Isomerization and Decomposition Reactions of Primary Alkoxy Radicals Derived from Oxygenated Solvents

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This paper presents quantum chemical studies of the unimolecular isomerization (1,5 H-shift) and decomposition (β C−C scission) reactions of a series of six oxygenated alkoxy radicals and 1-butoxy radical. The goal is to better understand the effects of ether, carbonyl, and ester functional groups on the reactivity of alkoxy radicals relevant to atmospheric chemistry. We also report the first quantum chemical study of the α-ester rearrangement: CH3C(O)OCH2O• → CH3C(O)OH + O. The six radicals are CH3OC(O)CH2O•, CH3C(O)OCH2O•, CH3CH2C(O)CH2O•, CH3C(O)CH2CH2O•, CH3OCH2CH2O•, and CH3CH2OCH2O•. All these radicals are, like 1-butoxy, primary alkoxy radicals with a methyl group δ− to the radical center. Calculations are carried out at the B3LYP/6-31G(d,p) and /6-311G(2df,2p) level of theory for all reactions. In addition, the G2(MP2,SVP) level of theory is used to study all isomerization reactions and selected decomposition reactions. Substituent effects on structure are very large and certainly significant for the fate of these radicals in the atmosphere; fates depend as much or more on the position of functional groups as their identity. We also make a preliminary examination of the effects of tunneling on the computed rate constants for the α-ester rearrangement and the 1,5 H-shift reaction of 1-butoxy. At 298 K, we find tunneling to increase the rate of the 1,5 H-shift reaction by a factor of 19−210, and the rate of the α-ester rearrangement by a factor of 1.3 to 6. The effects of tunneling have been neglected in most previous computational studies of the 1,5 H-shift reaction.