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.

Solvent effects on rates of photochemical reactions of rose bengal triplet state studied by nanosecond laser photolysis

Abstract: Bimolecular rate constants of energy transfer, electron-transfer, and H-atom abstraction reaction have been determined by following the transient absorption bands of the triplet states of rose bengal [ 3 ( RB 2− ) ∗ ] at about 1020 nm with various substrates using nanosecond laser flash photolysis. For electron acceptor (1,4-benzoquinone), electron-transfer takes place via an exciplex. Exciplex formation was found to be predominant in high viscosity solvents rather than low viscosity solvents. For electron donor (3,3′,5,5′-tetramethylbenzidine), the rate constants are independent of solvent viscosity, indicating that the contribution to an exciplex formation is small. The H-atom abstraction reactions of 3 ( RB 2− ) ∗ with H-donors show that the rate constants in water are higher than those in ethanol and in THF. The negative slopes of the Hammett plots indicate that the reaction center of [ 3 ( RB 2− ) ∗ ] has a highly electrophilic nature, which increases with solvent polarity, suggesting the polar transition states.

Kinetics and Mechanism of the Oxidation of 10-Methyl-9,10-dihydroacridine by Chromium(VI,V,IV): Electron vs Hydrogen Atom vs Hydride Transfer

Abstract: The oxidation of AcrH2 to AcrH+ by hydrogen chromate ions in H2O/MeCN (4:1, v/v) is a chain reaction that is strongly inhibited by oxygen. The initiation reaction between AcrH2 (or AcrD2) and H2CrO4 forms AcrH2•+ (or AcrD2•+) and occurs by a one-electron mechanism, kH = kD = 4.6 × 102 L mol-1 s-1 (25 °C). The Cr(V) produced along with AcrH• (from the acid ionization of AcrH2•+) are chain-carrying intermediates. The propagating reaction between AcrH2 and Cr(V), k = 1 × 108 L mol-1 s-1, is of key importance since it is a branching reaction that yields two chain carriers, AcrH• and CrO2+, by hydrogen atom abstraction. The same partners react competitively by hydride ion abstraction, to yield Cr3+ and AcrH+, k = 1.2 × 107 L mol-1 s-1, in the principal termination step. The reaction of CrO2+ and AcrH2, kH = 1.0 × 104 and kD = 4.8 × 103 L mol-1 s-1, proceeds by hydride ion transfer. The Cr2+ so produced could be trapped as CrOO2+ when O2 was present, thereupon terminating the chain. AcrH2 itself reacts with Cr2...

Paramagnetic Resonance Study of Liquids during Photolysis. V. Acid Amides and an Imide

Abstract: Paramagnetic resonance spectra of free radicals formed by abstracting hydrogen from simple and N‐methyl substituted formamides and acetamides and from succinimide have been studied. The hydrogen abstraction was brought about by ultraviolet irradiation of a solution of the amide containing either hydrogen peroxide or acetone. Deuterium substitution showed that the hydrogen bonded to carbon is abstracted from formamide resulting in a ``non‐π'' radical. A methyl hydrogen is abstracted from N‐methylformamide and from N,N‐dimethylformamide. The former gives a single radical while the latter gives a pair of rotational isomers. These results are discussed in relation to early NMR studies. A methyl hydrogen is abstracted from acetamide. A relaxation‐time measurement on acetamide indicated that linewidths are limited by the thermal relaxation time which is consistent with a number of observations for other radicals that are discussed. The hydrogen abstracted from N‐methylacetamide appears to be from the N‐methyl group, and a hydrogen bonded to carbon is abstracted from succinimide. Hyperfine couplings and g values are given for all of the above radicals. N,N‐dimethylacetamide gives a mixture of radicals which are probably rotational isomers.

Carbon Dioxide: A Waste Product in the Catalytic Cycle of α-Ketoglutarate Dependent Halogenases Prevents the Formation of Hydroxylated By-Products

Abstract: We present the first density functional theory study on α-ketoglutarate dependent halogenases and focus on the mechanism starting from the iron(IV)-oxo species. The studies show that the high-valent iron(IV)-oxo species reacts with substrates via an initial and rate determining hydrogen abstraction that is characterized by a large kinetic isotope effect (KIE) of 26.7 leading to a radical intermediate. This KIE value is in good agreement with experimental data. The reaction proceeds via two-state reactivity patterns on competing quintet and septet spin state surfaces with close lying hydrogen abstraction barriers. However, the septet spin radical intermediate gives very high barriers for hydroxylation and chlorination whereas the barriers on the quintet spin state surface are much lower. The calculations give extra information regarding the nature of the intermediates and a prediction of a new low-energy mechanism starting from the radical intermediate, whereby a waste product from an earlier step in the c...

Hydrogen Atom Adducts to the Amide Bond. Generation and Energetics of the Amino(hydroxy)methyl Radical in the Gas Phase

Abstract: The amino(hydroxy)methyl radical (1) represents the simplest model for hydrogen atom adducts to the amide bond. Radical 1 was generated in the gas phase by femtosecond electron transfer to protonat...