Proton dissociation constant

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

The proton dissociation constants, of two series of 3,7-bis(arylazo)-2,6-diphenyl-1 //-irnidazo[l,2-7]pyrazoles, in the ground state and the excited state were determined by the spectrophotometric method and utilizing the Forster energy cycle, respectively. These constants were correlated by the Hammett equation and the results of such correlations with spectral data indicated that both series of compounds exist in solution almost exclusively in the l//-bis-(arylazo) tautomeric form A <2002T2875> (Scheme 3). 

In aquatic chemistry, the unitary proton level of the proton dissociation reaction is expressed by the logarithm of the reciprocal of the proton dissociation constant i.e. p = - log K here, a higher level of proton dissociation corresponds with a lower pK. When the pKy of the adsorbed protons is lower than the pH of the solution, the protons in the adsorbed hydronium ions desorb, leave acidic vacant proton levels in adsorbed water molecules, and form hydrated protons in the aqueous solution. Fig. 9-22 shows the occupied and vacant proton levels for the acidic and basic dissociations of adsorbed hydronium ions and of adsorbed water molecules on the interface of semiconductor electrodes. 

A temperature change can have an effect on the degree of ionization of various moieties on a protein as well as on substrates and effectors. Since proton dissociation constants values) are thermodynamic parame-... 

Fast atom bombardment mass spectrometry has been utilized for the quantitative determination of ionic species, in glycerol/water solutions, which are produced by chemical and enzymic reactions. It is shown that reaction constants can be determined in this manner and that they can be accurately related to those determined by other methods used in the analysis of aqueous solutions. The reactions studied include proton dissociation constants for organic acids, an enzyme equilibrium constant, and enzyme rate constants using natural substrates. 

The negative logarithm of the proton dissociation constant for atenolol was determined as pK = =9.6.(7) a... 

The protonation reactions of surface hydroxyls which account for the development of surface charge are often represented as the dissociation of a diprotic acid site. Thus in water three surface sites can be present, =S—OH =S—OH0 and =S—O. The proton dissociation constants are represented by... 

The low value for A l for carijonic acid is due in part to the existence of some of the un iontzed acid in the form of dissolved CO molecules rather than HgCOa. The proton dissociation constant for the molecular species HgCO is about 2 x 10 ... 

In the triple layer model, values for the intrinsic protonation and dissociation constants, as well as values for tlie intrinsic surface complexation constants for the background electrolyte, can be obtained from hnear, double, or electrokmetic extrapolations to zero surface charge and zero and infinite electrolyte concentration. Values of intrinsic protonation-dissociation constants and intrinsic surface complexation constants for background electrolytes obtamed for the triple layer model using the various extrapolations are compiled in Goldberg (1992). Use of graphical extrapolation methods has been criticized because the triple layer parameter values obtained are not unique (Koopal et al., 1987). 

Values for protonation-dissociation constants of generic surface sites can also be predicted using solvation and crystal chemical theory (Sverjensky and Sahai, 1996). Using tlie standard states defined above (Sverjensky, 2003), the values of log and log can be calculated from log 8 which are... 

Another standardized database for the diffuse layer model was developed for montmorillonite by Bradbury and Baeyens (2005). Surface complexation constants for strong and weak sites and cation exchange were fit to adsorption data for various metals using constant site densities and protonation-dissociation constants in a nonelectrostatic modeling approach. Linear free energy relationships were developed to predict surface complexation constants for additional metals from their aqueous hydrolysis constants. 

Use the definition of the proton dissociation constants for oxide surfaces, Kx and Ki, to show that PZC, defined in Chapter 3, is given by the equation... 

In the dissociative model (Fig. 4.2) a phosphorus-oxygen bond is broken, yielding an alcohol or alkoxide (depending on pH) and a metaphosphate ion (PO ). The metaphosphate ion is unstable and highly susceptible to nucleophilic attack. The rate of reaction is dependent on the proton dissociation constant (pKJ of the leaving group, but independent of the basicity of the nucleophile (Wijesekera, 1992). 

M. Sramko, M. Smiesko, and M. Remko, Accurate aqueous proton dissociation constants calculations for selected angiotensin-converting enzyme inhibitors, J. Biomol. Struct. Dyn. 25 (2008), pp. 599-608. 

Equilibrium constants for the reactions of cysteine derivatives with formaldehyde, and proton dissociation constants for the eight species of cysteine which exist in solution within the pH range 0—14, provide essential background information for evaluating the complex pH-rate profile for the formation of L-thiazolidine-4-carboxylic acid (18) from L-cysteine and formaldehyde. ... 

To reduce the number of adjustable parameters, the A- and B-type proton dissociation constants are given, respectively, by... 

Intrinsic proton dissociation constant for type A sites (Equation 13.2)... 

Atalay, Y. B., R. F. Carbonaro, and D. M. Di Toro. 2009. Distribution of proton dissociation constants for model hnmic and fulvic acid molecules. Environmental Science Technology A3, no. 10 3626-3631. 

In order to introduce three chelating chains on the lower rim of calix[4]arenes, two general methodologies have been developed for the selective synthesis of calix[4]arene monoalkyl ethers [22]. The synthesis of trihydroxamate siderophores was achieved through the sequence of reactions reported in Scheme 1. The convergence of the three chelating chains was established by and NMR [23]. The pKa values of compound 10 were evaluated in ethanol/water 9 1 the first proton dissociation constant... 

Steady-state kinetic parameters such as kc t and Am can vary when they are studied as a function of pH. After one corrects for ionizations of the substrate and controls for possible effects on the native structure of the enzyme, variations in kcut and Am can often be assigned to ionizations of aciddrase groups at the active site of the enzyme. The term kc t/Kyi reflects the proton dissociation constants of the free enzyme, provided that the proton transfers remain fast relative to other steps in the pathway. In the simple one-intermediate kinetic sequence expanded to implicate two ionizations, the term ko t/Ku would display pA and pA), the term would reflect pA and pA(,. The pH dependence of the oat parameter affords information about the substrate-bound state. 

Table 4.2 Proton dissociation constants for several compounds in organic solvents...

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