The Rattling and Rotation Behaviours of the Hydrated Excess Proton in Water

The rattling and rotation behaviours of the hydrated excess proton (H⁺) in water are investigated using the density functional theory–quantum chemical cluster model (DFT-CM) method. The rattling pathways for the target proton ^*H⁺ between two adjacent O atoms in the form of Zundel configurations with symmetrical solvation environments are obtained. The zero-point contribution reduces the reaction energy barrier and enables the rattling to occur spontaneously at room temperature. The rotational behaviour of ^*H⁺ in the form of ^*H⁺·H₂O^* is found. Upon ^*H⁺·H₂O^* rotation, ^*H⁺ changes its position accompanied by concerted displacement of surrounding solvent water molecules and the breaking and formation of hydrogen bonds. The “^*H⁺·H₂O^* rotating migration mechanism” is proposed for the proton transfer mechanism in water — the same ^*H⁺ migrates via ^*H⁺·H₂O^* rotation through void in solvent water, rather than different protons hopping along water hydrogen bond chains as known as the Grotthuss mechanism.