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Atmospheric Chemistry of Criegee Intermediates. Unimolecular Reactions and Reactions with Water Bo Long, Junwei Lucas Bao, and Donald G. Truhlar J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b08655 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016
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Journal of the American Chemical Society revised for JACS, Sept. 28, 2016
Atmospheric Chemistry of Criegee Intermediates. Unimolecular Reactions and Reactions with Water Bo Long, a , b Junwei Lucas Bao,b and Donald G. Truhlarb ∗ a
College of Information Engineering, Guizhou Minzu University, Guiyang, 550025, China b
Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA
Abstract: Criegee intermediates are produced in the ozonolysis of unsaturated hydrocarbons in the troposphere, and understanding their fate is a prerequisite to modeling climate-controlling atmospheric aerosol formation. Although some experimental and theoretical rate data are available, they are incomplete and partially inconsistent, and they do not cover the tropospheric temperature range. Here we report quantum chemical rate constants for the reactions of stabilized formaldehyde oxide (CH2OO), acetaldehyde oxide (syn-CH3CHOO, and anti-CH3CHOO) with H2O and for their unimolecular reactions. Our results are obtained by combining post-CCSD(T) electronic structure benchmarks, validated density functional theory potential energy surfaces, and multi-path variational transition state theory with multidimensional tunneling, coupled-torsions anharmonicity, and high-frequency anharmonicity. We consider two different types of reaction mechanisms for the bimolecular reactions, namely (i) addition-coupled hydrogen transfer and (ii) double hydrogen atom transfer (DHAT). First we show that the MN15-L exchange-correlation functional has kJ/mol accuracy for the CH2OO + H2O and syn-CH3CHOO + H2O reactions. Then we show that, due to tunneling, the DHAT mechanism is especially important in the syn-CH3CHOO + H2O reaction. We show that the dominant pathways for reactions of Criegee intermediates depend on altitude. The results we obtain eliminate the discrepancy between experiment and theory under those conditions where experimental results are available, and we make predictions for the full range of temperatures and pressures encountered in the troposphere and stratosphere. The present results are an important cog in clarifying the atmospheric fate and oxidation processes of Criegee intermediates, and also they show how theoretical methods can provide reliable rate data for complex ∗
Corresponding author:
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atmospheric processes. 1. Introduction Criegee intermediates1 are carbonyl oxides formed in the ozonolysis of unsaturated hydrocarbons; their atmospheric fate plays a critical role in determining the oxidative efficacy of the atmosphere and its capacity for free radical generation and secondary organic aerosol formation.2,3,4,5 In the atmosphere, Criegee intermediates undergo bimolecular reactions with atmospheric molecules such as H2O, SO2, and HCOOH and unimolecular isomerization and decomposition to yield OH.2 Inferences about the roles of Criegee intermediates were originally based on indirectly estimated rate constants, and there has been no consensus on whether the dominant reaction of stabilized Criegee intermediates in the troposphere is with water molecules.2,6 Furthermore, the unimolecular reactions of Criegee intermediates must be considered when estimating the atmospheric fate of Criegee intermediates and the production of atmosphere-cleansing OH.7,8,9 In addition to their role as sources of OH, exploring the reactions of Criegee intermediates with water and their unimolecular reactions is especially critical because of the abundance of water in the atmosphere and because the bimolecular reactions of stabilized Criegee intermediates in the atmosphere are key processes in the formation of aerosols,10,11 which scatter sunlight and affect earth's radiative balance.8,9 In particular, the oxidation of sulfur dioxide via stabilized Criegee intermediates is an important source of sulfuric acid,12,13,14,15,16,17,18 which plays an important role in aerosol nucleation. Given the importance of Criegee intermediates, it is very exciting that new experimental methods have been developed to directly measure bimolecular rate constants for reactions of stabilized Criegee intermediates with atmospheric compounds.19,20,21,22,23,24,25 Elucidating the kinetics of bimolecular reactions between Criegee intermediates and atmospheric molecules and the kinetics of their unimolecular reactions are essential for fully estimating the atmospheric fates of Criegee intermediates and for making chemical models of the atmosphere more reliable. However, direct rate data for the reactions of Criegee intermediates with H2O are still scarce and are partially inconsistent. Moreover, the rate constants of their unimolecular reactions are even more limited and complicated because they depend on temperature and
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pressure. Throughout this article, we will give rate constants in units of cm3 molecule−1 s−1 and s−1 for bimolecular reactions and unimolecular reactions, respectively. Welz et al.20 estimated the CH2OO + H2O reaction rate as < 4 × 10−15, Chao et al.26 estimated
