Eigen complex

Eigen complex — In 1954, M. Eigen (Nobel Prize in Chemistry 1967) proposed from thermodynamic mea-... 

Hydronium (HiO+) ion water, - Eigen complex, -> proton, -> Zundel complex... 

Structural diffusion is provided by various complexes bare hydronium ion, Eigen complexes, and - Zundel complexes. Structural diffusion of bare hydronium ion and Eigen complexes occurs by proton hops between two water molecules. Two or more protons and several water molecules are involved in the structural diffusion of Zundel complexes. The contribution of mechanisms to the overall mobility depends on the temperature. Eigen and Zundel complexes prevail at room temperature, whereas bare hydronium ions dominate at high temperatures. Excess proton mobility of water has Arrhenius-like (-> Arrhenius equation) temperature dependence with the - activation energy about 0.11 eV. 

If the hydroxonium ions migrated only hydrodynam-ically A° 85 cm2 would be expected, which value can easily be derived by using the -> Stokes law and the known values of the -+ self-diffusion coefficient of water, the radius of the ion and the - viscosity. (See also proton, - Eigen complexes, - Zundel complexes, charge transfer, - Dahms-Ruff theory.)... 

See also -> Eigen complex, -> water, - prototropic charge transport, -> Grotthuss mechanism. 

Figure 10.4. Model systems to study proton transport in slab pores. Top rigid slab with embedded 803 head groups. Bottom slab boundary in a Nafion side-chain model. Proton complexes are coloured according to internal structure blue H30 -like (Eigen complexes), green H502 -like (Zundel complexes).

The system, for which proton-transfer reactions are investigated best, is very simple and complex at the same time liquid water. Numerous theoretical studies - mainly based on different types of molecular-dynamics simulations - have been published in the last decades that try to reveal the secrets behind the proton-transport properties of water. Generally, these studies make use of an excess proton which might be solvated in two different ways either as a so-called Eigen ion (or Eigen complex) H9O/ or as a so-called Zundel ion (Zundel complex) H502i In the first, the excess proton is complexed by... 

Fig. 1 In the Eigen ion H9O/ (shown on the left) the positive charge is formally located at the central hydronium ion, which is the center of the Eigen complex. In the Zundel ion H5O2 the proton is complexed by only two water molecules (shown on the right). Hydrogen bonds are depicted as dashed lines.

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