Proton transfer reaction energetics

It may be instructive to again consider the energetics of a proton transfer reaction of the type involved in the first step of the examples above, in solution. Under the influence of a possible general base as the proton acceptor and a possible metal ion assisting as a catalyst we can write... 

The present level of theoretical work can be seen in a recent paper by Sevilla s group. They have employed DFT to study the details of proton transfer reactions in Gua Cyt base pairs [57]. Using the DFT functional B3LYP with the 6-31 + G(d) basis set on the entire Gua Cyt molecule (19 heavy atoms), the results are presented which show that it is energetically favorable for the N1 proton of guanine to transfer to the N3 of cytosine after reduction of cytosine or after oxidation of guanine. 

For the stronger proton acceptors (ammonia, monoethylamine, and piperidine) a relation between the B proton affinities and the spectral shifts of the S3 <- S0 states of phenol(B) or naphthol(B) shows a linear dependence for proton affinities lower than a limit value situated around 10.4 eV (s240 kcal mol-1) for both phenol or naphthol molecules. Above this limit, the spectral shift is much larger and is different for phenol and 1-naphthol (see Figure 4-17). Nevetheless, this limit seems to correspond with the energetical limit of the proton transfer reaction. 

This simple neutralization reaction always favors the weaker base. Thus the most acidic H is deprotonated, even if the acid has an adjacent CH. Because of the higher electronegativity of heteroatoms, protons on heteroatoms are usually lost before protons on carbon. The base n-butyllithium deprotonates the nitrogen of diisopropyl amine. The adjacent CH is about 14 orders of magnitude less acidic. Always check the eq to be sure your proton transfer is energetically reasonable. The Xgq for the reaction below is highly favorable, 10(50 - 36) = 10+14. 

CI) species from strong acids produced by EI of a suitable bath gas (H2, CH4...) due to a lower internal energy of the ions controlled by the energetics of proton-transfer reaction... 

Marcus R. A. (1975), Energetic and dynamical aspects of proton transfer reaction in solution , Faraday Symp. Chem. Soc. 10,60-68. 

Table 2. The geometrical (in A and degrees) and energetic (kcal/mol) parameters for two tautomeric forms of the simple enaminones the results for transition states of corresponding proton transfer reactions are also included there is also the difference in energies between the considered system and the most energetically stable system (hence 0 designates the most stable system)...

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