Hydrogen abstraction barrier

DFT calculations on the ruthenium-mediated hydroxylation show that the low-spin reaction trajectory is preferred throughout, in accord with general expectations for the behavior of second-row transition metals The ruthenium analog was found to be more electrophilic than its iron complex, having lower hydrogen abstraction barriers. Thus, the data for the iron and ruthenium porphyrin systems is in accord with the predictions of theory that a radical rebound process is viable for iron which has an accessible high-spin state but not for ruthenium which is always low-spin. 

The hydrogen abstraction barriers in alkane hydroxylation shown in Table 2.1 exhibit a high sensitivity to the donor property of the alkane and the C-H bond energy. They range from 26.7,... 

Fig. 8. Correlation between hydrogen abstraction barrier (AE ) and BDEch with data taken from Ref. (52).

Among the seven reaction pathways, the intramolecular hydrogen abstraction from the methyl group in the MPP radical by the radical peroxy oxygen atom leading to 2-hydroperoxybenzyl radical (HPB, pathways I, Scheme 2.15) is one of the kinetically most favored since its activation enthalpy is only 26.5 kcal/mol. This benzylic radical is fairly unstable and rapidly generate the o-QM by OH radical loss through a very low activation barrier (7.6 kcal/mol). 

One of the simplest chemical reactions involving a barrier, H2 + H —> [H—H—H] —> II + H2, has been investigated in some detail in a number of publications. The theoretical description of this hydrogen abstraction sequence turns out to be quite involved for post-Hartree-Fock methods and is anything but a trivial task for density functional theory approaches. Table 13-7 shows results reported by Johnson et al., 1994, and Csonka and Johnson, 1998, for computed classical barrier heights (without consideration of zero-point vibrational corrections or tunneling effects) obtained with various methods. The CCSD(T) result of 9.9 kcal/mol is probably very accurate and serves as a reference (the experimental barrier, which of course includes zero-point energy contributions, amounts to 9.7 kcal/mol). 

The Hammond Postulate applies only if both forward reactions are fast. Obtain energies for the transition states leading to 1 -propyl and 2-propyl radicals (propane+F end andpropane+F center). Draw an energy diagram for each hydrogen abstraction reaction (place the diagrams on the same axes). Do these diagrams indicate that use of the Hammond Postulate is justified Calculate the barrier for each reaction, and calculate the relative concentrations of 1-propyl and 2-propyl radicals that would form at 298 K if each reaction were irreversible. Use equation (2). How does this (kinetic) ratio compare to the equilibrium (thermodynamic) ratio of these radicals ... 

Valence Bond Modeling of Barriers in the Nonidentity Hydrogen Abstraction Reactions, X" + H—X X —H + X (X fX = CH3, SiH3, GeH3, SnH3, PbH3). 

The rates of many chemical reactions axe limited by an energy barrier AEb which must be surmounted [24]. A well-studied case [14,15,17,25] is the hydrogen abstraction... 

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