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

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 ⁻ (H ₂ O) _{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 _N 2, 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 CH ₂ OH + I ⁻ is predicted to be the dominant product channel. This is in sharp contrast to conventional closed‐shell ion reactions where the S _N 2 pathway prevails. Adding water molecules to O ⁻ leads to two exothermic S _N 2 products, CH ₃ O and CH ₃ OH + I ⁻ . Remarkably, the formed CH ₃ O, CH ₂ OH, and CH ₃ OH 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.