Modeling the Ground- and Excited-State Unimolecular Decay of the Simplest Fluorinated Criegee Intermediate, HFCOO, Formed from the Ozonolysis of Hydrofluoroolefin Refrigerants.

Hydrofluoroolefins (HFO) are fourth-generation refrigerants designed to function as efficient refrigerants with no ozone depletion potential and zero global warming potential. Despite extensive studies on their chemical and physical properties, the ground- and excited-state chemistry of their atmospheric oxidation products is less well understood. This study focuses on the ground- and excited-state chemistry of the simplest fluorinated Criegee intermediate (CI), fluoroformaldehyde oxide (HFCOO), which is the simplest fluorinated CI formed from the ozonolysis of HFOs. HFCOO contains syn- and anti-conformers, which have Boltzmann populations of, respectively, 87 and 13% at 298 K. For both conformers, the calculated ground-state reaction energy profiles associated with cyclization to form fluorodioxirane is lower than the equivalent unimolecular decay path in the simplest CI, H2COO, with anti-HFCOO returning a barrier height more than half of that of H2COO. The excited-state dynamics reveal that photoexcitation to the bright S2 state of syn-HFCOO and anti-HFCOO is expected to undergo a prompt O-O fission─with the former conformer expected to dissociate with an almost unity quantum yield and to form both O (1D) + HFCO (S0) and O (3P) + HFCO (T1) products. In contrast, photoexcitation of anti-HFCOO is expected to undergo an O-O bond fission with a non-unity quantum yield. The fraction of photoexcited anti-HFCOO that dissociates is predicted to exclusively form O (1D) + HFCO (S0) products, which is in sharp contrast to H2COO. The wider implications of our results are discussed from both physical and atmospheric chemistry perspectives.

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

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 the bathochromic 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.

Comparing the Excited State Dynamics of CH2OO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation.

Ab initio molecular dynamics studies of CH2OO molecules following excitation to the minimum-energy geometry of the strongly absorbing S2 (1ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S0) state. Trajectories propagated for 100 fs from the minimum-energy region of the S2 state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S2 and S1 (1nπ*) states at geometries close to the S2 minimum, which enables population transfer to the optically dark S1 state. Greater than 80% of the excited population undergoes O-O bond fission on the S2 or S1 potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH2 moiety is twisted relative to the COO plane. Trajectory analysis also reveals recurrences in the O-O stretch coordinate, consistent with the resonance structure observed at the red end of the parent S2-S0 absorption spectrum, and a small propensity for out-of-plane motion after nonadiabatic coupling to the S1 PES that enables access to a conical intersection between the S1 and S0 states and cyclization to dioxirane products.