Grotthuss mechanism

The Grotthuss mechanism is now a general name for the proton-hopping mechanism. In liquid water the solvation of the excess proton is idealized by two forms: the H9O4+ (Eigen cation) or H5O2+ (Zundel cation). While the transport mechanism is believed to involve the inter-conversion between these two solvation structures, the details of the hopping and transport mechanism is still debated. Currently there are two plausible mechanisms: • Eigen to Zundel to Eigen (E–Z–E), on the basis of experimental NMR data, • Zundel to Zundel (Z–Z), on the basis of molecular dynamics simulation. The calculated energetics of the hydronium solvation shells were reported in 2007 and it was suggested that the activation energies of the two proposed mechanisms do not agree with their calculated hydrogen bond strengths, but mechanism 1 might be the better candidate of the two. By use of conditional and time-dependent radial distribution functions (RDF), it was shown that the hydronium RDF can be decomposed into contributions from two distinct structures, Eigen and Zundel. The first peak in g(r) (the RDF) of the Eigen structure is similar to the equilibrium, standard RDF, only slightly more ordered, while the first peak of the Zundel structure is actually split into two peaks. The actual proton transfer (PT) event was then traced (after synchronizing all PT events so that t=0 is the actual event time), revealing that the hydronium indeed starts from an Eigen state, and quickly transforms into the Zundel state as the proton is being transferred, with the first peak of g(r) splitting into two. For a number of important gas phase reactions, like the hydration of carbon dioxide, a Grotthuss-like mechanism involving concerted proton hopping over several water molecules at the same time has been shown to describe the reaction kinetics. This Grotthuss-like concerted proton transfer seems to be especially important for atmospheric chemistry reactions, like the hydration of sulfur oxides, and other reactions important for ozone depletion.{{ cite journal|title=Toward elimination of discrepancies between theory and experiment: The gas-phase reaction of N2O5 with H2O|first=Andreas F.|last=Voegele|author2=Tautermann, Christofer S.|author3=Loerting, Thomas|author4=Liedl, Klaus R.|journal=Physical Chemistry Chemical Physics|year=2003|volume=5|issue=3|pages=487–495|doi=10.1039/b208936j cite journal|title=Reactions of HOCl + HCl + nH2O and HOCl + HBr + nH2O|first=Andreas F.|last=Voegele|author2=Tautermann, Christofer S.|author3=Loerting, Thomas|author4=Liedl, Klaus R.|journal=Journal of Physical Chemistry A|year=2002|volume=106|issue=34|pages=7850–7857|doi=10.1021/jp0255583 cite journal|title=Reactions of HOBr+ HCl+ nH2O and HOBr+ HBr+ nH2O|first=Andreas F.|last=Voegele|author2=Tautermann, Christofer S.|author3=Loerting, Thomas|author4=Liedl, Klaus R.|journal=Chemical Physics Letters|year=2003|volume=372|issue=3–4|pages=569–576|doi=10.1016/S0009-2614(03)00447-0 ==The anomalous diffusion of protons==

The Grotthuss mechanism, along with the relative lightness and small size (ionic radius) of the proton, explains the unusually high diffusion rate of the proton in an electric field, relative to that of other common cations whose movement is due simply to acceleration by the field. Random thermal motion opposes the movement of both protons and other cations. Quantum tunnelling becomes more probable the smaller the mass of the cation is, and the proton is the lightest possible stable cation. Thus there is a minor effect from quantum tunnelling also, although it dominates at low temperatures only. ==Possible alternative mechanism==

Some evidence from theoretical calculations, supported by recent X-ray absorption spectroscopy findings, has suggested an alternative mechanism in which the proton is attached to a "train" of three water molecules as it moves through the liquid. ==References==