Hydrogen atom abstraction

About: Hydrogen atom abstraction is a research topic. Over the lifetime, 7059 publications have been published within this topic receiving 151781 citations.

Ab initio study of the reactions between methane and OH, H, and 3O

Abstract: The primary processes in methane combustion include hydrogen abstraction by radicals to form CH{sub 3}. Ab initio molecular orbital calculations have been used to study three important reactions contributing to the formation of the methyl radical: CH{sub 4} + X {yields} CH{sub 3} + HX, where X = OH, H, and {sup 3}O. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the HF/6-31G** and UMP2/6-31G** levels. Barriers and heats of reaction have been estimated by fourth-order Moeller-Plesset perturbation theory with spin projection (PMP4(SDTQ)) using the 6-311G** basis set.

Theoretical studies on cytochrome P-450 mediated hydroxylation: a predictive model for hydrogen atom abstractions

Abstract: The semiempirical quantum chemical method, AM1, was used to develop a predictive model for cytochrome P-450 hydrogen abstraction reactions Initially, a general rank-order correlation was observed between the calculated stability of a radical and the tendency of a carbon-hydrogen bond to undergo hydrogen atom abstraction by cytochrome P-450 Of several oxygen radicals studied, the p-nitrosophenoxy radical has the most appropriate transition-state symmetry for use as a model for P-450-mediated hydrogen atom abstractions

Adsorption and Dissociation of Ammonia on Graphene Oxides: A First-Principles Study

Abstract: The interactions of ammonia with graphene oxides (GO) were studied by density functional theory calculations. Our results indicate that the adsorption of NH3 on GO is generally stronger than that on graphene because of the presence of diverse active defect sites, such as the hydroxyl and epoxy functional groups and their neighboring carbon atoms. These surface oxygen sites can form OH···N and O···HN hydrogen bonds with NH3 and enhance charge transfers from NH3 to the graphene oxide. The dissociation of the adsorbed NH3 into the chemisorbed NH2 or NH species through the H atom abstractions leads to hydroxyl group hydrogenation and ring-opening of epoxy group. The reactions of NH3 with the hydroxyl and epoxy groups are predicted to be exothermic with different energy barriers, depending on the oxidation species and the atomic arrangement of these groups. The hydroxyl group exhibits relatively higher reactivity toward hydrogen abstraction from the adsorbed NH3 than the epoxy group in GO with a single oxygen ...

Evidence for an Alternative to the Oxygen Rebound Mechanism in C–H Bond Activation by Non-Heme FeIVO Complexes

Abstract: The hydroxylation of alkanes by heme FeIVO species occurs via the hydrogen abstraction/oxygen rebound mechanism. It has been assumed that non-heme FeIVO species follow the heme FeIVO paradigm in C–H bond activation reactions. Herein we report theoretical and experimental evidence that C–H bond activation of alkanes by synthetic non-heme FeIVO complexes follows an alternative mechanism. Theoretical calculations predicted that dissociation of the substrate radical formed via hydrogen abstraction from the alkane is more favorable than the oxygen rebound and desaturation processes. This theoretical prediction was verified by experimental results obtained by analyzing iron and organic products formed in the C–H bond activation of substrates by non-heme FeIVO complexes. The difference in the behaviors of heme and non-heme FeIVO species is ascribed to differences in structural preference and exchange-enhanced reactivity. Thus, the general consensus that C–H bond activation by high-valent metal–oxo species, inclu...

Hydrogen Atom Abstraction by a High-Valent Manganese(V)−Oxo Corrolazine

Abstract: High-valent metal-oxo complexes are postulated as key intermediates for a wide range of enzymatic and synthetic processes. To gain an understanding of these processes, the reactivity of an isolated, well-characterized Mn(V)-oxo complex, (TBP8Cz)MnVO (1), (TBP8Cz = octakis(para-tert-butylphenyl)corrolazinato(3-)) has been examined. This complex has been shown to oxidize a series of substituted phenols (4-X-2,6-t-Bu2C6H2OH, X = C(CH3)3 (3), H, Me, OMe, CN), resulting in the production of phenoxyl radicals and the MnIII complex [(TBP8Cz)MnIII] (2). Kinetic studies have led to the determination of second-order rate constants for the phenol substrates, which give a Hammett correlation ((log k''x/k''H) vs sigmap+) with rho = -1.26. A plot of log k versus BDE(O-H) also reveals a linear correlation. These data, combined with a KIE of 5.9 for 3-OD, provide strong evidence for a concerted hydrogen-atom-abstraction mechanism. Substrates with C-H bonds (1,4-cyclohexadiene and 9,10-dihydroanthracene) are also oxidized via H-atom abstraction by 1, although at a much slower rate. Given the stability of 1, and in particular its low redox potential, (-0.05 V vs SCE), the observed H atom abstraction ability is surprising. These findings support a hypothesis regarding how certain heme enzymes can perform difficult H-atom abstractions while avoiding the generation of high-valent metal-oxo intermediates with oxidation potentials that would lead to the destruction of the surrounding protein environment.