Equilibrium protolytic

The kinetic consequence of the non-participating ligand was also noticed in the autoxidation reactions catalyzed by Ru(III) ion, Ru(EDTA) (1 1) and Ru(IMDA) (1 1) (EDTA = ethylenediaminetetraace-tate, IMDA = iminodiacetate) (24,25). Each reaction was found to be first order in ascorbic acid and the catalysts and, owing to the protolytic equilibrium between HA /H2A, an inverse concentration dependence was confirmed for [H+]. Only the oxygen dependencies were different as the Ru(III)-catalyzed reaction was half-order in [02], whereas the rates of the Ru(III)-chelate-catalyzed reactions were independent of [02]. In the latter cases, the rate constants were in good agreement with those... 

E. Seclaman, A. Sallo, F. Elenes, C. Crasmareanu, C. Wikete, S. Timofei and Z. Simon, Hydrophobicity, protolytic equilibrium and chromatographic behaviour of some monoazoic dyes. Dyes Pig., 55 (2002) 69-77. 

Forster s cycle (50MI1) (method 1 in Table VIII, also known as the thermodynamic method ). This cycle is particularly important because it can be used even when the protolytic equilibrium is not reached in the excited state. On the other hand, it has two important limitations (i) the frequencies of the 0-0 transitions in absorption or emission are necessary and (ii) ionization entropy changes are assumed to be the same in the ground and in the excited states. The experimental difficulties involved in determining the 0-0 transition frequencies have led to the use of the frequencies of the absorption maxima (procedure a), emission maxima (procedure b), or the average therefrom (procedure c). 

Weller s method (55MI1) is based upon the determination of intensities or fluorescence quantum yields under steady-state conditions. Its main limitation is the condition of attainment of the protolytic equilibrium in the excited state. 

State, the charge density on oxygen is leduced and a reasonable charge density is now found on ortho- and meta-positions. Therefore, excited phenol is more acidic and is also ortho-meta-directing towards substitution fa benzene ring. The protolytic equilibrium constant pK for the reaction is 10.0 and 5.7 in the ground and excited states, respectively, a difference 6f 4.3 pK units. 

The deprotonation time is therefore often so long that protolytic equilibrium cannot become established during the lifetime of singlet excited states (say... 

This question of equilibration of the protonation and deprotonation processes leads to another fundamental problem in the case of excited state reactions between which states can a protolytic equilibrium be at all established A molecule has only one ground state, so there can be no ambiguity about the thermal protolytic equilibrium which connects of course the ground states of the acid and base forms. However, there are many excited states of both these forms, excited states which can differ greatly in electron distribution (e.g. mr and 7T7T states) or even in multiplicity (e.g. singlet and triplet states). 

One thing is clear there can be no acid-base equilibrium between states of different multiplicities thus it is correct to consider only the pK of the singlet state , or the pA of the triplet state . However, the question of the protolytic equilibrium between an mr singlet and a tttt or charge transfer (CT) singlet remains open. This problem is illustrated in Figure 4.48 for the case of 4-hydroxybenzophenone, in which there is a reversal in the order of mr and CT states between the acid and base forms. Excitation of the protonated molecule in ethanol for example leads to the ground state deprotonated form, but the detailed mechanism of this process is not known. 

Annular tautomerism inherent to NH-unsubstituted tetrazoles and tetrazolium ions governs many chemical and physicochemical properties of these compounds. Since the 1950s, this type of protolytic equilibrium has been investigated by various teams. In this section, we analyze the results of experimental and theoretical studies on the annular tautomerism of NH-tetrazoles and their conjugate acids. The results of investigations published within the last decade are considered from the viewpoint of the classical concepts based on earlier research. 

This mechanism presents the closest analogy to protolytic equilibrium. Combination of equations (37) and (44) yields... 

Evidently this does not give an entirely satisfactory description of the experimental results, even though the maximum discrepancy between theory and experiment amounts to only ca. 0-05 pK unit. These experiments represent the only existing study where the dependence of a protolytic equilibrium constant on n has been related only to independently measured fractionation parameters. ... 

The case of phosphoric acid was selected by Salomaa et al. (1964a) to exemplify a protolytic equilibrium in which both the acid and base form involved contain more than a single equivalent hydrogen atom. The appropriate form of the general equation (32) is thus... 

The inflection point of the fluorescence intensity curves against acidity gives a first approximation to the p/f/Sj )-value but this involves the assumption that the protolytic equilibrium is established within the lifetime of the S state and that the fluorescence lifetimes of B and BH+ are equal. These assumptions are less likely to hold when only one form is fluorescent and Lasser and Feitelson (1973)... 

R. J. Sturgeon and S. G. Schulman. Electronic absorption spectra and protolytic equilibriums of doxombicin direct spectrophotometric determination of microconstants. /. Pharm. Sci., 66, 958-61 (1977)... 

If it is accepted that reactions catalyzed by acids or bases involve at some stage a slow acid-base reaction, then the Bronsted relation (cf. Sec. II.4) assumes a reasonable aspect. The constants used to express the strengths of the catalyzing species are usually defined with reference to an equilibrium with some standard acid-base system such as the solvent, but they could in principle be defined in terms of the (hypothetical) protolytic equilibrium between the catalyst and the substrate. The Bronsted relation then amounts to a parallelism between the rates and equilibrium constants of a series of similar reactions. The general form of the relation can in fact be inferred without any reference to a molecular interpretation. Suppose that we have any acid-base equilibrium... 

One of the most important aspects of hydrochemistry is the determination of the relative representation of the particular components of a protolytic equilibrium involving distribution coefficients 6. Graphical illustration of the relationship S = /(pH) can be expressed by either non-logarithmic or logarithmic distribution diagrams. Using these diagrams it is possible to read off the values of the concentrations of the particular species. 

MHD2 would, in its turn produce HD. The only way in which HD formation from MHD2 could be avoided would be if the trihydride were in a sense asymmetric, with one M-H bond chemically distinct from the other two and hence unable to reductively-eliminate H2 with either of the other M-H bonds. It is not difficult to envisage such a situation brought about by stereochemical restrictions. However, such a species is without precedent in simple chemical systems. It seems more likely that the observations with the various dihydrogen isotopomers dictate that only a dihydridic state is produced. Point (ii) demonstrates that any metal hydride is not involved in a protolytic equilibrium as shown in Equation (15), or at least that this exchange process is slow. 

In aqueous neutral solutions, 47 has two emission bands corresponding to the fluorescence of the neutral = 353 nm) and anionic = 420nm) forms [168]. Fluorescence was quenched by I in water and in the presence of y-CD, the quenching was the same in the presence of a-and P-CD, the quenching was depressed [167]. Addition of a- and p-CD increased the fluorescence yield and lifetime of the neutral form of 47 and depressed the yield of the anionic form [142, 166, 167]. No variation was induced by y-CD. A depression of the proton dissociation of the included molecule explains these results. In fact, the fcj and fc, rate constants found for the protolytic equilibrium... 

The structural formulae of the guest molecules used in this experiment are shown in Fig. 1(a) and 1(b). These azo compounds show the following protolytic equilibrium in aqueous solution,... 

Quantitative comparison of indicator color changes provides a direct empirical measurement of the acidity of the medium without appreciably changing it. For an uncharged indicator, neglecting concentrations of solvent and counter ions, we consider the protolytic equilibrium... 

If now we wish to determine the unknown strength of a weak base B, which has suitable indicator properties for measuring the appearance of its protonated form BH+, we follow a similar treatment. For its protolytic equilibrium... 

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