Chloride ions quenching

FIGURE 15-4 Spreadsheet and plot to determine quenching constant for chloride ion quenching of quinine. 

Chloride ion is known to quench the intensity of quinine s fluorescent emission. For example, the presence of 100 ppm NaCI (61 ppm Ch) gives an emission intensity that is only 83% of that without chloride, whereas the presence of 1000 ppm NaCI (610 ppm Ch) gives a fluorescent emission that is only 29% as intense. The concentration of chloride in urine typically ranges from 4600 to 6700 ppm Ch. Flow is an interference from chloride avoided in this procedure ... 

Rearrangements may also be observed in these carbocations if they have the appropriate stmctnral featnres. It does not matter how the carbocation is prodnced, subsequent transformations will be the same as we have seen where rearrangements are competing reactions in nucleophilic substitution. Thus, electrophilic addition of HCl to 3,3-dimethylbut-l-ene proceeds via protonation of the alkene, and leads to the preferred secondary rather than primary carbocation (see Section 8.1.1). However, this carbocation may then undergo a methyl migration to produce the even more favonrable tertiary carbocation. Finally, the two carbocations are quenched by reaction with chloride ions. The prodnct mixture is found to contain predominantly the chloride from the rearranged carbocation. 

Specifically Netzel et al. ( - ), in studies of face-to-face , covalently-linked MgP-P dimers, found evidence for the formation within 6 psecs of a low-lying, relatively long-lived intramolecular CT state of the type MgP -P" in polarizable or highly polar solvents and in solvents where chloride ion coordinates with the magnesium ion of the MgP-macrocycle. These workers also observed the formation of benzoquinone anion radicals as stable photoproducts of the CT formation process when the experiments were carried out in the presence of benzoquinone ( ). This approach provides a more direct test for the formation of an intramolecular CT state, and the results are in sharp contrast to those typically observed when porphyrin Ktt, ) states are quenched in the presence of benzoquinone (23). 

Because of the much greater driving forces potentially available in reactions between substrates and excited state molecules, difficult—but valuable—electron transfer reactions, such as the oxidation of water or chloride ion, may be accessed through excited state photochemistry. The question of how to separate hole-electron pairs generated in a quenching reaction, how to provide kinetic pathways to lead these two highly reactive species far apart from each another, and how to couple in some useful chemistry are currently of interest. 

The choice of the mobile phase is very important, as fluorescence is sensitive to fluorescence quenchers. Highly polar solvents, buffers, and halide ions quench fluorescence. The pH of the mobile phase is also important to fluorescence efficiency for example, quinine and quinidine only display fluorescence in strongly acidic conditions, whereas oxybarbiturates are only fluorescent in a strongly alkaline solution [67,68]. Due to the stability of the chromatographic sorbents, the use of very acidic or basic mobile phase may not be possible. One alternative is to alter the effluent pH postcolumn. Postcolumn addition of sulfuric acid has been used for the assay of ethynodiol diacetate and mestranol in tablets [69]. Another example is the determination of tetracycline antibiotics in capsules and syrup where EDTA and calcium chloride were added to enhance fluorescence [70]. 

The photoreduction of 2- and 3-chloroanisole in alcoholic solvents has been studied and is considered to be best accounted for by invoking methoxyphenyl radicals which abstract hydrogen atoms from the solvent378.4-Chloroanisole probably reacts partly via a homolytic cleavage, but in view of the results of quenching and sensitization experiments, another pathway consists of electron transfer from the solvent (ROH) to excited aryl halide, followed by dissociation of the radical anion into chloride ion and aryl radical. 

A novel Os and Ru bis(bipyridyl) containing an amide macrocyclic receptor has been shown to detect the presence of anions by both electrochemical and optical methodologies. Photophysical studies have clearly shown that the rate constants of the energy transfer process responsible for the quenching of the luminescent ruthenium excited state significantly decreases in the presence of chloride ions. ... 

A palladium(II)-catalyzed three component coupling reaction was established by Lu, who performed the intermolecular carbopalladation involving propargyl alcohols and alkenes, and this was followed consecutively by allylic chloride insertion to the C-Pd bond and its quenching by p-heteroatom elimination in the presence of an excess of chloride ions. An example is shown below <03TL467>. 

In a similar study, Fe-labeled Fe(H20)6 was allowed to exchange with Fe(H20)e + in the presence of chloride ions [72]. The reaction was quenched by the addition of 2,2 -bipyridine which complexes Fe, and the activity of Fe in the complex was determined by scintillation counting. It was found that most of the exchange takes place by the reaction of Eq. 66... 

Coumarins are another much investigated group. This period of review has included excited state interactions of coumarin with nucleotide bases and quenching by chloride ions A Fluorescence lifetimes of angular furocoumarins in several solvents have been measured and correlated with triplet yields 55 Structural and solvent effects on the fluorescence properties of benzodihydropyranones have been examined also. ... 

The effect of the heavy-atom substituents, bromine and iodine, on the electron-donor aniline in the electron-transfer reaction with thiopyronine triplet has been investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching and radical yields are presented in Table 30. The results are analysed in terms of decay constants of an intermediate triplet exciplex. The influence of an external heavy-atom effect on the phosphorescence spectra of quinoxaline and 2,3-dichloroquinoxaline has also been reported. The influence of chloride ion on the decay rate of Methylene Blue triplet in 0.01 M acid in the... 

The fluorescence oi quinine sull.iic tsee Figure 1.5 3) quenched by high concentraiii iis of chloride ion. The folli>wing fluorescence signals A were obiained as a liinelion of the conccnlration of ehloriile ion. 

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