Hydrogen atom transfer barriers

Fig. 41. Empirical correlation between 0-0 distance, barrier height and hydrogen-atom transfer distance in OH-O fragment.

In none of these examples has the potential for removal of an electron approached the ionization potentials of the metals. Although traditional treatments attribute the potentials of Eqs. (10.1), (10.15), and (10-17)—(10-21) to the removal of electrons from the metals, coupled with large ionic solvation energies, this requires a pathway with the ionization potential as a kinetic barrier. Furthermore, the spontaneous reaction of iron with acidified water is driven by the formation of Fe—OH2+ and H—H covalent bonds that facilitate hydrogen-atom transfer from water (rather than electron transfer from iron) ... 

To summarize this section, the theoretical calculations [2] strongly support the mechanism proposed for galactose oxidase. It was shown that the proton transfer step proposed to initiate the oxidation of the substrate is very fast and just slightly exothermic. The rate-limiting hydrogen atom transfer step has a calculated barrier of feasible 13.6 kcal/mol. The proposed short-lived ketyl radical intermediate has been localized, and it was argued that the subsequent... 

Explicit tunneling effects are required to accommodate the kinetics of SLO, and may be equally important in many other hydrogen atom transfer reactions. Many H transfer reactions are characterized by very large inherent chemical barriers, such that movement through, rather than over, the barrier may dominate the reaction pathway as the lowest energy path for conversion of reactants to products. 

Pu, J., Teuhiae, D. G. (2004) Benchmark results for hydrogen atom transfer between carbon centers and validation of electronic structure methods for bond energies and barrier heights,/. Phys. Chem. A 108, 2475-2486. 

Hydrogen atom transfer, 148-149 barriers, 149 Hydrogen bond, 82 Hydrogen bonding in, 50 aimnonia complexes, 138 aqueous HF, 138... 

It should be noticed that in the reaction step of Equation 10.8, the electron moves, separated from the proton. We cannot be dealing with hydrogen atom transfer, since the moving electron is in a orbital, which cannot mix with H Is. The rate of this reaction depends on both the activation barrier for PT and the activation barrier and coupling for ET. The calculation of rate will be discussed later. 

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