Intervalence transfer absorption band

One experimental source of these essential data (for mixed-valence systems) is the intervalence transfer absorption band. The moments of the envelope (particularly the first and second moments) can be interpreted to yield values for x and (from a study of temperature dependence) for an effective coupling frequency. The study of effects of solvent variation can also lead to separation of x into intramolecular and environmental components. ... 

This ion provides a model for study of intervalence transfer absorption bands characteristic of trapped valence species that correspond to the process... 

Mixed-valence chemistry was reviewed in the late 1960 s both by Robin and Day (4) and by Hush (5). Their work provided the beginnings of a theoretical background for understanding the properties of mixed-valence compounds including the low energy absorption bands which have been termed Intervalence Transfer (IT) or Metal-Metal Charge Transfer (MMCT) bands. 

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... 

The electronic spectra of a variety of transition metal-bearing oxide and silicate minerals have been measured at high pressures and/or elevated temperatures. Trends for absorption bands originating from crystal field (CF), metal-metal intervalence charge transfer (IVCT) and oxygen —> metal charge transfer (OMCT) transitions are summarized in tables 9.2,9.3 and 9.4, respectively. 

The difficulties of interpretation presented by some of these complexes are well illustrated by the sequence of ferrocene derivatives (VIII to X). The spectrum of VIII shows a near-infrared absorption as expected for a mixed valence complex, but the band has a shoulder, and at low temperatures is resolved into two bands. Complex IX has a well-defined band almost certainly ascribable to intervalence transfer, but complex X with a similar molecular structure has no such... 

For class I systems, 0 = 0 and the mixed valence absorption bands observed generally fall at frequencies larger than 27 000 cm , except when the system contains a colored ion as a constituent. Intervalence interactions (see Intervalence Transfer Transition) can provide a source for intensification of such ligand field transitions, however, in class I systems. The mixed valence electronic transition is given by equation (2), where has the same meaning for the excited state as a does for the ground state. Hence a = /J = 0 for true class I behavior. 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

Ferrocenyl derivatives of 73, namely (Co3(CO)9 L (/i3-CFc)] (n = 0-3, L = phosphine or phosphite) undergo reversible reduction at the tricobalt center to detectable radical anions (n = 0), and oxidation at the iron center to give isolable cations (182). For the trisubstituted derivatives [ = 3, L = P(OMe)3 or P(OPh)3], the monocations appear to be mixed-valence species in which the two redox sites are weakly interacting low-energy absorption bands, assigned to intervalence transfer transitions, have been observed. The dications (Co3(CO)6 P(OR)3 3(/i3-CFc)] (R = Me or Ph) are also isolable, oxidation having occurred at both possible redox sites (183). 

Coloured ion-pair associates have been observed between [Mo(CN)s] and the metal ions Fe, Cu, U02, and VO. The long-wavelength absorption band is attributed to an intervalence CT transition, and from the solvent dependence of this band the interionic distances have been estimated. A Mo e.s.r. signal has been observed in frozen aqueous glycerol solutions at 93 K and has been ascribed to the photochemically induced electron transfer ... 

MV clusters usually exhibit characteristic absorption bands within the infrared or visible spectral ranges. These bands are related to light-induced transfer of the extra electron between metal ions. This absorption is called intervalence. 

The main difference in the spectra concerns the presence of an intense absorption band at around 700 nm, which can be attributed to Intervalence Charge-Transfer between Mo5+ and Mq6+ in the Keggin unit [22,23], thus indicating the presence of a partially reduced POM for the catalyst which has been unloaded after reaction under isobutane-rich conditions (spectrum... 

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