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.

Production of cold formaldehyde molecules for study and control of chemical reaction dynamics with hydroxyl radicals

Abstract: We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde H2CO and the hydroxyl radical OH can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of H2CO. We have decelerated a molecular beam of H2CO essentially to rest, producing molecules at 100 mK with a density of 10 6 cm �3 .

Hydrogen bonding in MM2

Abstract: Hydrogen bonding is qualitatively accounted for in the early versions of the MM2 program, but not quantitatively. Experimentally, the hydrogen bonds are somewhat shorter and stronger than calculated by MM2. This has been corrected now in MM2(87), by reducing the van der Waals radius of the hydrogen involved in hydrogen bonding for that interaction only, and by increasing the van der Waals' attraction between the hydrogen and the various electronegative atoms to which it can hydrogen bond by about 1–3 kcal/mol, depending on the particular atoms involved. It is now possible to reproduce reasonably well ab initio calculations on simple amides and the methanol dimer, and experimental data on compounds in which a hydroxyl hydrogen is hydrogen bonded to an alcohol, an alkyl chloride, or to a carbon–carbon double bond.

Theoretical Study of N-Demethylation of Substituted N,N-Dimethylanilines by Cytochrome P450: The Mechanistic Significance of Kinetic Isotope Effect Profiles

Abstract: The mechanism of N-demethylation of N,N-dimethylanilines (DMAs) by cytochrome P450, a highly debated topic in mechanistic bioinorganic chemistry (Karki, S. B.; Dinnocenczo, J. P.; Jones, J. P.; Korzekwa, K. R. J. Am. Chem. Soc. 1995, 117, 3657), is studied here using DFT calculations of the reactions of the active species of the enzyme, Compound I (Cpd I), with four para-(H, Cl, CN, NO2) substituted DMAs. The calculations resolve mechanistic controversies, offer a consistent mechanistic view, and reveal the following features: (a) the reaction pathways involve C-H hydroxylation by Cpd I followed by a nonenzymatic carbinolamine decomposition. (b) C-H hydroxylation is initiated by a hydrogen atom transfer (HAT) step that possesses a "polar" character. As such, the HAT energy barriers correlate with the energy level of the HOMO of the DMAs. (c) The series exhibits a switch from spin-selective reactivity for DMA and p-Cl-DMA to two-state reactivity, with low- and high-spin states, for p-CN-DMA and p-NO2-DMA. (d) The computed kinetic isotope effect profiles (KIEPs) for these scenarios match the experimentally determined KIEPs. Theory further shows that the KIEs and TS structures vary in a manner predicted by the Melander-Westheimer postulate: as the substituent becomes more electron withdrawing, the TS is shifted to a later position along the H-transfer coordinate and the corresponding KIEs increases. (e) The generated carbinolaniline can readily dissociate from the heme and decomposes in a nonenzymatic environment, which involves water assisted proton shift.

Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene

Abstract: RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650 K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100 atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2 + C2H2 and C6H4C2H3 + C2H2 reactions in the entire 500–2500 K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H + C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T