Diffusion control photolysis

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Since these values are of the same order of magnitude, Hammond assumed that the quenching was diffusion controlled with kq = 2 x 109 liters/mole-sec in (3.21). With this value for kv, the rates of the other processes can be determined and are shown in Table 3.1. As we shall see, these values agree well with those obtained by Linschitz using the technique of flash photolysis. 

A mixture of 2-naphthoylazide and sensitizer 2-isopropylthioxanthone ( r = 65 kcal/mol) was irradiated (k > 385 nm), conditions under which the sensitizer absorbed >95% of the light. Laser flash photolysis experiments demonstrated that the triplet sensitizer is quenched by 2-naphthoylazide at a diffusion controlled rate. 

McClelland and co-workers verified the absolute magnitude of the k Q values for 76h, 76n, and 76o from measurements of the effect of d-G on the rate constants for disappearance of these ions that had been generated by laser flash photolysis. " They provided additional data, included in Table 3, for 75g and 75p-75v. Their results confirm that k -c reaches an apparent diffusion-controlled limit of ca. 2.0 x 10 M s for the... 

Since these isotope effects have been interpreted in terms of a physical transition state, it is instructive to contrast this phenomenon with studies in which the absence of an isotope effect was used to demonstrate a physical rate determining step [97]. The most relevant example is rotational-diffusion control of radical disproportionation in the solid-state photolysis of azobisis-obutyronitrile (AIBN). Since there is normally a primary isotope effect on the disproportionation of cyanoisopropyl radicals to methacrylonitrile and isobutyronitrile, the absence of such an effect in the solid-state photolysis of... 

The extension of equilibrium measurements to normally reactive carbocations in solution followed two experimental developments. One was the stoichiometric generation of cations by flash photolysis or radiolysis under conditions that their subsequent reactions could be monitored by rapid recording spectroscopic techniques.3,4,18 20 The second was the identification of nucleophiles reacting with carbocations under diffusion control, which could be used as clocks for competing reactions in analogy with similar measurements of the lifetimes of radicals.21,22 The combination of rate constants for reactions of carbocations determined by these methods with rate constants for their formation in the reverse solvolytic (or other) reactions furnished the desired equilibrium constants. 

Values of for chloride ions have been determined by combining a rate constant for solvolysis ksoiv (for reactions for which the ionization step is ratedetermining) with a rate constant for the reverse reaction corresponding to recombination of cation and nucleophile. The latter constant may be found (a) by generating the cation by photolysis and measuring directly rate constants for reactions with nucleophiles or (b) from common ion rate depression of the solvolysis reaction coupled with diffusion-controlled trapping by a competing nucleophile used as a clock. 

In a series of papers, Chateauneuf and Brennecke (Roberts et al., 1993b, 1995) reported absolute rate constants for the dimerization of benzyl radical determined using laser flash photolysis (LFP). The termination rate constant (2 kj) was diffusion-controlled and, thus, decreased slightly with increasing pressure (e.g., 2 fcT = 4 x 1010 and 2 x 1010 s 1 at 80 and 100 bar,... 

Roberts, C. B. Zhang, J. Brennecke, J. F. Chateauneuf, J. E. Laser Flash Photolysis Investigations of Diffusion-Controlled Reactions in Supercritical Fluids. J. Phys. Chem. 1993a, 97, 5618-5623. 

The dynamics of reversible onium ion formation has been studied by generating carbenium ions in the presence of nucleophiles using pulse radiolysis or flash photolysis, and following the rate of disappearance of the carbenium ions by UV. As discussed in Chapter 2, the kinetics of reaction of various electrophiles with nucleophiles obey a general reactiv-ity/selectivity relationship. The rates of reaction of various nucleophiles with carbenium ions are summarized in Table 9. These rates often approach diffusion controlled limits (k 10 ° mol-,-L-sec l). The rates are slower for less nucleophilic and less electrophilic compounds, and are particularly slow with sterically hindered amines such as lutidine (2,6-dimethylpyridine) [63]. Solvent effects are minimal when the reactions are diffusion controlled, although tributyl amines react slower with carbenium ions in more nucleophilic dichloroethane than in methylene chloride. 

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