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.

Full dimensional time-dependent quantum dynamics study of the H + NH3 --> H2 + NH2 reaction.

Abstract: A rigorous full dimensional time-dependent wave packet method has been developed for the reactive scattering between an atom and a tetra-atomic molecule. The method has been applied to the hydrogen abstraction reaction H+NH(3)-> H(2)+NH(2). Initial state-selected total reaction probabilities are investigated for the reactions from the ground vibrational state and from four excited vibrational states of ammonia. The total reaction probabilities from two lowest "tunneling doublets" due to the inversion barrier for the umbrella bending motion of NH(3) and from two pairs of doubly degenerate vibrational states of NH(3) are also inspected. Integral cross sections and rate constants are calculated for the reaction from the ground state with the centrifugal-sudden approximation. The calculated results are compared with those from the previous seven dimensional calculations [M. Yang and J. C. Corchado, J. Chem. Phys. 126, 214312 (2007)]. This work shows that the full dimensional rate constants are a factor of 3 larger than the corresponding seven dimensional calculated values at T=200 K and are overall smaller than those obtained from the variational transition state theory in the whole temperature region. The work also reveals that nonreactive NH bonds of NH(3) cannot be treated as spectators due to the fact that three NH bonds are coupled with each other during the reaction process.

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

Abstract: The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, 13C and 1H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1-phenyl-hydroxymethylene-4-diphenyl-hydroxymethyl-2,5-cyclohexa-diene and three kinds of α-alkylbenzhydrols have been detected and identified for the first time by 13C and 1H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state 3(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000

Photodimerization of cyclohexene and methane by decatungstate anions: molecular orbital theory

Abstract: A molecular orbital study has been made of the photodimerization of cyclohexene by excited W{sub 10}O{sub 32}{sup 4{minus}}, yielding 3,3{prime}-dicyclohexene to explain the observations of Yamase. The overall process is radical monomer coupling. Radical formation by an adiabatic H transfer from cyclohexene to O{sup {minus}} on the O 2p to W 5d charge transfer photoexcited oxyanion is described. This process is highly activated, just as on metal oxide surfaces, because of the stability which comes when an electron from the CH bond reduces the hole in the oxyanion O 2p band during H abstraction. Calculated H abstraction activation energies for olefinic, {alpha}, and {beta} CH bonds in cyclohexene and for a CH bond in CH{sub 4} are higher, but they are low enough that in the case of CH{sub 4} it can be suggested that dimerization could be looked for in future experiments using photoactivated oxyanions. Based on the calculated electronic structures, it is possible to envision a nonadiabatic electron transfer to O{sup {minus}} during mild thermal collisions between a CH bond and the clusters yielding an organic radical cation followed by proton transfer at a later time. Polar solvents would enhance the probability for this mechanism.

Unique products of the reaction of isoprene with atomic chlorine: Potential markers of chlorine atom chemistry

Abstract: GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO.13, PAGES 1615-1618, JULY 1, 1997 Unique products of the reaction of isoprene with atomic chlorine: Potential markers of chlorine atom chemistry Trent Nordmeyer, Weihong Wang, Mark L. Ragains, and Barbara J. Finlayson-Pitts University of California, Irvine, Department of Chemistry, Irvine, California Chet W. Spicer, Robert A. Plastridge Atmospheric Science and Applied Technology Department, Battelle, Columbus, Ohio Abstract. The contribution of atomic chlorine to the chemistry of marine regions as well as the Arctic at ground level at polar sunrise is the subject of a number of recent studies. However, identifying the specific chlorine atom precursors has proven difficult. One potential approach is the measurement of definitive products of chlorine atom reactions, for example with biogenic hydrocarbons. We report here product studies of the chlorine atom reaction with isoprene using ppm concentrations at one atmosphere air and 298 K in a NOx-free system using atmospheric pressure ionization-mass spectrometry (API-MS) as well as GC-MS. 1-chloro-3-methyl-3-butene-2-one (CMBO) is identified as a unique product of this reaction, and there is evidence of the formation of three additional isomers of CMBO as well. Methyl vinyl ketone (MVK) is formed in small yields (9 + 5 %), consistent with earlier studies of this reaction in which an upper yield of 13% was reported. The stable product expected from allylic hydrogen atom abstraction (measured in earlier kinetic studies to be 15% of the total reaction), 2-methylene-3- butenal, is also tentatively identified using API-MS. Assuming that similar chemistry occurs at the ppb-ppt levels found in the atmosphere, identification of CMBO and/or its isomers in field studies could provide strong evidence of chlorine atom chemistry in low NO x environments where there are also sources of isoprene. identity is not known have also been measured recently at ground level in the Arctic at polar sunrise [Impey et al., 1997]. The sources of these compounds are not known, but may include reactions of NaC1 and NaBr as well as other sea salt components such as MgC12 6H20 [Langer et al., 1997] with various oxides of nitrogen and perhaps ozone, accompanied by a recycling mechanism [Barfie et al., 1988; McConnell et al., 1992; Fan and Jacob, 1992; Finlayson-Pitts, 1993; Graedel and Keene, 1995; LeBras and Platt, 1995; Mozurkewich, 1995; Tang and McConnell, 1996; Sander and Crutzen, 1996; Vogt, Crutzen and Sander, 1996]. Another potential approach to investigating chlorine atom production in the troposphere is to identify and measure any unique chlorine-containing products of its reactions with organics. For example, significant quantities of isoprene are produced by deciduous trees and shrubs in coastal areas [e.g. see Guenther et al., 1995]; in addition, there appears to be a source over the oceans since it has been shown to be generated by phytoplankton in seawater [Bonsang et al., 1992; Moore et al., 1994; Milne et al., 1995; McKay et al., 1996]. Isoprene emitted in coastal areas or in the marine boundary layer may be oxidized by 03, NO3 (at night), OH (during the day) as well as C1 atoms at dawn. If the Cl-isoprene reaction gives unique chlorine- containing products, they could serve as markers for chlorine atom chemistry. The kinetics and mechanism of reaction of chlorine atoms with Introduction The role of sea salt particles in the chemistry of the troposphere has been recognized for a number of years [Cicerone, 1981]. More recently, the potential for generation of photochemically active chlorine-containing products which subsequently photolyze to generate chlorine atoms has been of great interest [Finlayson-Pitts, 1993; Graedel and Keene, 1995; Keene et al., 1996; Behnke et al., 1997]. isoprene have been studied recently [Ragains and Finlayson-Pitts, 1997]. As expected for a di-unsaturated alkene, the reaction is fast, with a rate constant of (4.6 + 0.5) x 10 © cm 3 molecule -• s -• (2 ,) at 298 K and one atmosphere pressure. A small but significant fraction, 15 + 4 % (2 ,), of the overall reaction proceeds by abstraction of an allylic hydrogen, which is expected to lead to the formation of 2-methylene-3-butenal as a stable product in air. The remaining 85% of the reaction must proceed Observations in air to date provide evidence primarily for the via the initial addition of the chlorine atom to one of the double existence of C12 in marine areas. For example, Keene and bonds, followed by reaction of the resulting alkyl radical with 02. coworkers [Keene et al., 1993; Pszenny et al., 1993] measured This suggests the potential for formation of unique chlorine- inorganic chlorine-containing compounds other than HC1 using a containing products characteristic of this reaction. mist chamber. While individual compounds were not identified, We report here laboratory studies identifying 1-chloro-3- they were hypothesized to include C12 and possibly other species methyl-3-butene-2-one (CMBO): such as HOC1. Recently, Spicer and coworkers (1996) specifically identified C12 at concentrations up to 150 ppt for the CH3 O first time in coastal marine areas using atmospheric pressure ionization mass spectrometry (API-MS). One or more C•C photolyzable chlorine and bromine atom precursors whose Copyright 1997 by the American Geophysical Union. Paper number 97GL01547. 0094-8534/97/97GL-01547505.00 CH 2 CH2CI and three of its isomers as unique products of the gas phase chlorine atom reaction with isoprene in air at room temperature. In addition, we have tentatively identified the expected abstraction product, 2-methylene-3-butenal.