Hydration spheres, reorganization

In any case, the hydration of Tl is very weak as compared to Tl q (see later discussion), in agreement with the hydration enthalpies for these ions 335 and 4117 kJ mole S respectively (15). This difference is probably an important reason for the slow electron exchange between the two thallium ions because of the necessarily large reorganization energy of their hydration spheres (cf Section IV,B). 

The elementary process is the transfer of an electron from an electron donor orbital on the reductant (e.g., Fe " ") to the acceptor orbital of the oxidant (e.g., Fe +). The rate of the electron transfer is very high it takes place within 10 seconds however, the reorganization of the bonds may range from 10 to 10 seconds, the reorientation of the solvent dipoles (e.g., water molecules in the hydration sphere) needs seconds and the duration... 

The reservations made in section 7.1 while discussing a similar problem and connected with the possible peculiarities of the reorganization of the first hydrate sphere do not play a significant role in this case, since the reacting ions are deprived, to a large extent (and sometimes even completely), of a direct contact with water molecules. 

A. 14.5 The Born approximation assumes water is a continuum and that the rigid spheres with a radius equal to the crystallographic radius, but neither of these is correct when an ion is dissolved in water. Water reorganizes around the ion adding a hydration sheath and increasing its effective radius. Including these factors, as well as some others, would make the theory more accurate. 

The inner-sphere reductions of [Co(NH3)5(SCONHR)] and [Co(NH3)5 (OCSNHR)] by Gr involve attack at the remote oxygen and sulfur atoms, respectively, with a subsequent isomerization of the 0-bonded ehromium(III) product in the former reaction. The unusually rapid reactions of the S-bonded cobalt(III) complexes are attributed to a structural tran -effect on the Co—N bond length, reducing the reorganization energy needed to form the transition state. A kinetic study of the Cr reduction of [Co(NH3)5(pyruvate)] reveals that the rate of reduction is dependent on the nature of pyruvate ligand, with the keto form about 400 times as reactive as the hydrated form. An inner-sphere mechanism has be postulated for the Cr reduction of [Co(NH3)5(pyridine N-oxide)] on the basis of the rate and activation parameters. The outer-sphere Cr reduction of [Co(sepulchrate)] is catalyzed by halide ions, with the ion-pair formation constants for [Co(sep), estimated to be 5.5, 2.3, and 1.7 M" for Cl", Br", and I", respectively. ... 

Another way to explain the value of E for an electrode reaction is to retain the large effective radius of an HaO ion but to take into account, as was shown by Kharkats[232], the possibility of increasing the reorganization energy by introducing an ion into a low-permittivity interlayer adjoining the electrode. However, since such an interlayer is much thinner than the diameter of a hydrated hydrogen ion, even such an explanation retains the assumption that the primary solvation sphere of an H30 ion is considerably reduced. 

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