Rate constants, photochemical reaction

The reaction of thermally and photochemically generated tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

The protonation of photochemically generated singlet nitrenes in aqueous solution has recently been used to study a wide range of nitrenium ions." Data on the rate constants for reaction of 75g, 75n, and many other nitrenium... 

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 ... 

The effect of substituents on the rate constant for reaction of H with benzene was also examined. Correlation with Hammett s substituent constants was possible and showed that the hydrogen atom behaves as a slightly electrophilic reactant (Neta, 1972a, and references therein see also Brett and Gold, 1971, 1973 and photochemical studies by Pryor et al., 1973). 

Vapor-phase rate constant for reaction with photochemically produced hydroxy radicals... 

I.r. lasers based upon photochemically excited chloroethylenes,153 yielding vibrationally excited HC1, have been discussed in great detail, and a simple model has been developed to explain vibronic state distributions. There is insufficient space here to do credit to the arguments outlined in this extensive article, but interested readers are recommended to study it. The reactions of vibrationally excited HC1 with and H atoms have also been reported.154 Absolute rate-constants for reactions of chlorine atoms with HX (where X = Br or I) [reaction (73)] have been determined as 4.56 x 10 and 5.76 x 10111... 

Quantum yields for production of D2 on photolysis of Cr2+ in D20 solution are lower than those for H2 liberation from H20.38 From pulse radiolysis experiments Cohen and Meyerstein39 have determined the rate constant for reaction (10) (1.5 x 10 dm3 mol-1 s-1) and they have compared their results with those obtained in earlier photochemical studies of Cr2+. 

Instead of the simple hjq>othetical reaction in equation 12.4, organic molecules normally exhibit a number of photochemical and photophysical processes that compete with each other. A minimal complication is illustrated in equation 12.8, in which the photoexcited molecule both fluoresceses and forms product. The rate constant for fluorescence is kf, and the rate constant for reaction is k. ... 

The photochemical reactivity of metal complexes is quite varied. As an example, Cr(III) complexes such as Cr(NH3)6, [Cr(NH3)5X], and related species undergo photosolvolysis, often with a quantum yield as high as 0.4 or more, in contrast to then-great thermal stability. Taking into account the short lifetime of the excited state, the pseudo-first-order rate constant for reaction is in the range 10 -10 s, which contrasts with the ground state rate constant of 8 x at 25 °C. Thus, an... 

What are photochemical reactions Describe rate constant of reactions alongwith its determination and types of photochemical reactions. 

A recurring theme in this article has been the close links between the reaction and nonreactive relaxation of excited species. For the interpretation of competitive experiments, such as bulk photochemical studies on hot atom reactions, as well as chemical and photochemical activation experiments on unimolecular reactions, more accurate and detailed information about the energy-transfer processes are required. In other more direct experiments, for example, those in which fluorescence or chemiluminescence is observed, it is often difficult to determine whether it is reaction or relaxation by the active species which predominates. As we have seen, a powerful method of obtaining detailed rate constants is to apply the equations derived from the principle of microscopic reversibility to the results of experiments on exothermic processes. In favorable, nearly thermoneutral, cases, a detailed rate constant for reaction can then be compared with the rate constant for total removal obtained directly. 

Uncertainties in Photochemical Models. The ability of photochemical models to accurately predict HO concentrations is undoubtedly more reliable in clean vs. polluted air, since the number of processes that affect [HO ] and [H02 ] is much greater in the presence of NMHC. Logan et al (58) have obtained simplified equations for [HO ] and [HO2 ] for conditions where NMHC chemistry can be ignored. The equation for HO concentration is given in Equation E6. The first term in the numerator refers to the fraction of excited oxygen atoms formed in R1 that react to form HO J refers to the photodissociation of hydrogen peroxide to form 2 HO molecules other rate constants refer to numbered reactions above. 

The dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

It is important to point out at this point that the rate constant k and the quantum yield for a photochemical reaction are not fundamentally related. Since the quantum yield depends upon relative rates, the reactivity may be very high (large kr), but if other processes are competing with larger rates, the quantum yield efficiency of the reaction will be very small. That there is no direct correlation between the quantum yield and the rate is clearly seen from the data in Table 1.2 for the photoreduction of some substituted aromatic ketones in isopropanol ... 

The rate of reaction is dependent upon both the rate constant and the concentration of reactant molecules. Photochemical reactions occur through transformations of molecules which have a new distribution of electron density due to light excitation. The steady-state concentration of these excited molecules is given by... 

It should be noted that this expression is a general one that can be used for any photochemical reaction that can be quenched. It is commonly called the Stern-Volmer equation. This equation predicts that if the proposed mechanism is correct, the data, when plotted as 4>a0/4>a vs. [Q], should be linear with an intercept equal to unity and a slope equal to kqr. Linear plots were indeed observed out to large d>°/d> values. Assuming a value of 5 x 10 M 1 sec-1 for the quenching rate constant,(7) the data presented in Table 4.1 were obtained. 

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