Charge-transfer-to-solvent spectra

The absorption spectra of anions are very sensitive to the composition of solvents in which they are embedded. In general, they are solvated, i.e. they are surrounded by a solvent shell. The molecules composing the solvent shell constantly exchange position with those in the bulk of the solvent. In these transitions, an electron is ejected not into the orbitals of a single molecule, but to a potential well defined by the group of molecules in the solvation shell. Such transitions are known as charge- transfer-to-solvent (CTTS) transitions. 

Special cases of charge-transfer spectra are the so-called charge-transfer-to-solvent (CTTS) spectra [17, 68]. In this type of CT transitions, solute anions may act as electron-donors and the surrounding solvent shell plays the role of the electron-acceptor. A classical example of this kind of CTTS excitation is the UV/Vis absorption of the iodide ion in solution, which shows an extreme solvent sensitivity [68, 316]. Solvent-dependent CTTS absorptions have also been obtained for solutions of alkali metal anions in ether or amine solvents [317]. Quantum-mechanical molecular simulations of the CTTS spectra of halide ions in water are given in reference [468]. 

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. 

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. 

Figure 5.2 shows ATR-FUV spectra of pure water as well as 5 % NHj, 5 % HCl, and 10 % H2O2 aqueous solutions obtained with a quartz ATR probe. A hidden absorption peak near 180 nm in the absorption spectrum of 5 % NH3 aqueous solution appears to be a band due to the n a transition of NH3 [17], and a band due to the charge transfer to solvent (CTTS) of Cl is also observed near 180 nm [18]. An aqueous solution of H2O2 demonstrates a broader featureless absorption... 

Franck and Scheibe (8) were the first to interpret the absorption spectra of halide ions in solution as due to a charge transfer to the solvent (CTTS). The solvation configuration around the ion cannot change during the transition. Therefore the energy required for the transition at the band maximum ... 

Properties of the microenvironment of soluble and cross-linked polymers were studied by the shift of bands in the electron spectra of solvatochromic reporter molecules embedded in polymer chains. Generally, the charge-transfer (CT) absorption spectra and emission spectra of a number of compounds were used to correlate solute-solvent interactions with physical and chemical properties of interest. The energy of the band maxima of these chromophores is quite solvent sensitive and is linearly correlated with empirical solvent polarity parameters. The observed shift of the maximum of the solvatochromic reporter embedded in the polymer chains, compared with a low-molecular weight analog in the same solvent, was interpreted in terms of a change in the polarity of the microenvironment of the polymer in solution. 

An intramolecular charge transfer toward C-5 has been proposed (77) to rationalize the ultraviolet spectra observed for 2-amino-5-R-thiazoles where R is a strong electron attractor. Ultraviolet spectra of a series of 2-amino-4-p-R-phenylthiazoles (12) and 2-amino-5-p-R-phenylthiazoles (13) were recorded in alcoholic solution (73), but, reported in an article on pK studies, remained undiscussed. Solvent effects on absorption spectra of 2-acetamido and 2-aminothiazoles have been studied (92). 

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Solvatochromic pareuaeters, so called because they were Initially derived from solvent effects on UV/visible spectra, have been applied subsequently with success to a wide variety of solvent-dependent phenomena and have demonstrated good predictive ability. The B jo) scale of solvent polarity is based on the position of the intermolecular charge transfer absorption band of Reichardt s betaine dye [506]. Et(io> values are available for over 200 common solvents and have been used by Dorsey and co-%rarkers to study solvent interactions in reversed-phase liquid chromatography (section 4.5.4) [305,306]. For hydrogen-bonding solvents the... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The solvent effects on charge-transfer spectra between Me3Sn-NCS and I2 was investigated. Onsager s theory of dielectrics was used to estimate the stabilisation energy of excited states257. 

The micellar surface appears to be less polar than water, based largely on shifts in fluorescence or charge-transfer spectra (Section 1). Although it may not be reasonable to apply bulk solvent parameters such as Z or dielectric constant to submicroscopic species such as micelles, the spectral and kinetic evidence are self-consistent. An additional point is that these reactions have... 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

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