title: On the recombination of hydronium and hydroxide ions in water.
abstract: The recombination of hydronium and hydroxide ions following water ionization is one of the most fundamental processes determining the pH of water. The neutralization step once the solvated ions are in close proximity is phenomenologically understood to be fast, but the molecular mechanism has not been directly probed by experiments. We elucidate the mechanism of recombination in liquid water with ab initio molecular dynamics simulations, and it emerges as quite different from the conventional view of the Grotthuss mechanism. The neutralization event involves a collective compression of the water-wire bridging the ions, which occurs in approximately 0.5 ps, triggering a concerted triple jump of the protons. This process leaves the neutralized hydroxide in a hypercoordinated state, with the implications that enhanced collective compressions of several water molecules around similarly hypercoordinated states are likely to serve as nucleation events for the autoionization of liquid water.
authors.affiliation: Department of Chemistry and Applied Biosciences, Eidgenössiche Technische Hochschule Zurich and Università della Svizzera Italiana, via G. Buffi 13, CH6900 Lugano, Switzerland. ali.hassanali@phys.chem.ethz.ch
authors.forename: Ali
authors.initials: A
authors.lastname: Hassanali
authors.forename: Meher K
authors.initials: MK
authors.lastname: Prakash
authors.forename: Hagai
authors.initials: H
authors.lastname: Eshet
authors.forename: Michele
authors.initials: M
authors.lastname: Parrinello
mesh_terms: D002621:Chemistry / Q000379:methods; D003201:Computers; D049452:Green Fluorescent Proteins / Q000737:chemistry; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000737:chemistry*; D007477:Ions; D008958:Models, Molecular; D008968:Molecular Conformation; D056004:Molecular Dynamics Simulation; D009861:Onium Compounds / Q000737:chemistry*; D011522:Protons; D012984:Software; D014867:Water / Q000737:chemistry*
chemical_list: D006878:Hydroxides; D007477:Ions; D009861:Onium Compounds; D011522:Protons; D014867:Water; D049452:Green Fluorescent Proteins; C027727:hydronium ion; C031356:hydroxide ion