Solute quenching

Precipitation hardening consists of solutioning, quenching, and aging. Solutioning entails heating above the solvus temperature in order to form a homogeneous soHd solution. 

Absorption and emission spectra of six 2-substituted imidazo[4,5-/]quinolines (R = H, Me, CH2Ph, Ph, 2-Py, R = H CH2Ph, R = Ph) were studied in various solvents. These studies revealed a solvent-independent, substituent-dependent character of the title compounds. They also exhibited bathochromic shifts in acidic and basic solutions. The phenyl group in the 2-position is in complete conjugation with the imidazoquinoline moiety. The fluorescence spectra of the compounds exhibited a solvent dependency, and, on changing to polar solvents, bathochromic shifts occur. Anomalous bathochromic shifts in water, acidic solution, and a new emission band in methanol are attributed to the protonated imidazoquinoline in the excited state. Basic solutions quench fluorescence (87IJC187). 

Figure 7.46 Fluorescence quenching of doxorubicin by DNA [597] (a) doxorubicin in aqueous solution, quenched immediately on addition of DNA (b) doxorubicin fluorescence not affected by vesicles (c) Doxorubicin preequihbrated with vesicles, and then subjected to DNA. The fraction bound to the outer membrane leaflet is immediately quenched by the DNA. (d) Same as (c), but multilamellar vesicles used. The left arrow represents a 5-min interval and applies to the first three cases the right arrow represents 30-min interval and applies to (d) only. [Reprinted from Ronit Regev and Gera D. Eylan, Biochemical Pharmacology, vol. 54, 1997, pp. 1151-1158. With permission from Elsevier Science.]...

Quenched Mixed solutions quenched after a predetermined time controlled by the distance between the mixer and quencher and the flow rate. Tedious but leisurely analysis. Essential for the batch method used in rapid isotopic exchange and low temperature epr monitoring. A 10-20 ms resolution. Large volumes of reactants used ( 5 ml). Commercially available. 

Heavy atoms in solution quench fluorescence by colliding with excited molecules so that their energy is dissipated, e.g. chloride or bromide ions in solution cause collisional quenching. 

X 109 and 8. 9 x 109 s 1 for Ndm and Ybm, respectively. Both the above-described in-termolecular mechanism, as well as an intramolecular pathway in the ternary complex with aad which forms in solution, are responsible for the observation of NIR luminescence in these systems. Addition of water to the toluene solutions quenches the NIR luminescence, while it enhances the visible CL emission of the corresponding solution of Eum and Tbm (Voloshin et al., 2000c). Neodymium and ytterbium tris(benzoyltrifluoroacetonates) display the same CL as tta complexes, although for Ybm its intensity is about 2.5 times lower than for the tta chelate. On the other hand, almost no CL is detected for acetylacetonate complexes (Voloshin et al., 2000a). Thermal or photochemical decomposition of aad also triggers CL from [Pr(dpm)3] and Pr(fod)3], both in the visible (from the 3Pi, 3Po, and 1D2 levels) and in the NIR at 850 nm ( Do -> 3F2 transition) and 1100 nm ( D2 3F4 transition). The excited... 

Besides the formation of an elastic network, amylose gelation is also characterized by the development of opacity, which is generally attributed to chain aggregation.197,400 401 For a polydisperse amylose preparation (DP 3080, 2.4% solution, quenched to 32°C), the increase in turbidity slightly preceded the onset of G development.197 Crystallization, as detected by x-ray diffraction (intensity of the 100 diffraction peak),... 

Calvert et al. have further shown that l,l -azoisobutane in hexane solution quenches the phosphorescence (and not the fluorescence) of biacetyl with a rate coefficient of 2 x 10 l.mole sec" at, which is only slightly lower than the diffusion controlled rate coefficient. In gas phase experiments energy transfer has been demonstrated from triplet biacetyl and acetone with near collision efficiency... 

When two CH2 molecules combine, it seems reasonable to assume the superposition of two identical charge distributions of this kind, with a common Z-axis. An alternative is to assume that the two carbon atoms line up with respective electronic configurations of C(p p+i) and C p p i) as in 7(b)E. The azimuthal distribution of the angular momentum vectors of such a pair is in opposite sense as shown in Figure 20-7(e). The actual distribution of electron density in the ethylene molecule is conjectural. The obvious guess that anti-parallel 2p solutions quench the angular momentum, is favoured, but not confirmed, by chemical intuition. 

In a typical spinel, solid solutions, quenching, and redox equilibria can very rapidly complicate the simple analysis presented here. Additional Reading contains a number of references that the interested reader can consult. 

Example of application of solute quenching in protein studies One of the main aims in biophysical studies of the stracture and function of proteins is to identify the protein domains which are responsible for the interaction of the entire protein with physiologically relevant binding partners. Proteins usually contain several tryptophan residues, which may be distributed among the different protein domains. Since each of these tryptophan residues is located in a distinct environ-... 

The P-lactam 128 thus prepared was treated with LDA at — 78 °C in THF as solvent and the resulting solution quenched with fourfold excess of an electrophile to furnish the expected trans 3,4-disubstituted P-lactam suitable for further chemical manipulation. For instance, the trans 3-(l-hydroxyethyl) P-lactam 129 was obtained in 98% yield as an equimolar mixture of dia-stereomers epimeric about the hydroxyl group. The mixture of epimers, after protection of the hydroxyl group as acetoxy derivative, was subjected to low... 

In a standard hydrogen exchange experiment (see Section 2.3.4 for variations in experimental workflow), the sample is deuterated for a predetermined time (milliseconds and longer times), and the reaction is slowed down several orders of magnitude by the addition of an acidic quench solution. Quenched reactions can be subjected to direct mass spectrometry analysis or immediately flash frozen in liquid nitrogen and stored at -80°C in multiple aliquots for future analysis. These flash-frozen samples need to be thawed prior to mass spectrometry. It is important to note that freeze-thaw cycles contribute to signal loss due to back-exchange (this is described further in Section 2.3.8), which... 

A correlation between the rate constants k and free enthalpy change AG of electron transfer was studied by Hashimoto and Thomas [127] for quenching of excited singlet states of both pyrene and N-ethylcarbazol and of the triplet state of N-methylphenothiazine by a number of metal ions and for back electron transfer reactions in micellar sodium taurocholate and sodium dodecylsulfate solutions. Quenching rate constants were determined from Stern-Volmer plots obtained for lifetimes of excited states at high concentration of micelles, where the exponential decay... 

The exit rate constants of the excited anions after the photoprotolytic dissociation of l,4-dichloro-2-naphthol within decylsulfate, dedecylsulfate, and cetylsulfate micelles were measured with a fluorescence quencher hardly penetrating the micelles, - the nitrate ion [121]. The addition of nitrate into the solution quenched the fluorescence of those anions which escape from the micelles within the lifetime of the excited state only. The exit rate constant of the naphtholate anion increases with increasing length of the hydrocarbon radical in the micelle-forming surfactant. The exit rate is thus controlled by the lowering of the micelle polarity (i.e. by the free energy of the exit process) rather than by the micelle size or the distance that the anion must diffuse. Perhaps one can establish a kind of correlation between the rate constant of this process and its free energy as was done for photochemical electron transfer [126] and proton transfer [156,157]. 

The precatalyst [ (ArNCMe)2CH Ca p-H](THF)]2 has been shown to react with ketones to yield the corresponding dimeric p-diketiminato calcium alkoxide products derived from insertion of the ketone into the Ca-H bond. These latter species have been shown to be catalyticaUy competent for the hydrosilylation of ketones. Stoichiometric preparations included reaction of [ (ArNCMe)2CH Ca p-H](THF)]2 with benzophenone, acetophenone, cyclohexanone, 1,3-diphenylacetone, and 2-adamantone to yield the corresponding alkoxides in 9-73 % following crystallization from hydrocarbon solutions. Quenching the reaction mixtures with MeaSiCl showed, in addition to the expected reduction products, moderate to significant... 

Scrubbing Solution Quenched Boiler Stack Gas — 110-115 43-46 — E E E plus fly ash, aeration... 

The complex Cu(2,9-Me2-phen>2 is luminescent in the solid state. When the complex is dissolved in methanol, acetone, acetonitrile, or water, however, no luminescence is observed at ambient temperature. In noncoordinating solvents such as dichloromethane, luminescence is observed with quantum yields in the range, and lifetimes in the 20-3(X) ns range. When Lewis acids that can act as donor ligands to the complex are added to these dichloromethane solutions, quenching of the luminescence is observed with quenching rate constants in the range of 10 -10 s" The luminescence is also quenched by the addition... 

The colloid solutions described above (Ag nanotriangles, citrate-reduced Ag, and citrate-reduced Au) were dialyzed against pH=5 ultrapure water for 48 hours before further modification. Based on the known size and density of the metallic cores, 1 mM aqueous (3-aminopropyl)trimethoxysilane (390 iL) was added to 10 mL of the dialyzed colloid solution. After 15 minutes, the pH of the colloid/(3-aminopropyl)trimethoxy-silane solution was raised to 11.5 by adding aqueous potassium hydroxide. Finally, the silica shell began to form after 27% sodium silicate (2.1 xL) was added to the basic solution. Quenching of the shell growth was accomplished by adding 40 mL of absolute ethanol to the nanoparticle solution. 

Employing high-temperature solution heat treatment, which is termed quench-annealing or solution quenching. 

---