Proton transfer activation energy

A noticeable feature of Figure 11.8 is that the activation barrier is always small the maximum is of the order of 0.25 eV. This is comparable to proton transfer activation energies in water, suggesting that the proton transfer process to an adsorbed OH is not so different from those encountered in proton hopping... 

Proton exchange among the three possible protonation sites in mesitylene (the —C—H sites) appeared to occur without intervention of an acid molecule to carry the proton. The activation energy of this process has been measured as 10 kcal mole-1. In hexamethylbenzene the C-methyl positions are protonated and the proton jump process has the same activation energy. In pentamethylbenzene the proton is always located on the —C—H site. Later results of MacLean and Mackor (1962) are more comprehensive. Intermolecular proton transfer via a solvent molecule has been established for mesitylene, anisole and m-xylene, but in hexamethylbenzene the transfer is always intramolecular. High activation energies of at least 8 kcal mole-1 were measured for proton transfers from the carbonium ion and this was associated with a weak interaction between... 

Figure A3.8.3 Quantum activation free energy curves calculated for the model A-H-A proton transfer reaction described 45. The frill line is for the classical limit of the proton transfer solute in isolation, while the other curves are for different fully quantized cases. The rigid curves were calculated by keeping the A-A distance fixed. An important feature here is the direct effect of the solvent activation process on both the solvated rigid and flexible solute curves. Another feature is the effect of a fluctuating A-A distance which both lowers the activation free energy and reduces the influence of the solvent. The latter feature enliances the rate by a factor of 20 over the rigid case.

Proton transfers from strong acids to water and alcohols rank among the most rapid chemical processes and occur almost as fast as the molecules collide with one another Thus the height of the energy barrier the activation energy for proton transfer must be quite low... 

Steps 2 and 4 are proton transfer reactions and are very fast Nucleophilic addi tion to the carbonyl group has a higher activation energy than dissociation of the tetra hedral intermediate step 1 is rate determining... 

This mechanism can reduce the overall activation energy of the reaction in at least two ways. The partial transfer of a proton to the carbonyl oxygen increases the electrophilicity of the carbonyl. Likewise, partial deprotonation of the amino group increases its nucleophilicity. 

The concerted nature of proton transfer contributes to its rapid rate. The energy cost of breaking the H—Cl bond is partially offset by the energy released in forming the new bond between the transfened proton and the oxygen of the alcohol. Thus, the activation energy is far- less than it would be for a hypothetical two-step process in which the H—Cl bond breaks first, followed by bond formation between FF and the alcohol. 

No proton transfers were observed in linear oligomers (catemers) of pyrazoles 8 in the solid, a fact which was understandable because such rearrangements would require a very high activation energy [97JCS(P2)101]. A possible exception to this rule is a catemer 8f, for which slow proton transfer was observed in the solid state [97JCS(P2)1867]. 

Many computational studies in heterocyclic chemistry deal with proton transfer reactions between different tautomeric structures. Activation energies of these reactions obtained from quantum chemical calculations need further corrections, since tunneling effects may lower the effective barriers considerably. These effects can either be estimated by simple models or computed more precisely via the determination of the transmission coefficients within the framework of variational transition state calculations [92CPC235, 93JA2408]. 

An interesting point that emerges from Fig. 5.6 is the relation between Ag and (AAgsol)w. p. As seen from the figure, the lowering of the activation energy for the reaction is almost linearly proportional to the stabilization of the ionic resonance form (AAg )w. p. An enzyme which is designed to accelerate a proton transfer between A and B will simply stabilize the (B 1—H A-) state more than water. 

Proton transfer reactions, 143-144, 144 activation energy, 149,164 all-atom models for, 146-148 Cys 25-His 159 in papain, 140-143 computer program for EVB calculations, 150-151... 

Figure 5-3. Active site and calculated PES properties for the reactions studied, with the transferring hydrogen labelled as Hp (a) hydride transfer in LADH, (b) proton transfer in MADH and (c) hydrogen atom transfer in SLO-1. (i) potential energy, (ii) vibrationally adiabatic potential energy, (iii) RTE at 300K and (iv) total reaction path curvature. Reproduced with permission from reference [81]. Copyright Elsevier 2002...

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