Elucidating atmospheric oxidation mechanisms is necessary for estimating the lifetimes of atmospheric species and understanding secondary organic aerosol formation and atmospheric oxidation capacity. We report an unexpectedly fast mechanistic pathway for the unimolecular reactions of large stabilized Criegee intermediates, which involves the formation of bicyclic structures from large Criegee intermediates containing an aldehyde group. The barrier heights of the mechanistic pathways are unexpectedly low – about 2–3 kcal/mol – and are at least 10 kcal/mol lower than those of hydrogen shift processes in large syn Criegee intermediates; and the calculated rate constants show that the mechanistic pathways are 10⁵-10⁹ times faster than those of the corresponding hydrogen shift processes. The present findings indicate that analogous low-energy pathways can now also be expected in other large Criegee intermediates and that oxidative capacity of some Criegee intermediates is smaller than would be predicted by existing models.

Criegee intermediates (i.e., carbonyl oxides with two radical sites) are known to be important atmospheric reagents; however, our knowledge of their reaction kinetics is still limited. Although experimental methods have been developed to directly measure the reaction rate constants of stabilized Criegee intermediates, the experimental results cover limited temperature ranges and do not completely agree well with one another. Here we investigate the unimolecular reaction of acetone oxide [(CH₃)₂COO] and its bimolecular reaction with H₂O to obtain rate constants with quantitative accuracy comparable to experimental accuracy. We do this by using CCSDT(Q)/CBS//CCSD(T)-F12a/DZ-F12 benchmark results to select and validate exchange-correlation functionals, which are then used for direct dynamics calculations by variational transition state theory with small-curvature tunneling and torsional and high-frequency anharmonicity. We find that tunneling is very significant in the unimolecular reaction of (CH₃)₂COO and its bimolecular reaction with H₂O. We show that the atmospheric lifetimes of (CH₃)₂COO depend on temperature and that the unimolecular reaction of (CH₃)₂COO is the dominant decay mode above 240 K, while the (CH₃)₂COO + SO₂ reaction can compete with the corresponding unimolecular reaction below 240 K when the SO₂ concentration is 9 × 10¹⁰ molecules per cubic centimeter. We also find that experimental results may not be sufficiently accurate for the unimolecular reaction of (CH₃)₂COO above 310 K. Not only does the present investigation provide insights into the decay of (CH₃)₂COO in the atmosphere, but it also provides an illustration of how to use theoretical methods to predict quantitative rate constants of medium-sized Criegee intermediates. Significance Obtaining rate constants and tropospheric lifetimes of Criegee intermediates is of critical significance for atmospheric modeling. We report calculations that can produce quantitative rate constants by variational transition state theory with corner-cutting tunneling based on implicit potential energy surfaces calibrated against high-level calculations. Demonstration of this capability is particularly significant because the calculations can fully cover the atmospheric conditions, which is hard to do by experiment. The present results also show the usefulness of the M11-L and MN15-L density functionals in describing this nominally multireference system. Modern quantum chemistry is now accurate enough to be used for atmospheric chemistry, and the present methods and strategies can be also be used to study other atmospherically important reactions of other medium-sized molecules.