Fe2* -* Fe3* charge transfer

Fe2 - Fe3 Charge Transfer. Partial reduction of Fe3 clays results in Fe2 and Fe3 cations occupying edge-sharing Fe06 coordination polyhedra. Such a condition allows for optically (and perhaps thermally) induced intervalence charge transfer (IVCT) transitions ... 

To account for these results for (Mg,Fe)Si03 perovskites, various charge transfer mechanisms have been proposed (Li and Jeanloz, 1990 Hirsch and Shankland, 1991 Sherman, 1991). Lattice defects permitting Fe3+ ions to exist in the perovskite structure give rise to oxygen — Fe and Fe2+ — Fe3+ charge transfer transitions, the latter being facilitated by the close proximity (279 pm) of the A sites (Fe2+) to the B sites (Fe3+) in the perovskite structure. The opacity of the hydrous phase D indicates that extensive electron delocalization may occur in its crystal structure. 

Sherman, D. M. (1987a) Molecular orbital (SCF-Xa-SW) theory of metal-metal charge transfer processes in minerals. I. Applications to Fe2+ -+ Fe3 charge transfer and electron delocalization in mixed-valence iron oxides and silicates. Phys. Chem. Minerals, 14, 355-64. 

Figure 12. Molecular orbital diagram for an FegOjg cluster used to understand the orbitals involved in Fe Fe3 charge transfer. The absorption band observed near 13,000 cm 1 in the spectra of mixed-valence silicates is due to the transition from the Fe2 (t2g)- Fe3+(t2g) orbitals. A transition state calculation for that energy in the cluster presented here gives 10,570 cm"1 in fair agreement with experiment.

Purple acid phosphatases (PAPs) catalyze the hydrolysis of phosphate monoesters with mildly acidic pH optima (5-7) utilizing a binuclear metal center containing a ferric ion and a divalent metal ion. PAPs are also characterized by their purple color, the result of a tyrosine (Tyr) to Fe3+ charge transfer transition at about 560nm.113 All known mammalian PAPs are monomeric and have a binuclear Fe3+-Fe2+ center, whereas the kidney bean and soybean enzymes are dimeric and have an Fe3 + -Zn2+ center in each subunit. The X-ray structures for kidney bean PAP114 and the PAP115 from rat bone reveal that despite a sequence similarity of only 18%, they share very similar catalytic sites. The structure of the kidney bean PAP shows the two metal ions at a distance of 3.1 A, with a monodentate bridging Asp-164. These and other residues involved in metal coordination can be seen in Fig. 21. 

Charge transfer reactions represent an important category of electrochemical behavior. As already pointed out above, an appropriate investigation of kinetic parameters of electrochemical reactions in aqueous electrolytes suffers from the small temperature range experimentally accessible. In the following, some preliminary results using the FREECE technique are presented for the Fe2+/Fe3+ redox reaction and for hydrogen evolution at various metal electrodes. 

There are two kinds of charge-transfer reactions at electrodes. An electron-transfer reaction is the first kind and is exemplified by the reduction of Fe3 to Fe2+ at the interface. The ions in the layer hardly move while the electron comes from the electrode or leaves the ions in the layer of solution adjacent to the electrode and gpes to the electrode. The charge transfer is dominated by means of electrons transferring from electrode to ions and vice versa. 

Evidence for the Fe2+/Fe3+ redox cycle was provided later by ESR measurements [205], while recent experiments with deuterium-labelled butene indicate that C—H cleavage is involved in the rate-controlling step [138]. In agreement with the views of Schuit [281], chemisorption of the olefin on an anion vacancy is assumed, but O- is postulated as the active oxygen species rather than O2-. An argument in favour of O" is that otherwise much more, and rather complicated, elementary reaction steps are required to account for the transfer of charge. 

Two points may be made at this stage. First, the quantity of charge transferred between phases in order to establish an equilibrium potential difference is normally so small that the actual change in composition of the solution is negligible. For example, one can show that when a 1 cm2 platinum electrode is immersed in a Fe2+/Fe3+ solution, a net reduction of between 10-9 and 10-,° moles of Fe3+ takes place. Second, and as will be stressed later, the kinetics of the charge transfer process are very important, since if rates are slow, it may not be possible for a true equilibrium to be established. 

It is characteristic that the iron in this compound is present in two different oxidation states Fe2+ (here in square brackets) and Fe3+ (here on the outer left). The interaction between these two different iron ions also gives rise to the blue color of this compound (Charge-Transfer-Complex). The actual composition can be quite variable, depending on the stoichiometry on formation and the presence of impurities, in which case the color varies between dark blue and greenish-blue tones. 

Optical Spectra. The optical properties of smectites have been studied by various workers (32,37-40), and involve several different types of electronic transitions. One important type of transition is the intervalence charge transfer (IT), which is observed in the optical spectra of minerals containing both Fe2 and Fe3 in their... 

Vivianite, Fe2+3(P04)2.8H20, is the classic example of a mineral showing an intervalence charge transfer transition (Wherry, 1918 Bums, 1981). Vivianite has a diagnostic indigo-blue colour and a well characterized Fe2+ —> Fe3+ IVCT absorption band in the polarized spectra illustrated in fig. 4.12 and is the datum with which electron interaction parameters for other minerals are compared. The chemical formula of vivianite is not indicative of a mixed-valence compound. However, the pale-green colour of newly cleaved vivianite crystals or fleshly... 

For a variety of minerals listed in table 4.2, Fe2+ —> Fe3 IVCT transitions have been assigned to a range of energies spanning 9,700 cm-1 to 18,500 cm-1. These data enable two widely held beliefs to be examined first, that the charge transfer energies should decrease with decreasing separation between the inter-... 

Homogeneous charge transfer can take place between chemically similar redox species of one redox couple, e.g., Fe3+ and Fe2+ ions in solution or ferrice-nium and ferrocene moieties in poly(vinylferrocene) films, the electron transfer (- electron hopping or electron exchange reaction) can be described in terms of second-order kinetics and according to the - Dahms-Ruff theory [viii-x] it may be coupled to the isothermal diffusion ... 

The mechanism of electron transfer reactions in metal complexes has been elucidated by -> Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in -> photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, - cytochrome (Fe3+/Fe2+), ferredoxin (Fe3+/Fe2+), - nicotine-adenine-dinucleotide (NAD+/NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

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