On the recombination of hydronium and hydroxide ions in water

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 be...

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Bibliographic Details
Published in:Proceedings of the National Academy of Sciences - PNAS 2011-12, Vol.108 (51), p.20410-20415
Main Authors: Hassanali, Ali, Prakash, Meher K, Eshet, Hagai, Parrinello, Michele
Format: Article
Language:English
Subjects:
Autoionization
Chemistry - methods
Compression
Computers
Green Fluorescent Proteins - chemistry
Hydrogen Bonding
Hydrogen bonds
Hydrogen-Ion Concentration
Hydroxides
Hydroxides - chemistry
Ionization
Ions
Liquids
Models, Molecular
Molecular Conformation
molecular dynamics
Molecular Dynamics Simulation
Molecular modelling
Molecules
neutralization
Nucleation
Onium Compounds - chemistry
pH effects
Physical Sciences
Protons
Recombination
Simulation
Software
Solvents
Trajectories
Water
Water - chemistry
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Summary: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.
ISSN:0027-8424
1091-6490
DOI:10.1073/pnas.1112486108