Showing papers on "Hydrogen atom abstraction published in 1994"
TL;DR: In this paper, 24 different local and gradient-corrected density functional methods were used in a study of the reaction H+H2→H2+H. The results were compared to those obtained by ab initio methods and experiment.
328 citations
TL;DR: In this paper, an alumina-supported manganese oxide catalysts (2-8.4 wt% Mn) were characterized by in situ infrared (IR) spectroscopy and temperature-programmed reaction and desorption (TPRD) in relation to the selective catalytic reduction (SCR) of NO with NH 3.
147 citations
TL;DR: The reaction of H atoms with solid CO molecules was studied in this article, where the solid CO film deposited on the cold head of the cryocooler was reacted with H atoms by spraying the plasma-activated hydrogen gas over the surface of the sample.
140 citations
TL;DR: The latter process was found to mediate the photoperoxidation of linoleic acid through a type I mechanism, as evidenced by the inhibition produced by the radical scavengers butylated hydroxyanisole and reduced glutathione.
Abstract: Irradiation of ketoprofen in neutral aqueous medium gave rise to 3-ethylbenzophenone as the major photoproduct. Its formation is justified via protonation of a benzylic carbanion or hydrogen abstraction by a benzylic radical. Minor amounts of eight additional compounds were isolated. Four of them are derived from the benzylic radical: 3-(1-hydroperoxyethyl)benzophenone, 3-(1-hydroxyethyl)benzophenone, 3-acetylbenzophenone and 2,3-bis-(3-benzoylphenyl)butane. The other four products involve initial hydrogen abstraction by the excited benzophenone chromophore of ketoprofen: 1,2-bis-(3-ethylphenyl)-1,2-diphenyl-1,2-ethanediol, 2-(3-benzoylphenyl)-1-(3-ethylphenyl)-1-phenylpropan-1-ol, alpha-(3-ethylphenyl)phenylmethanol, 1,2-bis-[3-(2-hydroxycarbonylethyl) phenyl]-1,2-diphenyl-1,2-ethanediol. The latter process was found to mediate the photoperoxidation of linoleic acid through a type I mechanism, as evidenced by the inhibition produced by the radical scavengers butylated hydroxyanisole and reduced glutathione. The major photoproduct, which contains the benzophenone moiety but lacks the propionic acid side chain, also photosensitized linoleic acid peroxidation. Because lipid peroxidation is indicative of cell membrane lysis, the above findings are highly relevant to explain the photobiological properties of ketoprofen.
138 citations
112 citations
TL;DR: The nature of the oxygen species catalyzing C-H bond cleavage through the measurement of oxygen-18 isotope effects as a function of substrate structure is addressed, finding values of 18(V/K) which decrease from 1.0281 +/-0.001 to 1.0216 +/- 0.0003 as the rate of the C- H bond Cleavage step decreases from 680 to 2 s-1.
Abstract: Previous studies of dopamine beta-monooxygenase (D beta M) have implicated the formation of a substrate-derived benzylic radical via a hydrogen atom abstraction mechanism [Miller & Klinman (1985) Biochemistry 24, 2114]. We now address the nature of the oxygen species catalyzing C-H bond cleavage through the measurement of oxygen-18 isotope effects as a function of substrate structure. Using deuterium isotope effects, together with experimental O-18 isotope effects with protonated and deuterated substrates, it has been possible to calculate intrinsic O-18 isotope effects. Since the D beta M mechanism includes many steps which may involve changes in bond order at dioxygen, e.g., the reversible binding of O2 to the active-site copper and its reductive activation to a copper-hydroperoxide species, the intrinsic O-18 isotope effect is expected to be the product of two terms: (1) an overall equilibrium O-18 isotope effect on steps leading from O2 binding to the formation of the intermediate which catalyzes C-H bond cleavage and (2) a kinetic O-18 isotope effect on the C-H bond cleavage step. Thus, the magnitude of a single O-18 isotope effect measurement cannot reveal the nature of the bonding at oxygen during substrate activation. In the present study we have measured the change in O-18 isotope effect as a function of substrate structure and reactivity, finding values of 18(V/K) which decrease from 1.0281 +/- 0.001 to 1.0216 +/- 0.0003 as the rate of the C-H bond cleavage step decreases from 680 to 2 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)
104 citations
TL;DR: In this article, the authors reported related chemistry of the Tp[double prime]Co-moiety (Tp[ double prime] = hydridotris (3-tert-butyl-5-methylpyrazolyl) borate), including the observation of an intermediate and evidence for a tunneling contribution to hydrogen atom abstraction from the ligand.
Abstract: The activation of dioxygen by coordination to metals plays an important role in living organisms as well as artificial oxidation catalysts. We have previously described the dioxygen complex Tp[prime]Co[sup II](O[sub 2]) (Tp[prime] = hydridotris (3-tert-butyl-5-methylpyrazolyl)borate) and its reaction with Tp[prime]Co[sup I](N[sub 2]) to yield Tp[prime]Co[sup II]OH, the apparent product of hydrogen atom abstraction by a highly reactive cobalt - oxo complex. Herein we report related chemistry of the Tp[double prime]Co-moiety (Tp[double prime] = hydridotris (3-isopropyl-5-methylpyrazolyl) borate), including the observation of an intermediate and evidence for a tunneling contribution to hydrogen atom abstraction from the ligand. 16 refs., 1 fig.
96 citations
TL;DR: The results establish that linoleic acid is oxidized within the heme crevice by reaction with the ferryl oxygen rather than a protein radical, and indicate, furthermore, that hydrogen abstraction and oxygen addition occur in an antarafacial manner and suggest a specific model for binding of linolesic acid within the myoglobin active site.
93 citations
83 citations
TL;DR: The results are consistent with a mechanism that involves stepwise oxidation of the alcohol to a carbon radical alpha to the alcohol function, followed by oxygen rebound to yield the gem-diol, dehydration of which gives the carbonyl product.
Abstract: The mechanisms of oxidation of primary and secondary benzylic alcohols to the corresponding carbonyl compounds by purified rabbit liver cytochrome P450 forms 2B4 and 2E1 in a reconstituted enzyme system has been examined by linear free energy relationships, intramolecular and steady-state deuterium isotope effects, and the incorporation of an O2-derived oxygen atom or solvent-derived deuterium. The kcat and Km values were found to be relatively insensitive to the presence of electronic perturbations at the para position. The Hammett reaction constants for the oxidation of benzyl alcohols by P450s 2B4 and 2E1 are -0.46 and -0.37, respectively, and with 1-phenylethyl alcohols the corresponding reaction constants are -1.41 and -1.19, respectively. With [1-2H1]benzyl alcohol, P450s 2B4 and 2E1 show similar intramolecular deuterium isotope effects of 2.6 and 2.8, respectively, whereas with [1-2H2]benzyl alcohol under steady-state conditions, the deuterium isotope effects on the catalytic constants are 2.8 and 1.3, respectively. No significant isotope effect on the catalytic constant was noted for either form of P450 with 1-phenylethyl alcohol. In D2O, acetophenone formed by either form of P450 from 1-phenylethyl alcohol does not contain a deuterium atom at the methyl group, whereas under an atmosphere of 18O2 approximately 30% of the labeled oxygen is incorporated into the carbonyl group with either form of the cytochrome. The results are consistent with a mechanism that involves stepwise oxidation of the alcohol to a carbon radical alpha to the alcohol function, followed by oxygen rebound to yield the gem-diol, dehydration of which gives the carbonyl product.(ABSTRACT TRUNCATED AT 250 WORDS)
78 citations
TL;DR: Results indicate that there is extensive C--H bond stretching in the transition state for the rate-controlling step in P450-catalyzed hydroxylation of 1CH3, which is consistent with the formation of a radical intermediate via a hydrogen atom abstraction from the methyl group by the catalytically active iron-oxo center.
Abstract: The ultrafast radical clock probe trans-1-methyl-2-phenylcyclopropane (1CH3) and its mono-, di-, and trideuteriomethyl analogues were oxidized by phenobarbital-induced rat liver microsomal enzymes. This cytochrome P450-catalyzed hydroxylation of 1CH3 gave three products: the alcohol trans-(2-phenylcyclopropyl)methanol (2), the rearranged alcohol 1-phenylbut-3-en-1-ol (3), and the phenol trans-2-(p-hydroxyphenyl)-1-methylcyclopropane (4). The identification of both the unrearranged and rearranged products of oxidation, 2 and 3, is consistent with the formation of a radical intermediate via a hydrogen atom abstraction from the methyl group by the catalytically active iron-oxo center. Hydroxylation of three deuteriomethyl forms of 1CH3 produced the analogous deuterated products, although in different amounts of each. Perdeuteration of the methyl group (1CD3) disfavored oxidation at the methyl group and caused an increase in the oxidation of the phenyl ring (metabolic switching). By comparing the amounts of alcohols and phenol formed from the individual, noncompetitive oxidation of 1CH3 and 1CD3 the overall (i.e., combined primary and secondary) deuterium kinetic isotope effect (DKIE) was found to be 12.5. Intramolecular DKIEs for 1CHD2 and 1CH2D were 2.9 and 13.2, respectively. From these results, the primary and secondary DKIEs were calculated to be 7.87 and 1.26, respectively, values that indicate that there is extensive C--H bond stretching in the transition state for the rate-controlling step in P450-catalyzed hydroxylation of 1CH3.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this paper, the products from the thermal degradation of poly(ethylene glycol) (PEG) in a nitrogen atmosphere have been studied by GC/MS, and the results show that PEG degrades in nitrogen by a series of competitive intermolecular and intramolecular processes.
TL;DR: In this article, the processes of atomic hydrogen adsorption and abstraction on a diamond surface determine the fraction of sites available for reaction with carbon containing species during growth, using high resolution electron energy loss spectroscopy.
Abstract: The processes of atomic hydrogen adsorption and abstraction on a diamond surface determine the fraction of sites available for reaction with carbon containing species during growth The relative efficiencies of hydrogen atom adsorption and abstraction on a polycrystalline diamond surface were determined at surface temperatures of 80 and 600 °C using high resolution electron energy loss spectroscopy Abstraction is seen to occur 005 times as efficiently as adsorption on a diamond surface at 80 °C This is interpreted to indicate that the activation barrier to abstraction is higher than in analogous gas phase reactions No change in either the adsorption or abstraction rate is seen for a diamond surface at 600 °C indicating that hydrogen atoms do not accommodate the surface during the reaction Thus, abstraction proceeds via a generalized Eley–Rideal mechanism
TL;DR: The findings suggest that fatty acids with doubly allylic hydrogen atoms react preferentially by a hydrogen atom abstraction reaction rather than by the addition of nitrogen dioxide to a double bond.
Abstract: The reaction of methyl linoleate with low levels of nitrogen dioxide in a carrier gas, such as helium or air, at nitrogen dioxide concentrations ranging from 2 to 228 ppm was studied and the products formed were monitored. In both aerobic and anaerobic conditions, low concentrations of nitrogen dioxide reacted with methyl linoleate predominately to form allylic products. When a 1∶1 mixture of methyl palmitate/methyl linoleate was layered over an aqueous buffer and a nitrogen dioxide stream was passed from underneath, so that the stream passed through the aqueous layer before contacting the organic layer, allylic products again predominated. In the absence of air, the allylic products consisted of allylic nitro and nitrite derivatives of linoleate, whereas in the presence of air, allylic hydroperoxides were the principal products. The findings suggest that fatty acids with doubly allylic hydrogen atoms react preferentially by a hydrogen atom abstraction reaction rather than by the addition of nitrogen dioxide to a double bond.
TL;DR: In this paper, the authors studied the catalytic liquid-phase oxidation of aqueous solutions of substituted phenols (p -chlorophenol and p -nitrophenol) in a differential, liquid-full operated fixed bed reactor.
TL;DR: In this article, 1-bromo-2-methyl-2propyl ether derivatives of alcohols were treated with Bu 3 SnH(D) to obtain 1,5-hydrogen abstraction reactions.
TL;DR: A palladium precursor and SnCl 2 as cocatalyst were used under 4 MPa of carbon monoxide for the catalytic alkoxy carbonylation of several monoterpenes into C 11 esters.
TL;DR: More than one mechanistic pathway appears to operate in soluble methane-monooxygenase-catalysed reactions and, in some cases, the pathway chosen may be dictated by the substrate.
Abstract: The data from the deuterium isotope experiments in this study show that the primary kinetic isotope effect for methane oxidation catalysed by soluble methane monooxygenase from Methylococcus capsulatus (Bath) is very small, 7. A mechanistic pathway in which a substrate radical is formed from hydrogen atom abstraction by a ferryl species is believed to operate for CH4, the toluene –CH3 group and similar alkanes. Direct oxygen atom addition, rather than H atom abstraction, is indicated for aromatic ring oxidations in benzene and toluene and for styrene oxide formation from styrene. Thus, more than one mechanistic pathway appears to operate in soluble methane-monooxygenase-catalysed reactions and, in some cases, the pathway chosen may be dictated by the substrate.
In the soluble methane-monooxygenase-catalysed oxidation of toluene the rates of: (a) substrate dissociation from the enzyme-substrate complex, (b) product formation and (c) product release (benzyl alcohol and p -cresol) from the enzyme-product complex are comparable in magnitude. Therefore all three of these steps are partially rate–determining in the soluble methane monooxygenase catalytic cycle for toluene oxidation.
TL;DR: In this article, the Eley-Rideal mechanism was used to identify the hydrogen abstraction reaction instrumental in low-pressure diamond synthesis and related processes, and the results showed that the hydrogen recombination reaction proceeds via an Eleyrideal mechanism with an apparent cross section of 0.05 A2.
TL;DR: In this paper, a single-crystal X-ray structural analysis of K5H2[V15O36(CO3)]·14.081 for 4182 independent data with I > 5σ(I) showed that the pentadecavanadate encapsulates CO32− with approximate D3h symmetry and formally contains eight VIV and seven VV centres.
Abstract: Prolonged photolysis of aqueous solutions containing [V4O12]4– and MeOH at pH 9 (pH adjusted by use of K2CO3) led to the formation of K5H2[V15O36(CO3)]·1 4.5H2O. A single-crystal X-ray structural analysis of dark green crystals of K5H2[V15O36(CO3)]·14.5H2O [triclinic, space group Pa= 12.361 (4), b= 18.149(5), c= 11.415(4)A, α= 93.44(3), β= 107.76(3), γ= 93.54(3)°, Z= 2, R= 0.081 for 4182 independent data with I > 5σ(I)] showed that the pentadecavanadate encapsulates CO32– with approximate D3h symmetry and formally contains eight VIV and seven VV centres. The co-ordination geometries at the vanadium centres in the anion consist of twelve (VIV, VV) VO5 square pyramids and three VIVO6 octahedra. Each of the VO6 octahedra co-ordinates the CO32– oxygen atoms with V–O bond lengths 2.2616(8)–2.3710(7)A. The photochemical encapsulation of other anions such as Cl–, Br–, NO3– or PO33– in the [V15O36]5– spherical cluster shell is observed in a similar way, which is applicable to the photofixation of CO2. The photoinduced electron transfer to the O → V ligand-to-metal charge-transfer (l.m.c.t.) excited state of [V4O12]4– from alcohols such as MeOH, EtOH or PriOH was investigated with chemically induced dynamic electron polarization (CIDEP) with 100 ns time resolution. The formation and decay of α-hydroxyalkyl radicals, following pulsed-laser excitation of the polyoxometalate was also observed for [Mo7O24]6– and [W10O32]4–. Observation of an emissive CIDEP ESR pattern for the α-hydroxyalkyl radicals revealed that the reaction precursor is the excited triplet state of the O → M (M = V, Mo or W) l.m.c.t. which undergoes hydrogen abstraction from alcohols to yield the one-protonated reduced polyoxometalate (for example [V4O12H]4–) and the α-hydroxyalkyl radical. The absence of distortion in the ESR polarization of α-hydroxyalkyl radicals and the external magnetic field effect indicated that the interaction between [V4O12H]4– and α-hydroxyalkyl radicals is extremely weak in water.
TL;DR: In this article, a novel method of producing atomic hydrogen and the active carbon species necessary for diamond chemical vapor deposition (CVD) has been demonstrated, which starts with the generation of atomic chlorine from the thermal dissociation of molecular chlorine in a resistively heated graphite furnace at temperatures from 1300-1500°C.
Abstract: A novel method of producing atomic hydrogen and the active carbon species necessary for diamond chemical vapor deposition (CVD) has been demonstrated. This method starts with the generation of atomic chlorine from the thermal dissociation of molecular chlorine in a resistively heated graphite furnace at temperatures from 1300–1500°C. Atomic hydrogen and the carbon precursors are subsequently produced through rapid hydrogen abstraction reactions of atomic chlorine with molecular hydrogen and hydrocarbons at the point where they mix. It was found that the quality of the diamond deposits depends on both substrate temperatures and mole ratios. Substrate temperatures are found to be ~150° lower than for a hydrogen/hydrocarbon hot filament system for similar growth rates.
TL;DR: In this paper, the mechanism of the reaction of hydroperoxyl radical with ozone has been studied as a function of temperature by measuring product branching ratios using isotopic labeling, and it was shown that the two channels can be distinguished by detecting the OH product with an isotopically specific method.
Abstract: The mechanism of the reaction of hydroperoxyl radical with ozone has been studied as a function of temperature by measuring product branching ratios using isotopic labeling. In these studies H 18 O 2 was reacted with unenriched ozone in order to measure the relative importance of hydrogen abstraction and oxygen abstraction by ozone. In either case hydroxyl radical is a product. However, with isotopic labeling the two channels can be distinguished by detecting the OH product with an isotopically specific method: H 18 O 2 +O 3 → 18 OH+O 2 + 18 OO; H 18 O 2 +O 3 →OH+O 2 + 18 O 2
TL;DR: In this article, a global mechanism was proposed involving active species of the remote nitrogen plasma (CRNP) in initiation step of hydrogen abstraction and the nitrogen triplet state molecule N2(A3Σ) in methyl abstraction on SiO· type of free radical.
Abstract: Cold remote nitrogen plasma (CRNP) selectively reacts with silane-terminated organosiloxane compounds such as 1.1.3.3-tetramethyldisiloxane to give polymeric layers. Deposition rate measurements, FT-IR and Raman spectroscopy were performed. The chemical composition of the deposited film is closely dependent on the reactive gas composition and its flowing conditions. An original effect of dioxygen addition in the nonionic reactive media is pointed out: dioxygen addition leads to a fast and highly hydrocarbonated polymer formation with a nitrogen fixation in a silazane structure. Polymerization is described by a model where SiO· type of radicals are the critical reactant. A global mechanism is proposed involving active species of the CRNP in initiation step of hydrogen abstraction and the nitrogen triplet state molecule N2(A3Σ) in methyl abstraction on Si· type of free radical. Dioxygen adjunction appears to limit the methyl abstraction steps. The efficient direct oxygen reaction on free radicals leads to an increase of the SiO· radical density and, consequently of the average length of the growing polymeric fragments. Nitrogen fixation, involving oxygenated species, is discussed. Under defined conditions, a highly hydrophobic polymeric film is obtained with a volumic mass of 1.34g/cm3 and a deposition rate of about 12 mg/cm2 h corresponding to a growth rate of 200 A/s. © 1994 John Wiley & Sons, Inc.
TL;DR: In this paper, the reaction of H 2 + CH 4 → CH 3 + CH 2 was investigated in a flow system between 348 and 421 K. Hydrogen atoms were generated in a microwave discharge, introduced to the reactor through a movable in...
Abstract: The reaction: H + CH4 → CH3 + H2 has been investigated in a flow system between 348 and 421 K. Hydrogen atoms were generated in a microwave discharge, introduced to the reactor through a movable in...
TL;DR: In this paper, the second-order rate constant of oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin (OFeIVT2MPyP) was analyzed using the modified Hammett equations.
Abstract: The reaction of oxoiron(IV) tetra(2-N-methylpyridyl)porphyrin (OFeIVT2MPyP), generated from iron(III) tetra(2-N-methylpyridyl)porphyrin and tert-butyl hydroperoxide, with 3-cyanophenol in aqueous solution (pH 7.7) shows first-order dependence on the concentration of the phenol and the oxidant. The pH dependence of the measured second-order rate constant (pH 7.7–8.6) indicates that the phenol, and not the phenolate ion, is the substrate oxidised by OFeIVT2MPyP.Substituent effects on the second-order rate constant were obtained from the oxidation of phenol and six monosubstituted derivatives and these data were analysed by Hammett and modified Hammett equations. The ρ values obtained, in conjunction with the results from an EPR study of the oxidation of the water soluble hindered phenol, Trolox C, and the oxidation of [O-1H2]-4-fluorophenol, suggest that the rate-determining step in these reactions involves hydrogen atom abstraction from the phenol by the oxoiron(IV) species.The Hammett analyses of rate data from the oxidation of phenols by horseradish peroxidase compound II have been reexamined and compared with those from the present study. This leads to the conclusion that the enzymatic process involves a rate determining electron transfer from the phenol to the oxo-haem.