Grotthuss mechanism
Encyclopedia
The Grotthuss mechanism is the mechanism by which an 'excess' proton
or protonic defect diffuses through the hydrogen bond
network of water molecules or other hydrogen-bonded liquids through the formation or cleavage of covalent bond
s.
In his 1806 publication “Theory of decomposition of liquids by electrical currents”, Theodor Grotthuss proposed a theory of water conductivity. Grotthuss envisioned the electrolytic reaction as a sort of ‘bucket line’ where each oxygen atom simultaneously passes and receives a single hydrogen atom.
It was an astonishing theory to propose at the time, since the water molecule was thought to be OH not H2O and the existence of ions was not fully understood.
On its 200th anniversary, his article was reviewed by Cukierman.
Although Grotthuss was using an incorrect empirical formula
of water, his description of the passing of protons through the cooperation of neighboring water molecules proved to be remarkably fortuitous.
Currently there are two plausible mechanisms:
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.
of the proton relative to other the typical ionic diffusion of other cations (Table 1) which is due simply to random thermal motion i.e. Brownian motion
. 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.
Table 1
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
or protonic defect diffuses through the hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
network of water molecules or other hydrogen-bonded liquids through the formation or cleavage of covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s.
In his 1806 publication “Theory of decomposition of liquids by electrical currents”, Theodor Grotthuss proposed a theory of water conductivity. Grotthuss envisioned the electrolytic reaction as a sort of ‘bucket line’ where each oxygen atom simultaneously passes and receives a single hydrogen atom.
It was an astonishing theory to propose at the time, since the water molecule was thought to be OH not H2O and the existence of ions was not fully understood.
On its 200th anniversary, his article was reviewed by Cukierman.
- OH
OH
OH ? O-
HO
HO 
H+
Although Grotthuss was using an incorrect empirical formula
Empirical formula
In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
of water, his description of the passing of protons through the cooperation of neighboring water molecules proved to be remarkably fortuitous.
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:
- 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 dynamicsMolecular dynamicsMolecular dynamics is a computer simulation of physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a period of time, giving a view of the motion of the atoms...
simulation.
The calculated energetics of the hydronium
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
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
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the 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.
The anomalous diffusion of protons
The Grotthuss mechanism, along with the relative lightness and small size of the proton, explains the unusually high diffusionDiffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
of the proton relative to other the typical ionic diffusion of other cations (Table 1) which is due simply to random thermal motion i.e. Brownian motion
Brownian motion
Brownian motion or pedesis is the presumably random drifting of particles suspended in a fluid or the mathematical model used to describe such random movements, which is often called a particle theory.The mathematical model of Brownian motion has several real-world applications...
. Quantum tunnelling
Quantum tunnelling
Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
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.
Table 1
| Cation | Mobility / cm2 V−1 s−1 |
| NH4+ | 0.763×10−3 |
| Na+ | 0.519×10−3 |
| K+ | 0.762×10−3 |
| H+ | 3.62×10−3 |