Acid dissociation constants , theoretical calculations

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... 

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. 

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 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. 

The closeness of fit may be gauged from the experimental and theoretical rate vs. concentration curves for hydrolysis of p-nitrophenyl carboxylates catalysed by quaternary ammonium surfactant micelles (Figure 3). The shape of the curve is satisfactorily explained for unimolecular, bimolecular, and termolecular reactions. An alternative speculative model is effectively superseded by this work. Romsted s approach has been extended in a set of model calculations relating to salt and buffer effects on ion-binding, acid-dissociation equilibria, reactions of weakly basic nucleophiles, first-order reactions of ionic substrates in micelles, and second-order reactions of ionic nucleophiles with neutral substrates. In like manner the reaction between hydroxide ion and p-nitrophenyl acetate has been quantitatively analysed for unbuffered cetyltrimethylammonium bromide solutions. This permits the derivation of a mieellar rate constant km = 6-5 m s compared to the bulk rate constant of kaq =10.9m s . The equilibrium constant for ion-exchange at the surface of the micelle Xm(Br was estimated as 40 10. The... 

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