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Barrierless Reactions with Loose Transition States Govern the Yields and Lifetimes of Organic Nitrates Derived from Isoprene
Citation:
Piletic, I., E. Edney, AND L. Bartolotti. Barrierless Reactions with Loose Transition States Govern the Yields and Lifetimes of Organic Nitrates Derived from Isoprene. JOURNAL OF PHYSICAL CHEMISTRY A. American Chemical Society, Washington, DC, 121(43):8306-8321, (2017).
Impact/Purpose:
The mechanisms of NOx reactivity, persistence and transport in the atmosphere is of paramount importance in the production of other significant tropospheric pollutants such as ozone and secondary organic aerosol. In particular, the production of organic nitrates is significant due to their potential to at least temporarily sequester some NOx emissions. Numerous kinetic studies of organic nitrate formation from peroxy radical reactions with NOx have revealed their sensitivity to pressure, temperature and molecular size although the studies have provided a relatively large range of values. The results from this work provide values for several crucial parameters of isoprene oxidation (organic nitrate yields, lifetimes, NOx recycling) used in atmospheric models to determine ambient concentrations of NOx, ozone and PM2.5. Because isoprene is the dominant unsaturated VOC in the atmosphere, the impact of its chemistry on modeling several EPA criteria pollutants cannot be understated. Because of the sensitivity of computational chemistry, this work provides a comprehensive chemical interpretation for the reported results and forms a scientific foundation to help understand the fate of other emitted VOCs in the atmosphere. The data may be used by developers and users of atmospheric models in the Agency’s regulatory and program offices as well as atmospheric scientists to build upon knowledge in the field. In a society facing complex and entangled environmental, economic and social issues, computational chemistry is a vital tool that delivers accurate, cost-effective and efficient data that may be used to reduce harmful pollution concentrations.
Description:
The chemical reaction mechanism of NO addition to two β and δ isoprene hydroxy–peroxy radical isomers is examined in detail using density functional theory, coupled cluster methods, and the energy resolved master equation formalism to provide estimates of rate constants and organic nitrate yields. At the M06-2x/aug-cc-pVTZ level, the potential energy surfaces of NO reacting with β-(1,2)-HO-IsopOO• and δ-Z-(1,4)-HO-IsopOO• possess barrierless reactions that produce alkoxy radicals/NO2 and organic nitrates. The nudged elastic band method was used to discover a loosely bound van der Waals (vdW) complex between NO2 and the alkoxy radical that is present in both exit reaction channels. Semiempirical master equation calculations show that the β organic nitrate yield is 8.5 ± 3.7%. Additionally, a relatively low barrier to C–C bond scission was discovered in the β-vdW complex that leads to direct HONO formation in the gas phase with a yield of 3.1 ± 1.3%. The δ isomer produces a looser vdW complex with a smaller dissociation barrier and a larger isomerization barrier, giving a 2.4 ± 0.8% organic nitrate yield that is relatively pressure and temperature insensitive. By considering all of these pathways, the first-generation NOx recycling efficiency from isoprene organic nitrates is estimated to be 21% and is expected to increase with decreasing NOx concentration.
