Activation Energy For Hydrogen Abstraction Research Articles

Prediction of Activation Energies for Hydrogen Abstraction by Cytochrome P450

We have estimated the activation energy for hydrogen abstraction by compound I in cytochrome P450 for a diverse set of 24 small organic substrates using state-of-the-art density functional theory (B3LYP). We then show that these results can be reproduced by computationally less demanding methods, for example, by using small organic mimics of compound I with both B3LYP and the semiempirical AM1 method (mean absolute error of 3-4 kJ/mol) or by calculating the bond dissociation energy, without relaxation of the radical (B3LYP) or estimated from three-point fit to a Morse potential (AM1; errors of 4 and 5 kJ/mol, respectively). We can assign activation energies of 74, 61, 53, 47, and 30 kJ/mol to primary carbons, secondary/tertiary carbons, carbons with adjacent sp(2) or aromatic groups, ethers/thioethers, and amines, respectively, which gives a very simple and predictive model. Finally, some of the less demanding methods are applied to study the CYP3A4 metabolism of progesterone and dextromethorphan.

Activation energy for hydrogen abstraction from methane over Li-doped MgO: A density functional theory study

Hydrogen abstraction from methane over Li-doped MgO is studied by means of density functional theory. The generalized synchronous transit method is applied to determine the transition state of the reaction. This method allows a transition state search that is more comprehensive compared with previous studies. The convergence of the calculated activation barrier with respect to cutoff energy, k-point mesh, vacuum layer thickness, and number of ionic layers in the crystal slab is examined. The activation barrier is calculated to be 0.745+/-0.01 eV (71.9+/-1.0 kJ/mol).

The C–H bond dissociation enthalpies of haloethers and its correlation with the activation energies for hydrogen abstraction by OH radical: A DFT study

The C–H bond dissociation enthalpies for 12 haloether molecules were computed from the results of density functional theory calculations using B3LYP exchange-correlation functional in conjunction with two basis sets, such as 6-311G(d,p) and 6-311++G(2d,p). The trend in reactivity towards hydrogen abstraction by OH radical has been discussed in the light of the C–H bond dissociation enthalpies. Good correlation has been observed between the activation energies for the hydrogen abstraction from haloethers by OH radical and the corresponding C–H bond dissociation enthalpy values.

A DFT Study on the C−H Bond Dissociation Enthalpies of Haloalkanes: Correlation between the Bond Dissociation Enthalpies and Activation Energies for Hydrogen Abstraction

The C−H bond dissociation enthalpies of halomethanes were computed from the results of density functional theory (DFT) calculations at the B3LYP level with various basis sets, such as 6-311G(d,p) and 6-311++G(3df,2p). Reasonably accurate C−H bond dissociation enthalpies were obtained even at the B3LYP/6-311G(d,p) level when ROB3LYP method was used for radicals. Applying the same procedure, the C−H bond dissociation enthalpies for a series of haloethanes were also calculated. Good correlation has been observed between the activation energies for the hydrogen abstraction from haloalkanes by OH radical and the corresponding C−H bond dissociation enthalpy values.

Polarity of Free Radicals in Hydrogen Abstraction Reactions

Abstract The electrophilicity of the CF3 radical in hydrogen abstraction reactions was studied theoretically by the CNDO/2 method. The activation energy for hydrogen abstraction by the CF3 radical from aliphatic hydrocarbons decreases with an increase in the electron density on the hydrogen atom to be abstracted. The origin of the electrophilicity of the CF3 radical can be well understood by the stabilization energy due to delocalization of electrons (SEDE) between the radical and substrate. The electrophilic stabilization energy denned by the SEDE from substrate to the radical governs the relative reactivities of aliphatic hydrocarbons, CH4<CH3CH3<CH3CH2CH3<(CH3)3CH, which corresponds to the order of the increase in the electron density. The results indicate that the radical is an electrophile if the relative reactivities of substrates are governed by the electrophilic stabilization energy, and a nucleophile when the reactivities are governed by the nucleophilic stabilization energy (the SEDE from the radical to substrate). SEDE was calculated for the reactions of the OH and the CH3 radicals with CH4and CH3CN; the results indicated that the OH radical is electrophilic and the CH3 radical nucleophilic. These polarities of radical reagents are discussed in connection with their physicochemical properties such as ionization potentials and electron affinities.

Hydrogen abstraction from organosilicon compounds. The reactions of fluoromethyl radicals with tetramethylsilane. Polar effects in gas phase reactions

AbstractThe photolysis of 1,1,3,3‐tetrafluoroacetone has been reinvestigated as a source of CHF2 radicals at temperatures up to 578°K, and the following rate constant ratio was determined for the reactions magnified imageθ= 2.303 RT in kcal/mole. 1,1‐Difluoro‐ and 1,1,3,3‐tetrafluoroacetone were photolyzed in the presence of tetramethylsilane, and Arrhenius parameters were measured for the hydrogen abstraction reactions: R + Me4Si → RH + Me3SiCH2 R T(°K) E (kcal/mole) log A (mole−1cc sec−1) log k (500°K) (mole−1cc sec−1) CH2F 473–586 12.00 ± 0.30 11.68 ± 0.12 6.44 CHF2 416–526 10.18 ± 0.33 11.65 ± 0.15 7.21 By comparing with previous data on the CH3 and CF3 reactions, the activation energies were interpreted in terms of the enthalpy changes for the reactions and a polar effect operative between the attacking fluoromethyl radicals and the substrate which tends to reduce the activation energy progressively as the fluorine content of the radical increases. This polar effect, which decreases the activation energy for hydrogen abstraction from SiMe4 along the series CH3, CH2F, CHF2, and CF3, is in marked contrast to the polar repulsion between the fluorinated radicals and SiHCl3 which was tentatively proposed to explain the increase in activation energy observed in the same series of reactions with SiHCl3.

The Photolysis of Acetone: Diffusion of Methyl Radicals from the Light Beam and the Activation Energy of Hydrogen Abstraction

The Photolysis of Acetone: Diffusion of Methyl Radicals from the Light Beam and the Activation Energy of Hydrogen Abstraction