CTTS transition

Two examples of shift data with nonuniform curvature (unsymmetrical shift data) which can be fitted by this approach are shown in Figures 14 and 15. The study of iodide solvation by uv CTTS and 127I chemical shifts is the only one where both approaches have been used on the same ion (44). The accuracy of the results is questionable but 127I peaks are very broad and measurement of shifts difficult (5). The treatment leading to Equation 53 is not valid for a CTTS transition, and its application is intuitive, but a difference in wave number shifts rather than wavelengths should be plotted. However, the discrepancy resulting is numerically trivial. 

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. 

CTTS transitions in coordination compounds result in a radial movement of electron density from the metal to the surrounding solution medium. The energies of these transitions generally are very sensitive to environmental parameters such as solvent polarity, temperature and the presence of salts.104 This sensitivity has been used in a diagnostic sense to identify CTTS bands in the spectra of anionic cyanide complexes105 and 1,2-dithiolene complexes of Ni, Pd and Pt.106 Hydrated cations such as Cr2+(aq) and Fe2+ (aqj exhibit absorption bands that are sometimes referred to as CTTS in character. Since the solvent occupies the first coordination sphere of the metal, however, the distinction between CTTS and CTTL transitions in these systems becomes obscured. 

The radius thus calculated from the theory of Smith and Symons does not correspond to any known property of halide ions. However, when the acceptable physical model of Franck and Platzman is combined with the concept of a variable radius, as proposed by Smith and Symons, both absolute value and environmental effects can be accounted for. This was done in the theory of Stein and Treinin (18, 19, 47), using an improved energetic cycle to obtain absolute values of r, the spectroscopically effective radius of the cavity containing the X ion. These values were then found to correspond to the known partial ionic radii in solution, as did values of dr/dT to values obtained from other experiments. The specific effects of temperature, solvents, and added salts could be used to differentiate between internal and such CTTS transitions where the electron interacts in the excited state strongly with the medium. These spectroscopic aspects of the theory were examined later in detail and compared with experiment by Treinin and his co-workers (3, 4, 32, 33, 42,48). 

CTTS transitions, in which the oriented solvent participates are absent in the spectrum of the aromatic solutes but the lifetime of the excited state, as shown by the existence of fluorescence is longer, probably 10 9 sec. During this extended lifetime, sufficient reorganization of the solvent may occur to enable the excited electron to be trapped, and to allow for the first relative diffusive displacement of the gemini, which is necessary for the observed kinetics to develop. The temperature ef-... 

From the preceding it is apparent that the nature of the first excited state has been clarified for CTTS transitions of simple anions, and that both for these and for intramolecular processes following on primary excitation the events are known with some certainty up to the stage of asymmetrization. Also, after the pair of solvated electron and parent species have been established by at least one diffusive translation (so that at least one solvent molecule separates them) all steps following have been elucidated. There remains the question of the detailed nature of the processes in the time interval between 10 u and 10 n to 10-9 sec. after primary absorption. 

Full details subsequent to high energy electronic excitation of Fe(CN)6- have not been elucidated, but some data do support the notion that a Fe(CN)4- - solvent CT (CTTS) excited state is responsible for the ejection of the electron. Shirom and Stein177) have postulated the reasonable mechanism indicated in Scheme 17. Despite the fact that there appears to be an absorption associated with the CTTS transition there is no necessity to invoke direct population of the CTTS excited species as it could be populated by relaxation of the 1Tiu excited Fe(II)- CN- CT state. 

Another important type of electronic transition is the charge transfer to solvent (CTTS) transition. Recently, it has also been found5 that electronic transitions may occur between MO s which are localized on different ligands of the same complex. 

In addition to the inner-sphere (or intramolecular) CT transitions, outer-sphere CT (OSCT) transitions are observed. These OSCT interactions include transitions between complex and solvent (CTTS transitions), complex and its ion-pair partner (IPCT transitions), and complex and a non-bonded quencher. Frequently, OSCT excitation leads to photochemical reactions [27, 30,50, 91, 92] (see Figure 6.6). As a consequence, the complex undergoes oxidation at the expense of the solvent... 

Table 20 Rate constants for the anion-induced fluorescence quenching ( /Cq), lifetimes (tq), half-wave potentials of aromatic molecules ( 1/2), oxidation potentials [ (X /X )], and CTTS transition energies [ (ctts)] of inorganic anoins, and excited singlet energy levels ( a ) in 50% Et0H-H20 solutions at 300 K... 

Ag ) and (Ag (CN)2" ) are both autoionizing whereas the ground states are not. Assuming that Ag°(EDTA) behaves similarly we can estimate from the CTTS transition energy that the redox potential is close to -2.2 Vnhe-... 

Another proposed solvent scale is based on the energy (in kcal mol" ) of the maximum of the CTTS transition of iodide ion. There is no simple correlation between this scale and Z values, which has been... 

Figure 32 Simple representation of a CTTS transition for iodide ions in water.

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