Reaction spectra, absorbance time diagrams

Even though applications are given in the next chapter, in this section the principle approach of the first steps of photokinetic analysis are discussed. By these means the necessity for spectral data becomes obvious. 

All the recorded spectra result in an overlay of spectra representing the progress of the photoreaction. This sequence of spectra is called a reaction spectrum. An example is given in Fig. 4.16 for the photoisomerisation of trans- to c. r-stilbene with consecutive rearrangement to phenanthrene. Since the dihydrophenanthrene is very unstable at the chosen conditions and time domain, it is not observable as an intermediate as in flash photolysis experiments [60,61]. In liquids and solids, absorption spectra do not permit the characterisation of the individual reactants. Therefore a reaction spectrum merely informs the photochemist about the complexity of a reaction at first glance. In addition, wavelengths can be selected which best represent the 

If a reaction spectrum exhibits isosbestic points, the reaction is assumed to be a single step process. During complex reactions, time intervals can be found during which quasi-isosbestic points are recorded. In both cases the assumption of a simple reaction can be erroneous because of similar absorption spectra of reactants or lack of spectral resolution of the spectrometer. Therefore the simplicity of any reaction has to be proven by other methods. In any case an irradiation wavelength close to an isosbestic point or exactly at the wavelength of the isosbestic point reduces the expenditure in evaluation as discussed in Section 3.3.1 (Example 3.31). 

In consequence without further knowledge such types of consecutive and parallel reactions cannot be distinguished. To evaluate the rate laws derived in Chapters 2 and 3, the concentrations of the reactants have to be known. For this reason the next step is to get the concentrations dependent on time out of the reaction spectrum or the absorbance-time curves. For further details see Sections 5.1 and 5.3.3. 

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To measure the rate of dissolution of solutes and rate of chemical reactions in supercritical CO2, Hunt et al. designed a high-pressure fiberoptic cell connected to a CCD (charge-coupled device) array UV-Vis spectrometer (20). The CCD array spectrometer allows rapid measurement of the UV-Vis spectra of solutes (e.g., one spectrum per second) in supercritical CO2. Rate information can be derived from data regarding variation of absorbance with time obtained from the fiberoptic reactor. A schematic diagram of the high-pressure fiberoptic reactor is shown in Figure 2. The fiberoptic reactor was used to measure the rate of formation of metal nanoparticles in a water-in-supercritical CO2 microemulsion (20). The reactor was also used to measure the rate of dissolution of ferrocene in supercritical CO2 (20). 

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