Protons dissociation

Consider first a proton dissociation reaction for a group AH being a part of a small molecule... 

On the other hand, if activity decreases sharply as pH is raised, activity may depend on a protonated group, which may act as a general acid, donating a proton to the substrate or a catalytic water molecule (b). At high pH, the proton dissociates and is not available in the catalytic events. 

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

The salt effect is very strong in polyconjugated polyelectrolytes. Figure 15 is a graph of the proton dissociation energy vs. the dissociation degree of PPA of different structures. Also, the graphs for poly(methacrylic acid) and a copolymer... 

Fig. 15. Energy of proton dissociation (Ez) from Z times ionized polyelectrolyte molecules as function of the degree of dissociation (a). (A) - PPAL (1), PPAS (2), PPA (3), polyfmethacrylic acid) (4), copolymer of acrylic acid with ethylenesulfonic acid (50 50) in aqueous solutions (5), (B) - PPAL (1), PPAS (2), PPA in the presence of NaCl (3) ( ) INaClj = 0 (X) fNaCll = 0.25 mmol/1 (o) 0.50 mmol/1...

However, the addition of even small volumes of alkali leads to the screening of these groups, with a subsequent decrease of the proton dissociation energy at low dissociation degrees. This complies with the salt effect (Fig. 15). 

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. 

We close this section with a note on the influence of pH on reduction potentials. Many redox reactions are pH-dependent, which can be understood with reference to the simple model in Figure 13.4, in which a redox compound in its oxidized state has a pK,t for proton dissociation that is different from (i.e., lower than) the corresponding value for its reduced state the positive charge of Xox is higher than that of Xred, so it is more difficult for Xox to accept a proton (i.e., its pKa is lower). The °(pH) is now... 

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). 

If only the acid/base proton dissociation of complex MOH is considered, Scheme 4 is simplified to include only steps (a) and (b) therein. According to this model, (I) is the primary deprotonation pathway in acidic medium, while (III) is of importance in basic media. On the other hand, direct proton transfer (II) can occur around neutral pH values. 

Figure 13 Snapshots of the simulations at 75 GPa (left) and 115GPa (right). The temperature for both is 2000 K. At 75 GPa, the water molecules are starting to cluster, and at 115 GPa, a well-defined network has been formed. The protons dissociate rapidly and form new clusters (at 75 GPa) or networks of bonds (at 115 GPa).

In general, semiconductor electrodes adsorb in aqueous solutions water molecules, hydronium ions, and hydroxide ions in addition to various solute ions. As a result, the dissociation-association equilibria of the adsorbed hydronium ions and water molecules produce, in the proton dissociation-association reactions of Eqns. 9-69 and 9-70, the acidic and basic proton levels, respectively, on the electrode interface as shown in Fig. 9-21 ... 

Reactions in Eqns. 9-69 and 9-70 8me the dissociation processes of (1) the acidic protons from adsorbed hydronium ions and (2) the basic protons from adsorbed water molecules on the electrode interface, respectively. Eqn. 9-71 gives the equilibrium constants, and K, of these proton dissociation reactions ... 

Fig. 9-21, Proton levels of adsorbed hydronium ions and of adsorbed water molecules on semiconductor electrodes (a) acidic proton dissociation of adsorbed hydronium ions, (b) basic proton dissociation of adsorbed water molecules. S = semiconductor surface atom.

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. 

For compound semiconductors, the adsorbed proton level differs with different constituents in the semiconductor thus, the distinction between the acidic and basic proton levels, pKi and pKt, is greater than in the case of elemental semiconductors. For example, on metal oxide electrodes, the acidic proton dissoci-... 

Paddison, S. J. and Elliott, J. A. 2006. On the consequences of side chain flexibility and backbone conformation on hydration and proton dissociation in perfluorosulfonic acid membranes. Physical Chemistry Chemical Physics 8 2193-2203. 

The majority of the molecular-scale information concerning the effects of structure and local chemistry on proton dissociation and separation in PEM fragments alluded to previously " were initially determined using HE theory and split valence local basis sets. Refinements to the equilibrium configurations were made using both Mailer-Plesset (MP) perturbation schemes and hybrid density functional theory (described below). 

Tables 4.S-4.7 and Fig. 4.6 list organic acids commonly used as metal extractants. When the acids are not protonated, dissociated, polymerized, hydrated, nor form adducts, the distribution ratio of the acid HA is constant in a given solvent extraction system ...

The system dihydrophenazine-phenazine shows a combination of redox and proton dissociation equlibria in aqueous solution summarised in Scheme 6.8. Phe-... 

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-... 

The larger molecular entity need not be a macromolecule. Consider the proton dissociation steps of the simplest amino acid, glycine. The dissociation of the first proton (i.e., the most acidic proton) can be represented by... 

A solvent kinetic isotope effect (SKIE) of 0.44 from solvolysis in CD3CN-H2O versus CD3CN-D2O mixtures was in line with predicted values for the protonation-dissociation mechanism for which the SKIE should be between 0.48 and 0.33 (Figure 18a... 

In common with all protonic dissociation, this reaction pathway assumes that all k species of the same charge (k=j + i +1) are equivalent and takes advantage of the assumed independence of the macroscopic dissociation constants Kh K2,.Kj+i. Moreover, this dis-... 

Equation 13.22, however, does not tell the whole story about metal ion hydrolysis. Not only can further proton dissociations occur... 

Experimentally it has been found that primary and secondary amines react by solvolysis, while only the tertiary amines generally produce reduction, if reduction is observed. It thus seemed appropriate to study the reaction of niobium (V) halides with pyridine, where proton dissociation need not be considered and any reaction would necessarily lead to a simple adduct of pyridine or reduction of the metal halide. In this work, reduction of the niobium(V) halides was observed, and the reaction products were characterized. Elucidation of the pyridine oxidation products has permitted an interpretation of the reaction mechanism in terms of the two-electron reduction of niobium(V) by the pyridine molecule. 

A more general method proposed by Forster is a thermodynamic calculation based on a cycle called after his name (Figure 4.12). If AH and AH are enthalpies of proton dissociation in the ground and the excited states, respectively, then... 

Kinetic studies have shown that axial ligands in the complexes Na3[Cr(TPPS)(H20)2], CrQ(TPP)L (L = monodentate base) and [Cr(Schiff s base)(H20)2]+ are unusually labile,1113 1253 1254 and equilibrium constants for proton dissociation from coordinated water in the first complex1252 have been determined. 

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