Solvation protonated hydrates, water molecule

The H entry into a metal fiom an aqueous electrolyte is believed to involve the same surface-bulk transfer step as in the gas phase, but the preliminary adsorption step is a more complex process because more H sources are involved in aqueous solution, allowing more possible H surface reactions, and also because of the specificity of the electrolyte-metal interface. Whereas H adsorption in the gas phase occurs by dissociative adsorption of gaseous H2 on the free sites of a bare metallic surface, H adsorption in aqueous solution may occur either chemically by dissociation of dissolved H2 or electrochemically from solvated (hydrated) protons or water molecules it takes place on a hydrated surface and thus implies the displacement of adsorbed water molecules or specifically adsorbed ions and local reorganization of the double layer [20] competition with the adsorption of oxygen species formed from the dissociation of water may also occur [21-23], The adsorbed H layer is also in interaction with surrounding water molecules, i.e., it is hydrated [8c,24,25],... 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

In acid-catalyzed reactions, the distinction between single-species and complex catalysis is not always clear-cut. The actual catalyst is the solvated proton, H30+ in aqueous solution, and H20 (or a molecule of the nonaqueous solvent) may thus appear as a co-product in the first step and as a co-reactant in the step reconstituting the original solvated proton, possibly also in other additional steps, e.g., if the overall reaction is hydrolysis or hydration. Moreover, the acid added as catalyst may not be completely dissociated, and its dissociation equilibrium then affects the concentration of the solvated proton. At high concentrations this is true even for fairly strong acids such as sulfuric, particularly in solvents less polar than water. Such cases are better described as acid-base catalysis (see Section 8.2.1). 

The relative importance of each of these three reactions is likely a strong function of water content. In order for the bulk-like reaction (Eq. 16a) to take place, one must be at high degrees of hydration, where there is bulk-like water within the membrane. From electronic stracture calculation we are aware that a minimum of three water molecules is required for the dissociation of protons from the sulfonic acid end group, it is likely that the reaction in Eq. (16c) is important only at very low water contents. The reaction in which oxygen atoms of the sulfonate groups act as part of the solvation shell, (Eq. 16b) is likely relevant across a range of intermediate hydration levels. 

Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically [13-16], and also in a few alcohols [17,18]. Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the recombination processes are complex and depend on the solvent. The central problem in the theory of geminate ion recombination is to describe the relative motion and reaction between the two particles with opposite charges initially separated by a distance rg. In water, the primary products of solvent radiolysis are the hydrated electron e ", the hydroxyl radical OH and the hydronium cation H3O+ ... 

From the definition of acidity in solution as presented in Eq. (7.4), it is clear that in order to compute AG° one must know the proton solvation energy. However, from the previous discussion on the structural models for the hydrated proton one may anticipate some difficulties. For example, how many water molecules should be considered in the calculation of the proton solvation energy In other words How large should one take the H (H20) cluster One reasonable approach should be to examine the convergence of... 

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