Geminate neutralization

A recently proposed nonhomogeneous kinetics model (5) has been moderately successful in the interpretation of the rates of reactions of ions produced during the y-radiolysis of dielectric liquids. A wide range of dielectric liquids has been tested, from hydrocarbons (5,14, 20) to alcohols (16,17) and water (18). The same model explains the variation in the yield of free ions—i.e., ions that have escaped geminate neutralization—from one liquid to another (6). The model is still in a rather crude form and has many shortcomings. Its only strength is the degree to which it provides a quantitative interpretation of experimental results. 

With the continuing improvement in the time resolution of pulsed radiation equipment, it should soon be possible to measure the lifetimes of a significant portion of the ions that undergo geminate neutralization. This information will be valuable in determining the distribution of ion separation distances. 

Thus, some pairs of ions undergo geminate neutralization and others become free ions. The free ions diffuse at random in the liquid and ultimately meet other free ions from other spurs and undergo neutralization ... 

The yield Ggn of the ions that undergo geminate neutralization is given by... 

Consider the decrease of the number of ions in a liquid as a function of time after an instantaneous pulse of radiation. The ions that undergo geminate neutralization move together under the influence of their mutual electric field. The relative velocity v of a pair of ions with respect to each other is negative and is given by the negative product of the electric field strength between them and the sum of their mobilities ... 

The effect of neutralization of the free ions must be superimposed on Equation VII. Since the lifetimes of the free ions are greater than those of the ions that undergo geminate neutralization, this can be done as follows. Reaction 4 obeys simple second order kinetics, so the fraction Ffi(t) of the free ions that survive at time t is given by... 

The value of Ffi(t) depends upon the initial free ion concentration (see Equation VIII), which in turn depends on the irradiation dose delivered in the instantaneous pulse (see Equation IX). If such a large dose were used that Ffi(t) became appreciably less than unity before geminate neutralization had been completed, this would indicate the occurrence of spur overlap and would alter the calculation somewhat. 

The coulombic attraction between the ion and electron tends to draw them back together. When it succeeds the reaction is called geminate neutralization. 

Geminal difluorides usually require a strong base for conversion to vinylic fluorides, but some aluminum oxides such as neutral y-alumina or Woelm alumi-... 

Mozumder and Magee, 1967), the success of the geminate pair model in kinetics (Warman et al., 1969 Rzad et al, 1970) and in free-ion yield determination (Hummel and Allen, 1966 Freeman, 1963a, b) has been rationalized on the basis that in multi-ion pair spurs all but the final e-ion pair would quickly neutralize due to intense internal coulombic interaction. Therefore, emphasis is laid in this section on the geminate pair with two caveats ... 

Liquid carbon dioxide is decomposed efficiently by ionizing radiation79. The decreased radiation stability of the liquid phase compared to the gas phase has been attributed to the much smaller contribution of ion-molecule reactions to radiolysis in the condensed phase, where an efficient geminate charge neutralization process is likely to minimize the occurrence of such processes. Ion-molecule reactions are probably responsible for the rapid reoxidation observed in the gas phase. The yields of CO, 02 and 03 from the y-radiolysis of liquid C02 can be... 

Acylals (geminal diacetates) are frequently used as protecting groups for aldehydes because of their stability to neutral and basic conditions [8]. In addition, the acylal functionality can be converted into other useful functional groups [9]. For example a novel synthesis of chiral allylic esters has been developed using palladium-catalyzed asymmetric allylic alkylation of gem-diesters [10]. The allylation of... 

Through exothermic dissociation of a neutral excited state in molecule by electron transfer to an adjacent molecule. This process leads to the generation of geminately bound electron-hole pairs as precursors of free positive and negative charges in an organic solar cell. 

Competing reactions may introduce mechanistic ambiguities Ketone or quinone triplet states abstract hydrogen atoms, forming neutral radicals. Also, many radical cations are proton donors and radical anions are comparably strong bases. Thus, geminate radical ion pairs may generate neutral radicals by proton transfer. 

Let us consider now annealing of neutral defects whose annihilation is stimulated by elastic interaction, characterized by the energy U(r) = -a/r3 (ct > 0) - equation (3.1.4). In the case where all geminate pairs are created at the same distance l, we have [71, 76]... 

Geminal difluorides usually require a strong base for conversion to vinylic fluorides, but some aluminum oxides, such as neutral y-alumina or Woelm aluminum oxide, result in ready de-hydrofluorination under unexpectedly mild conditions. Just stirring the geminal difluorides 49 and 51 with a large excess of alumina at room or slightly elevated temperature affords products 50 and 52, respectively.84-85... 

Several neutral titanium complexes have been shown to catalyse intramolecular hydroamination reactions of alkenes. The corresponding pyrrolidine and piperidine products were formed in up to 97% yields. However, only the geminally disubstituted aminoalkenes were successfully cyclized (Thorpe-Ingold effect).56... 

At the time of writing this review, the major question remains in the understanding of the reasons for absence of the M.I.R. in most photo-induced e.t. processes. Observations of the M.I.R. in thermal charge shift and charge recombination reactions have now become commonplace, especially in intramolecular e.t. as well as in formally bimolecular geminate ion pair neutralizations — here the molecules which form the ion pair may form a supermolecule , something like an exciplex. 

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