Microhydration Affects the Competing Product Channels of Oxygen Anion Radical Reacting With Methyl Iodide in Gas Phase

Abstract

Oxygen anion O⁻ is an important radical in atmospheric, combustion, and biological chemistry. How solvation affects its reaction with haloalkane remains elusive. Here, we report a quantum chemistry calculation on the potential energy profiles of microhydrated O⁻(H₂O)_{n = 0–3} anions and methyl iodide in the gas phase. The reactive nature of O⁻ leads to multiple product channels, including the halogen abstraction (XA), S_N2, proton transfer (PT), hydrogen atom transfer (HAT) pathways, and three post-HAT pathways, namely the oxygen insertion (OxIn), HAT&PT, and HAT&XA pathways. Higher hydration levels increase both barriers and reaction enthalpies across pathways, but the differential shifts reorder the barrier heights. The OxIn pathway leading to CH₂OH + I⁻ is predicted to be the dominant product channel. This is in sharp contrast to conventional closed-shell ion reactions where the S_N2 pathway prevails. Adding water molecules to O⁻ leads to two exothermic S_N2 products, CH₃O and CH₃OH + I⁻. Remarkably, the formed CH₃O, CH₂OH, and CH₃OH species, with or without water molecules and I⁻ ions, can convert to each other via proton and hydrogen atom transfer. This work elucidates how individual solvent molecules affect the competing radical ion–molecule pathways, offering mechanistic insights into radical-induced C─H bond activation in hydrocarbons.