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.

Periodic Density Functional Theory Study of Methane Activation over La2O3: Activity of O2-, O-, O22-, Oxygen Point Defect, and Sr2+-Doped Surface Sites

Abstract: Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at , , , point defect, and Sr2+-doped surface sites on La2O3(001). The results show that the anionic species is the most active surface oxygen site. The overall reaction energy to activate methane at an site to form a surface hydroxyl group and gas-phase •CH3 radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site is −102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr2+/La3+ exchange at the surface. The O--like nature of the surface site is reflected in a calculated hydrogen binding energy of −109.7 kcal/mol. Calculations indicate that surface peroxide ( ) sites can be generated by adsorption of O2 at surface oxygen vacancies, as well as by dissociative adsorption of O2 across the closed-shell oxide surface of La2O3(001). The overall reaction energy and appa...

Protein radical involvement in biological catalysis

Abstract: or is also warranted. While it is generally accepted that thiols are capable of transferring a hydrogen atom to carbon-centered radicals because of the relative weakness of their bond dissociation energies (91 + 1.5 kcallmol) (33, 89), their role as hydrogen atom abstractors is contraindicated by most commonly held beliefs. Hydrogen atom abstraction of the 31-H of a nucleoside by a thiyl radical is allowed on thermodynamic grounds because the C-H homolytic bond dissociation energy is -91 kcallmole. Akhlaq et al recently devised a method to detect thiyl radical-mediated hydrogen atom abstractions (34), which are usually masked in the presence of high RSH concentrations due to thiol-mediated repair. Akhlaq et al took either cis-2,5 Me2THF or trans-2,5 Me2THF and measured the production of a mixture of cis and trans 2,5 Me2THF (Eq. 4). Thiols in this case, as in the earlier studies H'C>(J0 CH, 103/M/, -aD CH, RS . H .====-H3C • + RSH H 108/M/s H 4. by Huyser & Kellogg (35), do in fact induce a chain reaction. Thus, the feasibility of thiyl radicals mediating hydrogen atom abstractions has reason­ able chemical precedent. The final step in the postulated mechanism, thiyl radical abstraction of a hydrogen atom from 51 deoxyadenosine, is thermo­ dynamically unfavorable by 1 8 kcal. The driving force for this reaction would have to be provided by reformation of the C-Co bond to produce AdoCbI. This proCI!SS has recently been shown to be exothermic by 25 to 30 kcallmole (91). The establishment of the role of any postulated thiol in an enzymatic reaction will be facilitated by the cloning, sequencing, and expression of any of these genes and use of site-directed mutagenesis methods to replace the cysteine residues. Efforts are presently under way with both RTPR and ethanolamine ammonia lyase to investigate the putative role of thiyl radicals in the chemistry . An interesting paper of Hartmanis & Stadtman provides further credibility to the postulated role of protein radicals (36). Specifically, a dioldehydrase that does not require AdoCbl has been partially purified from Chlostridium glycolicum and shown to possess a protein radical that is destroyed by reduction with hydroxyurea concomitant with loss of dioldehydrase activity. The properties of this radical are similar to those observed in E. coli RDPR. In addition to the binuclear iron center, tyrosyl radical, and AdoCbl­ dependent systems discussed above, recent studies from Follmann' s labora­ tory indkate that ribonucleotide reductase isolated from Brevibacterium ammoniagenes requires manganese for activity (37) . Based on recent chemi­ cal model systems from Lippard's and Wieghardt' s laboratories, it is reason­ able to postulate the existence of a novel binuclear Mn(III)-dependent system

Benzophenone triplet properties in acetonitrile and water. Reduction by lactams

Abstract: The technique of time resolved laser flash spectroscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile. New estimates have been obtained for the triplet extinction coefficient, unimolecular and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams in both solvents. Evidence for hydrogen abstraction from water by a triplet of benzophenone is found. Correlation between rate constants of hydrogen abstraction from lactams by the lowest benzophenone triplet and their ionization potential is discussed. Reversibility of these hydrogen abstractions is considered.