Grotthus type

Horne has studied the kinetics of exchange in aqueous perchlorate media at temperatures down to —78 °C by the isotopic method ( Fe) and dipyridyl separation. The same rate law in these ice media as in aqueous solution was observed, although the acid dependence was small. Horne concluded that the same exchange mechanism occurs in solid and liquid solvent. Evidence for a Grotthus-type mechanism has been summarised. ... 

The broad outline of the mechanism of catalysis of ester hydrolysis by hydroxide ion is not in doubt. The reaction is well known to involve acyl-oxygen cleavage, and seems invariably to be of the second order, being first order in both ester and hydroxide anion. General base catalysis in the usual sense is not a possibility, the partial removal of a proton from water cannot generate a species more reactive than hydroxide ion, so direct nucleophilic attack must be involved. (However, if it is accepted that the high ionic nobility of the hydroxide ion in water is explained by a Grotthus-type mechanism... 

Ab initio calculations at the CIS, CASSCF and MRMP2 (second-order multireference Mpller-Plesset perturbation theory) levels have revealed that the reaction proceeds by H atom transfer via a series of Grotthus-type translocation steps [15]. Figure 3.35 gives an overview of these computational results. 

Delocalized H+ counterions are denoted with a subscript f, while H+ species which transfer between tbe film and bulk solution during the redox reaction are identified by the subscripts s. Thus, for each electron injected into the film there is a simultaneous transfer of one proton, i.e. Hs +, from the solution bulk into the hydrous oxide material, while at the same time there is a transfer locally of 1.5 protons into the ligand sphere of the central metal ion for each electron added to the latter. Proton transport is likely to occur via a Grotthus-type mechanism in these films and is much more likely than OH movement as suggested by other authors [144]. 

Most work on fast proton-transfer reactions has been carried out in water or hydroxylic solvents, in which transmission of protons through the solvent by a Grotthus-type process is an important feature of the mechanism. It is thus of interest to study these reactions in aprotic solvents, in which such proton transmission is impossible. 

The Grotthus-type diffusion mechanism was used to explain the proton diffusion process in imidazole chains [160]. The protonic defects cause local rather than long-range disorder by forming (... Him - (HIniH) - ImH. ..) and (ImH) configurations. At 117 °C, the proton-transfer step is fast, with a time scale ofO.lps the reorientation step is rate-determining and the corresponding time scale is approximately 30 ps. 

Ability to transport protons between the charged sites with a Grotthus type mechanism, in the presence of water. 

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