Herbert P. Broida Prize Talk: Spectroscopic and dynamical probes of atmospheric reaction pathways*

Alkene ozonolysis is a primary oxidation pathway for alkenes, the most abundant organic compounds in the Earth’s troposphere after methane, and also an important source of atmospheric hydroxyl radicals (OH). Alkene ozonolysis takes place through a complicated reaction pathway with multiple intermediates and barriers on the way to OH products. A carbonyl oxide species, known as the Criegee intermediate (RR'COO), represents a critical branching point on the pathway that controls the products formed in this important class of reactions. In this laboratory, the simplest Criegee intermediate CH₂OO and methyl-, dimethyl-, ethyl-, and vinyl-substituted Criegee intermediates are generated by alternative synthetic schemes. Recent studies have focused on characterizing the Criegee intermediates utilizing infrared and ultraviolet spectroscopic methods, and examining their unimolecular and bimolecular reactions under laboratory and atmospheric conditions. Infrared ‘fingerprint’ and electronic spectra reflecting π-conjugation of the Criegee intermediates are obtained, along with time-resolved studies of their unimolecular decay to OH radical products. Remarkably, the conformation and nature of the substituents (R, R') of the Criegee intermediates are found to have a profound effect on their reaction dynamics and subsequent chemistry in the atmosphere.