Half quenching concentration

From Eqs. (2) and (10) the intrinsic yields qDm are equal to the measured fluorescence yields y (C) at infinite fluor (or quencher) concentrations a sufficient condition that [M] [M] (or [Q] [Q]y2) is established by the complete quenching of molecular fluorescence (Eqs. 1 and 9), as in pure liquids at moderate temperatures or for solutions of fluor in liquid quenchers. Alternatively the intrinsic yields may be computed from the measured yields at the half-quenching concentration [M]i/2 or [Q]y2 (Eqs. 2 and 10), or, following Hirayama and Lipsky,140 from linear plots of y against y which yield qM and qD as intercepts according to the relationship... 

H-abstraction intramolecular, 378 Half-value concentration, 181 Half-band width, 69 Half quenching concentration, 173 Hamiltonian operator, 65 perturbing, 67 Hammet equation, 110 He-Ne laser, 318 Heavy atom perturbation, 70 external, 145 intermolecular, 71 intramolecular, 71... 

A° values can be obtained experimentally from eq. 23, which defines the half-quenching concentration, permitting calculation of a corresponding value. These are shown In Table 1 for a variety of donor-acceptor pairs. 

The quenching constant can also be calculated from the condition of 50% quenching. If [Q],/2 is the concentration of the quencher when the solution is half-quenched, then... 

NATA and DHE, respectively. These values imply that quench concentrations near 03M are required to quench one-half of the fluorophores by a static p x)ces8. Such a weak association suggests that the fluorophores and quenchers do not actually form a ground-state complex. Instead, it seems that the apparent static con nent is due to the quencher being adjacent to the fluorophore at the moment of excitation. These closely spaced fluorophore-quencher pairs are immedi ely quenched and thus appear to be dark com dcxes. 

The reduction of Co(lll) by Fe(II) in perchloric acid solution proceeds at a rate which is just accessible to conventional spectrophotometric measurements. At 2 °C in 1 M acid with [Co(IlI)] = [Fe(II)] 5 x 10 M the half-life is of the order of 4 sec. Kinetic data were obtained by sampling the reactant solution for unreacted Fe(Il) at various times. To achieve this, aliquots of the reaction mixture were run into a quenching solution made up of ammoniacal 2,2 -bipyridine, and the absorbance of the Fe(bipy)3 complex measured at 522 m/i. Absorbancies of Fe(III) and Co(lll) hydroxides and Co(bipy)3 are negligible at this wavelength. With the reactant concentrations equal, plots of l/[Fe(Il)] versus time are accurately linear (over a sixty-fold range of concentrations), showing the reaction to be second order, viz. 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

In BaS04 Ag (Fig. 5.58) three bands have been detected red, green and UV. The red one peaking at 650 nm at room temperature has a half-width of 150 nm and decay time of 75 ps and its spectrum is similar to the broad orange band in natural barite. The decay time of this band in artificially activated barite is shorter compared to the natural one, which may be connected with higher Ag content with resulting concentration quenching. Ag" is very big ion with ionic radius of 142 pm for coordination number 8, which is close to those of Ba " ". 

This polarimetric method was made even more general by utilizing chiral HPLC techniques. The L-UNCAwas dissolved in the solvent at a concentration of 0.33 M at 20 °C. The tertiary amine (1.5 equiv) was added at time zero. The solution was allowed to stand for an experimentally determined delay time, during which the only process that can occur was epimerization, since there is no nucleophile present. The delay time was determined after carrying out several experiments with different delay times and chosen so as to fall within or just after the first half-life for racemization. At the end of the delay period, benzylamine was added. Benzylamine is a very powerful nucleophile that reacts virtually instantly (regardless of the type of activation) with the resulting mixture of l- and d-UNCAs to form the benzyl amides and quench the epimerization process. Thus, a snapshot of the ratio of l/d activated intermediates at the instant of benzylamine addition was obtained by measurement of the l/d ratio of the benzyl amide products. 

Solutions of peroxide were prepared by oxidizing to the required extent, quenching the oxidation by cooling, and adding an excess of an inert diluent such as toluene. More than half the toluene was then pumped off while the oxidate was kept at — 20°C. After this procedure had been repeated twice, solutions of peroxide in toluene could be prepared in which the residual chloroprene concentration was about 0.5% (w./w.) of the peroxide. Complete removal of solvent gave faintly yellow viscous peroxidic material which was mildly explosive at room temperature. 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

Base catalysis—hydrolysis. Pohl studied the hydrolysis in aqueous solutions of a series of trialkoxysilanes of R Si(OCH,CH,OCH,), structures in which R was an alkyl or a substituted alkyl group [42]. The reactions were followed using an extraction/quenching technique. Silanes were studied at concentrations ranging from 0.001 to 0.03 M and pHs adjusted from 7 to 9. The hydrolysis was found to be first order in silane. The order in water was not determined because the reactions were carried out in a large excess of water (water was the solvent). The rate constants for the hydroxide anion catalyzed hydrolysis reactions and reaction half-lives are reported in Table 1. 

A simplified theory of FRET is sufficient to describe affinity sensors used in fluorescence transduction of glucose concentrations. A key quantity that describes the potential FRET interaction between a donor-acceptor pair is the Forster distance, Ro, the distance at which half the donor molecules are quenched by the acceptor molecules. Ro is proportional to several parameters of the fluorophores, in accordance with Ro = K6 Jx2n 4 cf>DJ l], where K is a constant. The variable k2 refers to the relative spatial orientation of the dipoles of D and A, taking on values from 0 to 4 for completely orthogonal dipoles and collinear and parallel transitional dipoles k2 = 4,... 

---