Insights into the Ultrafast Photodissociation Dynamics of Isoprene-Derived Criegee Intermediates.

Abstract

Isoprene is the most abundant nonmethane 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 (CH2 OO), 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, which leads to dissociation to form aldehyde/ketone + oxygen products. Here, we compare the excited state chemistry of CH2 OO, MVK-oxide and MACR-oxide in order to ascertain how increasing molecular complexity affects their photodynamics. In CH2 OO, vertical excitation to the S2 state leads to prompt O-O bond fission with a unity quantum yield. Branching into both the O(1 D) + H2 CO (S0 ) and O (3 P) + H2 CO (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 nonunity quantum yield. In the latter case, ca. 10% and 25% of trajectories remain as the parent MVK-oxide and MACR-oxide molecules, respectively. Additionally, at most only 5% of the dissociating trajectories form O (3 P) + MVK/MACR (T1 ) products, with a greater fraction forming O (1 D) + MVK/MACR (S0 ) products (cf. CH2 OO). This latter observation coupled with the greater fraction of undissociated trajectories aligns with thebathochromic shift in the electronic absorption of the MACR-oxide and MVK-oxide (cf. CH2 OO). We discuss the implications of the results in a broader context, including those that are relevant to the atmosphere.