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... 

Table 10.1. Energy Changes for Isodesmic Proton-Transfer Reactions of Substituted Benzenes"...

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]. 

The approach presented above is referred to as the empirical valence bond (EVB) method (Ref. 6). This approach exploits the simple physical picture of the VB model which allows for a convenient representation of the diagonal matrix elements by classical force fields and convenient incorporation of realistic solvent models in the solute Hamiltonian. A key point about the EVB method is its unique calibration using well-defined experimental information. That is, after evaluating the free-energy surface with the initial parameter a , we can use conveniently the fact that the free energy of the proton transfer reaction is given by... 

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... 

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

A proton transfer reaction involves breaking a covalent bond. For an acid, an H — X bond breaks as the acid transfers a proton to the base, and the bonding electrons are converted to a lone pair on X. Breaking the H — X bond becomes easier to accomplish as the bond energy becomes weaker and as the bonding electrons become more polarized toward X. Bond strengths and bond polarities help explain trends in acidity among neutral molecules. 

Numbers used in this cycle AG° for addition of water to give a monoanionic adduct, Table 1.7 AG° for proton transfer reactions, based on pAT, values estimated by the method of Branch and Calvin.) The dissociative energy is taken from Table 1.7. 

Similarly, Ervin and co-workers have measured acidities of organic molecules by measuring the energy for endothermic proton transfer reactions between acids and anionic bases." " Alternatively, it is possible to use competitive CID of proton-bound dimer ions." Nominally, these are relative approaches for measuring acidities, as the measured acidities depend on the properties of the reference acids or bases. However, it is usually possible to select references with very accurately known acidities (such as HE, HCN, or HCl), such that the accuracy of the final measurement depends predominantly on the accuracy of the threshold energy determination. 

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. I. Fixed proton donor—acceptor separation. J Phys Chem A... 

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