Search for: All records

When volatile alkenes are emitted into the atmosphere, they are rapidly removed by oxidizing agents such as hydroxyl radicals and ozone. The latter reaction is termed ozonolysis and is an important source of Criegee intermediates (CIs), i.e., carbonyl oxides, that are implicated in enhancing the oxidizing capacity of the troposphere. These CIs aid in the formation of lower volatility compounds that typically condense to form secondary organic aerosols. CIs have attracted vast attention over the past two decades. Despite this, the effect of their substitution on the ground and excited state chemistries of CIs is not well studied. Here, we extend our knowledge obtained from prior studies on CIs by CF3 substitution. The resulting CF3CHOO molecule is a CI that can be formed from the ozonolysis of hydrofluoroolefins (HFOs). Our results show that the ground state unimolecular decay should be less reactive and thus more persistent in the atmosphere than the non-fluorinated analog. The excited state dynamics, however, are predicted to occur on an ultrafast timescale. The results are discussed in the context of the ways in which our study could advance synthetic chemistry, as well as processes relevant to the atmosphere. 

Criegee intermediates are amongst the most fascinating molecules in modern-day chemistry. They are highly reactive intermediates that find vital roles that range from atmospheric chemistry to organic synthesis. Their excited state chemistry is exotic and complicated, and a myriad of electronic states can contribute to their photodissociation dynamics. This manuscript reports a multi-state direct dynamics (full-dimensional) study of the photoinduced fragmentation of the simplest Criegee intermediate, CH2OO, using state-of-the-art CASPT2 trajectory surface hopping. Energy- (excitation-) transfer is observed between the separating O and H2CO fragments at separations that, traditionally, might be viewed as the classically asymptotic region of the potential energy surface. We suggest that such long-range energy exchange accounts for the unusual and non-intuitive total kinetic energy distribution (TKER) in the O(1D) + H2CO(S0) products observed following photoexcitation of CH2OO. The present results also reveal the interplay between seven singlet electronic states and dissociation to yield the experimentally observed O(1D) + H2CO(S0) and O(3P) + H2CO(T1) products. The former (singlet) products are favored, with a branching ratio of ca. 80 %, quantifying the hitherto unknown product branching ratios observed in velocity map imaging experiments. To the best of our knowledge, such long-range energy transfer between fragment pairs originating from a common parent – at classically asymptotic separations approaching those more typically reminiscent of Forster-type Energy Transfer – has not been recognized hitherto in the case of a molecular photodissociation.