Free energy of protonation, and

East ALL, Smith BJ and Radom L ( 1997) Entropies and free energies of protonation and proton-transfer reactions. Journal of the American Chemical Society 119 9014-9020. 

The relative stabilities of 1-phenylvinyl cations can be measured by determining the gas-phase basicity of the corresponding alkynes. The table below gives some data on free energy of protonation for substituted phenylethynes and 1-phenylpropynes. These give rise to the corresponding Yukawa-Tsuno relationships. 

In the gas phase the free energy of protonation of A,N-dimethylvinylamine (equation 48) is — 220.0 kcal mol-1 64. As discussed in Section II.A.4, protonation occurs at Cp, as shown506 112,113. The free energies of protonation of methyl vinyl ether and methyl vinyl sulfide are —198.8 and —198.6 kcal mol-1, respectively these substances also are protonated at C/14. Far more basic than any of these is the enolate of acetaldehyde (vinyloxide-) for which C-protonation is accompanied by AG°(g) = —359 kcal mol-1 115. Thus, the gas-phase reactivity order is enolates > enamines en-olethers enols vinyl sulfides. (For the present purpose, we ignore the real, but smaller, differences in reactivity between enols, their ethers and vinyl sulfides.)... 

A second class [ 16] is one that may enjoy increased interest in the future because of the presence of one of its members in the first industrial IL process [lb], and also because of the new finding that its members can have aqueous solution-like conductivities [17] and can serve as novel electrolytes for fuel cells [18]. This class is closely related to the first but differs in that the cation has been formed by transfer of a proton from a Br0nsted acid to a Br0nsted base. The process is reversible, depending on how large the free energy of proton transfer is. When the gap across which... 

Table 23 The free energy of the reaction of eqn (19) in the gas phase, the free energy of solvation, and the free energy of the reaction in an aqueous solution, together with the thereby obtained pA a value in comparison with experimental values (denoted Exp) for the acids given by HA. and pK,J were calculated by considering other proton exchange reactions. All energies are in kcal/mol and the results are from ref. 80...

Despite all the problems inherent to QM/CM approaches, some extremely interesting and perceptive work has been described in the literature recently in which all sorts of approaches have been used, improvements introduced and results obtained ([351, 372] and references therein). The study of enzyme catalysed reaction mechanisms, the calculation of relative binding free energies of substrates and inhibitor, and the determination of proton transfer processes in enzymatic reactions, are all good examples of enzyme-ligand interactions studies. Even though Warshel s EVB method [349] probably remains the most practical QM/CM approach for the study of enzyme catalysis, very useful work has been reported on enzyme catalysed reactions ([381] for an excellent review-[238, 319, 382-384]). This is a consequence of the accuracy of QM to treat the active site and inhibitor/substrate and the viability of classical mechanics to model the bulk of the enzyme not directly involved in the chemical reaction. 

Kelly, C.P., Cramer, C.J., Truhlar, D.G. Aqueous solvation free energies of ions and ion-water clusters based on an accurate value for the absolute aqueous solvation free energy of the proton. J. Phys. Chem. B 2006,110(32), 16066-81. 

Hess measured the apparent equilibrium constant for reduction of cytochrome by ferroeyanide as a function of pH 185), and found that the actual reduction step did not involve a proton and was entirely independent of pH. The free energy of reduction, and hence the reduction potential, change with pH only because the ratio of amounts of state III and IV changes with pH. The observed decrease of cytochrome reduction potential of 60 mV per pH unit above pH 8 is exactly what would be calculated from the simple Nernst equation. 

Standard Free Energies of Proton Transfer and Solvation in Aqueous Aci< General Description. ... 

STANDARD FREE ENERGIES OF PROTON TRANSFER AND SOLVATION IN AQUEOUS ACID... 

From the preceding it follows that the half-wave potential measured in DCP will only in rare cases approximately equal the standard potential. The requirements for this are (i) no side reactions (equilibria) of the reduced or oxidized form (esp. no protonation reactions), (ii) no amalgamation, or a dissolution in mercury with negligible Gibbs free energy of amalgamation, and (iii) no strong deviation of the activity coefficient ratio from unity. 

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