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.
|Original language||English (US)|
|Number of pages||6|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|State||Published - Jun 12 2018|
Bibliographical noteFunding Information:
ACKNOWLEDGMENTS. We thank Prof. Kirk A. Peterson from Washington State University for information about CCSD(T)-F12b/cc-pV5Z-F12 calculations and Prof. Jan M. L. Martin from Weizmann Institute of Science for helpful discussion of MW2-F12 extrapolation. This work was supported in part by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contracts DE-AC02-06CH11357 and DE-SC0015997; in part by the National Natural Science Foundation of China Grant 41775125; and by the Science and Technology Foundation of Guizhou Province, China Grants 350 and 1080. Computations were performed using resources of Minnesota Supercomputing Institute and the National Energy Research Scientific Computing Center.
- Atmospheric chemistry
- Density functional theory
- Direct dynamics