Computational Studies of Intramolecular Hydrogen Atom Transfers in the β-Hydroxyethylperoxy and β-Hydroxyethoxy Radicals

Abstract

The beta-hydroxyethylperoxy (I) and beta-hydroxyethoxy (III) radicals are prototypes of species that can undergo hydrogen atom transfer across their intramolecular hydrogen bonds. These reactions may play an important role in both the atmosphere and in combustion systems. We have used density functional theory and composite electronic structure methods to predict the energetics of these reactions, RRKM/master equation simulations to model the kinetics of chemically activated I, and variational transition state theory (TST) to predict thermal rate constants for the 1,5-hydrogen shift in I (Reaction 1) and the 1,4-hydrogen shift in III (Reaction 2). Our multi-coefficient Gaussian-3 calculations predict that Reaction 1 has a barrier of 23.59 kcal/mol, and that Reaction 2 has a barrier of 22.71 kcal/mol. These predictions agree rather well with the MPW1K and BB1K density functional theory predictions but disagree with predictions based on B3LYP energies or geometries. Our RRKM/master equation simulations suggest that almost 50% of I undergoes a prompt hydrogen shift reaction at pressures up to 10 Torr, but the extent to which I is chemically activated is uncertain. For Reaction 1 at 298 K, the variational TST rate constant is approximately 30% lower than the conventional TST result, and the microcanonical optimized multidimensional tunneling (muOMT) method predicts that tunneling accelerates the reaction by a factor of 3. TST calculations on Reaction 2 reveal no variational effect and a 298 K muOMT transmission coefficient of 10(5). The Eckart method overestimates transmission coefficients for both reactions.