Enthalpy of ionization

The ionization of a series of 4-substituted pyridines has been studied, and both equilibrium acidities (pK ) and enthalpies of ionization have been recorded at 25°C ... 

Fig. 5. Differential enthalpy of ionization of crosslinked MA-DMEG copolymers as a function of the degree of ionization of carboxylic groups. Crosslinking agent (DMEG) content (mol%) 1)3 2) 7.5 5) 12...

Tris(2-aminoethyl)amine (tren) and hexamethyl tren form pentacoordinate complexes with a bound water. The enthalpies of ionization have been determined.242 These results dispute the data from the systems that led to the view that solvent structure mediates the pAia of zinc bound water in zinc hydrolytic enzymes. The effect was shown to be entirely enthalpic. 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

The lattice enthalpy, Aiatt//m, is the molar enthalpy change accompanying the formation of a gas of ions from the solid. Since the reaction involves lattice disruption the lattice enthalpy is always large and positive. Aatom//m and Adiss//m are the enthalpies of atomization (or sublimation) of the solid, M(s), and the enthalpy of dissociation (or atomization) of the gaseous element, X2(g). The enthalpy of ionization is termed electron gain enthalpy, Aeg//m, for the anion and ionization enthalpy, Ajon//m, for the cation. 

Table 18 shows the free energies and enthalpies of ionization of some nitroalkanes. 

D. Free Energies and Enthalpies of Ionization in Aqueous Solution.201... 

The enthalpies of ionization corresponding to Eqs. (4), (3"), and (11) can be determined by means of the temperature effect on the respective standard free energy changes (70MI3) or by calorimetric techniques. 

It also includes the enthalpy of ionization of the hydrogen atom (equal to, but opposite in sign to, the electron attachment enthalpy of the gaseous proton), the enthalpy of atomization of dihydrogen and the enthalpy of hydration of the proton. The enthalpy of formation of the cation is estimated by use of the equation ... 

Imidazole groups in solution have enthalpies of ionization of about 30 kJ/mol (7 kcal/mol), whereas carboxylic acids have negligible enthalpies of ionization. But the changes in the enthalpy of the solvating water molecules also make important contributions to these values, and so the solution values cannot be extrapolated to partly buried groups in proteins. 

The difference in the pKa values of m- and p-nitrophenol in aqueous medium has aroused interest. In earlier work (Hepler et al., 1965), the enthalpies of ionization were found to be almost identical, and the difference in pATa was attributed to an entropy effect. Since m-nitrophenol had the more negative entropy of ionization, this appeared to be consistent with a greater degree of solvent orientation around its conjugate base as expected since the negative... 

A thermochemical method for comparing pKa values of weak acids in DMSO has been described by Arnett et al. (1973). A linear relationship is observed between AGf and AHf for 30 weak acids in DMSO over a range of 20 pKa units. Thus AHf, the enthalpy of ionization in DMSO, is a useful guide to relative acid/base strength (Arnett and Scorrano, 1976). The relationship implies, of course, that AS° is linearly related to AHf. Correlations have been drawn between acidities in DMSO and intrinsic gas-phase acidities (Arnett et al., 1975 Bordwell et al., 1975). 

The fluorine ion donor ability of Xep4 has been shown to be less than that of Xep2 and XeFe XeFg > Xep2 Xep4. These findings are in agreement with the enthalpies of ionization derived from photoionization studies (equations 9-11). 

The enthalpy of ionization of XePe is more than 0.6 eV lower than that anticipated on the basis of Xep2 and Xep4 data. This is compatible with the greater stability of XeFs+ salts compared with XeF+ and Xep3+. It seems that the pseudooctahedral geometry of the Xep5+ ion is especially favorable. 

Johnson, F.A., Lewis, S.D., Shafer, J.A. (1981). Perturbations in the free energy and enthalpy of ionization of histidine-159 at the active site of papain as determined by fluorescence spectroscopy. Biochemistry 20 52-8. 

Figure 1. Relationship between the enthalpy of ionization (aHo) and pKa for major classes of acidic functional groups...

The tabulated enthalpy of ionization (AH,) of the functional group is calculated from the experimentally determined enthalpy of neutralization (AH) using the equation AH = AH + AH, where AHy = + 55.8 kJ/mole is the enthalpy of ionization of water. 

At 65 °C, fcs [rate expression (8)] has the value 9.1 x 10 sec" n = 2.2). The apparent activation energy is 25 kcal.mole or, reckoning this to be a reaction of HCIO, 15.1 kcal.mole when corrected for enthalpy of ionization. A mechanism is not established beyond the guess that the first step might be... 

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