Absorbance diagrams

The photoinduced and thermal isomerization reactions are nearly perfectly reversible, and side reactions are virtually absent. In de-aerated hydrocarbon solution, azobenzene can be irradiated for days with near UV or visible radiation without any change of absorbance after the photostationary state is established. Under air, the only side reaction is a very slow oxidation to azoxybenzene. This can be checked without much effort by Mauser diagnostics (Section 1.2.2.3). For most azobenzenes, the application of absorbance diagrams gives perfectly straight lines, indicating that the isomerization is the only reaction (Figure 1.4). This fact warrants the use of azobenzene as a convenient actinometer. ... 

Related double-helical metal complexes have been used to investigate the role of cooperativity in self-assembly. Ligands (14a) and (14b) were allowed to react with CUBF4 to form the corresponding helicates (15a) and (15b). The formation of helicates (15a) and (15b) was confirmed in solution by spectrophotometric titration. Two sharp isosbestic points are seen in the spectrum, and the excess absorbance diagram is linear even at a very tow... 

In the first examples, spectrometry is used as an excuse to revisit some of the problems and methods encountered in earlier chapters, such as the determination of the pKa of an indicator dye, and multicomponent analysis. We also illustrate the still little-known absorbance-absorbance diagrams. 

Fig. 4.18. Absorbance diagram of the photochemical reaction of frans-stilbene.

If only C absorbs the absorbances in the absorbance diagram are also situated on a zero point line. The measurement starts in the zero point and ends at c . One can determine from the diagram y, = c/c , if the section Al is intersected with the ratio y,. to 1. The resulting point is connected with the related point P of measurement. Then one has determined the parallel to the section AB. If this parallel is a tangent to the curve in A one of the reactions 1-4 will take place. If the parallel is not a tangent, one of the reactions 5-10 is going to happen. 

Point 1 is the intersection of the tangent at the points A and V of the curve of measurement. It can be determined in any case. Because of the affine transformation it is always valid that the curve of measurement intersects the triangle A with the ratio of the two eigenvalues. If it is known that a simple consecutive reaction 1 takes place, the points V and C coincide. In addition if k, > 2, the point 1 relates to the comer B. However, if k, < 2, then point B will coincide with point 2. The position of this point can always be determined using the absorbance diagram with... 

By application of absorbance diagrams of higher order and mass spectrometry of the reactants the following mechanism is assumed... 

These diagrams allow the determination of the number and the appearance of fluorescent compounds. The variety of the forms of these diagrams is manifold and more characteristic than in absorbance diagrams. As can be seen in Fig. 5.38, even for the same mechanism the fluorescence diagrams are extremely dependent on the wavelength of observations. 

A prerequisite for the evaluation mentioned is knowledge about the reaction mechanism. Linear absorbance diagrams proved the photoisomerisation taking place as in solutions. However, the siloxane matrix has to be fresh. Different types of siloxanes were tested, some photochemically polymerised, others fabricated by a catalyst induced process. In the latter case the Pt-catalyst must not overcome a concentration limit otherwise it influences the azobenzene photoreaction. Approximate evaluations at low absorption (assuming a irradiation intensity independent of the volume element) do not offer appropriate results because of measurement problems. Therefore a transformation of the time scale has been used, discussed in Section 5.7.3. 

Dihydroindolizines can be embbeded in siloxanes out of an ether solution during the process of polymerisation. Absorbance diagrams allow a qualitative comparison with those obtained in solution. In principle the quantum yield of the photoreaction is reduced for a large number of derivatives. The l,2-di-cyano-4-methyl-aceto derivative no longer exhibits a backward thermal reaction from the betaine because of steric hindrance. Even though irradiation was chosen co-linear with the measurement direction and thus diffusion is not involved, the combinations of differential equations according to Section 3.4 cause numerical problems at evaluation. Therefore only approximate calculations can be done simplifying the mechanism to estimate reaction constants [153]. 

Rank analysis allows the determination of the number of linear independent steps of reaction. Graphical approaches are preferable since deviations of a correct determination of this number become better visible than fuzzy results which make the decision delicate. Absorbance diagrams are constructed at different orders. 

Next absorbance diagrams are plotted in various combinations. A typical graph demonstrates curves, which means the reaction contains more than one linear independent step of reaction. 

The absorbance diagram of the stilbene photoreaction is shown schematically below. Since the flrst step is an equilibrium, the absorbance line from c/.r-stilbene to phenanthrene does not form a tangent to the measured absorbances. The point B is determined from the absorption coefflcients of the c 5-stilbene at both the wavelengths 250 and 292 nm and the initial concentration Oq. Point C is obtained by the absorbance coefficients of phenanthrene and the initial concentration of fra/u-stilbene. The lines JITai b Kq correspond to the calibration curves for the three reactants absorbance/ concentration. 

The absorber diagrammed below removes benzene from contaminated air. 

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