Charge transfer to solvent bands

With some metal complexes, e.g. Fe(CN)6", where a clear CTTS (charge transfer to solvent) band is evident, photoexcitation can cause direct photoionisation and the creation of the solvated electron. 

This earlist as well as the simplest of all the methods involves photolysis of transition metal ions such as Cu+, Fe2+, Eu2+ and others in their CITS (Charge Transfer To Solvent) bands in acidic media. Systems of this type for photoproduction of H2 reported till date19-33) are collected in Table 2.1. General features of this type of systems can be summarised as follows ... 

A pulse radiolysis of Ag solution was studied, and the behavior of formed silver atom (Ag ) and dimer cation (AgJ) was measured [80]. The absorption band for dimer shows a significant red shift with increasing temperatures, which implies to the CTTS (charge transfer to solvent) character of the band. 

A photoprocess rather common with inorganic compounds is the formation of solvated electrons, e [ in organic solvents and eat in aqueous solutions.43,44 The photoprocess is most commonly observed with anions whose absorption spectrum exhibits a characteristic charge transfer to solvent, CTTS, band in the ultraviolet. It is the typical photoprocess of the halide anions shown in Equations 6.89 and 6.90 where X = Cl, Br, and I-. 

The CTTS band can also be found in the absorption spectrum of some polyatomic anions together with transitions to the excited states described above43 44 In the case of SCN, an intense absorption band with 2max = 225 nm (s = 3.5 x 103 M 1 cm-1) has been assigned to a charge transfer to solvent transition. The wavelength-dependent photochemistry of SCN induces, however, the formation of solvated electrons according to Equation 6.89 and the detachment of S (Equation 6.91) in a parallel process. 

Our interest in the photooxidation of Fe " in aqueous solution derives from our more general interest in the effects of solvents on electronic transitions, particularly those in which strong specific interactions with solvent molecules are present (42,45,46). We proceed by performing electronic structure calculations, hquid structure simulations, and spectroscopic calculations for mechanisms 1, 3, and 4, investigating the nature of the photochemical processes of aqueous Fe. In particular, we first require the gas-phase absorption frequencies and intensities of the Fe (H20)6 complex, using both ab initio and semi-empirical (INDO-MRSCI) techniques. Second, we need to determine the structure of water around the Fe " ion in solution. Third, we need to determine the solvent shifts of the absorption bands to evaluate transition energies in solution. This will lead to an estimation of relative importance of all but the charge transfer to solvent process (mechanism 2), calculation of which is beyond die capacity of our present computational facihties. The potential surfaces em-... 

The interactions of halide ions (X , X = F, Br, I, Cl) with polar solvent molecules correspond to specific solvent-cage effects. In aqueous solutions, the ground state of halide ions is localized in solvent cavities and exhibits a strong absorption band in the ultraviolet. This electronic absorption spectrum is the signature of a charge transfer to solvent (CTTS), for which an electron interacts... 

An alternative is to photoionize a solute in water at wavelengths where water transmits—i.e., A > 2000A.—or illuminate a substrate at wavelengths within a suitable charge-transfer-to-solvent absorption band. 

Upon dissolving of salts characteristic bands appear, which have to be attributed to charge-transfer-to-solvent transitions (CTTS) being dependent on the solvent polarity. As these are allowed by the selection rules in most cases, the intensity of CT bands is generally rather strong and depends on the extent of overlap of the xp, and xp i wave functions. 

The compounds dissolve readily in the monomer. The electronic spectra of the resulting solutions display a near-UV absorption band that is a charge-transfer-to-solvent (metallocene - - cyanoacrylate) transition band. Irradiation at this wavelength causes electron transfer or one-electron oxidation of the metallocene to the corresponding metallocenium eation, This is accompanied by a reduction of the ethyl cyanoacrylate to its radieal anion and anionic polymerization of the electrophilic monomer. 

Fig. 6.7. Chemical shifts (in ppm) at 23°C for Cl , Br and 1 ions in CH OH (o), H2O (A), CH3CN ( ) and (CH3)2 NCHO ( ). The shifts are given with the ions in water as references, with a positive shift denoting a shift to higher field. The shifts are plotted versus the wave-length of the charge transfer to solvent absorption band. (From Ref. [dZS])...

Finally, as already mentioned, it seems clear that the solvent molecules can be involved in the charge-transfer process. Detailed discussions of charge-transfer spectra in transition metal complexes quite often label the corresponding bands quite separately, giving them the label CTTS—charge transfer to solvent. So, the fact that Fel3 has recently been prepared in non-aqueous media suggests that the solvent—water—is not always the mere spectator that it was implicitly assumed to be above. 

Since ligands are directly exposed to the solvent it is not surprising that electrons which reside in high-energy ligand orbitals can be ejected to the solvent. The formation of solvated electrons is thus a typical reaction of MLCT states. In some cases this reaction seems to be induced by CTTS (charge transfer to solvent) excitation [6,37,38,121]. However, CTTS absorptions are often difficult to identify and to distinguish from other bands. In any case, the formation of solvated electrons can be frequently related to the presence of MLCT states. Solvated electrons are detected by ESR or visible spectroscopy. They are stable in low-temperature matrices... 

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