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.

Mechanistic Studies on the Transformation of Ethanol into Ethene over Fe‐ZSM‐5 Zeolite

Abstract: Ethanol, through the utilization of bioethanol as a chemical resource, has received considerable industrial attention as it provides an alternative route to produce more valuable hydrocarbons. Using a density functional theory approach incorporating the M06-L functional, which includes dispersion interactions, a large 34T nanocluster model of Fe-ZSM-5 zeolite in which T is a Si or Al atom is employed to examine both the stepwise and concerted mechanisms of the transformation of ethanol into ethene. For the stepwise mechanism, ethanol dehydration commences from the first hydrogen abstraction of the ethanol OH group to form the ethoxide-hydroxide intermediate with a low activation energy of 17.7 kcal mol(-1). Consequently, the ethoxide-hydroxide intermediate is decomposed into ethene through hydrogen abstraction from the ethoxide methyl carbon to either the OH group of hydroxide or the oxygen of the ethoxide group with high activation energies of 64.8 and 63.5 kcal mol(-1), respectively. For the concerted mechanism, ethanol transformation into the ethene product occurs in a single step without intermediate formation, with an activation energy of 32.9 kcal mol(-1).

An ab initio/Rice-Ramsperger-Kassel-Marcus study of the hydrogen-abstraction reactions of methyl ethers, H3COCH3−x(CH3)x, x = 0–2, by ˙OH; mechanism and kinetics

Abstract: A theoretical study of the mechanism and kinetics of the H-abstraction reaction from dimethyl (DME), ethylmethyl (EME) and iso-propylmethyl (IPME) ethers by the ˙OH radical has been carried out using the high-level methods CCSD(T)/CBS, G3 and G3MP2BH&H. The computationally less-expensive methods of G3 and G3MP2BH&H yield results for DME within 0.2–0.6 and 0.7–0.9 kcal mol−1, respectively, of the coupled cluster, CCSD(T), values extrapolated to the basis set limit. So the G3 and G3MP2BH&H methods can be confidently used for the reactions of the higher ethers. A distinction is made between the two different kinds of H-atoms, classified as in/out-of the symmetry plane, and it is found that abstraction from the out-of-plane H-atoms proceeds through a stepwise mechanism involving the formation of a reactant complex in the entrance channel and product complex in the exit channel. The in-plane H-atom abstractions take place through a more direct mechanism and are less competitive. Rate constants of the three reactions have been calculated in the temperature range of 500–3000 K using the Variflex code, based on the weak collision, master equation/microcanonical variational RRKM theory including tunneling corrections. The computed total rate constants (cm3 mol−1 s−1) have been fitted as follows: k(DME) = 2.74 × T3.94 exp (1534.2/T), k(EME) = 20.93 × T3.61 exp (2060.1/T) and k(IPME) = 0.55 × T3.93 exp (2826.1/T). Expressions of the group rate constants for the three different carbon sites are also provided.

Fe(VI)-Mediated Single-Electron Coupling Processes for the Removal of Chlorophene: A Combined Experimental and Computational Study

Abstract: Potassium ferrate [Fe(VI)] is a promising oxidant widely used in water treatment for the elimination of organic pollutants. In this work, the reaction kinetics, products, and mechanisms of the antimicrobial agent chlorophene (CP) undergoing Fe(VI) oxidation in aqueous solutions were investigated. CP is very readily degraded by Fe(VI), with the apparent second-order rate constant, k, being 423.2 M-1 s-1 at pH 8.0. A total of 22 oxidation products were identified using liquid chromatography-quadrupole time-of-flight-mass spectrometry , and their structures were further elucidated using tandem mass spectrometry. According to the extracted peak areas in mass spectra, the main reaction products were the coupling products (dimers, trimers, and tetramers) that formed via single-electron coupling. Theoretical calculations demonstrated that hydrogen abstraction should easily occur at the hydroxyl group to produce reactive CP· radicals for subsequent polymerization. Cleavage of the C-C bridge bond, electrophilic substitution, hydroxylation, ring opening, and decarboxylation were also observed during the Fe(VI) oxidation process. In addition, the degradation of CP by Fe(VI) was also effective in real waters, which provides a basis for potential applications.

A quantum chemical study of the self-directed growth mechanism of styrene and propylene molecular nanowires on the silicon (100) 2×1 surface

Abstract: We use density functional theory to investigate the self-directed growth mechanism of molecular nanowires on the Si (100)-2×1 monohydride surface from the molecular precursors styrene (H2C=CH–C6H5) and propylene (H2C=CH–CH3). The reaction is initiated using a scanning tunneling microscope tip to create a Si dangling bond on the surface. This dangling bond then attacks the C=C π bond to form a Si–C bond and a C radical. Next, the C radical abstracts a H atom from a neighboring surface site, which results in a new Si dangling bond to propagate the chain reaction. For the case of H2C=CH–C6H5 the predicted hydrogen abstraction barrier of 18.0 kcal/mol from a neighboring dimer along the dimer row for C–H bond formation is smaller than H2C=CH–C6H5 desorption energy of 22.6 kcal/mol. On the other hand, for the case of H2C=CH–CH3 the predicted hydrogen abstraction barrier of 10.8 kcal/mol for C–H bond formation from a neighboring dimer is significantly larger than H2C=CH–CH3 desorption barrier of 2.7 kcal/mol. Consequently, the predicted barriers indicate that the self-directed growth of nanowires on (100) silicon using styrene occurs while a self-directed chain reaction using propylene should not occur, in agreement with experimental observations.

Structure of Dihydrochalcones and Related Derivatives and Their Scavenging and Antioxidant Activity against Oxygen and Nitrogen Radical Species

Abstract: Quantum mechanical calculations at B3LYP/6-31G** level of theory were employed to obtain energy (E), ionization potential (IP), bond dissociation enthalpy (O-H BDE) and stabilization energies (ΔE iso ) in order to infer the scavenging activity of dihydrochalcones (DHC) and structurally related compounds. Spin density calculations were also performed for the proposed antioxidant activity mechanism of 2,4,6-trihydroxyacetophenone (2,4,6-THA). The unpaired electron formed by the hydrogen abstraction from the phenolic hydroxyl group of 2,4,6-THA is localized on the phenolic oxygen at 2, 6, and 4 positions, the C 3 and C 6 carbon atoms at ortho positions, and the C 5 carbon atom at para position. The lowest phenolic oxygen contribution corresponded to the highest scavenging activity value. It was found that antioxidant activity depends on the presence of a hydroxyl at the C2 and C4 positions and that there is a correlation between IP and O-H BDE and peroxynitrite scavenging activity and lipid peroxidation. These results identified the pharmacophore group for DHC.