Nature Abstract

Abstract

The excess proton in water has many anomalous properties, most notably its unexpectedly high mobility. Such unusual characteristics, together with the difficulty of extracting unambiguous information from experiment, have fueled a century-long scientific debate that remains unresolved. Starting with de Grotthuss, who nearly 200 years ago ingeniously introduced the concept of ``structural diffusion,'' various mechanistic explanations, such as thermally induced hopping, proton tunneling, or solvation effects have been put forward. The more recent literature has been mainly polarized around two seemingly opposing structural models for the hydrated proton. Eigen proposed the formation of an H₉ O₄⁺ complex, wherein an H₃O⁺ core is strongly hydrogen bonded to three H₂ molecules. Zundel, in contrast, supported the notion of an H₅O₂⁺ complex, in which the proton is shared between two H₂ molecules. In this study, a powerful computer simulation technique is applied, which includes time-independent equilibrium thermal and quantum fluctuations of all nuclei and determines internuclear interactions from the electronic structure. The key findings are: (I) The hydrated proton forms a fluxional defect, with H₉O₄⁺ and H₅O₂⁺II) This defect can become deloclized over several hydrogen bonds due to quantum fluctuations. (III) Solvent polarization induces a small transfer barrier, which is washed out by zero-point motion. This can be classified as a ``low-barrier hydrogen bond,'' where tunneling is negligible and the simplest of transition state concepts to not apply. (IV) The rate of structural diffusion is determined by thermally induced hydrogen-bond breaking in the second solvation shell.