Proton solvation energy

To avoid the issue of the proton solvation energy, the pK value of the group of interest is calcnlated by comparing the above terms for a suitable reference compound, for which the pAT valne is experimentally known (for calculation of pK of His291, for example, the model compound 4-methylimidazole will be used) then the shift of pK, value is calculated pAT ... 

B. The Solvated Proton The Proton Solvation Energy The Solvation of Anions... 

From the definition of acidity in solution as presented in Eq. (7.4), it is clear that in order to compute AG° one must know the proton solvation energy. However, from the previous discussion on the structural models for the hydrated proton one may anticipate some difficulties. For example, how many water molecules should be considered in the calculation of the proton solvation energy In other words How large should one take the H (H20) cluster One reasonable approach should be to examine the convergence of... 

In some circunstances, it might be convenient to take the hydronium ion as the basic unit for a proton in solution. In those cases, what is needed is the hydronium solvation free energy, AG° i (HgO ). Table VII.IO shows the results of theoretical calculations (da Silva [44b]), unless otherwise speci-hed) obtained with different continuum solvation models, according to the Ben-Naim [106] definition of the solvation process. Note that the only experimental value available (Pearson [107]) was obtained assuming AG/oi (H ) = —259.5 kcal/mol, which differs considerable from the more recent experiments of Tissandier et al. [104]. However, if one adopts Tissandier s result for the proton solvation energy, Pearson s scheme furnishes AG/oi (H30 ) = —106.16 kcal/mol, a value in close agreement with some of the theoretical results shown in Table VII.IO. 

Ml 31 B. E. Conway and M. Salomon, Classical H/D Isotope Effect in Proton Conductance, and the Proton Solvation Energy, in Chemical Physics of Ionic Solutions, Chapter 24 (B. E. Conway and R. G. Barradas, eds.), Wiley, New York (1966)... 

The Dissociation of a Molecule into Ions. The Removal of Ions from a Metal Surface. The Removal of Ions from the Surface of an Ionic Crystal. The Solvation Energy of an Ion. Work Done against Electrostatic Forces. Molecules and Molecular Ions Containing One or More Protons. Proton Transfers. The Quantities D, L, Y, and J. Two Spherical Conductors. 

Polar solvents have no effect on the rate constant of the reaction R02 + RH [56], This means that the solvation energies of the peroxyl radical R02 and TS R02 HR are very close. A different situation was observed for the reaction of cumylperoxyl radical with benzyl alcohol (see Table 7.10). The rate constant of this reaction is twice in polar dimethylsulfoxide (s = 33.6) than that in cumene (a 2.25). It was observed that the very important property of the solvent is basicity (B), that is, affinity to proton. A linear correlation... 

C. Interpretation of Equilibrium Data For pyridine, quinoline, isoquinoline, and acridine, the differences in the 7r-electronic energies of the protonated form and the free base have been calculated at several levels of sophistication, from the HMO method to the Cl SCF method.191 The A W values obtained are linearly interrelated, which supports the view that the HMO data are meaningful. In this connection, considerable attention has been paid to the role of solvation energies in acido-basic equilibria.161... 

A proton becomes a much stronger acid in an aprotic solvent such as acetonitrile (MeCN) than it is in water, since the solvation energy for proton in an aprotic solvent is much less than in water [39]. Thus, acid catalysis on electron transfer is expected to be much more efficient in MeCN than in water. In fact, the luminescence of [Ru(bpy)3]2 + ... 

Thus in the case that BH+ is H30+, Equation (14) becomes an identity reaction, for which there is no medium effect, and m =0. On the other hand, if BH+ is a protonated aniline, m = 1. These values provide fixed points on a scale of solvation energy changes associated with proton transfer between H30+ and the protonated base under study. 

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