Spectral changes

Typical spectral changes during the progress of the cited oxidation reactions are shown in Figs. 12.1-12.4. A gradual decrease in the absorbance of chromium (VI] at 350 nm, its absorption maximum, with time can be observed in these figures, which may indicate the reduction of the chromium (VI] oxidant. The spectrum of the formed chromium (III] after the reaction completion is shown in Fig. 12.4, which may confirm the reduction of chromium ion from hexavalent to trivalent state. 

The keen trials have been made in order to detect either the formed unstable Cr (IV) or Cr(V) transient species, spectrophoto-metrically. Unfortunately, all of these attempts have failed. This fact may be attributed to either tbe fast disappearance of the intermediates by their following fast reactions or the small molar extinction coefficients of these intermediates under the experimental conditions used of low Cr (VI) concentration. 

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If the perturbations thus caused are relatively slight, the accepted perturbation theory can be used to interpret observed spectral changes (3,10,39). The spectral effect is calculated as the difference of the long-wavelength band positions for the perturbed and the initial dyes. In a general form, the band maximum shift, AX, can be derived from equation 4 analogous to the weU-known Hammett equation. Here p is a characteristic of an unperturbed molecule, eg, the electron density or bond order change on excitation or the difference between the frontier level and the level of the substitution. The other parameter. O, is an estimate of the perturbation. 

Hahde complexes of Cu with nitrogen base ligands are known to exhibit another form of reversible spectral change known as fluorescence thermochromism. The example of Cu4l4(Py)4 from Table 1 is typical and shows red shifting ia the visible emission spectmm while the sample is both cooled and irradiated with a 364 nm ultraviolet source (7). 

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

When acetone is treated with hydroxylamine in aqueous solution near neutral pH, the carbonyl UV absorption intensity decreases very rapidly this fast spectral change is followed by a much slower absorption increase that is due to the appearance of the oxime product. This suggests that, at such pH values, the initial addition is very rapid and the second step, dehydration of the carbinolamine, is the rds. Figure 5-12 is a plot of the apparent first-order rate constant against pH for this reaction. As the pH is decreased from neutrality, the rate increases, indicating that the rds... 

With imines, salts formation is accompanied by characteristic spectral changes (153) (a) a bathochromic shift in the ultraviolet region by as much as 50 m/i, according to compound type and to properties of any auxochrome present, and (b) a high frequency shift of the... 

The spectral changes which occur in increasingly acid solutions of polyaza-heterocycles may indicate a second ionization. This event, however, can readily be distinguished from dehydration by measuring the spectra in anhydrous dichloroacetic acid, provided that the pKa value for the anhydrous species is above 1. Anhydrous dichloroacetic acid has a Hammett acidity function (Hq) of — 0.9 (as determined using o-nitroaniline as the solute), and the ultraviolet spectrum of a base with a p > 1 would be that of the anhydrous cation in this 2 A. Albert and W. L. F. Armarego, J. Chem. Soc. 4237 (1963). 

The smallness or the spectral changes observed between corresponding pairs of cations and neutral molecules enables the main features of the spectra of unstable species such as the hydrated neutral molecule or the anhydrous cation of pteridine to be predicted from the spectra of the hydrated cation and anhydrous neutral molecule, respectively. In this way, suitable wavelengths can readily be selected at which hydration and dehydration will produce big changes in the optical density. 

For illite and kaolinite with decreasing solution concentration (Figure 5) there are two important changes. The relative intensity for inner sphere complexes increases, and the chemical shifts become substantially less positive or more negative due to the reduced Cs/water ratio, especially for the outer sphere complexes. Washing with DI water removes most of the Cs in outer sphere complexes and causes spectral changes parallel to those caused by decreasing solution concentration (data not shown). 

Fig. 1.7 Spectral change of the in vitro firefly bioluminescence by pH, with Photinus pyralis luciferase in glycylglycine buffer. The normally yellow-green luminescence (Amax 560 nm) is changed into red (Xmax 615 nm) in acidic medium, accompanied by a reduction in the quantum yield. From McElroy and Seliger, 1961, with permission from Elsevier.

Fig. 7.2.3 Spectral changes of Odontosyllis luciferin caused by various reagents (Shimomura et ai, 1963d). The peak wavelengths (nm) of the absorption, luminescence and fluorescence spectra are shown in parentheses.

Fig. 7.2.5 Spectral change of Odontosyllis luciferin in 50 mM NaOH in air A, at zero time B, after 15 min C, after 30 min and D, after 80 min (Xmax 385 nm). From Shimomura et ai, 1963d, with permission from John Wiley Sons Ltd.

Fig. 8.4 Absorption spectrum of dinoflagellate luciferin, and the spectral changes caused by luminescence reaction after the addition of luciferase, in 0.2 M phosphate buffer, pH 6.3, containing 0.1 mM EDTA and BSA (O.lmg/ml) (Nakamura et al., 1989). Reproduced from Hastings, 1989, with permission from the American Chemical Society and John Wiley Sons Ltd.

A merocyanine dye, l-ethyl-4-(2-(4-hydroxyphenyl)ethenyl)pyridinium bromide (M-Mc, 2), exhibits a large spectral change according to the acid-base equilibrium [40, 41]. The equilibrium is affected by the local electrostatic potential and the polarity of the microenvironment around the dye. Hence, this dye is useful as a sensitive optical probe for the interfacial potential and polarity when it is covalently attached to the polyelectrolyte backbone. 

Spectral changes on adsorption are of three types appearance of inactive fundamentals (often coincident with infrared absorptions—see Table IX), shifts in Raman line positions for active vibrations, changes in relative peak intensities, and changes in half-bandwidths. The first three types of change have been reported for centrosymmetric adsorbates. 

CO oxidation reaction. The spectral changes in Cluster C are followed hy Cluster B reduction with a rate constant that is similar to the steady-state value. On the other hand, the rate of formation of the characteristic EPR signal for the CO adduct at Cluster A is much slower. Its rate constant matches that for acetyl-CoA synthesis, hut is several orders of magnitude slower than CO oxidation. Therefore, it was proposed that the following steps are involved in CO oxidation (1) CO hinds to Cluster C, (2) EPR spectral changes in Cluster C are accompanied hy oxidation of CO to CO2 hy Cluster C, (3) Cluster C reduces Cluster B, and (4) Cluster B couples to external electron acceptors (133). 

Isocyanides [RNC] (174, 175) are isoelectronic with CO and have been extensively used as CO analogs in studies of heme proteins (176-180). W-Butyl isocyanide (N-BIC) behaves as a CO analog at both the CODH and ACS active sites (181). N-BIC competes with CO in the CO oxidation reaction, is a sluggish reductant, and causes EPR spectral changes at Clusters A, B, and C similar to those elicited by CO. 

FIGURE 4.2.3 UV-vis spectral changes and AAbs obtained by heating bixin in water ethanol (8 2) at 92°C. Inset shows kinetic profile at several wavelengths, with the solid lines representing the fitting of experimental data from the sum of two exponential functions. From Rios, A.O., Borsarelli, C.D., and Mercadante, A.Z., J. Agric. Food Chem., 53, 2307, 2005. With permission. 

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