Photochemical reactions, dissociation

Photochemical reactions, dissociation of trihalometh-anes, 52, 132 rearrangement, 52, 53 Phthalic acid, reduction, 50,... 

Alkene-metal complexes are usually prepared by a process by which some other ligand is dissociated from the metal. Both thermal and photochemical reactions are used. 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

It is the same rate as given in thermal reaction given by equation (3.31). Thus, both in thermal and photochemical reactions, the primary process is the dissociation of Br2 molecules. In spite of the chain mechanism, the quantum yield of H2 and Br2 reaction is very small, i.e. about 0.01 at ordinary temperature, although it increases as the temperature is raised. The reason is that the reaction immediately after initiation following the primary state, i.e. 

From the foregoing discussion it might seem fruitless to utilize MNP to investigate photochemical reactions. However, the monomer is transparent between ca. 270 and 550 nm, and by irradiating reaction mixtures in this window excellent results have been obtained without complications from DTBN formation (Leaver et al., 1969 Leaver and Ramsay, 1969a,b Torssell, 1970). This expedient is unfortunately not infallible, there being good evidence that aromatic ketones can photosensitize MNP dissociation (Ikeda et al., 1978). 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

Fluctuations in fluorescence intensity in a small open region (in general created by a focused laser beam) arise from the motion of fluorescent species in and out of this region via translational diffusion or flow. Fluctuations can also arise from chemical reactions accompanied by a change in fluorescence intensity association and dissociation of a complex, conformational transitions, photochemical reactions (Figure 11.10) (Thompson, 1991). 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

We report the experimental observation of laser control over a branching photochemical reaction. The reaction studied is the 2-photon dissociation of... 

The concentration of bromine atoms in the dark reaction is known from the concentration of molecular bromine and the thermal dissociation constant of bromine, hence that prevailing at any stage of the photochemical reaction is found. 

Photochemomechanical systems have also been studied using gels and photochemical reactions such as photochromism of spiropyrans [20], trans-ds transition of azobenzene [21] and photo-dissociation of triphenylmethane leuco cyanide [22]. It is an attractive approach to utilize a molecular level conformational change. 

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

Cis-trans isomerization can take place either photochemically or in the dark, but the reaction pathways are quite different. In the light-induced process the reaction goes through a tetrahedral intermediate formed from the triplet excited state, whereas the dark reaction involves a dissociation of the complex, followed by recombination. In the latter case the presence of free glycine is demonstrated by the use of radioactive tracers no free glycine appears in the photochemical reaction. 

---