Unimolecular Decay of Criegee Intermediates to OH Radical Products: Prompt and Thermal Decay Processes

Abstract

Alkene ozonolysis is a primary oxidation pathway for anthropogenic and biogenic alkenes emitted into the troposphere. It is also an important source of atmospheric hydroxyl (OH) radicals, often called the atmosphere's detergent. Alkene ozonolysis takes place through a highly exothermic reaction pathway with multiple intermediates and barriers prior to releasing the OH radical products. This Account focuses on a key reaction intermediate with a carbonyl oxide functional group (-COO), known as the Criegee intermediate, which is formed along with a carbonyl coproduct in alkene ozonolysis reactions. Under atmospheric conditions, the initially energized Criegee intermediates may promptly decay to OH products or be collisionally stabilized prior to thermal decay to OH radicals and other products. Alternatively, the stabilized Criegee intermediates may undergo bimolecular reactions with atmospheric species, including water vapor and sulfur dioxide, which can lead to nucleation and growth of aerosols. The dimethyl-substituted Criegee intermediate, (CH)COO, is utilized in this Account to showcase recent efforts to experimentally measure and theoretically predict the rates for prompt and thermal unimolecular decay processes of prototypical Criegee intermediates under laboratory and atmospheric conditions. The experimental laboratory studies utilize an alternative synthesis method to efficiently generate Criegee intermediates via the reaction of iodoalkyl radicals with O. Infrared excitation is then used to prepare the (CH)COO Criegee intermediates at specific energies in the vicinity of the transition state barrier or significantly below the barrier for 1,4-hydrogen transfer that leads to OH products. The rate of unimolecular decay is revealed through direct time-domain measurements of the appearance of OH products utilizing ultraviolet laser-induced fluorescence detection under collision-free conditions. Complementary high-level theoretical calculations are carried out to evaluate the transition state barrier and the energy-dependent unimolecular decay rates for (CH)COO using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which are in excellent accord with the experimental measurements. Quantum mechanical tunneling through the barrier, incorporated through Eckart and semiclassical transition state theory models, is shown to make a significant contribution to the unimolecular decay rates at energies in the vicinity of and much below the barrier. Master equation modeling is used to extend the energy-dependent unimolecular rates to thermal decay rates of (CH)COO under tropospheric conditions (high pressure limit), which agree well with recent laboratory measurements [ Smith et al. J. Phys. Chem. A 2016 , 120 , 4789 and Chhantyal-Pun et al. J. Phys. Chem. A 2017 , 121 , 4 - 15 ]. Again, tunneling is shown to enhance the thermal decay rate by orders of magnitude. The experimentally validated unimolecular rates are also utilized in modeling the prompt and thermal unimolecular decay of chemically activated (CH)COO formed upon ozon...

Mechanistic and kinetics investigations of oligomer formation from Criegee intermediate reactions with hydroxyalkyl hydroperoxides

“…1 Values in parentheses correspond to no consideration of the BSSE correction, values in brackets correspond to CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(2df,2p), and values in braces correspond to CCSD(T)/CBS//B3LYP/6-311+G(2df, 2p). troposphere, which generates HO-CH 2 OO-H as the dominant product (Kumar et al, 2014;Lewis et al, 2015). Figure 2 shows the schematic PES for the reaction of CH 2 OO with HO-CH 2 OO-H, with the optimized geometries of all stationary points on this PES given in Fig.…”

“…3.4 PES for the reaction of (CH 3 ) 2 COO with HO−(CH 3 ) 2 COO−H The dimethyl-substituted Criegee intermediate, (CH 3 ) 2 COO, is generated in the ozonolysis of 2,3-dimethyl-2-butene (Lester and Klippenstein, 2018;Drozd et al, 2017). Long et al (2018) proposed that the unimolecular decay of (CH 3 ) 2 COO is the predominant pathway above 240 K, whereas it can compete with the reaction (CH 3 ) 2 COO + SO 2 below 240 K. Drozd et al (2017) elucidated the fact that tunneling for both the thermal and prompt unimolecular decay of (CH 3 ) 2 COO is significant.…”

“…Alkenes are the most abundant volatile organic compounds (VOCs) in the atmosphere after methane and primarily originate from anthropogenic and biogenic sources (Lester and Klippenstein, 2018;Guenther et al, 2000). Alkene ozonolysis produces a carbonyl oxide (also called Criegee intermediates; CIs) and a carbonyl moiety (Donahue et al, 2011a;Aplincourt and Ruiz-López, 2000;Johnson and Marston, 2008;Welz et al, 2012;Criegee, 1975;Taatjes et al, 2013), which is thought to be an important source of radicals, whose subsequent reactions lead to the formation of hydroperoxides, organic peroxides, and secondary organic aerosol (SOA) (Donahue et al, 2011a;Becker et al, 1990;Kroll and Seinfeld, 2008;Hallquist et al, 2009;Tobias and Ziemann, 2001).…”

“…Li et al, 2014). The thermal unimolecular decay of vibrationally excited CIs is thought to be an important nonphotolytic source of atmospheric hydroxyl (OH) radicals, particularly in low light conditions, urban environments, and heavily forested areas (Lester and Klippenstein, 2018;Foreman et al, 2016;Kidwell et al, 2016;Green et al, 2017;Zhang and Zhang, 2002;Cremer et al, 2001;Anglada et al, 2002). The OH radical is one of the most powerful oxidants that participates in the atmospheric photochemical oxidation of VOCs (Gligorovski et al, 2015); it thus contributes to tropospheric ozone formation by being involved in the production of organic peroxy radicals (RO 2 ), which, in turn, facilitate the cycling of NO to NO 2 (Zhang and Zhan, 2002;Gligorovski et al, 2015).…”

The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and Its Relevance in Solar Geoengineering Models

“…It is noteworthy that another largely studied oxidant species in the troposphere are the Criegee Intermediates (CI). [41,42] They are carbonyl oxide RR'COO species, which are formed by reactions of unsatured organic compound with ozone. However, the role of the CI in the oxidation chemistry is not so relevant in the stratosphere, where there is a low concentration of organic compounds, and low pressures that hinder the stabilization of the CI.…”