Transition states for H atom transfer reactions in the CH2CH2OH radical: The effect of a water molecule

Abstract

H atom transfer reactions in the 2-hydroxyethyl radical (CH₂CH₂OH), formed by addition of an OH radical to ethylene, have been computationally evaluated both in the gas-phase and in the presence of a single water molecule, and the corresponding transition state (TS) structures determined and discussed. The considered isomerization reactions of CH₂CH₂OH include a [1,3] hydrogen shift to form CH₃CH₂O (reaction (1)) and a [1,2] hydrogen shift to form CH₃CHOH (reaction (2)). The presence of a single water molecule in two-step processes was found to significantly reduce the energy barrier for these rearrangements by providing alternative mechanisms that avoid the strained TSs found in the unimolecular gas phase and in the single-step water-assisted synchronous reactions. In addition, the effect of one water molecule in the single-step [1,3] hydrogen rearrangement of vinyl alcohol (reaction (3)), the product of one hydrogen atom abstraction from CH₂CH₂OH, has also been considered and discussed. Unlike reactions (1) and (2), where the water-assisted single-step processes yielded energy barriers higher than the gas-phase reactions, the TS system for reaction (3) has an energy barrier well below the energy barrier for the single-step gas-phase mechanism. While the computed activation energies for the lower activation energy reaction paths are still significant and there may be other reactions for the CH₂CH₂OH radical that are more energetically favorable, this study suggests that gas-phase chemistry can be significantly altered in the unique solvated environment of a gas-phase cluster.