Energies in Aqueous Solution

So far we have considered only those dissociation processes which take place in the gas phase. This chapter is concerned with dissociation processes, or more strictly with ionization processes in aqueous solution. This introduces a number of additional variabh factors. For example, the ionization process CHaCOOHiaq) 

CH3COO (aq) + H+(aq), can be thought of as occurring in a number of separate stages  

For a homologous series of acids, RCOOH, the ionization heat wdll, therefore, be affected by the dissociation energy of the oxygen-hydrogen bond (b), the electron affinity of the radical RCOO (c), and the hydration energy of the ion RCOO (d). From a study of the variations of ionization heats, within a homologous series, it may be possible to suggest which of these factors is important. 

Hitherto, the many studies of ionization processes in solution have been confined mainly to determination of dissociation constants, from which free energies of ionization can be calculated. Relatively few investigations have been concerned with heats and entropies of ionization. Some heats and entropies of ionization have been calculated from electrochemical data by Everett and Wynne-Jones and by Harned and Owen. There have been few data available for members of homologous series of compounds. 

In a recent series of papers, Laidler and his co-workers have measured the heats of neutralization of a number of such series, n- 

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Cramer C j and D G Truhlar 1992. AM1-SM2 and PM3-SM3 Parametrized SCF Solvation Models for Free Energies in Aqueous Solution. Journal of Computer-Aided Molecular Design 6 629-666. 

The data in Table 2.1 suggest that the O-benzylated adduct cannot be isolated since it is less stable than reactants. The N3-benzylated adduct should be generated faster, but it should also decompose under mild conditions into free reactants, because the activation free energy in aqueous solution for the decomposition into free QM and methylcytosine is only 21.4 kcal/mol.14 In other words, these data suggested that the QM-N3-cytosine conjugate could act as QM-carrier, few years before the experimental data related to the stability of QM-conjugates became available.4... 

C. J. Cramer and D. G. Truhlar, AM1-SM2 and PM3-SM3 parameterized SCF solvation models for free energies in aqueous solution, J. Comput.-Aided Mol. Design 6 629 (1992). 

Solubilities and Aqueous Activity Coefficients of Organic Liquids Solubilities and Aqueous Activity Coefficients of Organic Solids Solubilities and Aqueous Activity Coefficients of Organic Gases Illustrative Example 5.1 Deriving Liquid Aqueous Solubilities, Aqueous Activity Coefficients, and Excess Free Energies in Aqueous Solution from Experimental Solubility Data... 

P 5.1 Calculating Aqueous Activity Coefficients and Excess Free Energies in Aqueous Solution from Experimental Solubility Data... 

Calculate the aqueous activity coefficients,, and the excess free energies in aqueous solution, G (in kJ mol1), of (a) w-decane (n-C ioH22), (b) 2,3,7,8-tetra-chlorodibenzo-p-dioxin, and (c) bromomethane (CH3Br) at 25°C using the data provided in Appendix C. 

The equilibrium constants Ka, K, and h2o are conveniently summarized in Scheme 10 in the form of a cycle similar to that shown above for the a-phenethyl and t-butyl cations (Schemes 1 and 4). It is worth noting that P h2o measures the stability of the double bond relative to the alcohol (hydrate). If p fR was converted to HIA, p h2o in the cycle would be replaced by the energy of hydrogenation. The latter provides the conventional measure of double bond stability, save that here free energy in aqueous solution is measured rather than the more usual heat of hydrogenation in the gas phase. 

Table 4.3 Dimerization free energy in the gas phase (1 atm reference state), difference in hydration free energy between dimers and monomers and dimerization free energy in aqueous solution (1 M reference state) for selected complexes. The gas phase optimum geometry was used in all cases. All value are in kcal mol-1...

Metal-X Bond Energies. In aqueous solutions at pH 0 the silver electrode is oxidized reversibly ... 

Table 13-2. Vertical electronic n —> 7T transition energy of acetone in gas phase, Egas, and aqueous solution, Ewat, in units of eV. Excitation energy in aqueous solution was obtained from the combined QM/MM calculations treating acetone molecule at the quantum mechanical level of theory as indicated in the first column and using the polarizable potential for water molecules as a statistical average over 1200 molecular configurations extracted from classical MD simulation. The solvent shift in excitation energy, AE (in cm-1), is evaluated as a difference between excitation energies in water and in vacuum...

In summary, there is little distinction between the syn and anti lone pairs of the car-boxylate oxygen atom with regard to forming H-bonds with proton donors. When a proton has approached closely enough to form a covalent bond, the syn position is favored, but only in the gas phase. The syn and anti conformers of the carboxylic acid are close in energy in aqueous solution. Even in the gas phase, the preference for the syn configuration of the isolated carboxylic acid can be eliminated when it forms a H-bond, due to more favorable electrostatic interactions between the partner and the anti geometry of the carboxyl. 

Likewise, reaction 10 for which regrettably the thermochemistry is hmited to Gibbs energies in aqueous solution is exoergic by 30 kJmol . ... 

Several picoseconds after photon adsorption, the electrons end up in a localized solvated state at distances of l-5nm from the electrode surface. The electron s hydration energy in aqueous solutions is about 1.5-1.6eV. 

This reaction is highly endothermic because a bond is broken and charges are separated both of these processes require energy. In aqueous solutions, both ions are further solvated by water molecules, such as... 

This mechanism is rather unconventional in assuming the existence of OH-radicals in solution and hindrance of their chemical reaction with the electrode surface. This contradicts the generally recognized instability of OH- radicals in aqueous solutions. In spite of some early efforts to calculate their free energy in aqueous solution and to consider them as bulk phase inter-... 

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