Pulse nonaqueous solvents

In nonaqueous solvents, such treatment with dry hydrohahc acids is the only way to cleave the /r-oxo dimer nonoxidatively. However, the /x-oxo dimers of water-soluble porphyrins are readily cleaved by Tewis bases such as hydroxide, imidazole, histidine, and pyridine. Both the equilibria and kinetics of such reactions have been reported. In addition, /x-oxo dimers of water-insoluble Fe porphyrins in dichloromethane can be oxidatively cleaved to yield PFe p2 and (probably) an Fe species. Studies of the picosecond decay of the excited state of (TPPFe)20 in benzene following a 532- or 355-nm 25-ps pulse suggest that the intermediate state is a photodissociated pair, (TPP -)Fe -l-TPPFe — (0 ), and a small amount of disproportionation reaction products, TPPFe -f TPPFe = O. ... 

In the following section some pulse radiolysis investigations are described in which the radiolysis of nonaqueous solvents has been used to generate one-electron redox reagents and, in some cases, to characterize their involvement in electron-transfer reactions. 

Among the methods that have considerable promise but that are yet to be significantly exploited are pulse electrochemical techniques, impedance methods, flow-injection analysis, the use of nonaqueous solvents in the sensor, the combined use of chemometrics and multi-electrode measurements for analysis of complex mixtures, the use of ultramicroelectrodes in applications outside the clinical and biological areas, and rapid deaeration of flow systems. 

A significant modification to the simple mixing of solutions is the slow and controlled release of one of the reactants. For example, Ohtaki et al. prepared CdS nanoparticles in nonaqueous solvents, where one of the reactants (S ) was made available slowly and uniformly via the controlled hydrolysis of P2S5 (114). The CdS particles were, on average, 6 nm in diameter, had a size distribution standard deviation of 1.2 nm, and formed stable suspensions under the protection of polymers (Figure 14). A similar strategy was used by Meisel and coworkers (115) and, more recently, by Yin et al. (116) in the preparation of CdS nanoparticles, where the slow release of S was achieved through the use of pulse radiolysis. 

One of the problems in electrocatalysis is that electrochemical reactions are generally carried out in aqueous or nonaqueous solution. Thus, the solvent may intervene in the over-all reaction. In addition, it is necessary to carry out the reaction under highly purified conditions. Otherwise, impurities in the solution may affect the kinetics of the reaction concerned, so that mechanism studies become difficult. For gas phase reactions, though impurity concentrations are generally lower than in electrochemical reactions, one uses high-vacuum techniques for purification. Electrochemical purification techniques— pre-electrolysis or adsorption of impurities near the potential of maximum adsorption—are often simpler. The activation of a poissoned catalyst is often difficult or impossible. An electrocatalyst can often be reactivated in situ, by pulse techniques (cf. Section VII,D). 

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