Hydrogen atom transfer activation energy

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...

Enthalpies, Activation Energies, and Rate Constants of Intramolecular Hydrogen Atom Transfer in Peroxyl Radicals Calculated by the IPM Method [36], (Ais Increment of Polar Interaction in the Transition State)... 

The intramolecular hydrogen atom transfer occurs with lower activation energies in comparison with the intermolecular transfer (see the values of Ee for both types of reactions in Table 6.11). The values of the activation energies of intramolecular radical H-atom abstraction calculated by the IPM method are given in Table 6.15. 

Abramovitch, R.A., Abramovitch, D.A., Iyanar, K. and Tamareselvy, K., Transfer hydrogenation, atom economy activation, palladium, chemistry Application of microwave-energy to organic-synthesis - improved technology Tetrahedron Lett., 1991, 32, 5251-5254. 

It seems obvious to assume a priori that the facility of the transfer reaction depends on the strength of the broken and newly formed bond between radical and hydrogen atom transferred. The dependence between the activation energy E and dissociation energy D of the broken bond R —H ... 

For Equations (21a) and (21b) to apply, internal hydrogen atom transfer in the surface intermediates is an elementary step in the chain-growth reaction. Calculations by Ciobica et al. (90) indicate that the activation energies for C—H bond cleavage or C—H bond formation of adsorbed alkylidene or alkenyl intermediates are less than 50 kj/mol. 

The possible range of values of kA can be estimated. For hydrogen atom transfer between radicals, the pre-exponential factor A may be taken to be 10 cm. Vmolecule sec. and the activation energy may vary from 0 to 15 kcal./mole. Temperatures of interest may vary from 273 to 1000°K. Thus EfRT may vary from 0 to about 20, and a reasonable range of rate constant is... 

Biological systems activate ground-state dioxygen (302) for controlled energy transduction and chemical syntheses via electron-transfer and hydrogen-atom-transfer reduction to O2 -, HOO-, and HOOH. These reduction products are further activated with metalloproteins to accomplish oxygen atom-transfer chemistry. Conversely, green plants via photosystem II facilitate the oxidation of water to 02 (Chapter I),l... 

The conformity of given reaction parameters to those obtained from empirical correlations for reactions of the same type for one reactant with other partners and/or even with participation of other species. For instance, the Polanyi-Semenov equation is a good approximation for activation energy of hydrogen atom transfer between two species (radicals or molecules) (see, for instance, Kondratyev and Nikitin, 1974). If the parameters for some reaction seriously... 

The oxidation and snbsequent reduction of Tyrz are important at all stages of this process but the question of how the tyrosine is reduced has been a major issue. The two leading hypotheses are illustrated in Scheme 1. Pathway A involves a hydrogen atom transfer (HAT) but has been criticised in favour of a more conventional electron transfer (ET) process, pathway B. DFT calculations were used to try and determine which pathway is more likely. However, the detailed structure of the active site was unknown at that time. There is now a fairly low resolution structure available but in any case, trying to model the Mn4 system would be a daunting computational task. Instead, the important feature of the reaction was identified [16] as the oxidation of Mn(III) to Mn(IV) which experimentally has an activation free energy of about 12 kcal mol ... 

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