autogeneration and maintenance of kinetic models as a future‐proof approach for atmospheric model development. There is an overarching need for the evaluation and extension of structure‐activity relationships (SARs) that predict the properties and reactions of the many multifunctionalized compounds in the atmosphere that are at the core of detailed mechanisms, but for which no direct chemical data are available. Here, we discuss the experimental and theoretical data needed to support the development of mechanisms and SARs, the types of SARs relevant to atmospheric chemistry, the current status and limitations of SARs for various types of atmospheric reactions, the status of thermochemical estimates needed for mechanism development, and our outlook for the future. The authors have recently formed a SAR evaluation working group to address these issues.

With the rise in production of natural gas, there is increased interest in homogeneous partial oxidation (POX) to convert methane to syngas (CO + H2), ethene (C2H4) and acetylene (C2H2). In POX, polycyclic aromatic hydrocarbons (PAH) are important undesired byproducts. To improve the productivity of such POX processes, it is necessary to have an accurate chemical mechanism for methane-rich combustion including PAH. A new mechanism was created to capture the chemistry from C0 to C12, incorporating new information derived from recent quantum chemistry calculations, with help from the Reaction Mechanism Generator (RMG) software. For better estimation of kinetics and thermochemistry of aromatic species, including reactions through carbene intermediates, new reaction families and additional data from quantum chemistry calculations were added to RMG-database. Many of the rate coefficients in the new mechanism are significantly pressure-dependent at POX conditions. The new mechanism was validated against electron-ionization molecular beam mass spectrometry (EI-MBMS) data from a high-temperature flow reactor reported by Kohler et al. In this work quantification of additional species from those experiments is reported including phenylacetylene (C8H6), indene (C9H8), naphthalene (C10H8) and acenaphthylene (C12H8) at many temperatures for several feed compositions. Comparison of the experimental species concentration data and the new kinetic model is satisfactory; the new mechanism is generally more accurate than other published mechanisms. Moreover, because the new mechanism is composed of elementary chemical reaction steps instead of global fitted kinetics, pathway analysis of species could be investigated step-by-step to understand PAH formation. For methane-rich combustion, the most important routes to key aromatics are propargyl recombination for benzene, reactions of the propargyl radical with the phenyl radical for indene, and hydrogen abstraction acetylene addition (HACA) for naphthalene.