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H Abstraction (h + abstraction)
Selected AbstractsThe reaction of OH with acetaldehyde and deuterated acetaldehyde: Further insight into the reaction mechanism at both low and elevated temperaturesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2006Philip H. Taylor The pulsed laser photolysis/laser-induced fluorescence technique has been used to conduct additional measurements of the gas-phase CH3CHO + OH reaction. These measurements were conducted to verify the complex temperature dependence previously observed by the authors and to acquire mechanistic information about the reaction mechanism in the form of primary kinetic isotope effects. Primary kinetic isotope effect measurements at temperatures of 297, 383, 600, and 860 K indicate that H abstraction from the acetyl group dominates that of the methyl group at low to modest temperatures (,600 K) and H abstraction from the methyl group dominates that from the acetyl group at higher temperatures (860 K). A bi-exponential © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 489,495, 2006 [source] Modification induced by alpha particle irradiationin Makrofol polycarbonateJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2008S. A. Nouh Abstract Makrofol DE 1-1 CC polycarbonate samples were exposed to alpha particles of initial energies at levels between 5.1 and 34 MeV. The modifications induced in polycarbonate samples due to the alpha particle irradiation have been studied through different characterization techniques such as X-ray diffraction (XRD), infrared spectroscopy, intrinsic viscosity, and color difference studies. The infrared spectroscopy indicated that the intensities of the characteristic absorption bands decrease with increasing the deposited alpha energy in the range 5.1,8.4 MeV, indicating that the degradation is the dominant mechanism at this range. At the same time, an increase in the OH groups was observed at the same energy range 5.1,8.4 MeV due to the degradation of carbonate group and the H abstraction from the polymer backbone to form hydroxyl groups. The degradation reported by IR spectroscopy enhanced the degree of ordering in the degraded samples as revealed by XRD technique. Additionally, this degradation decreases the intrinsic viscosity from 0.56 to 0.43 at 35°C, indicating a decrease in the average molecular mass. The non irradiated Polycarbonate polymer is nearly colorless. It showed significant darkness sensitivity towards alpha particle irradiation, indicated by an increase in the color intercept L* from 33.6 to 36.7. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Possible molecular hydrogen formation mediated by the radical cations of anthracene and pyreneJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2003Mutsumi Hirama Abstract Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H2 formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H2 is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H2. The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion,molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H2 formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H2 formation because deuteration lowers the energies of polyatomic PAH cations appreciably. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1378,1382, 2003 [source] Tunnelling corrections in hydrogen abstractions by excited-state ketonesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2010Monica Barroso Abstract Hydrogen abstraction from 1-phenylethanol by triplet acetophenone occurs from both CH and OH bonds. The reaction path of the Interacting-State Model (ISM) is used with the Transition-State Theory (TST) and the semiclassical correction for tunnelling (ISM/scTST) to help rationalizing the experimental kinetic results and elucidate the mechanisms of these reactions. The weak exothermicity of the abstraction from the strong OH bond is compensated by electronic effects, hydrogen bonding and tunnelling, and is competitive with the more exothermic abstraction from the ,-CH bond of 1-phenylethanol. The alkoxy radical formed upon abstraction from OH reacts within the solvent cage and the primary product of this reaction is 1-phenylethenol. The corresponding kinetic isotope effect is ca. 3 and is entirely consistent with a tunnelling correction ca. 9 for H abstraction. We therefore demonstrate that the tunnelling correction is the major contributor to the kinetic isotope effect. Copyright © 2010 John Wiley & Sons, Ltd. [source] Photohydroxylation of 1,4-Benzoquinone in Aqueous Solution RevisitedCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2004Justus von Sonntag Dr. Abstract In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH2 which are formed with a quantum yield of ,=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (,max=282 and ,410 nm) which is the precursor of these products increases nonlinearly (k=(2,3.8)×106 s,1) with increasing Q concentration ((0.2,10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q2* (K=5500±1000,M,1). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)3, Q2* decays by electron transfer and water addition yielding benzosemiquinone .QH and . OH adduct radicals .QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical .Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of .QH. On the post-millisecond time scale, that is, when .QH has decayed, Ph(OH)3 is oxidized by Q yielding HOQ and QH2 as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)3 is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H2PO4, giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that . OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the . OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q2* oxidizes DMPO (k ,1×108,M,1,s,1) to its radical cation which subsequently reacts with water. Q*/Q2* react with alcohols by H abstraction (rates in units of M,1,s,1): methanol (4.2×107), ethanol (6.7×107), 2-propanol (13×107) and tertiary butyl alcohol (,0.2×107). DMSO (2.7×109) and O2 (,2×109) act as physical quenchers. [source] Cyanomethylidyne: A Reactive Carbyne RadicalCHEMPHYSCHEM, Issue 3 2006Jian Wang Abstract The cyanomethylidyne radical (CCN) has been a long-standing subject of extensive structural and spectroscopic studies. However, its chemical reactivity has received rather little attention. Recently, we studied the reaction of CCN with the simplest alkane, CH4, which follows a mechanism of carbyne insertion,dissociation rather than that of direct H abstraction proposed by a recent experimental study. However, we are aware that alkanes like CH4 bear no electron lone pairs and thus are not ideal diagnostic molecules for distinguishing between the carbyne-insertion and H-abstraction mechanisms. Hence, we chose a series of ,-bonded molecules HX (X=OH, NH2, and F) which bear electron lone pairs and are better diagnostics for carbyne-insertion behavior. The new results at the CCSD(T)/6-311+G(2df,p)//B3LYP/6-311G(d,p)+ZPVE, CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p)+ZPVE, G2M(CC1), and MC-QCISD//B3LYP/6-31G(d)+ZPVE levels definitively confirm the carbyne-insertion behavior of the CCN radical towards HX. In addition, we make the first attempt to understand the reactivity of the CCN radical toward ,-bonded molecules, using the CCN+C2H2 model reaction. This reaction involves carbenoid addition to the CC bond without a potential-energy barrier to form a C3 three-membered cyclic intermediate followed by H extrusion. Therefore, the reactions of CCN with both ,- and ,-bonded molecules conclusively show that CCN is a reactive carbyne radical and may be more reactive than the well-known CN radical. Future experimental studies, especially on product characterization, are strongly desired to test our proposed carbyne mechanism. The studied reactions of CCN with CH4,, NH3,, H2O, and C2H2 could be of interest to combustion science and astrophysics, and they could provide efficient routes to form novel cyano-containing molecules in interstellar space. [source] | ||||||||||