Color solute induced

Explanations for the phenomenon offered in the literature are largely unsatisfactory. McKay (80) observed that MB solutions turned red only in the presence of strongly basic amines (pKjj < 5) or in other bases (ethoxide, hydroxide), but that they remained blue in weakly basic solvents such as pyridine or quinoline. McKay suggested that Lewis-acid (dye)/base (amine) complexes might be formed, but rejected this explanation in favor of color changes induced by tight ion pair formation between the dyes and the ionic bases such as ethoxide. 

When a solution of NaF in HF was added gradually to the stable lilac-colored solution of U3+ in BF3-HF (Sec. 11.3.4.2) a precipitate was observed which was green UF4 flecked with black particles of metallic U and the supernatant solution was colorless, i.e. it contained no U(III) or U(IV) species. Under favorable experimental conditions some of the metallic uranium was deposited as a bronze-colored mirror on the walls of the synthetic sapphire reaction tube, as shown in the original reference [59]. Base-induced disproportionation had occurred. 

The spiropyran-modified poly(L-glutamic acid) PGA-2 undergoes a coil helix transition upon exposure to visible light in hexafluoro-2-propanol solution. In the dark, the polypeptide, containing 30-80 mol% chromophore units in the open charged form, adopts a random coil conformation. Irradiation causes isomerization in the side chains, as indicated by complete bleaching of the colored solution (see Scheme 5.4). The formation of the colorless and uncharged spiropyran form induces spiralization of the polypeptide chain. The coil helix transition can be followed with the aid of CD spectra, as shown in Fig. 5.4. 

Acids, bases, and salts are among the most common and important solutes found in solutions. Until late in the 19th century, these substances were characterized by such properties as taste and color changes induced in certain dyes. Acids taste sour bases, bitter and salts, salty. Litmus, a dye, is red in the presence of acids and blue in the presence of bases. These and other observations led to the correct conclusions that acids and bases are chemical opposites, and that salts are produced when acids and bases react with each other. Today, acids and bases are defined in more precise ways that are useful when studying their characteristics. 

Perhaps the most widely studied imperfection phenomenon is that of color centers induced in alkali halide crystals by a -irradia-tion or by introducing excess alkali metal. The best known of these is the F center which is now widely accepted to consist of a halide ion vacancy with an electron trapped in it. As yet there has been no definitive treatment of this center from first principles. The usual zeroth-order treatment approximates the solution as a particle in the box giving (l/l ). Using F-peak energies and lattice parameters for several alkali halides Mollwo showed that this is an approximately correct relationship. A more detailed analysis of the data by Ivey gave a relationship of the form... 

Another typical example for solute-induced pitch shifts is the color change produced in (cholesteric) mixtures of cholesteryl derivatives upon addition of cis-or fram-isomers of azobenzenes or stilbenes. In a cholesteric mixture of cholesteryl chloride and cholesteryl nonanoate (70 mole percent) cis azobenzene produces a red shift, while the trans-isomer induces a blue shift [143]. For cholesteric mixtures well above the cholesteric to smectic transition the shift in the reflection... 

Electron-transfer activation. The observation of intense coloration upon mixing the solutions of hydroquinone ether MA and nitrogen dioxide at low temperature derives from the transient formation of MA+ cation radical, as confirmed by the spectral comparison with the authentic sample. The oxidation of MA to the corresponding cation radical is effected by the nitrosonium oxidant, which is spontaneously generated during the arene-induced disproportionation of nitrogen dioxide,239 i.e.,... 

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