Direct photolysis first-order rate constant

First-Order Rate Constant for Quantification of Direct Photolysis Illustrative Example 15.3 Estimating the Photolysis Half-Life of a Weak Organic Acid in the Well-Mixed Epilimnion of a Lake Determination of Quantum Yields and Chemical Actinometry Advanced... 

First-Order Rate Constant for Quantification of Direct Photolysis... 

Similarly, the near-surface first-order rate constant k° for direct photolysis is given by ... 

Some reactions are difficult to study directly because the required instrumentation is not available or the changes in standard physical properties (light absorption, conductivity etc.) typically used in kinetic measurements are too small to be useful. Competition kinetics can provide important information in such cases. In some situations, the chemistry itself makes direct measurement inconvenient or even impossible. This is the case, for example, in studies of slow reactions of free radicals. Because of the ever-present radical-depleting second-order decomposition reactions, slow reactions of free radicals with added substrates are possible only at very low, steady-state radical concentrations. The standard methods of radical generation (pulse radiolysis and flash photolysis) are not useful in such cases, because they require micromolar levels of radicals for a measurable signal. The self-reactions usually have k > 10 M s , so that the competing reactions must have a pseudo-first-order rate constant of lO s or higher (or equivalent, if conditions are not pseudo-first order) to be observed. Competition experiments, on the other hand, can handle much lower rate constants, as described later for some reactions of C(CH3)20H radicals with transition metal complexes. 

How is the first-order direct photolysis rate constant of a given compound defined What is(are) the prerequisite(s) that direct photolysis can be expressed by a first-order rate law ... 

Photolysis pseudo-first order reaction rate constant for direct photolysis k = 0.009 min1 with t, = 76.8 min. in dilute aqueous solution (Peijnenbuig et al. 1992). 

Photolysis measured pseudo-first-order reaction rate constant k = 0.014 min- for direct photolysis in aqueous solutions with = 50.7 min. (Peijnenburg et al. 1992). 

Quenching rates of 0( D) with Xe, Oj, O3, CO, CO, Nj, NO, N O, NOj, Hj, H2O, and CH4 have been measured (10, 262). Rate constants arc on the order of 10" cm molec sec , that is, the reactions proceed with almost unit collision efficiency. Noxon (745) and Clark and Noxon (216) have measured the absolute emission intensity at 6300 A in the steady state photolysis of O2 and CO2 at 1470 A. From the rate of production ofO( D) Noxon was able to obtain a rateconstant of6 x 10 cm molec sec for 0( D) quenching by O2. Gilpin et al. (398) were the first to detect 0( D) atoms directly and measure their decay following the flash photolysis ofO, in the Hartley band. The 6300 A emission disappeared within 200 /isec after the flash at an O3 pressure of 3 mtorr. From the exponential decay curve integrated over 600 shots, the authors have obtained a rate constant of 2.5 1 X 10 cm molec sec for the reaction 0( D) O3. Rate con-... 

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