Showing papers on "Hydrogen atom abstraction published in 1996"
TL;DR: The evidence is interpreted to favor a 1-electron pathway with N,N-dialkylamines with P450 2B1 as well as HRP and several biomimetic models and to lower isotope effects with para electron-withdrawing substituents, due to an apparent shift in rate-limiting steps.
160 citations
TL;DR: In this paper, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution was determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical.
Abstract: Nanosecond laser flash photolysis studies have been carried out of the kinetics of inter- and intramolecular phenolic hydrogen abstraction by alkoxyacetophenone, 5-alkoxyindanone, and 4-alkoxybenzophenone triplets in acetonitrile and benzene solution. Information on the geometric requirements for abstraction by carbonyl n,π* and π,π* triplets is derived from the results for a series of ketones which contain a para-phenolic moiety attached via a para-oxyethyl linkage. For all of these compounds, the deuterium kinetic isotope effect on the triplet lifetime in acetonitrile solution indicates that triplet decay is determined by the rate of intramolecular abstraction of the remote phenolic hydrogen, which yields the corresponding phenoxyl−hemipinacol biradical. The biradicals have also been detected, and are about an order of magnitude longer-lived than the triplet in each case. For three of the compounds, the rates of the intramolecular process follow the same trend as that observed in the rates of bimolecula...
126 citations
TL;DR: It is proposed that molecular oxygen is not productively bound by lipoxygenase but rather interacts directly with the substrate radical lipoxygenicase to form the hydroperoxyl radical of linoleate.
Abstract: Molecular oxygen is generally unreactive toward covalent bonds, due to spin conservation rules; a major role for oxygen-utilizing enzymes is, therefore, to activate dioxygen through a change in electronic configuration. In an effort to understand how lipoxygenase catalyzes lipid hydroperoxidation under conditions of catalytic turnover, kinetic deuterium isotope effects have been measured as a function of oxygen concentration. The properties of oxygen binding to lipoxygenase have also been pursued. The results presented herein show that, under steady state conditions, atmospheric oxygen enters the reaction pathway only after abstraction of hydrogen from substrate. Furthermore, it has not been possible to detect any form of lipoxygenase capable of binding molecular oxygen in the absence of activated substrate. We propose that molecular oxygen is not productively bound by lipoxygenase but rather interacts directly with the substrate radical lipoxygenase to form the hydroperoxyl radical of linoleate. A mechan...
118 citations
115 citations
TL;DR: In this article, the authors used pulsed laser photolysis/pulsed laser-induced fluorescence to determine products of the following reactions: CH3S + O2 → products.
Abstract: The technique of pulsed laser photolysis/pulsed laser-induced fluorescence was used to determine products of the following reactions: OH + CH3SCH3 → products (1) and CH3S(OH)CH3 + O2 → products (5). The non-adduct-forming channel of reaction 1 was observed to proceed via hydrogen abstraction (branching ratio, Φabs = 0.84 ± 0.26) at 298 K. Direct formation of CH3S from reaction 1 was not observed (ΦCH3S < 0.04). A small channel for producing CH3 and CH3SOH in the non-adduct-forming route could not be ruled out. HO2 was found to be produced from reaction 5 with a yield of 0.50 ± 0.15 at 234 and 258 K. An upper limit for the production of OH from reaction 5 was determined to be ΦOH < 0.05. The atmospheric implications of the results from this and the accompanying paper are discussed.
113 citations
TL;DR: The rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis as mentioned in this paper.
Abstract: Absolute rate constants for hydrogen atom abstraction by alkoxyl radicals from phenol, aniline, and diphenylamine have been measured in 14 solvents at room temperature by laser flash photolysis. For all three substrates the rate constants decline as the solvent becomes a stronger hydrogen-bond acceptor (HBA). Thus, on changing the solvent from CCl4 to CH3CN the rate constants decline by factors of 148, 7.1, and 4.7 for PhOH, PhNH2, and Ph2NH, respectively. The kinetic solvent effect for phenol correlates rather well with Abraham's scale of relative HBA activities of the solvents we have employed as measured as solutes in CCl4 solvent. This correlation is not quite so good with aniline and it is almost nonexistent for diphenylamine. With all three substrates the “deviant” solvents produce higher rate constants than would be expected from the value of the solvent and, generally, these are the solvents in which steric hindrance to hydrogen-bond formation would appear probable. The kinetic data for the three ...
112 citations
TL;DR: The photochemical reactions induced by metal complexes with DNA are reviewed, with most emphasis on polypyridyl complexes [especially those of Ru(II), Rh(III), and Co(III]], porphyrins and uranyl ions.
Abstract: The photochemical reactions induced by metal complexes with DNA are reviewed, with most emphasis on polypyridyl complexes [especially those of Ru(II), Rh(III) and Co(III)], porphyrins and uranyl ions. In each case, where available, data on the binding of the complexes to DNA, the effect of such binding on the excited state lifetime and other properties, and the primary photochemical reactions are described. Particular emphasis is laid on reactions involving electron transfer. Evidence for direct oxidation of the DNA bases, production of DNA-damaging reactive species, direct hydrogen abstraction from the DNA ribose and formation of adducts between the metal complex and DNA are summarised. The use of metal complexes as photophysical and photochemical probes for studying the structure and conformation of nucleic acids and possible photo-therapeutic applications are discussed.
102 citations
TL;DR: The 9,10-dehydroanthracene biradical, a model for the p-benzyne-type biradicals implicated in DNA cleavage by the enediyne antitumor antibiotics, was prepared by photodissociation of a propellane in solution.
Abstract: The 9,10-dehydroanthracene biradical, a model for the p-benzyne-type biradicals implicated in DNA cleavage by the enediyne antitumor antibiotics, was prepared by photodissociation of a propellane in solution. Trapping products characteristic of biradicals, e.g. anthracene-d2, are found. The rates of hydrogen abstraction by the biradical from acetonitrile and isopropyl alcohol are measured directly by flash photolysis/transient absorption spectroscopy, giving second-order rate constants of kMeCN,abstr = (1.1 ± 0.2) × 103 M-1 s-1 and ki-PrOH,abstr = (6.5 ± 0.6) × 103 M-1 s-1 at room temperature, which are 100−200 times lower than the corresponding rate constants for phenyl or 9-anthryl radical. A second decay route for the biradical is found, and assigned, based on thermochemical, kinetic, and trapping arguments, to a retro-Bergman reaction that converts the 9,10-dehydroanthracene biradical into the ring-opened 3,4-benzocyclodeca-3,7,9-triene-1,5-diyne. Although the retro-Bergman reaction is relatively fast...
96 citations
TL;DR: In this article, a trihydridoruthenium(IV) complex was obtained by treating RuH2(H2)2(PCy3)2 with 3 equiv of an alkene bearing σ-donor substituents such as CH2CH(SiEt3)/CH2CHtBu.
96 citations
TL;DR: In this paper, the photoreduction of benzophenone (BP) takes place in the interlayer between the LDPE films, and is a three-stage process: surface photoresduction, secondary photoresuction, and solid-state photoresde.
Abstract: A bulk surface photografting process which is conducted in the interface between two polymer films was designed and investigated. The bulk surface photografting is a highly efficient process. With this method, the surface of hydrophobic polymers can be made hydrophilic in less than 2 s and a grafted layer 5 μm thick can be obtained in 30 s. The kinetic investigation shows that the bulk surface photografting polymerization involves a four-step reaction process: induction period, surface initiation, successive polymerization, involves a four-step reaction process: induction period, surface initiation, successive polymerization, and solid-phase crosslinking. The photoreduction of benzophenone (BP) takes place in the interlayer between the LDPE films and is a three-stage process: surface photoreduction, secondary photoreduction, and solid-state photoreduction. With regard to the photoreduction of the initiator caused by hydrogen abstraction, the kinetic studies show that the photoreduction rate has a first-order dependence on the BP concentration. The activation energy of the hydrogen abstraction reaction is about 28.5 KJ (6.8 kcal)/mol. With regard to the photografting polymerization reaction, the reaction order of the rate Rp with respect to the monomer is unity and 0.89 with respect to BP. This means that the termination reaction takes place mainly by combination of polymer chain free radicals and semipinacol free radicals from BP. The activation energy of the overall polymerization reaction is around 8.8 kJ (2.1 kcal)/mol. © 1996 John Wiley & Sons, Inc.
95 citations
TL;DR: In this article, the authors evaluated the reaction barrier for hydrogen abstraction reaction with hydrogen hydroxy radicals from hydrogen and methane molecules with the Gaussian 6-311G (2d,2p) basis set.
TL;DR: In this paper, the rate constants for hydrogen atom abstraction from two water-soluble α-tocopherol analogues by tert-butoxyl radicals were measured at room temperature in four solvents.
Abstract: Rate constants for hydrogen atom abstraction from two water-soluble α-tocopherol (α-TOH, vitamin E) analogues by tert-butoxyl radicals have been measured at room temperature in four solvents, inclu...
TL;DR: Support is provided for 5'-hydrogen atom abstraction from the deoxyribose ring as the initial event in the sequence-selective nicking by 02-derived and metal-associated oxidants, which may provide a biomarker for the reactivity of HOO in vivo.
Abstract: In previous studies, the ability of the hydrodioxyl (perhydroxyl) radical [HOO•, the conjugate acid of superoxide (O2•-)] to “nick” DNA under biomimetic conditions was demonstrated, and a sequence selectivity was observed. A background level of nonspecific nicking also was noted. This paper provides support for 5‘-hydrogen atom abstraction from the deoxyribose ring as the initial event in the sequence-selective nicking by O2•-/HOO•. Two experiments support the proposed mechanism. First, using a defined sequence 5‘-32P-labeled restriction fragment as the DNA substrate, only free (unalkylated) 3‘-phosphate is produced at the site of nicking. Second, using poly (dA)·poly (T) as the substrate, furfural is formed in the reaction from deoxyribose ring breakdown. Both results are consistent with 5‘-hydrogen atom abstraction for initiation of the site-selective nicking. Hydrogen atom abstraction at other sites of the deoxyribose ring and/or base oxidation and loss followed by strand scission likely are responsibl...
TL;DR: In this article, the rotational state distribution of HCl was determined using resonance enhanced multiphoton ionization (REMPI), center-of-mass scattering distributions were measured by the core extraction technique, and the average internal energy of the C2H5 product was deduced from the dependence of the core-extracted signal on the photolysis polarization.
Abstract: Photolysis of Cl2 initiates the title reaction at a sharply defined collision energy of 0.24±0.03 eV. Nascent product rotational state distributions for HCl (v=0) are determined using resonance enhanced multiphoton ionization (REMPI), center‐of‐mass scattering distributions are measured by the core‐extraction technique, and the average internal energy of the C2H5 product is deduced from the dependence of the core‐extracted signal on the photolysis polarization. The HCl product has little rotational excitation, but the scattering distribution is nearly isotropic. Although seemingly contradictory, both of these features can be accounted for by using the simple line‐of‐centers model presented to explain earlier results for the Cl+CH4 reaction. In contrast to the Cl+CH4 reaction, the data suggest that the Cl+C2H6 reaction proceeds through a loosely constrained transition‐state geometry. The reactions of atomic chlorine with ethane, C2H6, and perdeuteroethane, C2D6, yield virtually identical results. These findings, along with the low energy deposited by the reaction into the ethyl product (200±120 cm−1), demonstrate that the alkyl fragment acts largely as a spectator in this hydrogen abstraction reaction.
TL;DR: The mechanism of UV-induced cleavage of duplex DNA containing 5-bromodeoxyuridine under anaerobic conditions was investigated via product analysis and kinetic isotope effect measurements using gel electrophoresis as discussed by the authors.
Abstract: The mechanism of UV-induced cleavage of duplex DNA containing 5-bromodeoxyuridine under anaerobic conditions was investigated via product analysis and kinetic isotope effect measurements using gel electrophoresis. KIEs strongly suggest that strand breaks result from abstraction of the C2‘ hydrogen atom from the 5‘-adjacent nucleotide by deoxyuridin-5-yl (1). It is proposed that under anaerobic conditions the C2‘ radical is subsequently oxidized by the cation radical of adenine. The resulting carbocation undergoes 1,2-hydride migrations from C1‘ and C3‘ competitively, giving rise to cleavage products containing 3‘-phosphate and the novel, labile 3‘-ketone (4). The identity of the latter product is supported by electrospray mass spectrometric analysis of the crude photolysate. The results also suggest that the formation of alkaline labile lesions proceed via hydrogen atom abstraction from the C2‘ position by 1.
TL;DR: In this article, the authors studied the oxidation of aliphatic acids by radicals resulting from the protolysis of hydrogen peroxide and showed that the carboxylic group is slow to oxidize and the hydroxyl group of alphatic acids activates the adjacent CH bond and leads, after hydrogen atoms abstraction, O2 addition and HO2 removal, to the carbonyl group.
Abstract: The aim of this work was to study the oxidation of aliphatic acids by radicals resulting from the protolysis of hydrogen peroxide. Experiments carried out in dilute aqueous solutions (pH = 2.9−3.5) of mono and dicarboxylic acids with 1 to 4 carbon atoms showed (i) the oxidation of the carboxylic group is slow, (ii) the hydroxyl group of hydroxylated acids activates the adjacent CH bond and leads, after hydrogen atoms abstraction, O2 addition and HO2 removal, to the carbonyl group, (iii) a CC bond cleavage occurs after hydrogen atom abstraction and oxygen addition for unsubstituted dicarboxylic acids. Except for oxalic acid, the mechanism of oxidation into CO2 of the acids studied involves oxygen consumption and O2−/HO2 release.
TL;DR: In this paper, the effect of nitric oxide (NO) on 1-pentene was examined in a pressurized flow reactor facility at 6 atmospheres over the temperature range of 600-800 K and an equivalence ratio of 0.4.
TL;DR: In this paper, a free-radical mechanism for the oxidation of alkanes by aromatic peracids was proposed, which involves hydrogen abstraction from the OH group of peracides by an a...
Abstract: Evidences concerning a novel free-radical mechanism for the oxidation of alkanes by aromatic peracids are reported. The mechanism involves hydrogen abstraction from the OH group of peracids by an a...
TL;DR: Results show that 3MEI was formed in a cytochrome P450-catalyzed dehydrogenation reaction in which the rate-limiting step was presumably hydrogen atom abstraction from the 3-methyl position.
Abstract: The mechanism of 3-methyleneindolenine (3MEI) formation from 3-methylindole (3MI) in goat lung microsomes was examined using stable isotope techniques. 3MEI is highly electrophilic, and its production is a principal factor in the systemic pneumotoxicity of 3MI. Noncompetitive intermolecular isotope effects of DV = 3.3 and D(V/K) = 1.1 obtained after deuterium substitution at the 3-methyl position indicated either that hydrogen abstraction from the methyl group was not the initial rate-limiting step or that this step was rate-limiting and was masked by a high forward commitment and low reverse commitment to catalysis. An intramolecular isotope effect of 5.5 demonstrated that hydrogen atom abstraction was probably the initial oxidative and rate-limiting step of 3MI bioactivation or that deprotonation of an aminium cation radical, produced by one-electron oxidation of the indole nitrogen, was rate-limiting. However, a mechanism which requires deprotonation of the aminium cation radical is probably precluded ...
TL;DR: In this paper, it was shown that the quenching takes place in two parallel pathways, the major one involves the aromatic portion of toluene and leads to the recovery of the reactants, and the minor, productive path takes place by hydrogen atom abstraction from the methyl group, followed by the oxidation of PhCH2•.
Abstract: The visible light irradiation of aqueous solutions containing toluene, uranyl(VI) ions, and O2 results in the formation of benzaldehyde as a major product. Small amounts of PhCH2OH are also formed. The yields of benzaldehyde are 3 times greater for toluene-h8 than for toluene-d8, but the kinetic isotope effect for the quenching of the excited state *UO22+ by toluene is negligible (ktoluene-h8/ktoluene-d8 = 1.2). This and other evidence indicate that the quenching takes place in two parallel pathways. The major one involves the aromatic portion of toluene and leads to the recovery of the reactants. The minor, productive path takes place by hydrogen atom abstraction from the methyl group, followed by the oxidation of PhCH2•. Cumene, benzyl alcohol, and benzaldehyde react similarly.
TL;DR: In this paper, the authors examined the photoreaction of hexanucleotides d(GCABrUGC)2 in which one of the C2′ hydrogens of the deoxyribose moiety was stereospecifically deuterated.
TL;DR: In this article, the rate constants for quenching of the singlet and triplet-excited states of acetone and a cyclic azoalkane by the hydrogen donors tributyltin hydride, 1,4-cyclohexadiene, and 2-propanol have been determined by time-resolved spectroscopy.
Abstract: The rate constants for quenching of the singlet- and triplet-excited state of acetone and a cyclic azoalkane by the hydrogen donors tributyltin hydride, 1,4-cyclohexadiene, and 2-propanol have been determined by time-resolved spectroscopy. It is concluded, in variance with previous studies, that singlet-excited states are significantly more reactive than triplet-excited states and that the reactivity difference between the two states of different spin multiplicity increases (i) with decreasing reactivity of the hydrogen donor and (ii) with increasing singlet−triplet energy gap of the excited state. This result is corroborated by semiempirical calculations. The relative efficiency for photoreduction by tributyltin hydride, which was determined by monitoring the formation of tributyltin radicals upon flash photolysis, was found to be four times lower for singlet-excited acetone than for the triplet state. The discrepancy between higher reactivity but lower efficiency in the intermolecular interaction of n,π...
TL;DR: The spin trap nitrosodurene was used for the detection of radical intermediates formed by 50kHz ultrasound in argon-saturated n-alcohols, n-alkanes, cyclic ethers, toluene, N,N-dimethylformamide (DMF) and dimethylacetamide (DMA).
TL;DR: In this paper, the authors showed that the catalytic activity of Co(SB) depended on the nature of the SB ligand: the formal potential E° and steric factors seem to affect the reaction rate.
Abstract: Cobalt Schiff base complexes [Co(SB)] catalyze the oxidation of anilines (1) with tert-butyl hydroperoxide to give nitrobenzenes 2 and 4-(tert-butylperoxy)-2,5-cyclohexadien-1-imine derivatives 3 in yield distributions depending on the substitution mode of the substrate. 4-Alkyl- and 4-aryl-2,6-di-tert-butylanilines gave mixtures of 2 and 3, where the higher the bulkiness of the 4-substituent, the higher the yield of 2. With 2,4,6-trimethylaniline, the ratio of oxidations of the nitrogen and C-4 atoms was almost the same; but a hydrolyzed product 5 of the imine was obtained. 2,4,6-Triphenylaniline gave only 2. Nitrobenzene derivatives were also obtained from 2,6-dialkylanilines and 4-substituted anilines. The catalytic activity of Co(SB) depended on the nature of the SB ligand: the formal potential E° and steric factors seem to affect the reaction rate. Kinetic studies showed that the key step may involve hydrogen abstraction from the aniline, presumably by t-BuO• generated from homolytic decomposition o...
TL;DR: In this article, the triplet state of 1,8naphthalimide has been studied by the laser flash photolysis technique and various triplet-triplet parameters such as molar extinction coefficient, triplet yield and energy have been obtained.
Abstract: The triplet state of 1,8-naphthalimide has been studied by the laser flash photolysis technique. The triplet-triplet absorption spectrum and the decay kinetics of the triplet state have been investigated in benzene, acetonitrile and 2-propanol. It is shown that, unlike phthalimide, 1,8naphthalimide does not undergo photoinduced hydrogen abstraction reactions with protic solvents. Quantitative data on various triplet state parameters such as molar extinction coefficient of the triplet-triplet absorption, triplet yield and energy have been obtained. The behaviour of the compound towards some electron-rich substances have been studied and the rate constants have been evaluated.
TL;DR: Phenyl radicals with different positively charged, chemically inert substituents in the para position were generated by collision-activated dissociation of para-substituted iodobenzene ions in the gas phase inside a Fourier-transform ion cyclotron resonance mass spectrometer.
Abstract: Phenyl radicals with different positively charged, chemically inert substituents in the para position were generated by collision-activated dissociation of para-substituted iodobenzene ions in the gas phase inside a Fourier-transform ion cyclotron resonance mass spectrometer. The reactivity of the radicals was examined toward organic hydrogen atom donors. The findings parallel those made by others for neutral radicals in solution. For example, the efficiency of hydrogen atom abstraction increases in the expected order phenol < thiophenol < benzeneselenol. Inspite of great exothermicity, hydrogen atom abstraction by the charged phenyl radicals occurs at a small fraction of the gas-phase collision rate, suggesting the presence of a significant barrier on the reaction coordinate. Fluorine substitution on the phenyl ring was found to drastically enhance the reaction rate for all the substrates studied. For example, hydrogen abstraction from thiophenol by the (2,3,5,6-tetrafluoro-4-dehydrophenyl)trimethylphosp...
TL;DR: In this article, the authors applied density functional methods to the thermochemistry of methanol C−H and O−H bond activation by group 5 through group 8 transition metal (TM)−oxo model complexes.
TL;DR: In this paper, the potential energy surface related to the H + HNCO reactions has been explored by means of an ab initio molecular orbital method at the PUMP4/6-311++G(d,p) and QCISD(TC) levels.
Abstract: The potential energy surface related to the H + HNCO reactions has been explored by means of an ab initio molecular orbital method at the PUMP4/6-311++G(d,p) and QCISD(TC)/6-311++G(2df,2pd)+ZPE levels. The addition of H to N is confirmed to be the dominant reaction channel, giving the H2NCO radical as the primary intermediate and H2N + CO as the fragment products. The contribution of both C- and O-additions to H2NCO formation is negligible. The hydrogen abstraction requires an activation energy larger than that of N-addition but smaller than that of C- and O-additions. Using a quantum statistical Rice−Ramsperger−Kassel (QRRK) model, the kinetics of the N-addition and H-abstraction have been analyzed. The following rate constant expressions in the temperature range 300−3300 K and at a pressure of 1 atm are suggested: for H + HNCO → H2N + CO (via H2NCO*), K = 3.59 × 104T2.49 exp(−1180/T); for H + HNCO → H2NCO, K = 1.63 × 1011T-1.90 exp(−1390/T); for H + HNCO → H2 + NCO, K = 1.76 × 105T2.41 exp(−6190/T); an...
TL;DR: In this paper, a deuterium kinetic isotope effect greater than 3 was found in the catalytic cycle of cobalt porphyrin catalyzed chain transfer in methacrylate free radical polymerizations.
TL;DR: In this article, the S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1 naphthol and ammonia in the gas phase was analyzed.
Abstract: We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1‐naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1‐naphthol. The O–H...N heavy atom separation is R=2.86 A and the barrier to rotation of the NH3 group about its C3 axis is V3=39.9 cm−1 in the S0 state. Excitation of the complex to its S1 state increases the acidity of 1‐naphthol, decreases the heavy atom separation to R=2.72 A, and increases the torsional barrier to V3=46.5 cm−1. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S1 state.