Strong pairs

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

In the case of molten salts, no obvious model based on statistical mechanics is available because the absence of solvent results in very strong pair correlation effects. It will be shown that the fundamental properties of these liquids can be described by quasi-chemical models or, alternatively, by computer simulation of molecular dynamics (MD). 

Excited molecular complexes of the donor-acceptor type are called excimers if formed from identical molecules and exiplexes if originated from different molecules. From the theory, it is concluded that photochemical influence will more readily accelerate electron transfer in a weak donor-acceptor pair than in a strong pair (Juillard and Chanon 1983). An organic molecule in an electron-excited state is a more active oxidant or stronger reducer than the same molecule in a ground state. 

In 1998, Baker and Kirby conducted a 31P NMR investigation of electron exchange in the two-electron reduced heteropoly blue complex of [(P2Wi706i)2Th]18 (which contains an equilibrium mixture of oxidized, two-electron- and four-electron-reduced species) as a function of alkali metal counterion, concentration, and temperature. They interpreted their data in terms of Equation (8) in which the more strongly pairing alkali metals (M in Equation (8) = K+, Rb+, and Cs+ but not Li+) form an ion bridge between the two defect HPA units in the syn isomer. This interaction stabilizes the syn isomer and drives an apparent syn-anti equilibrium, Equation (8), to the left.118 The change in chemical shifts and other features of the 31P NMR spectra of these Th sandwich POM complexes as a function of the counterion defined a qualitative method to estimate the association of monocations with POM polyanions ... 

An old saw blade, one th is quiiethin, when cut up in strips will make springs for cheap woik. If too liard temper, it cannot very w ell he used until the temper be drawn to suit. They can bo bent into form with a strong pair of pliers. By warming the strips over a fire they can be leodily bent into shape. 

Spherical clusters and nuclei, as discussed in the above subsections, exist mainly for fermionic shell closures. The strong pairing interaction in nuclei restores spherical... 

Retention times of divalent metal cations can be manipulated by addition of the sodium salt of an anion with a strong pairing propensity. The retention times of the cations were all longer when iodide or thiocyanate was added to the sample (Figure 10.6). The elution order of the cations was entirely different from the order obtained in classical ion chromatography. 

Figure 22. The strong pairing between guanine and isoguanine as well as...

Figure 60. Whereas p-Ribo(G6) strongly pairs with p-RibcKCe), it does not show self-pairing this is in sharp contrast to both the homo-DNA (see fig. 21 in lecture 1) and the DNA soies.

Free radicals are involved with most significant atmospheric chemical phenomena and are of the utmost importance in the atmosphere. Because of their unpaired electrons and the strong pairing tendencies of electrons under most eircumstances, free radicals are highly reactive. The upper atmosphere is so rarefied, however, that at very high altitudes, radicals may have half-lives of several minutes, or even longer. Radicals can take part in chain reactions in which one of the products of each reaction is a radical. Eventually, through processes such as reaction with another radical, one of the radicals in a chain is destroyed and the chain ends, as shown by the reaction of two methyl (H3C ) radieals below ... 

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