Modeling the Ultrafast Photochemistry of Criegee Intermediates

Abstract:

Alkenes are a subset of anthropogenic and biogenic VOCs that are the second most abundant in the troposphere – after methane. They play a significant role in enhancing the oxidizing capacity of the troposphere and are a significant source of nucleation of particulates and for the growth of secondary organic aerosols (SOAs). Much of these atmospheric consequences arise via the oxidation of alkenes.

The major tropospheric alkene removal process is via ozonolysis, wherein the reaction of a given alkene with ozone forms a carbonyl oxide intermediate – known as the Criegee Intermediate (CrIs). CrIs are formed with excess internal energy and thus may undergo further reactions, such as unimolecular decay or isomerization. Alternatively, the highly vibrationally excited CrI may undergo vibrational energy relaxation through collisions with proximal molecules. The stabilized CrIs can then undergo unimolecular decay, react with several trace atmospheric species or may undergo UV-excitation. Theoretical and computational chemistry offers a deep mechanistic interpretation of these three major atmospheric loss processes of stabilized CrIs.

In this contribution we will present a systematic study of the electronic absorption spectra and multi-dimensional time-dependent quantum dynamics associated with the unimolecular photochemistry of a CrIs with varying molecular complexity. The excited-states dynamics involves several coupled potential energy surfaces and show rapid unimolecular decay on a sub-picosecond timescale with a rich variation in product state dissipation upon changes in molecular complexity of the CrI. The long time-scale dynamics reveal several products including CO and CO₂, as well as OH radicals formed from the decay of a vibrationally excited carboxylic acid. This latter product is of profound importance as it promotes the nucleation and subsequent growth of secondary organic aerosols.

The results shed light on the possible fates of conjugated CrIs – such as those derived from the ozonolysis of isoprene. The absorption spectrum of such CrIs are red-shifted (cf. the simpler CrIs) thus allowing for a greater removal probability via solar photolysis.