Grotthus

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. ... 

Grotthus, Th., Sur la decomposition de Teau et de corps qu elle tient en dissolution a Taide de Telectricite galvanique, Ann. Chim., 58, 54 (1805). 

Grotthus-Draper law radiation should be absorbed by the substance to perform a chemical change [199,200],... 

This principle is so simple that it has been given the title the first law of photochemistry, and was first expressed by Grotthus and Draper in the early 19th century. They stated it as the (hopefully) obvious truth Only light that is absorbed can have any photochemical effect . 

Grotthus chain theory phys chem An early theory used to explain the conductivity of an electrolyte. In which it was assumed that the cathode and anode attract hydrogen and oxygen respectively, and the molecules of the electrolyte are stretched out In chains between the electrodes, with decomposition occurring in molecules closest to the electrodes. grot-hus chan, the-3-re group CHEM I.Afamllyofelementswithsimilarchemical properties. 2. Acombina-tlon of bonded atoms that behave as a unit under certain conditions, for example. 

All of this suggests that the ion association explanation may be applied here to an essentially bimolecular (or associative) phenomenon. Considering the difference between hydroxide and any other reagent in water, apart from its basicity, one concludes that its mobility must play an important part. Whereas all the other reagents must be in a suitable position within the solvation shell before they can enter the complex, the hydroxide ion, by means of a Grotthus chain proton transfer, can be transmitted to any position where it is needed while the complex becomes activated. It can therefore be looked upon as an unsaturatable ion aggregate with hydroxide fully delocalized about the complex. Consequently, we do not observe any departure from the first-order dependence upon hydroxide concentration. This contribution to the reactivity will appear in the activation entropy rather than in the enthalpy term. 

The processes of photochemistry are the same for polymers and small molecules. The Grotthus-Draper law sfafes fhat no photochemical reactions can occur unless a photon of lighf is absorbed. This means, for example, thaf many commercial plastics transparent in the near UV can undergo photodegradation only as a result of the absorption of light by impurities. 

Just as in Faraday s law the amount of electrolysis is proportional to the current passed through an electrolytic cell, so in Grotthus law, the amount of photolysis is proportional to the absorbed energy of short enough wave-length to start chemical action. The departures from apparent proportionality can be explained in a similar way, the former by the phenomenon of residual currents or polarization, the latter, as shown by- P. Villard, by the existence of a limiting low exposure... 

The first law of photochemistry states Only that light which is absorbed by a system can cause chemipal change (Grotthus-Draper Law). 

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. 

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... 

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