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Photochemical mechanisms, distinction

The beginning and end points of a photochemical reaction pathway are the structures of the starting materials (substrates) and the isolated products. Elucidation of product structures can be carried out by conventional methods. Structure determination for products derived from labelled substrates, such as those with isotopic labels or with extra substituents, or from substrates with distinctive stereochemical features, can result in the elimination of certain mechanistic possibilities and provide support for others. Two key questions for photochemical mechanisms, as for thermal mechanisms, are whether or not a reactive intermediate (such as a biradical) lies on the reaction pathway, and if so, what are the rate constants for reaction steps subsequent to its formation. Questions that are peculiar to photochemical mechanisms mav be expressed ... [Pg.31]

FIGURE 1. A schematic photochemical mechanism, showing some of the possible elementary transformations. For the purpose of illustration, it is assumed that the states A and A2 have the same multiplicity, and correspond to the ground and lowest excited singlet states of most organic molecules. The state A] would then represent the lowest triplet state. Thus 21 and 11 are radiative transitions, fluorescence and phosphorescence, respectively, and 23 and 13 (intersystem crossing) and 22 (internal conversion) are nonradiative. All of 8, C, D, and F are chemical species distinct from A. Only vibrationally equilibrated electronic states are included in this mechanism (see discussion in Section III.A.l). [Pg.150]

The reactions initiated by atoms do not belong in the realm of photochemistry since under a given set of experimental conditions these reactions will be the same no matter how the atoms are formed. The classical work on reactions initiated by chlorine after absorption of light has shown that chain reactions may be started38. The thermal and photochemical hydrogen-bromine reactions lead to the same products with the same mechanism, the one distinction being that bromine atoms... [Pg.31]

Both thermal and photochemical processes take the form of a dehydrochlorination reaction which leads to discolouration as well as extensive changes in the internal structure of the polymer which has an unfavourable effect on the desirable electrical and mechanical properties. It has become apparent that considerable similarity exists between the two degradation processes and that it is neither easy nor desirable to make a vigorous distinction between the two. Information gained from e eriments on thermal degradation are often directly relevant to the analogous photochemical process. [Pg.208]

Over the past few years it has often been observed that the photochemical behaviour of adsorbed molecules is distinctly different to that of their gas phase counterparts. Even direct dissociations of molecules physisorbed on insulator substrates were found to have different dynamics to the analagous gas phase reaction, and exhibited a dependence on the coverage. This needs to be understood. For adsorbed molecules a new kind of "dissocation" is possible, namely desorption, Photolytic (non thermal) desorption has been reported from all kinds of substrate. On metal surfaces it is often found that the quantum yield for a direct photodissociation reaction is much lower than in the isolated molecule. This must be accounted for. Finally, the observation which has stimulated a great deal of research in surface photochemistry, photolysis is observable at energies where the gas phase molecules are transparent. It turns out that all of these interrelated effects can be interpreted by a delicate interplay of excitation mechanism and transient quenching. The fine details of course depend on particular adsorbate-substrate systems, which are described in section 4. [Pg.488]


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Photochemical mechanisms

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