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.

Theoretical study of the thermodynamics and kinetics of hydrogen abstractions from hydrocarbons.

Abstract: Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.

Hydrogen Abstraction Acetylene Addition and Diels-Alder Mechanisms of PAH Formation: A Detailed Study Using First Principles Calculations.

Abstract: Extensive ab initio Gaussian-3-type calculations of potential energy surfaces (PES), which are expected to be accurate within 1−2 kcal/mol, combined with statistical theory calculations of reaction rate constants have been applied to study various possible pathways in the hydrogen abstraction acetylene addition (HACA) mechanism of naphthalene and acenaphthalene formation as well as Diels−Alder pathways to acenaphthalene, phenanthrene, and pyrene. The barrier heights; reaction energies; and molecular parameters of the reactants, products, intermediates, and transition states have been generated for all types of reactions involved in the HACA and Diels−Alder mechanisms, including H abstraction from various aromatic intermediates, acetylene addition to radical sites, ring closures leading to the formation of additional aromatic rings, elimination of hydrogen atoms, H disproportionation, C2H2 cycloaddition, and H2 loss. The reactions participating in various HACA sequences (e.g., Frenklach's, alternative Fren...

Rate constants for hydrogen abstraction reactions of the sulfate radical, SO4−. Alkanes and ethers

Abstract: Rate constants have been determined for the reactions of SO4− with a series of alkanes and ethers. The SO4− radical was produced by the laser-flash photolysis of persulfate, S2O82−. For methane, only an upper limit of 1 × 106 M−1 s−1 could be determined. For ethane, propane, and 2-methylpropane, rate constants of 0.44, 4.0, and 10.5 × 107 M−1 s−1 were found. For ethyl and n-propyl ether, rate constants of 1.3 × 108 and 2.2 × 108 M−1 s−1 were found and for 1,4-dioxane and tetrahydrofuran, rate constants of 7.2 × 107 and 2.8 × 108 were obtained. The reaction of SO4− with allyl alcohol was also studied and found to have a rate constant of 1.4 × 109 M−1 s−1.

Formation of a Room Temperature Stable Fe V (O) Complex: Reactivity Toward Unactivated C−H Bonds

Abstract: An Fe(V)(O) complex has been synthesized from equimolar solutions of (Et4N)2[Fe(III)(Cl)(biuret-amide)] and mCPBA in CH3CN at room temperature. The Fe(V)(O) complex has been characterized by UV-vis, EPR, Mossbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C-H bond dissociation energies ranging from 99.3 kcal mol(-1) (cyclohexane) to 84.5 kcal mol(-1) (cumene). Linearity in the Bell-Evans-Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step.

Laser flash photolysis studies of the reactions of some 1,4-biradicals

Abstract: The technique of laser flash photolysis and equipment required for using the technique are briefly described The possibilities, use, and limitations of the laser photolysis technique to studies in the area of biradical reactions are discussed The biradicals discussed were generated as intermediates in the Norrish type II reactions and the photoenolization of o-alkyl-substituted carbonyl compounds Values for representative lifetimes for biradicals derived from these reactions are tabulated Hydrogen abstraction studies were carried out with molecules with very labile hydrogen atoms, and values for rate constants for these reactions are tabulated The interaction of biradicals with paramagnetic species such as oxygen, nitroxides, and copper(II) complexes were studied by the laser technique, and the reactions show characteristic behavior that reflects the operation of spin selection rules and the importance of intersystem crossing processes in biradical chemistry (BLM)