Grotthuss Mechanism - Proton Transport Mechanism and Proton-hopping Mechanism

Proton Transport Mechanism and Proton-hopping 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:

1. Eigen to Zundel to Eigen (E–Z–E), on the basis of experimental NMR data,
2. 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.

Addition: 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) 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 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.