From a localized H3O radical to a delocalized H3O+⋯e−solvent-separated pair by sequential hydration

Abstract

The impact of microhydration on the electronic structure and reactivity of the H₃O moiety is investigated by ab initio calculations. In the gas phase, H₃O is a radical with spin density localized on its hydrogen end, which is only kinetically stable and readily decomposes into a water molecule and a hydrogen atom. When solvated by a single water molecule, H₃O preserves to a large extent its radical character, however, two water molecules are already capable to shift most of the spin density to the solvent. With three solvating water molecules this shift is practically completed and the system is best described as a solvent-separated pair of a hydronium cation and a hydrated electron. The electronic structure of this system and its proton transfer reactivity leading to formation of a hydrogen atom already resemble those of a proton–electron pair in bulk water.