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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. 

Criegee intermediates (CIs) are of great significance to Earth’s troposphere – implicated in altering the tropospheric oxidation cycle and in forming low volatility products that typically condense to form secondary organic aerosols (SOAs). As such, their chemistry has attracted vast attention in recent years. In particular, the unimolecular decay of thermal and vibrationally-excited CIs has been the focus of several experimental and computational studies, and it now recognized that CIs undergo unimolecular decay to form OH radicals. In this contribution we reveal insight into the chemistry of CIs by highlighting the hitherto neglected multi-state contribution to the ground state unimolecular decay dynamics of the Criegee intermediate products. The two key intermediates of present focus are dioxirane and vinylhydroperoxide – known to be active intermediates that mediate the unimolecular decay of CH2OO and CH3CHOO, respectively. In both cases the unimolecular decay path encounters conical intersections, which may play a pivotal role in the ensuing dynamics. This hitherto unrecognized phenomenon may be vital in the way in which the reactivity of CIs are modelled and is likely to affect the ensuing dynamics associated with the unimolecular decay of a given CI.