Consecutive photoreaction

The consecutive mechanism gives two linear independent steps according to 

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When irradiated in the presence of norbornadiene and high-pressure synthesis gas, rhodium chloride is converted to a catalyst which is active for a variety of reactions. /2A/. The salt is probably converted photochemically to the rhodium norbornadiene complex 9. This dimer may undergo a consecutive photoreaction to give the monomeric hydrido complex 10, which is the actual catalyst for polymerisation, hydrogenation, and hydroformylation reactions. 

Photoexcitation of trans,trans-l,5-di-9-anthrylpentadienone 60 gives isomer 63 whose formation by two consecutive photoreactions, i.e geometrical isomerization to give the cis-trans-isomer 61, followed by geometrically favored intramolecular Diels-Alder addition, has been established by quenching experiments. In the presence of molecular oxygen, the photochem-... 

The light-induced conversion of acyclic divinylmethane reactants 1 to vinylcyclopropanes 2 (cf. Houben-Weyl, Vol. 4/5a, pp 413-433), also termed the Zimmerman rearrangement, usually occurs from the excited singlet state of the diene.The resulting vinylcyclopropane is often prone to undergo consecutive photoreactions and therefore these transformations are usually run to about 50% conversion. Under these conditions the material balance (1 - -2) is still > 95%. 

In several cases it has been shown that consecutive photoreactions of the initially formed bicyclohexenones can be totally avoided if light of wavelengths > 370 nm is used. Thus, irradiation of 2,4,6-tri-teri-butyl-4-methoxycyclohexa-2,5-dienone (5) at 410 nm in either hexane solution or as a solid " gives much higher yields of the bicyclohexenones than previously reported.""... 

All the equations used above can be taken in principle to describe photochemical reactions also. However, the relationships become more complicated, since the amount of light absorbed has to be considered. Furthermore the quantum yield can depend on the concentration which makes the formulae more complex. Therefore in a simple example of a consecutive photoreaction, the correlation is demonstrated. Details are discussed in Chapter 3. 

Both the photo-parallel reaction and the consecutive photoreaction, being physically sensitised, form two linear independent steps of the reaction. Their mechanisms and the reduced reaction scheme are listed in Table 3.5. The overall reaction schemes are derived in Appendix 6.6.2. 

Mechanisms and reaction schemes of parallel and consecutive photoreactions, physically sensitised... 

The consecutive photoreaction according to the details given in Table 3.5 and the reaction scheme in Appendix 6.6.2.2 gives the concentrations of the excited intermediates as... 

The amount of light absorbed is substituted according to eq. (1.36) and the local average according to eq. (3.32) is formed. The result is an explicit expression for the change of the degree of advancement x of the Arth partial reaction step with time. This relationship can be used to set up the differential equations for the concentrations a, of the reactants A, according to eq. (2.5). This procedure is discussed in Section 2.2.1 and its subsections. The results for photoreactions are compared to the thermal Jacobi matrices in Section 2.2.1.4. There the results for the two linear independent steps of a consecutive photoreaction... 

Thus, using eq. (2.5) and following the procedure in Section 2.2.1 one finds the three differential equations for the concentrations of the reactants in the chosen consecutive photoreaction... 

If time is eliminated from the explicit differential equations, the resulting differential equations give the relationships between the concentrations and the degrees of advancement, respectively. In many applications simple solutions are obtained, if the quantum yields of the partial steps of reaction do not depend on the intensity of irradiation and no slow dark reactions are involved. A typical example is the consecutive photoreaction treated above and discussed next. 

The consecutive photoreaction is discussed in Example 3.25 in Section 3.2.3. Instead of eq. (3.40) derived there, eq. (3.83) can be used. In this case the parameter x has a different meaning. As demonstrated above it becomes... 

Example 5.6 Initial slopes of a simple consecutive photoreaction... 

In Section 3.1.4.2 a consecutive photoreaction had been treated in Example 3.24. Using the law of conservation of mass... 

Example 5.9 Simple consecutive photoreaction For the consecutive photoreaction... 

The evaluation according to Sections 3.2.2 and 5.1.2.1 needs in general the measured values of many components A,-. If the a,(0) values are just known for a few components, the methods given can fail. The a(0) values can be evaluated by themselves for the given example of a consecutive photoreaction. However, the fe(0) and c(0) values need another set of measurements each. 

Example 5.12 Formal integration of a simple consecutive reaction In Example 2.19 in Section 2.2.1.4 the consecutive photoreaction... 

This means that the consecutive photoreaction examined in Section 5.1.2.1 (Example 5.9) can be analysed in the M system using difference equations. The kinetic constants k, and kj can be determined and correlated with the two steps of the reaction. Furthermore recursion equations can be used. By multiplication of eq. (5.14) by m and taking into account eq. (5.27) one finds... 

Example 5.19 Transformation concentration to absorbance for a consecutive photoreaction For the reaction... 

Fig. 5.8 is an example of an ED-diagram of a consecutive photoreaction which proceeds according to eq. (3.39) and fulfils eq. (3.40) with <1 [131,143]. The different combinations of wavelengths allow two linear areas to be realsied in which one of the partial steps of reaction dominates. However, the typical slopes of Fig. 2.7 of Section 2.4.2.2 given by... 

Fig. 5.9. Reduced ED-diagram of the consecutive photoreaction on 2-methyI-anthraquinone in alkaline methanolic solution, NaOH (0.1 moi f ) wavelength of irradiation is 313 nm, the intensity of the light source is / = 1.24 x KT Einstein s" cm", cone. ( ), a, = 5.81 x I(T ...

All the elements of the matrix are varied by this transformation in such a way that the calculation is only possible if ail the elements of the matrix Q are known. Since this procedure is a similarity transformation, the matrices and have the same eigenvalues. This means that the consecutive photoreaction treated in Section 5.1.2.1 (Example 5.9) results in... 

Example 5.22 Application of formal integration to a consecutive photoreaction... 

The concentrations are substituted by the degrees of advancement. One obtains the matrix eq. (5.100) for these quasi-linear consecutive photoreactions. The dependence on time of the two degrees of advancement is given by... 

This is rather generalised advice to treat photokinetic data. Some of these points are applied to the stilbene photoreaction as an example. The mechanism of a consecutive photoreaction is obvious. 

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