Studies with Accelerated Ions

A scintillator, sometimes known as the Daly detector, is an ion collector that is especially useful for studies on metastable ions. The principle of operation is illustrated in Figure 28.4. As with the first dynode of an electron multiplier, the arrival of a fast ion causes electrons to be emitted, and they are accelerated toward a second dynode. In this case, the dynode consists of a substance (a scintillator) that emits photons (light). The emitted light is detected by a commercial photon... 

The presence of such a mechanism was prohed hy varying the sweep rate as illustrated in Fig. 4. Caution must he exercised in drawing conclusions from such a study because, with hahde ions (and with some other unidentate species), bridging by the halide between the copper ion and the electrode surface may accelerate the rate of electron transfer and lead to erroneous conclusions. This type of mechanism has also been proposed by Palaniandavar and coworkers [121] for the Cu(II/I) complex with deprotonated salicylideneglycine in the presence of cytosine or cytidine in which the latter species tends to be coordinated only to the oxidized complex. 

Another interesting and important use of radiotracers in industry is in wear and corrosion studies. In studies of wear (tribology) one labels the part under study with a radionuclide. The radioactivity is concentrated on the surface undergoing wear by plating, diffusion, or ion implantation with a low-energy accelerator. The labeled part is put in service and, typically, one measures the radioactivity released in the lubricant as a measure of wear. Calibrations of the technique can be done to get absolute measures of wear. 

The effect of cations on the rate of hydrolysis of cycio-tri- and cvc/o-tetra-phosphates has also been studied.260 Alkali metal cations retarded the hydrolysis of both cyclo-phosphates in acidic solution but accelerated the reaction in basic solution. The catalytic activity followed the sequence Li+ > Na+> K+. Mg11, Ca11 and Ni11 retarded the hydrolysis of the cyclo-phosphates in the pH range 1.0-2.0 or 1.0-2.7, but accelerated the reaction at pH>3. Catalysis by Cu11 is observed above pH 2, while Alni has a marked catalytic effect at pH 1 and 2. A variety of other papers have appeared dealing with metal ion-promoted hydrolysis of polyphosphates.261-264 However, the mechanistic details of these reactions remain unclear and the catalytically active complexes remain undefined. 

Mg2+ is associated with a large number of enzymes involving the hydrolysis and transfer of phosphates. The MgATP complex serves as the substrate in many cases. As noted in Section 62.1.2.2.2, the interaction of Mg2+ with the ATP enhances the transfer (to a substrate or water) of the terminal phosphoryl group. The results of many studies with model compounds lead to the postulate of an SN2 mechanism for this reaction.125 Associative pathways allow greater control of the stereochemistry of the substitution, and the rates of such processes are accelerated more effectively by metal ions. 

A study of the effects of surfactants on the reaction of 2-(4-cyanophenoxy)quinoxa-line with hydroxide ions has shown that rate accelerations caused by cetyltrialkylam-monium chlorides may be due to reaction in premicellar aggregates or in micelles.54... 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

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