Acid dissociation constants , theoretical

The value of / has been determined by NMR measurements on the mixed solvent as 0.69+0.02 and is not to be interpreted as an independent fitting parameter. With / established, can be calculated from the acid dissociation constants in pure H20 and D20, and Equation 11.68 interpreted as a zero parameter theoretical prediction of... 

To extract acid dissociation constants from an acid-base titration curve, we can construct a difference plot, or Bjerrum plot, which is a graph of the mean fraction of bound protons, H, versus pH. This mean fraction can be measured from the quantities of reagents that were mixed and the measured pH. The theoretical shape of the difference plot is an expression in terms of fractional compositions. Use Excel SOLVER to vary equilibrium constants to obtain the best fit of the theoretical curve to the measured points. This process minimizes the sum of squares [nH(measured) -nH( theoretical) 2. 

Fig. 2 Theoretical influence of pH on the ratio of equilibrium constants FCapp/- pep and Kapp/Kest for peptide (solid line) and ester (dotted line) bond formation, respectively. pKi and pK2 are the acid dissociation constants of the carboxyl and amino groups, respectively, the values used for the calculation (pi i = 3.75, pK2 = 7.75) are representative of an internal peptide bond. Theoretical values calculated using the following equations Kapp/ffpep = 1/(1 + [H+]/fCi)(l + K2/[H+D with Kpep = [peptide]/[RC02-][R NH3+], Kapp/ffest = 1/(1 + Ki/[H+]) with Kest = [ester]/[RC02H][R OH]...

Juffer, A.H. (1998) Theoretical calculations of acid-dissociation constants of proteins, Bchemistry and Cell Biology 76, 198-209. 

Shibukawa et al. [109] published a new liquid chromatographic method for the determination of acid dissociation constants. On the basis of theoretical equations regarding the effect of background mobile phase ions on the retention of ionic analytes on a non-ionic polymer packing, they could determine simultaneously the dissociation constants (p/fa) and the charges of analyte molecules. They used chloride and perchlorate ions in the mobile phase as they exhibit large differences in the retention on the hydrophilic polymer packings used, so that the effect of the mobile phase electrolyte on the retention factor of an ionic analyte could be clearly evaluated. 

Theoretical Calculations of Acid Dissociation Constants A Review Article... 

Methods other than thermodynamic cycles are often used to calculate acid dissociation constants. Previous publications implement the theoretical relationship between pKa and structural property [6], bond valence methods and bond lengths [33], pKa correlations with highest occupied molecular orbital (HOMO) energies and frontier molecular orbitals [34], and artificial neural networks [35] to predict pKa values. In addition much work has been done using physical properties as quantitative structure-activity relationship (QSAR) descriptors, and regression equations with such descriptors to yield accurate pKa values for specific classes of molecules [36-47]. The correlation of pKas to various molecular properties, however, is often restricted to specific classes of compounds, and it is... 

In this text. Shields and Seybold carefully review the thermochemical principles involved in the measurement and prediction of acid dissociation constants. They further provide an extraordinarily useful comparison of several theoretical models that have been proposed, whether for more specific or more general applications, examining their relative utilities and ranges of applicability. Importantly, the authors tightly couple their analysis to the rich physical organic chemistry associated with all of the various functional groups that may serve as conjugate acids or bases, and they do so not only for the case of aqueous solution, but also for non-aqueous solvents. They wrap up with some welcome discussion of temperature effects, isotope effects, and other phenomena that can extend the utility of acid dissociation constants as reporters of electronic structure and reactivity. 

In 1934 Saxton and Meier [1] published a report in which they described measurements of the acid dissociation constants of benzoic acid and its three monochloro derivatives. The results showed an interesting pattern all of the chloro-derivatives were more acidic than the parent compound, with the ortlio-substituted compound most acidic and the meta-chloro derivative least acidic. However, the authors closed their report with the lament that at present, there is no adequate theoretical method of calculating the ionization of a weak electrolyte from its structure. ... 

From Alongi, K.S., and Shields, G.C., Theoretical calculations of acid dissociation constants A review article. Annual Reports in Computational Chemistry, pp. 113-138, .010. Copyright .010. with permission from F.lsevier. 

The initial goal of the kinetic analysis is to express k as a function of [H ], pH-independent rate constants, and appropriate acid-base dissociation constants. Then numerical estimates of these constants are obtained. The theoretical pH-rate profile can now be calculated and compared with the experimental curve. A quantitative agreement indicates that the proposed rate equation is consistent with experiment. It is advisable to use other information (such as independently measured dissociation constants) to support the kinetic analysis. 

In deriving theoretical equations of the current-potential (or time) curves of ion transfer of an acid we shall make essentially the same assumptions as the assumption 1-6 above. It is noted here that theoretical equations of the more general case, that is, of a dibasic acid, such as expressed by AH2 = AH + H, AH = A + H, can be derived [24], but are not included here, to save space. The formal formation constant, and formal dissociation constant,, in the a phase is defined by... 

Theoretical considerations show that the free energy of dissociation of an acid in water, and hence the dissociation constant, is governed by the algebraic sum of the free energies for the solution of the undissociated acid in water, for vaporisation of the acid, for the formation of a free proton and an anion from the molecule of acid in the gas phase, and for hydration of the proton and anion. Thus the true acidity, given by the third of these... 

Equations (8)—(16) indicate that substituents in the homocyclic ring of quinolinium, benzopyrylium, benzothiopyrylium, and isobenzothiopyrylium cations influence pKR for pseudobase formation in the heterocyclic ring via p values in the range 4.9-6.9. The particular type of charge neutralization upon pseudobase formation is the major determinant of the magnitude of p. The observed p values are considerably larger than p = 2.81 for the acid dissociation of ring-substituted anilinium ions (31)148 which can be considered as the simplest... 

Dippy, J. F., The dissociation constants of monocarboxylic acids Their measurement and their significance in theoretical organic chemistry, Chem. Rev. 25, 151-211 (1939). 

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