Metals extinction coefficient

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

Cryophotochemical techniques have been developed that (i) allow a controlled synthetic approach to mini-metal clusters 112), Hi) have the potential for "tailor-making small, bimetallic clusters (mini-alloy surfaces) 114,116), Hi) permit the determination of relative extinction-coefficients for naked-metal clusters 149), and iv) allow naked-cluster, cryophotochemical experiments to be conducted in the range of just a few atoms or so 112,150,151). 

Low-temperature, photoaggregation techniques employing ultraviolet-visible absorption spectroscopy have also been used to evaluate extinction coefficients relative to silver atoms for diatomic and triatomic silver in Ar and Kr matrices at 10-12 K 149). Such data are of fundamental importance in quantitative studies of the chemistry and photochemistry of metal-atom clusters and in the analysis of metal-atom recombination-kinetics. In essence, simple, mass-balance considerations in a photoaggregation experiment lead to the following expression, which relates the decrease in an atomic absorption to increases in diatomic and triatomic absorptions in terms of the appropriate extinction coefficients. 

The relative extinction-coefficients for Agi,2,s determined by pho-toaggregation procedures were found not to be strongly matrix-dependent (see Table VIII). Moreover, the results for Agj were in good agreement with those obtained by quantitative, metal-atom deposition-techniques. 

Phthalocyanine-based dyes are especially useful for CD-R, as the chromophore absorption band falls in the desirable spectral range, and they are noted for excellent photostability. Unlike cyanine dyes, phthalocyanines tend to have very poor solubility, particularly in solvents such as alcohols and aliphatic hydrocarbons (which do not attack polycarbonate and are therefore used for spin coating). Therefore, the main barrier to the wider use of these dyes is the relatively high cost of synthesizing soluble derivatives. Suitable modifications to the Pc core which have been developed, notably by Mitsui Toatsu, are shown in Scheme 7. The bulky R groups reduce undesirable molecular association (which in turn lower the extinction coefficient and hence reflectivity), whereas partial bromination allows fine-tuning of the film absorbance and reflectivity. The metal atom influences the position of the absorption band, the photostability, and the efficiency of the radiationless transition from the excited state.199 This material is marketed by Ciba as Supergreen.204... 

The photophysical properties (extinction coefficient, shifts in absorption and emission spectra, quantum yield, and lifetime) of a variety of probes are modified by pH changes, complexation by metal ions, or redox reactions. The resulting changes in photophysical parameters can be used to determine concentration of H+ and metal cations with suitably designed fluorophores. Most of these resulting sensors involve an equilibrium between the analyte, A, and the free probe (unprotonated or noncom-plexed by metal ion), Pf. If the stoichiometry of this reaction is 1 1, the reaction may be represented by... 

Table 3. The metal effect in some square planar metalloporphyrins [absorption maxima (nm), extinction coefficients (log e, given in parentheses) Unicam SP 800 B]...

Foster et al. have developed a method for determining technetium in dissolved nuclear fuel solutions. Tetrapropylammonium pertechnetate is doubly extracted from a basic medium into chloroform and the colored technetium (V) thiocyanate complex is formed in the chloroform phase by the addition of sulfuric acid, potassium thiocyanate and tetrapropylammonium hydroxide. The colored complex absorbs at 513 nm, has a molar extinction coefficient of 46,000 and is stable for several hours. Of more than 50 metals studied, none impairs measurements at ratios less than 100 to 1 mol with respect to technetium. Most anions do not disturb the determination of technetiiun. The standard deviation for a single determination is 0.09 fig over the range of 1 to 20 fig of technetium. 

Pertechnetate forms a blue complex and perrhenate a brownish-yellow complex with K4[Fe(CN) ] in presence of bismuth amalgam. This permits the spectrophotometric determination of both elements in the same solution . The adsorption maxima of the technetium and rhenium complexes are at 680 and 420 nm, respectively. The molar extinction coefficients are 10,800 for technetium and 4,000 for rhenium. Metals forming color or precipitates with K4[Fe(CN) ] must first be removed. 

In some cases large extinction coefficient allows sensitive detection Circular dichroism (CD) spectra Coordination geometry metal ligands Electron paramagnetic resonance (EPR) spectra... 

The spectra of the complexes LmM"+l R, containing a metal-carbon -bond, usually consist of several distinguishable bands the major bands with high extinction coefficients in the UV region are LMCT bands, followed sometimes by bands with mixed characters and the d-d bands in the visible region with low extinction coefficients. The location of the maximum of the LMCT transition is naturally strongly affected by the nature of the substituents R and by the redox potential of the central M(ra + 1) ions. 

The -organometallics show a reactivity pattern typical of an alkene bound weakly to a metal center. The moiety may be displaced easily by a variety of donor ligands such as CO or PR3 (R = OMe, Ph), and indeed many of the complexes are very air and water sensitive. Reactions can be monitored easily by the color change from the intense red or green of the adduct to the characteristic purple color of the free C60 molecule the other products tend to have much lower extinction coefficients [Eqs. (11) and (12)] (15, 26). 

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