Insights into the Ultrafast Photodissociation Dynamics of Isoprene Derived Criegee Intermediates

AbstractIsoprene is the most abundant non-methane volatile organic compound emitted into the troposphere by terrestrial vegetation. Reaction with ozone represents an important isoprene removal process from the troposphere and is a well-known source of Criegee intermediates (CIs), which are reactive carbonyl oxides. Three CIs, formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-oxide), and methacrolein oxide (MACR-oxide) are formed during isoprene ozonolysis. All three CIs contain strongly absorbing ππ* states, electronic excitation to which leads to dissociation to form aldehyde/ketone + oxygen products. Here, we compare the excited state chemistry of CH2OO, MVK-oxide and MACR-oxide in order to ascertain how increasing molecular complexity affects their photodynamics.In CH2OO, vertical excitation to the S2 state leads to prompt O-O bond fission with a unity quantum yield. Branching into both the O (1D) + H2CO (S0) and O (3P) + H2CO (T1) product channels is predicted, with 80% of trajectories dissociating to form the former product pair. Analogous vertical excitation of the lowest energy conformers of MVK-oxide and MACR-oxide also undergoes O-O bond fission to form O + MVK/MACR products – albeit with a non-unity quantum yield. In the latter case, ca. 10% and 20 % of trajectories remain as the parent MVK-oxide and MACR-oxide molecule, respectively. Additionally, at most only 5% of the dissociating trajectories form O (3P) + MVK/MACR (T1) products, with a greater frac...

Source: Photochemistry and Photobiology - October 30, 2022 Category: Science Authors: Ernest Antwi, Niamh A. Packer, Jordyn M. Ratliff, Barbara Marchetti, Tolga N. V. Karsili Tags: Research Article Source Type: research