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.

Arrhenius parameters for rearrangements of the neophyl, 1-indanylmethyl, 2-allylbenzyl, and 2-(2-vinylphenyl)ethyl radicals relative to hydrogen abstraction from tributylstannane

Abstract: The temperature dependence of rates of several radical rearrangements relative to abstraction of hydrogen atom from tributylstannane has been determined. The rearrangement of 2-phenyl-2-methylpropyl to 1-phenyl-2-methylprop-2-yl (neophyl rearrangement) relative to hydrogen abstraction is represented by log (r/M) = (2.48 +/- 0.08) - (8.13 +/-0.16)/theta, theta = 2.3 RT kcal/mol. The rearrangement of 2-allylbenzyl to 2-indanylmethyl exhibited log (r/M) = (2.32 +/- 0.16)- (10.47 +/- 0.36/theta, and the rearrangement of 2-allylbenzyl to 2-tetralyl is represented by log (r/M) = (2.18 +/- 0.28) - (12.47 +/- 0.60)/theta. The rearrangement of 1-indanylmethyl to 2-tetralyl is represented by log (r/M) = (2.65 +/- 0.12) - (11.21 +/- 0.24)/theta. Closure of 2-(2-vinylphenyl)ethyl to the five-member ring occurs with a 7.3 +/- 0.6 kcal/mol barrier and was preferred over closure to the six-member ring by 2.1 +/- 1 kcal/mol. From known or estimated rates of abstraction of hydrogen from tributylstannane, rates of rearrangement of the radicals are estimated.

Metal-Hydrogen Bond Cleavage Reactions of Transition Metal Hydrides: Hydrogen Atom, Hydride, and Proton Transfer Reactions

Abstract: Cleavage of the M-H bond of transition metal hydrides can result in delivery of a hydride (H−), hydrogen atom (H), or proton (H+) to an organic or organo-metallic substrate. Hydrogen atom transfer from a metal hydride to an olefin is an endothermic reaction; the rate at which different metal hydrides carry out this reaction is mainly dependent on the bond dissociation energy of the M-H bond. Hydrogen atom transfer from a metal hydride to a carbon-centered radical is an exothermic reaction in which the rate is largely determined by steric demands of the ligands of the metal hydride, with thermodynamics playing a less important role. Some neutral metal hydrides can function as hydride donors to carbocations. Anionic metal hydrides also exhibit hydridic reactivity, but in some cases the actual M-H bond cleavage step involves hydrogen atom transfer as a part of a radical chain reaction. Neutral metal hydrides can function as nucleophiles and as weakly coordinating ligands to coordinatively unsaturate...

Resist stripping in an O2+H2O plasma downstream

Abstract: Characteristics are reported for a resist stripping process downstream of an oxygen plasma to which water vapor is added. The effects of additive water vapor are an increase in atomic oxygen concentration in the plasma, a decrease in activation energy of ashing reaction, and protection of semiconductor devices from the sodium contamination from the resist. The atomic oxygen concentration was approximately doubled by mixing 10% H2O into the oxygen plasma. The activation energy of the ashing reaction to the resist made from novolak resin decreased from about 0.5 to 0.39 eV by the addition of water vapor of more than 1%. The activation energy of hydrogen abstraction from hydrocarbon molecules by an OH radical was lower than that by a ground state oxygen atom [O(3P)], which was the dominant ashing species in the oxygen plasma downstream, and that by an atomic hydrogen was higher than that by the ground state oxygen atom. Moreover, the activation energy in the downstream ashing of the oxygen plasma added to which was 1% water vapor was lower than that of the oxygen plasma to which 3% hydrogen was added, even though the relative concentration of atomic hydrogen in each plasma was equal. Therefore the decrease in the activation energy was probably due to the OH radical generated in the plasma and the downstream. Sodium atoms in the resist were blocked from entering into the semiconductor devices in the stripping process by use of the O2+H2O plasma downstream. Thus sodium was not removed and remained on the wafer surface after resist stripping. Also, by adding N2 or CF4 to the O2+H2O plasma, we can increase the ashing rate without losing the above characteristics.

Rates of OH Radical Reactions. I. Reactions with H2, CH4, C2H6, and C3H8 at 295 K

Abstract: A fast flash photolysis kinetic spectrophotometer capable of measuring rates of up to 105 s−1 is described. The rates of hydrogen abstraction from H2, CH4, C2H6, and C3H8 by OH radicals at 295 ± 2 ...

Selective Hydrogen Atom Abstraction from Dihydroflavonol by a Nonheme Iron Center Is the Key Step in the Enzymatic Flavonol Synthesis and Avoids Byproducts.

Abstract: The plant non-heme iron dioxygenase flavonol synthase performs a regioselective desaturation reaction as part of the biosynthesis of the signaling molecule flavonol that triggers the growing of leaves and flowers. These compounds also have health benefits for humans. Desaturation of aliphatic compounds generally proceeds through two consecutive hydrogen atom abstraction steps from two adjacent carbon atoms and in nature often is performed by a high-valent iron(IV)-oxo species. We show that the order of the hydrogen atom abstraction steps, however, is opposite of those expected from the C–H bond strengths in the substrate and determines the product distributions. As such, flavonol synthase follows a negative catalysis mechanism. Using density functional theory methods on large active-site model complexes, we investigated pathways for desaturation and hydroxylation by an iron(IV)-oxo active-site model. Contrary to thermochemical predictions, we find that the oxidant abstracts the hydrogen atom from the stro...