Abstract
The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric and combustion chemistry. This work used a combined ab initio/transition state theory (TST) method to study the reaction mechanisms and kinetics for hydrogen abstraction reactions by OH radical on C4–C6 alkenes. The elementary abstraction reactions involved were divided into 10 reaction classes depending upon the type of carbon atoms in the reaction center. Geometry optimization was performed by using DFT M06-2X functional with the 6-311+G(d,p) basis set. The energies were computed at the high-level CCSD(T)/CBS level of theory. Linear correlation for the computed reaction barriers and enthalpies between M06-2X/6-311+G(d,p) and CCSD(T)/CBS methods were found. It was shown that the C=C double bond in long alkenes not only affected the related allylic reaction site, but also exhibited a large influence on the reaction sites nearby the allylic site due to steric effects. TST in conjunction with tunneling effects were employed to determine high-pressure limit rate constants of these abstraction reactions and the computed overall rate constants were compared with the available literature data.
Highlights
The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric chemistry and combustion chemistry [1,2]
It was shown that the location of C=C double bond in long alkenes exhibited some unexpected trends among different structural alkenes [10], indicating that the C=C double bond may induce steric effects compared with small alkenes
Considering all possible abstraction reaction sites on these alkenes, there are 47 elementary abstraction reactions. These reactions were divided into 10 reaction classes based on the type of carbon atoms in the reaction centers and the RC-transition state theory (TST) [13,14]
Summary
The reaction of alkenes with hydroxyl (OH) radical is of great importance to atmospheric chemistry and combustion chemistry [1,2]. Detailed reaction mechanisms for modeling the combustion of butene isomers have been validated, and the success of the reaction classes are adopted for large alkenes [2,7,8]. It was shown that the location of C=C double bond in long alkenes exhibited some unexpected trends among different structural alkenes [10], indicating that the C=C double bond may induce steric effects compared with small alkenes This unexpected trend suggested that the rate constants of reaction classes for small alkenes may be improper for large alkenes. To continuously improve the accuracy of detailed reaction mechanisms for atmospheric and combustion chemistry, systematic theoretical studies of these reaction classes for long alkenes are crucial. Conventional transition state theory (TST), together with tunneling effects, are used to compute the high-pressure limit rate constants in the temperature range of 500–2500 K and the overall rate constants are compared with literature data
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