Photochemical after-effects

The photochemical after-effect, considering that after light is switched off, the light oxidation continues for some time (time of chain growth), also indicates the chain nature of the process. This after-effect was observed in the photochemical oxidation of... 

Photochemical processes are particularly suitable for grafting in homogeneous solutions but have disadvantages when applied to solid substrates of irregular shape, since it is virtually impossible to ensure uniform illumination of the surface. Fortunately with Mn2(C0)j and Re2(C0)j Q long-lived photochemical after-effects are readily obtained, and with their aid grafting... 

The effect of zearalenone on crop development may be connected to its influence on the status and functioning of the photosynthetic apparatus (Koscielniak et al. 2008). The after-effects of zearalenone on the growth of soybean and wheat plants, net photosynthesis and transpiration rates, stomatal conductance, photochemical efficiency of photosystem 2 and on final seeds yield were determined. Modifications in leaf area were more pronounced in soybean than in wheat, and this tendency increases in successive developmental phases. The net photosynthesis was stimulated during the juvenile phase and during that of the final one by about 13.6% (average) in soybean plants. Stimulation of transpiration was also observed after... 

If the primary process is followed by another reaction or if collisions with other molecules are necessary for the reaction, then the reaction rate will depend on concentration. If it depends only on the concentration of the absorbing material the reaction is of the first order, and if it depends on the square of the concentration it is of the second order. Examples of both are known but in photochemical reactions as in thermal reactions the over-all observed reactions frequently appear to follow neither the first nor the second order, usually on account of the existence of two or more reaction steps. Sometimes a. second reaction is slow in getting started because time is necessary to accumulate some of the product of the first reaction. This situation leads to a time-lag or induction period. Again the second reaction may continue after the primary photo-reaction has stopped, giving rise to after-effects. 

When radical generation is stopped abruptly, polymerization dies out at a rate proportional to the decreasing concentration of radicals. This is called the post or after effect. In a defined manner it can be brought about only in photochemically initiated polymerizations simply by switching off the light. In the course of polymerization decay, radicals are consumed by mutual termination... 

The pre- and after effect method [51 Bat 1] is analogous to the non-stationary state method except that the rate of a steady-state reaction is disturbed photochemically. 

Therm, of AIBN -I- methyl oleate -1- O2 Photochem. chlorobenzene after-effect 343 2k, = 5.6-10 M- s-> 80 Yuri... 

The predictions of simple rules such as Kaptein s and Muller s can be distorted by relaxation effects. These are particularly noticeable in photochemical experiments. In the pre-steady state (e.g., immediately after irradiation has begun), when build-up of the polarized signals is occurring, relaxation effects in the final product are relatively unimportant and observed spectra accord with the simple theory. Con-... 

Points 2-5 tend to indicate that electronic effects are important in the dimerization. More will be said about this after the kinetic data for the dimerization are added to the picture. First, however, let us see how preparative photochemical reactions are carried out. 

The cage effect described above is also referred to as the Franck-Rabinowitch effect (5). It has one other major influence on reaction rates that is particularly noteworthy. In many photochemical reactions there is often an initiatioh step in which the absorption of a photon leads to homolytic cleavage of a reactant molecule with concomitant production of two free radicals. In gas phase systems these radicals are readily able to diffuse away from one another. In liquid solutions, however, the pair of radicals formed initially are caged in by surrounding solvent molecules and often will recombine before they can diffuse away from one another. This phenomenon is referred to as primary recombination, as opposed to secondary recombination, which occurs when free radicals combine after having previously been separated from one another. The net effect of primary recombination processes is to reduce the photochemical yield of radicals formed in the initiation step for the reaction. 

A powerful explosion which occurred dining distillation of a 10-year-old sample of the alcohol was attributed to presence of peroxy compounds formed by autoxidation, possibly involving 2-butanone as an effective photochemical sensitiser [1], After a later explosion, it was found that the sample being distilled contained 12% of peroxide [2], A further incident involved a 12-year old sample which exploded at the end of distillation, and which also contained a high level of peroxide. Several other stock alcohols were found to contain much lower levels of peroxide than the 2-butanol, and recommendations on clean-up or disposal, depending on the level of peroxide, are made [3], A further report of an explosion at the end of laboratory distillation confirms the potential for peroxide formation on prolonged storage of 2-butanol [4],... 

The charge-transfer nitrations of the aromatic donors are generally carried out to rather low actinic conversions to avoid complications from light absorption by the nitroarene products, and in duplicate sets (with a dark control) to monitor simultaneously any competition from thermal processes. For example, the yellow solution of anisole and Me2PyN02 in acetonitrile at — 40°C is irradiated with the aid of the cut-off filter that effectively removes all excitation light with Aexc<400nm. After reasonable photochemical conversions are attained, the H NMR spectrum is found to be virtually identical to that of the reaction mixture obtained by electrophilic (thermal) nitration (60). 

Here, L total is the depth of the etched hole per pulse and is assumed to be the sum of photochemical and photothermal contributions, Tphoto and Thermal, respectively 0Ceff is the effective photon absorption coefficient of the medium and can vary with laser emission characteristics, e g., photon density Fis the incident laser fluence Fth is the medium s threshold fluence A and F are the effective frequency factor with units of pm/pulse and the effective activation energy with units of J/cm2, respectively, for the zeroth-order thermal rate constant F0, comparable in magnitude to Fth, is important only at low fluences.64 Equation (5) is obtained after assuming that the polymer temperature T in the laser-exposed region of mass mp and the thermal rate constant k are given, respectively, as... 

Table 14.1 Energy dependence of the secondary isotope effect ku/kc at zero pressure. Energies inkJ/mol (After Rabinovitch, B. S., Setser, D. W,Adv. Photochem. 3,1 (1964))...

After the pioneering studies of Haagen-Smit, an extensive scientific literature developed on the properties, measurement, and effects of photochemical smog. The attempt to control engine emission has had a profound effect on the automobile and petroleum industries. Estimated costs and associated benefits of automobile emission control each run into the billions per year. ... 

The photochemical isomerization 41B — 41A was observed in nonpolar solvents (83AJC1603). Equilibrium 1) was attained in CD3OD solution after several days, but following concentration and cooling of the solution, only the cyclic isomer 41B, solvated by methanol, was isolated in the solid state. Alcoholic solvents effectively quench the photochemical isomerization. 

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