Direct photolysis definition

One of the crucial points in the recent developments of coordination compound photochemistry has been the debate concerning the identity of the excited state(s) responsible for the photosolvation reactions that are obtained by irradiating Cr(III) complexes in their ligand field bands (5,8), Direct photolysis experiments revealed that the most likely candidates (see e.g,. Figures 2 and 3) are the lowest spin-allowed excited state ( T2ff in octahedral symmetry) and the lowest spin-forbidden excited state ( Eg). Such experiments, however, did not warrant any definite conclusion about the actual role of each of these two states (5). 

The photolysis of diazoalkanes both in the gas phase and in solution is a carbenoid reaction. Moreover, the results of EPR-spectroscopic investigations (Section IIB) demonstrate that triplet carbenes can be generated by irradiation of diazoalkanes. That the reactive intermediates in carbenoid reactions are free carbenes is usually taken as self-evident. While such an assumption is probably wholly justified in most cases, it is worth remembering that both in the gas phase and in solvents such as n-hexane, the electronic absorption spectra of simple diazoalkanes show definite fine structure (Bradley etal., 1964a). This implies that the photo-excited state is bonding (Hoffmann, 1966) and consequently may have a life-time long enough to enable it to react directly with another molecule... 

A number of studies, also by our group, have been devoted to the comparison of the kinetics of direct and sensitized photolysis in real or simulated surface waters (see e.g. [68-74]), but it is difficult to draw a definite conclusion because... 

While the significance of radicals in biological systems has been appreciated for decades, there is relatively little definitive experimental infonnation on the identity of the radicals and even less on the mechanisms by which they affect the physiology of living systems. The paucity of detailed information is a direct consequence of the fact that most radicals are highly reactive and, therefore, short-lived transient species. Despite the tremendous advances in spectroscopic and laser photolysis techniques, much less is known about radicals than about closed-shell species. The treatment of radicals by theoretical methods is, however, only marginally more difficult than that of closed-shell molecules. It is for these reasons that the numerous applications of quantum chemical techniques to radicals have proven to be complementary to experimental studies. 

The quantitative experiments by Wrighton et al. (106-108,111) have provided strong evidence for this general interpretation of the net photochemistry. More definitive, however, would be direct information, perhaps obtainable from laser flash photolysis studies, regarding the origins of the photolytically produced transients. 

Even complex chemical reaction mechanisms can be separated into several definite elementary reactions, i. e. the direct electronic interaction process between molecules and/or atoms when colliding. To understand the total process B-fot example the oxidation of sulfur dioxide to sulfate - it is often adequate to model and budget calculations in the climate system to describe the overall reaction, sometimes called the gross reaction, independent of whether the process A Bis going via a reaction chain A C D E. .. Z B. The complexity of mechanisms (and thereby the rate law) is significantly increased when parallel reactions occur A X beside A- C,E- X beside E F. Many air chemical processes are complex. If only one reactant (sometime called an educt) is involved in the reaction, we call it a unimolecular reaction, that is the reaction rate is proportional to the concentration of only one substance (first-order reaction). Examples are all radioactive decays, rare thermal decays (almost autocatalytic) such as PAN decomposition and all photolysis reactions, which are very important in air. The most frequent are... 

As with phenylcarbene (35) (c/. Scheme 5), the ring expansion of phenylnitrene (75) could occur directly on the singlet potential energy surface or via a bicycHc intermediate, in this case azirine 92. Such azirines were first detected in the matrix photolysis of 1- and 2-azidonaphthalene. For example, photolysis of 93 generated a photoproduct with an IR absorption at 1730 cm, which in turn gave a cyclic ketenimine [v(C=C=N) at 1926 cm ] on further photolysis. It was proposed that the intermediate with the 1730 cm IRband was the tricyclic azirine 94, which subsequently rearranged to 95. At the time of these experiments, however, reliable computations of IR transitions were not available, so a definitive identification of the azirines was scarcely possible. Nevertheless, a clear parallel was found here with the behavior of aryl carbenes, which give detectable cyclopropenes in the naphthalene series (c/. 44 and 45) but not with the monocyclic carbenes (see the brief discussion of this point in Section 14.4). 

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