Proton Hopping

Proton hopping - a.k.a. the Grotthuss Mechanism has been said to be "the fastest known chemical reaction". The effects of which were first noted by Theodor Grotthuss in 1805, who was investigating the decomposition of water under electrolysis.

In the process, an 'excess' proton (or its opposite, a proton-deficit) 'tunnels' through the hydrogen bond network of water molecules (or other hydrogen-bonded liquids). Leading to the formation, and simultaneous breaking, of covalent bonds with neighbouring molecules. On a local scale, this can occur in less than 2 picoseconds.

It has not yet been accurately explained.

Although well studied for over 200 years, excess proton solvation and transport remains to this day mysterious, surprising, and perhaps even misunderstood."

Source : J Phys Chem B. 111(17): 4300–4314.

There are currently two hypotheses :

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.

See: Wikipedia

Editor's clarification : The so-called 'Eigen' cation is H₉O₄⁺ and the 'Zundel' ion is H₅O₂⁺

It's also likely that some quantum physics effects are operating :

Despite its widespread importance, the quantum character of proton transfer has not been satisfactorily elucidated."

Source : Science, Vol. 275, Issue 5301, pp. 817-820

Note: Proton hopping can occur in any solution or structure where the water content exceeds 20%. Therefore it's probably in operation in the majority of biological systems.

Example : It has recently been suggested that proton hopping may be an important mechanism for the rapid operation of electro-chemical nerve impulses. Proton Hopping as the Nerve Conduction Message Current Computer Aided Drug Design, 2016;12(4): 255-258.

Also see: Water clustersplugin-autotooltip__plain plugin-autotooltip_bigWater clusters

Water has a long list of 'anomalous' physical (and chemical) properties (list here, archived from London Southbank University). Most are now fairly well understood - but there is currently no overall model to explain all of its unusual properties

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