Quenching reactions

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

The nonrelativistic 1,2 A conical intersection seam in the H2 + OH supermolecule has been well studied [28-30] because of its role in the non-adiabatic quenching reaction... 

Polymerizations are typically quenched with water, alcohol, or base. The resulting polymerizates are then distilled and steam and/or vacuum stripped to yield hard resin. Hydrocarbon resins may also be precipitated by the addition of the quenched reaction mixture to an excess of an appropriate poor solvent. As an example, aUphatic C-5 resins are readily precipitated in acetone, while a more polar solvent such as methanol is better suited for aromatic C-9 resins. 

The enolate of the 1,4-adduct, obtained after the stereoselective Michael addition step, as discussed in the previous sections, may be quenched in situ with various electrophiles. The fact that additional stereogenic centers may be formed via such tandem Michael addition/quench-ing procedures, giving products with high diastereoselectivity in many cases, extends the scope of these methods substantially. Furthermore these procedures occasionally offer the possibility of reversing the syn/anti diastereoselection. In the next sections pertinent examples of diastereoselective inter- and intramolecular quenching reactions will be discussed. 

For systems such as these, which consist of electron transfer quenching and back electron transfer, it is in general possible to determine the rates both of quenching and of the back reaction. In addition to these aspects of excited state chemistry, one can make another use of such systems. They can be used to synthesize other reactive molecules worthy of study in their own right. The quenching reaction produces new and likely reactive species. They are Ru(bpy)3+ and Ru(bpy)j in the respective cases just shown. One can have a prospective reagent for one of these ions in the solution and thereby develop a lengthy and informative series of kinetic data for the transient. 

Solutions were obtained, either analytically or numerically, on a computer. The quenched-reaction, kinetic model considered that the nucleation sequence of reactions evolves to some time (the quenching time) and then promptly halts. Both kinetic models yield a result having the same general form as the statistical model, namely,... 

A further interesting application of CLM involves the fluorescence quenching reaction between (5,10,15,20-tetraphenylporphyrinato) zinc (II) and methylviologen at a dode-cane-water interface. [62] This study demonstrated that the quenching reaction could occur only in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). The quenching efficiency depended on the concentration of SDS in the aqueous phase with a maximum value of 13.5%... 

The first experiments characterizing DNA-mediated CT over a precisely defined distance between covalently appended redox probes were reported in 1993 [95]. Remarkably, the luminescence of a photoexcited Ru(II) intercala-tor was quenched by a Rh(III) intercalator fixed to the other end of a 15-mer DNA duplex over 40 A away (Fig. 4). Furthermore, non-intercalating, tethered Ru(II) and Rh(III) complexes did not undergo this quenching reaction. In this way the importance of intercalative stacking for efficient CT was demonstrated. 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

Based on the above results, ultrasonic irradiation to ion-exchanged [Pd(NH3)4]2+-zeolite powders was performed in an aqueous solution containing 2-propanol. The reduction of [Pd(NH3)4]2+-zeolite to Pd°-zeolite was confirmed by XPS analyses. However, from XPS depth analyses of the prepared samples, it was suggested that the [Pd(NH3)4]2+ complexes in the zeolite pore were not sufficiently reduced even in the presence of 2-propanol. Presumably, the reductants formed from 2-propanol sonolysis could not easily diffuse into the zeolite nano-pore (size 1.2 nm) and/or reductants undergo recombination reactions and quenching reactions with the walls. In addition, the results of XPS spectral analyses of the sonochemically prepared Pd-zeolite powders indicated that the average size of the Pd clusters on the zeolite surface is roughly estimated to be less than 1 nm and composed of several dozen Pd atoms. 

Incubate the quenching reaction for 15 minutes at room temperature. 

Analyze the quenched reaction by SDS electrophoresis, Western blotting, and mass spec analysis. 

Neither the electronic absorption nor the emission spectrum of Re2Cl8 changes in the presence of the quenchers, and no evidence for the formation of new chemical species was observed in flash spectroscopic or steady-state emission experiments. The results of these experiments suggest that the products of the quenching reaction form a strongly associated ion pair, Re2Cl8 D+. 

The net quenching reaction in eq. 2, which leads to separated redox products capable of oxidizing and reducing water, relies on a series of electron transfer steps. The basic theme of this account is excited state and related electron transfer events which occur in such systems and the basis that we have for understanding them both experimentally and theoretically. 

In order to begin it is useful to consider Scheme 1, in which the quenching reaction in eq. 2 is considered in kinetic detail. 

Assuming that AE AG, the rate constants for the quenching reaction and its reverse (kj and k i in eq. 8) can be written using the theoretical results of eq. 5-7. In order to make the situation clear it is useful to consider two limiting cases. In the first limit, k i << back electron transfer to give the... 

The quenching of benzophenone phosphorescence has been used by Mar and Winnik (1981) as a photochemical probe of hydrocarbon chains in solution. The bimolecular reaction for quenching the triplet state of 4-methoxy-carbonylbenzophenone [24] by 1-pentene occurs at rates which are below the diffusion limit by two to three orders of magnitude. Consequently, the intramolecular quenching reactions of to-alkenyl esters of benzophenone-4-carbo-xylic acid [25] occurs under conformational control. In [25] the point of... 

The concentration of acetic anhydride at any time in the CSTR is determined by titration with sodium hydroxide. Because the titration procedure requires time (relative to the hydrolysis reaction time), it is necessary to quench the hydrolysis reaction as soon as the sample is taken. The quenching is achieved by adding an excess of aniline to the sample. The quench reaction is... 

The quenching reaction also forms acetic acid, but in a different stoichiometric ratio than the hydrolysis reaction. Thus it is possible to determine the acetic anhydride concentration at the time the sample was taken. 

In MCMv3.0 the quenching reaction of 0( D) with N2 has a rate coefficient of 2.58x 10-11 cm-3 molecule-1 s-1 at 298 K (Atkinson et al., 2001). Recently three groups reported a new rate coefficient of 3.09xl0-11 cm-3 molecule-1 s-1... 

Monoisotopic mercury photosensitization shows163 that HgO is not formed in the primary quenching reaction. 

In more recent work, Polanyi et a/.185 have observed infrared emission of vibrationally excited NO molecules (NOvlb) formed in the quenching of both Hg (3P,) and Hg (3P0) atoms, suggesting, by analogy with CO, that the primary quenching reactions are... 

Emnk M. R. (1991b) Fluorescence Quenching Reactions. Probing Biological Macromolecular Structures, in Dwey T. G. (Ed.), Biophysical and Biochemical Aspects of Fluorescence Spectroscopy, Plenum Press,... 

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