Kinetic data quenching

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

The reduction of Co(lll) by Fe(II) in perchloric acid solution proceeds at a rate which is just accessible to conventional spectrophotometric measurements. At 2 °C in 1 M acid with [Co(IlI)] = [Fe(II)] 5 x 10 M the half-life is of the order of 4 sec. Kinetic data were obtained by sampling the reactant solution for unreacted Fe(Il) at various times. To achieve this, aliquots of the reaction mixture were run into a quenching solution made up of ammoniacal 2,2 -bipyridine, and the absorbance of the Fe(bipy)3 complex measured at 522 m/i. Absorbancies of Fe(III) and Co(lll) hydroxides and Co(bipy)3 are negligible at this wavelength. With the reactant concentrations equal, plots of l/[Fe(Il)] versus time are accurately linear (over a sixty-fold range of concentrations), showing the reaction to be second order, viz. 

For iron(III) eomplexes, uic venues /vlh [Fe(aepa)2]BPh4 H2O and k = 6.7 x 10 s for [Fe(mim)2(salacen)]PF6 have been obtained [156, 166]. The rate constants derived from the line shape analysis of Mossbauer spectra thus vary between 2.1 x 10 and 2.3 x 10 s at room temperature, no significant difference between iron(II) and iron(III) being apparent. In addition, it is evident that the rates of spin-state conversion in solution and in the crystalline solid are almost the same. For iron(II) eomplexes, for example, the solution rates vary between /cjjl = 5 x 10 and 2 x 10 s , whereas in solid compounds values between kjjL = 6.6 x 10 and 2.3 x 10 s have been obtained. Rates resulting from the relaxation of thermally quenched spin transition systems are considerably slower, since they have been measured only over a small range of relatively low temperatures. Extrapolation of the kinetic data to room temperature is, however, of uncertain validity. 

Various kinds of information can be expected from the high pressure combustion and flame experiments Reaction kinetics data for conditions of very high collision rates. Results about combustion products obtained at high density and with the quenching action of supercritical water, without or with flame formation. Flame ignition temperatures in the high pressure aqueous phases and the ranges of stability can be determined as well as flame size, shape and perhaps temperature. Stationary diffusion flames at elevated pressures to 10 bar and to 40 bar are described in the literature [12 — 14]. 

Kinetic data have been reported for reduction of //-superoxo complexes by Fe2+,7 1 Mov,702 Co11703 and Ru11 complexes,704 and V2+, Cr2+ and Eu2+.705 These processes involve outer-sphere electron transfer and in some cases703,706 the Marcus theory has been applied to the rate constants obtained. Electron transfer quenching of the excited state of [Ru(bipy)3]2+ by various -superoxo cobalt(III) complexes leads to production of [Ru(bipy)3]3+ and the corresponding /z-peroxo species.706... 

For reactions that are slow at room temperature, one approach is to initiate reaction and confine the reaction to an autoclave. Following decompression the contents can be analysed to assess the progress of the reaction. This procedure was used to determine the first volume of activation for an inorganic reaction. Apparently only one elevated pressure was used to estimate the value.44 In more current practice repeating the process and arresting the reaction at different time intervals could lead to a reaction profile at a given pressure. The whole procedure would then be repeated at several different pressures, and kinetic data treated according to Equation (9). Obviously such a primitive method is to be avoided, as it is very labour intensive and wasteful of reactants. It would only be satisfactory if the analysis is very rapid compared with the reaction rate or the reaction can be quenched immediately upon decompression. 

Reaction Path During Quench. Unfortunately, there is a sparsity of high-temperature kinetic data for the H—C—N system. Hence, one is unable to predict with certainty the reaction path followed as the atomic-species H, C, and N, are cooled from 15,000°K. to 500°K. in 10 msec., for example. After an examination of the experimental results in the succeeding sections, it may be possible to infer the important steps in the reaction sequence. 

Fig. 2-1. Kinetic data may show that by quenching a reaction economic recoveries are feasible.

The kinetic data in Figure 2.2 indicate that carbene 10, under the conditions of our TRIR experiments, does not form ketene 11, in contrast to the observed reactivity of acyclic carbene 7. Platz and co-workers [94] determined that carbene 7 is separated from ketene 8 by a 3.4 kcal/mol barrier in hexafluorobenzene. It is likely that the higher singlet/triplet gap for carbene 10 relative to that of 7 raises the effective barrier to rearrangement. We find, by monitoring the lifetime of carbene 10 as a function of concentration of diazoester 9, that 10 is effectively quenched by 9 with /tdiazo=(5.0 0.5) x 10 M s in Freon-113. This observation suggests that a major decay route of the carbene under the conditions of our experiment is formation of azine 16. 

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