Hydration outer sphere

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

The surface behavior of Na is similar to that of Cs, except that inner sphere complexes are not observed. Although Na has the same charge as Cs, it has a smaller ionic radius and thus a larger hydration energy. Conseguently, Na retains its shell of hydration waters. For illite (Figure 6), outer sphere complexes resonate between -7.7 and -1.1 ppm and NaCl... 

According to the Marcus theory [64] for outer-sphere reactions, there is good correlation between the heterogeneous (electrode) and homogeneous (solution) rate constants. This is the theoretical basis for the proposed use of hydrated-electron rate constants (ke) as a criterion for the reactivity of an electrolyte component towards lithium or any electrode at lithium potential. Table 1 shows rate-constant values for selected materials that are relevant to SE1 formation and to lithium batteries. Although many important materials are missing (such as PC, EC, diethyl carbonate (DEC), LiPF6, etc.), much can be learned from a careful study of this table (and its sources). 

Neutron scattering has been used for studying the state of solvation of ions in aqueous solution (Enderby et al., 1987 Salmon, Neilson Enderby, 1988). These studies have shown that a distinct shell of water molecules of characteristic size surrounds each ion in solution. This immediate hydration shell was called zone A by Frank Wen (1957) they also postulated the existence of a zone B, an outer sphere of molecules, less firmly attached, but forming part of the hydration layer around a given ion. The evidence for the existence of zone B lies in the thermodynamics of... 

In the following, various Fe(III) compounds of R-substituted salicylaldehyde thiosemicarbazones will be discussed according to the criteria mentioned above, although it should be pointed out that a comparison of these materials may be rendered less meaningful due to the possible occurrence of different polymorphs. Moreover, upon variation of one substitution parameter, several other structural features may also be changed simultaneously. For instance, a change in outer-sphere cation or the introduction of a substituent at the salicylaldehyde moiety is frequently associated with increased hydration of the Fe(III) material. 

Cations attracted to colloid surfaces through their waters of hydration are said to be outer-sphere species, whereas those that interact directly with the oxygen atoms present on the surface are called inner-sphere species. Of the two, the latter species will be more strongly bonded and harder to extract than will be the outer-sphere species. 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

The electron transfer of hydrated redox particles at the outer Helmholtz plane is occasionally called the outer-sphere electron transfer, while the electron transfer of dehydrated and adsorbed redox particles on electrodes is called the inner-sphere electron transfer. 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

We consider a simple redox electron transfer of hydrated redox particles (an outer-sphere electron transfer) of Eqn. -1 at semiconductor electrodes. The kinetics of electron transfer reactions is the same in principal at both metal and semiconductor electrodes but the rate of electron transfer at semiconductor electrodes differs considerably from that at metal electrodes because the electron occupation in the electron energy bands differs distinctly with metals and semiconductors. 

TABLE 8-1. Preference for the conduction band mechanism (CB) and the valence band mechanism (VB) in outer sphere electron transfer reactions of hydrated redox particles at semiconductor electrodes (SC) Eo = standard redox potential referred to NHE c, = band gap of semiconductors. [From Memming, 1983.]... 

In addition to simple reactions of electron transfer (outer-sphere electron transfer) between an electrode and hydrated redox particles, there are more complicated reactions of electron transfer in which complexation or adsorption of redox particles is involved. In such transfer reactions of redox electrons, the redox particles are coordinated with ligands in aqueous solution or contact-adsorbed on the electrode interface before the transfer of their redox electrons occurs after the transfer of electrons, the particles are de-coordinated from ligands or desorbed from the electrode interface. 

We examine an electron transfer of hydrated redox particles (outer-sphere electron transfer) on metal electrodes covered with a thick film, as shown in Fig. 8-41, with an electron-depleted space charge layer on the film side of the film/solution interface and an ohmic contact at the metal/film interface. It appears that no electron transfer may take place at electron levels in the band gap of the film, since the film is sufficiently thick. Instead, electron transfer takes place at electron levels in the conduction and valence bands of the film. 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

Often, it is difficult to distinguish definitely between inner sphere and outer sphere complexes in the same system. Based on the preceding discussion of the thermodynamic parameters, AH and AS values can be used, with cation, to obtain insight into the outer vs. inner sphere nature of metal complexes. For inner sphere complexation, the hydration sphere is disrupted more extensively and the net entropy and enthalpy changes are usually positive. In outer sphere complexes, the dehydration sphere is less disrupted. The net enthalpy and entropy changes are negative due to the complexation with its decrease in randomness without a compensatory disruption of the hydration spheres. 

The extractabilities of metal-organic complexes depend on whether inner or outer sphere complexes are formed. Case 1, section 4.2.1, the extraction of ura-nyl nitrate by TBP, is a good example. The free uranyl ion is surrounded by water of hydration, forming U02(H20)f, which from nitric acid solutions can be crystallized out as the salt U02(H20)6 (N03), though it commonly is written U02(N03)2(H20)6. Thus, in solution as well as in the solid salt, the UOf is surrounded by 6 HjO in an inner coordination sphere. In the solid nitrate salt, the distance du.o(nitrate) between the closest oxygen atoms of the nitrate anions, (0)2N0, and the U-atom is longer than the corresponding distance, du-o(water), to the water molecules, OH2, i.e., du.o(nitrate) > 4u.o(water) thus the nitrate anions are in an outer coordination sphere. 

Molecular hydration in solution is described not only by the inner-sphere water molecules (first and second coordination spheres, see Section II.A.l) but also by solvent water molecules freely diffusing up to a distance of closest approach to the metal ion, d. The latter molecules are responsible for the so-called outer-sphere relaxation (83,84), which must be added to the paramagnetic enhancement of the solvent relaxation rates due to inner-sphere protons to obtain the total relaxation rate enhancement,... 

The second hydration sphere may be considered as "outer-sphere complexing , a phenomenon which is extremely important in all reactions inaqueoiis solution. Coordination of a third hydration sphere will... 

The strong hydration of halide ions in water is partly due to outer-sphere effects. Oxo anions in aqueous solution may complex with outer-sphere water molecules as in the hydrated chromate and permanganate ions ... 

Iron (III) is a stronger EPA than iron (II) so that the electron pair availability at the N atoms of the cyano groups is lower in the oxidized ion. In addition, iron(II) is a stronger n EPD than iron(III) which increases the electron-pair donor properties of the reduced form. Thus, outer-sphere hydration is considerably stronger for the ferrocyanide ion than for the ferricyanide ion. 

If organic solvents such as ketones or alcohols are added in considerable amounts, the redox potential is shifted towards more negative values (22) since, owing to the breakdown of the outer-sphere hydration, the stabilizing effect of water is no longer available (alcohols are known to have considerably weaker EPA properties). The outer-sphere hydration structure is more readily destroyed by the increasing donicity of the solvent that replaces water in the mixture. The destructive effect is more pronounced with the reduced species than with the oxidized one since the latter is less stabilized by hydration, and the redox potentials thus become more negative. 

This ion is readily hydrated since, in agreement with the functional principle, the oxygen atoms are capable of developing the EPD function to form outer-sphere complexes with water functioning as EPA ... 

Formation of an outer-sphere complex of the hydrated metal and the porphyrin. 

Levich (1970) suggested a molecular model to rationalize the phrase outer sphere activation." Thus, the lack in Marcus s original (1956) model of any explicit accounting for the influence of the first hydration layer led to the idea that the activation must arise outside the first sphere. ... 

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