CAS: Spectroscopy and Unimolecular Dynamics of Reaction Intermediates in Atmospheric Chemistry

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, Professor Marsha Lester of the University of Pennsylvania and her team are using molecular spectroscopic and theoretical methods to study the unimolecular decay dynamics of carbonyl oxide intermediates generated through ozonolysis of alkenes. These reaction intermediates are an important source of hydroxyl radicals, which result in the oxidation of most volatile organic compounds, and are precursors to new particle formation in the atmosphere. Professor Lester and her students will investigate novel rearrangement mechanisms of the carbonyl oxide intermediates that lead to different products. These studies could lead to a better understanding of the generation of hydroxyl radicals in the atmosphere and other products that may impact on new particle formation. These studies will provide a stimulating environment for training and development of next generation scientists working at the interface between physical chemistry and atmospheric science. Professor Lester and her students will combine infrared laser activation of carbonyl oxide intermediates with time-resolved detection of hydroxyl radical products utilizing laser-induced fluorescence to measure unimolecular decay rates and provide direct insight on transition state barriers leading to products. The planned experimental studies and theoretical modeling will examine unimolecular decay processes of carbonyl oxide intermediates via 1,3 ring closure mechanisms. Extension to carbonyl oxides with greater carbon chain length and/or heteroatom substitution will be explored, which may lead to new low energy pathways with the potential for ring closure and secondary ozonide formation. Additional studies will implement various laser spectroscopic techniques to identify and characterize alternative products resulting from intra-fragment roaming reactions in the unimolecular decay of carbonyl oxide intermediates. In this case, the separating hydroxyl radical can reorient and roam at long-range, potentially leading to a wide range of products via addition and/or abstraction reactions, which may impact the yield of hydroxyl radicals. 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.