RUI: The Tropospheric Oxidation of Biogenic Hydrocarbons: Probing Temperature Dependence and Autoxidation

With this award, the Environmental Chemical Sciences Program in the Division of Chemistry and the Atmospheric Chemistry Program in the Division of Atmospheric and Geospace Sciences are jointly funding Professor Keith T. Kuwata of Macalester College and Associate Professor Stacey A. Stoffregen of the University of Wisconsin-River Falls and their undergraduate research students to study chemical reactions of isoprene, the most abundant unsaturated organic compound in the atmosphere.

The fact that isoprene is unsaturated makes it prone to oxidation by reactive forms of oxygen such as hydroxyl radical (OH) and ozone. The Kuwata and Stoffregen laboratories use computational methods to predict all pathways by which isoprene is oxidized by OH and ozone. The ultimate project goals are to predict (1) the products of the isoprene + OH and isoprene + ozone reactions, (2) the rates of key steps in these reactions, and (3) how these rates vary with temperature.

These predictions will guide collaborators studying the same reactions experimentally and have the potential to improve the accuracy and precision of atmospheric chemistry models. The undergraduates working on the project receive research training in a variety of methods and typically present results at national scientific meetings and at twice-yearly meetings of the Midwest Undergraduate Computational Chemistry Consortium.

The chemical reactions under consideration pose significant theoretical challenges, with reactive intermediates often being chemically activated and possessing electronic structures with significant levels of both dynamic and static electron correlation. Predicting the fates of chemically activated species requires master equation simulations, with microcanonical rate constants being determined either by Rice-Ramsperger-Kassel-Marcus theory or variational transition state theory.

Use of a composite electronic structure method such as Weizmann-1 Brueckner Doubles theory provides an accurate treatment of dynamic correlation, while equation-of-motion spin-flip coupled-cluster theory provides a reasonable estimate of static correlation. The Kuwata and Stoffregen laboratories will apply these methods to study the competition between autoxidation and radical-terminating processes in the OH-initiated oxidation of isoprene, as well as study the generation of heretofore unmeasured hydroxycarbonyls in isoprene ozonolysis.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.