Intervalence optical transition

The heteropoly anions PV2Moio0 o and PV WioOj o are each shown to exist as mixtures of the five possible stereoisomers by P NMR spectroscopy. Controlled potential electrolytic reduction of HsPV ioOi o yields PV W Wio-(I) and PV2 Wio0 o (II) which were isolated as potassium salts. The ESR spectrum of anion I consists of superimposed 8- and 15-line components with (g) = 1.952 and (a) = 104.5 and 53 G. The relative intensities of the 8- and 15-line spectra are in quantitative agreement with the assumption that electron exchange between neighboring vanadium atoms (V-O-V) is rapid, but that in isomers with remote vanadium atoms (V-O-W-O-V, etc.) the electron is effectively trapped on a single vanadium. The ESR spectrum of anion II has a normal 15-line pattern arising from the triplet state. The intervalence optical transition in anion I occurs at 8.8 kK. 

The main spectroscopic consequence of the combined action of electron transfer and vibronic interaction is the occurrence of the so-called electron transfer optical absorption (intervalence band), which is shown by the arrows in Fig. 10. The shape and intensity of the intervalence band in the PKS model is defined by the ratio t /(v /cu). In the case of weak transfer the Franck-Condon transitions are almost forbidden, and at the same time, the Stokes shift can be significant. Therefore the MV dimers of Class I are expected to exhibit weak and wide intervalence bands. On the contrary, in the Class III compounds the Franck-Condon transition is allowed, and the Stokes shift is zero. For this reason, intervalence optical bands in delocalized MV dimers are strong and narrow. When the extra electron jumps over the spin cores in a multielecton MV dimer d" — > 1) [85-87] we are dealing with... 

Spectroelectrochemistry provides a convenient avenue to assess whether this conclusion is correct by allowing the isolation and spectroscopic smdy of the mixed valence state. Oxidation of the first metal center results in the formation of the mixed valence Ru(II)Ru(III) ion reflected in the grow-in of a new optical transition centered at 1700 nm (s = 2250 dm mol cm ) and identified as an intervalence charge transfer transition (IVCT). The optical characteristics of such transitions can be analyzed according to the Hush theory (21) and used to estimate the extent of electronic coupling between two metals across the intervening bridge. The full width at half maximum (FWHM) (Vj j) of the... 

The absorption spectrum of a supramolecular system can differ substantially from the sum of the spectra of the molecular components. Aside from those small shifts that can be dealt with in terms of perturbation of the spectra of the single components upon bridging, some totally new bands can be present in the spectrum of the supermolecule. These bands correspond to optical electron transfer transitions, commonly denominated charge-tranter and intervalence transfer transitions, in the organic [26] and inorganic [23-25] literature, respectively (process 7 in Fig. 4, eq 1). The factors that determine the spectroscopic characteristics of such bands... 

Mixed valency of this sort is the cause of the reflective, gold colour of Nao.3W03. In this system, like the MnfTc ion described above, electrons are excited optically following photon absorption from a ground-state electronic configuration to a vacant electronic state on an adjacent ion or atom. The colour is caused by a photo-effected intervalence transition between adjacent WVI and Wv valence sites ... 

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 ... 

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... 

Optical absorption spectroscopy No intervalence transitions in visible region One mixed-valence transition in visible region absorption bands intensify at elevated pressures and low temperatures One or more mixed-valence transitions in visible region temperature lowers intensity of absorption bands, pressure intensifies Opaque metallic reflectivity in visible region... 

The simplest model consists of two centres, one donor (D) and one acceptor (A), separated by a distance I and contains two electrons. Here we consider this simple system to illustrate some general relations between charge transfer, transition intensities and linear as well as non-linear optical polarizabilities. We will show below that the electro-optic parameters and the molecular polarizabilities may be described in terms of a single parameter, c, that is a measure of the extent of coupling between donor and acceptor. Conceptually, this approach is related to early computations on the behaviour of inorganic intervalence complexes (Robin and Day, 1967 Denning, 1995), Mulliken s model for molecular CT complexes (Mulliken and Pearson, 1969) and a two-form/two-state analysis of push-pull molecules (Blanchard-Desce and Barzoukas, 1998). 

Fig. 2.21. Polarized electronic (optical) absorption spectra of a vivianite crystal with zones (labeled 1, 2, 3) exhibiting three different degrees of oxidation. The arrows identify Fe + crystal-field bands at 8,300 and 11,400 cm , and the Fe + —> Fe intervalence charge-transfer transition at 15,800 cm (after Amthauer and Rossman, 1984 reproduced with the publisher s permission).

Optical electron transfer (often designated intervalence transfer, IT, in the case of binuclear mixed-valence transition metal complexes [6, 12]), occurring vertically from the equilibrium configuration of the initial state (Figure 2a). 

---