Spectroscopy and Dynamics of Reaction Intermediates in Atmospheric Chemistry

University Of Pennsylvania, Philadelphia PA

Investigators

Abstract

In this project funded by the Chemical Structure Dynamics and Mechanisms A and B Programs (CSDM-A and CSDM-B) of the Chemistry Division, Professor Marsha I. Lester and her students at the University of Pennsylvania is using sophisticated laser techniques to study intermediates formed in the atmospheric reactions of ozone (O3) with volatile organic molecules, especially alkenes (a class of molecules containing carbon-carbon double bonds) emitted by plants. The reactions of O3 with alkenes can produce a very reactive class of molecules called Criegee intermediates. Criegee intermediates are known to decay and release hydroxyl radical (OH) molecules, which are often called the atmosphere’s detergent because they can react (and deactivate) other potentially harmful pollutants in the air. Prof. Lester and her students generate these intermediates in the laboratory and then use an infrared laser pulse to excite the Criegee molecule and cause it to fall apart. Another technique called laser-induced fluorescence (LIF) is used to detect the production of OH radicals over time. The students engaged in this research are gaining valuable experience in modern laser technology, and analysis approaches for determining the identifying reaction pathways that create different products. The students are also utilizing theoretical models and computational tools to aid in the interpretation of the experimental data. This project focuses on reactive intermediates that give rise to hydroxyl radicals in the atmosphere. These intermediates originate from reaction of ozone with volatile organic compounds containing carbon-carbon double bonds, such as isoprene that is emitted in large quantities by plants. When formed in the atmosphere, these reactive intermediates have enough internal energy to spontaneously fall apart into an organic fragment and hydroxyl radical, in a process known as unimolecular decay, or react with other molecules. Professor Lester and her group generate these reactive intermediates with various substituents and conformations in the laboratory, and study their unimolecular decay processes to hydroxyl radicals using infrared action spectroscopy, LIF and theoretical modeling (Rice–Ramsperger–Kassel–Marcus (RRKM) theory with tunneling). The students working with Professor Lester gain experience is sophisticated experimental and theoretical methods that probe the physical properties of reactive intermediates in chemical reactions, and thus a significant broader impact of the project is its training of future scientists. 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.