Activation energy proton transfer reactions

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.

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. 

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... 

Lobaugh, J. and Voth, G. A. Calculation of quantum activation free energies for proton transfer reactions in polarsolvents, Chem. Phys. Lett., 198(1992), 311-315... 

Alonso et al. (2005) described anion-radical proton abstraction from prochiral organic acids. If the anion radicals were formed from homochiral predecessors, asymmetric deprotonation can be reached. However, low reactivity of the anion radical is required Slow proton transfer, that is, high activation energy of the reaction discriminates well between diastereoselective transition states. 

As shown in Figure 2.3, the activation energy of protonation (TSlt) is only 1.9kcal/mol and the proton-transfer reaction exhibits a very flat energy profile. 

Thus, if the ground state of the initial state (made up by H30+ + e ) moves up more titan Z does, the energy of activation for proton transfer gets less. Since the reaction velocity is proportional to e AE/RT, making the potential of the electrode more negative (the energy less negative) makes the reaction go faster. 

The nature of the methanol-zeolite interaction has been shown to be sensitive to a number of parameters and as such has proved to be a good benchmark for judging the reliability of quantum chemical methods. Not only are there a number of possible modes whereby one and two molecules interact with an acidic site (245), the barrier to proton transfer is small and sensitive to calculation details. Recent first-principles simulations (236-238) suggest that the nature of adsorbed methanol may be sensitive to the topology of the zeolite pore. The activation and reaction of methane, ethane, and isobutane have been characterized by using reliable methods and models, and realistic activation energies for catalytic reactions have been obtained. 

Polarisation effects should be relatively more important when the activation barrier is small or unimportant (i.e. the reactants diffuse together to form the encounter pair, as discussed in ref. 87b) than when the activation barrier is large (i.e. the reactants diffuse together and then have to obtain considerable energy to cross from the reactant to product potential energy surface). Clearly, much more work in this area is required and it is especially appropriate to consider these ideas when discussing proton transfer reactions. 

Quantum chemical DFT calculations at the B3LYP/6-31G(d) level have been used to study the enantioselective lithiation/deprotonation of O -alkyl and O-alk-2-enyl carbamates in the presence of (—)-sparteine and (—)-(f )-isosparteine.7 Complete geometry optimization of the precomplexes consisting of the carbamate, the chiral ligand, and the base (/-PrLi), for the transition states of the proton-transfer reaction, and for the resulting lithio carbamates have been performed in order to quantify activation barriers and reaction energies. 

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