Showing papers on "Hydrogen atom abstraction published in 1981"
273 citations
TL;DR: Some of the kinetic features of the nitrogen dioxide—initiated autoxidation of these unsaturated fatty acids have been studied, and the nitrite ion produced by the hydrogen abstraction mechanism would be expected to diffuse throughout the body, unlike nitrated lipids that would be confined to the pulmonary cavity.
Abstract: The reactions of nitrogen dioxide with cyclohexene have been studied as a model for the reactions that occur between nitrogen dioxide in smoggy air and unsaturated fatty acids in pulmonary lipids. As predicted from earlier studies at high nitrogen dioxide concentrations, this gas reacts with cyclohexene predominantly by addition to the double bond at nitrogen dioxide concentrations of 1 percent (10,000 parts per million) to 40 percent in nitrogen; in the presence of air or oxygen, this reaction initiates the autoxidation of the alkene. However, at concentrations below 100 parts per million in nitrogen, nitrogen dioxide reacts with cyclohexene almost exclusively by abstraction of allylic hydrogen; this unexpected reaction also initiates the autoxidation of the alkene in the presence of oxygen or air, but it leads to the production of nitrous acid rather than of a product containing a nitro group attached to a carbon atom. The nitrous acid can react with amines to produce nitrosamines. Moreover, the nitrite ion produced by the hydrogen abstraction mechanism would be expected to diffuse throughout the body, unlike nitrated lipids that would be confined to the pulmonary cavity. These findings have been confirmed with methyl oleate, linoleate, and linolenate; some of the kinetic features of the nitrogen dioxide—initiated autoxidation of these unsaturated fatty acids have been studied.
248 citations
TL;DR: In this paper, the absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 less than or equal to 298 K and 20 less than and equal to P less than 200 torr by using He, Ar, and SF/sub 6/ as diluent gases.
Abstract: Absolute rate constants for the reactions of the hydroxyl radical with benzene and toluene were measured within the temperature and pressure ranges 213 less than or equal to T less than or equal to 1150 K and 20 less than or equal to P less than or equal to 200 torr by using He, Ar, and SF/sub 6/ as diluent gases. To help elucidate the variations in reaction mechanism with temperature, we also studied OH reactions with deuterated benzene (C/sub 6/D/sub 6/) and with selectively deuterated toluenes (C/sub 6/H/sub 5/CD/sub 3/, C/sub 6/D/sub 5/CD/sub 3/, and C/sub 6/D/sub 5/CH/sub 3/). Three major reaction channels were characterized kinetically. At T less than or equal to 298 K, electrophilic addition of the OH radical to the aromatic ring is the dominant reactive pathway in all systems studied. At temperatures above 500 K, rapid decomposition of the thermalized adduct back to reactants diminishes the importance of the addition channel and leads to bimolecular reaction rate-constant values significantly lower than those measured near room temperature. At elevated temperatures, the ring hydrogen abstraction (for benzene) and side-chain hydrogen abstraction (for toluene) pathways are shown to be predominant. The measured bimolecular rate constants increase monotonically withmore » increases in temperature above 500 K, and kinetic separation of the two hydrogen abstraction modes for toluene is achieved.« less
180 citations
TL;DR: In this paper, the reaction mechanism of the OH. radical induced oxidation of methionine in aqueous solutions has been investigated by radiation chemical methods, mainly ns-ms pulse radiolysis.
Abstract: The reaction mechanism of the OH. radical induced oxidation of methionine in aqueous solutions has been investigated by radiation chemical methods, mainly ns-ms pulse radiolysis. The initial step is a competitive process between addition of the OH. radical to the sulfur atom and hydrogen abstraction at a 4:1 ratio in favor of the former. In strong acid solutions (pH approx. less than or equal to 3) the subsequent mechanism is practically identical with the oxidation mechanism of aliphatic thioethers. The intermediate of most interest in this case is a sulfur-centered radical cation complex with an absorption maximum at 480 nm. At higher pH in slightly acid, neutral, and basic solutions the mechanism is more complex, essentially involving a fast (k > 4 x 10/sup 6/ s/sup -1/) intramolecular and probably sterically assisted oxidation of the amino group by the primarily oxidized sulfur function. This process includes irreversible decarboxylation of the amino acid. Both the CO/sub 2/ and the remaining ..cap alpha..-amino radicals have been identified and their yields determined. The N-oxidation depends on the reduction potential of the amino group and takes place with both the basic and the zwitterionic form of methionine - in the latter case on accountmore » of the rapid proton exchange between the NH/sub 3//sup +/ and COO/sup -/ groups. Complete protonation of the carboxyl group in very acid solutions, or by the addition of powerful proton donors, e.g., H/sub 2/PO/sub 4//sup -/, or the introduction of an electron-withdrawing substituent such as in N-acetylmethionine lowers the reduction potential to such an extent that the oxidation site may effectively remain at the sulfur. A detailed mechanism with the associated reaction kinetics and some physicochemical properties of the intermediates are presented and discussed.« less
167 citations
162 citations
TL;DR: In this paper, an electron transfer from samarium to halides and hydrogen abstraction from THF is discussed. But the mechanism of this reaction is not discussed, except for the electron transfer in the presence of a proton donor.
127 citations
TL;DR: In this article, the authors identify the necessary components for a selective and active active molybdate catalyst, which are identified as a bifunctional site, composed of an active α-hydrogen abstraction (Bi) and a selective, O or NH inserting (Mo) species, and favorable solidstate and chemical properties which enhance the rapid donation of lattice oxygen and reconstitution of reduced sites by molecular oxygen.
Abstract: Selectivity in the oxidation of olefins over heterogeneous catalysts, an important requirement for efficient use of feed resources, has been achieved by the use of catalysts which produce an allylic intermediate via dissociative chemisorption. The development of catalysts for ammoxidation and oxidation of propylene has resulted in substantial increases in the supply of important monomers such as acrylonitrile, resulting in the discovery of new applications in the fibres and plastics industry.The best known and most studied ammoxidation and oxidation catalysts are those based on bismuth molybdate, which operate by a redox mechanism composed of a catalyst reduction cycle (olefin oxidation–selective product formation), and a catalyst reoxidation cycle (lattice-oxygen regeneration). Relative rates of reduction for a number of bismuth molybdate systems: multi-component > Bi2Mo2O9 > Bi2Mo3O12 > Bi3FeMo2O12 > Bi2MoO6, and the requirement for partial reduction to attain maximum selectivity are consistent with a mechanism in which coordinately unsaturated metal ions in complex shear domains are responsible for selective oxidation. The reoxidation process, which follows the order of decreasing rates: Bi2MoO6 > Bi2Mo2O9 > Bi2Mo3O12 > Bi3FeMo2O12≳ multicomponent system, is composed of a low-activation-energy surface reoxidation regime and a higher-activation-energy reoxidation regime involving bulk anion vacancies. The latter is strongly dependent on the structure of the catalyst. Substituent effects and allyl radical in situ studies have shown that selective oxidation occurs via rate-determining α-hydrogen-atom abstraction by oxygen associated with Bi centres to produce a radical-like π allyl molybdenum complex. For oxidation, subsequent reversible formation of a σ-O allyl molybdate (which can be formed in situ from allyl alcohol) is followed by a second hydrogen abstraction. This rate-determining step in the conversion of the σ species to acrolein is enhanced by the presence of bismuth in the catalyst. In ammoxidation, the analogous σ-N allyl molybdenum species is formed reversibly from the π complex after conversion of terminal MoO sites to MoNH via condensation with ammonia. The σ-N complex undergoes a second hydrogen abstraction to produce a reduced molybdenum 3-iminopropene complex, in a process of higher activation energy than the formation of acrolein from the corresponding σ-O complex. Reoxidation and a subsequent third hydrogen abstraction forms acrylonitrile and a reduced site. The reduced site is then reoxidized by lattice oxygen to complete the catalytic cycle.From this detailed work, the necessary components for a selective and active oxidation catalyst are identified as a bifunctional site, composed of an active α-hydrogen abstraction (Bi) and a selective, O or NH inserting (Mo) species, and favourable solid-state and chemical properties which enhance the rapid donation of lattice oxygen and reconstitution of reduced sites by molecular oxygen.
94 citations
TL;DR: In this article, the first observable species in the nanosecond region is Fe(OH) aq 2+ with λ max = 300 nm and ϵ 300 = 2400 M −1 cm −1.
89 citations
79 citations
TL;DR: In this paper, Ab initio electronic structure calculations have been used to characterize the equilibrium and saddle points for the addition of O(/sup 3/P) to HCCH on the two lowest triplet surfaces.
Abstract: Ab initio electronic structure calculations have been used to characterize the equilibrium and saddle points for the addition of O(/sup 3/P) to HCCH on the two lowest triplet surfaces. Structures, frequencies, and energetics are presented and, when possible, directly compared to those from experiment. The saddle point for abstraction of H from HCCH by O(/sup 3/P) has also been characterized for the two lowest triplet surfaces by analogy with recent ab initio calculations on similar abstraction reactions. Systematic errors in the calculated energetics are corrected by semiempirical arguments. Comparisons of the energetics are also made to semiempirical BAC-MP4 calculations. The final characterizations of the stationary points on the addition and abstraction surfaces are then used in RRKM calculations of the rate constants and product branching ratios. These calculations are compared to experiment. The comparisons show (1) abstraction is not an important process, (2) theory and experiment are in good agreement for the overall rate of reaction at temperatures below 1000 K but predict substantially lower rates than observed in high-temperature shock tube experiments, (3) the calculated branching ratio, production of H + HCCO over the total rate of reaction, is not small (certainly over 25% and most likely over 50%)more » with a relatively weak temperature dependence and no pressure dependence, and (4) the ground-state triplet surface (/sup 3/A') dominates both the overall rate constant and the branching ratio with the excited triplet surface (/sup 3/A') introducing only minor perturbations.« less
59 citations
TL;DR: An intermediate is formed from the reaction of ozone with PUFA that is stable at −78°C but decomposes to form radicals at about −45°C, tentatively identified as a trioxide (ROOOH, ROOOR, or R-CO-OOOH) on the basis of analogies and its temperature profile for decomposition to radicals.
TL;DR: In this article, several neopentyl halide complexes of zirconium have been prepared in order to test whether they could be induced to lose neOPentane and give neOPENTylidene complexes by adding phosphorus or nitrogen donor ligands.
TL;DR: In this paper, a number of small carboxylic acid esters and triglycerides were found to decay on warming to form alkyl radicals by using annealing.
Abstract: Reactions which occurred after electron attachment at 77K to a number of small carboxylic acid esters and triglycerides in an aqueous glass are reported. Most ester anions are found to decay on warming to form alkyl radicals by ..beta.. scission: RC(O/sup -/)OR' ..-->.. RCO/sub 2//sup -/ + R'.. The alkyl radical (R'.) produced by annealing is found to abstract hydrogen from the parent ester at an ..cap alpha..-carbon site, R'.+ R''CH/sub 2/CO/sub 2/R' ..-->.. R''CHCO/sub 2/R', or in the case of ethyl formate from the formate hydrogen, CH/sub 3/CH/sub 2/.+ HCO/sub 2/C/sub 2/H/sub 5/ ..-->.. C/sub 2/H/sub 6/ +.CO/sub 2/C/sub 2/H/sub 5/. Results found for the methyl formate anion suggest hydrogen abstraction by the anion itself may compete with alkyl radical formation. The anion of the triglyceride triacetin is found to undergo an analogous mechanism to the ester anions producing the propane diol diester radical, .CH/sub 2/CH(Ac)CH/sub 2/(Ac), Ac = acetate. This species subsequently abstracts hydrogen from the parent compound to produce the ..cap alpha..-carbon radical, .CH/sub 2/CO/sub 2/R. Results found after annealing the tripropionin radical anion give evidence for abstraction from the ..cap alpha.. carbon in the propionate side groups producing CH/sub 3/CHCO/sub 2/R. Studies of a ..gamma..-irradiated estermore » (ethyl myristate) and two triglycerides (tripalmitin and tristearin) yield results which suggest that the mechanism of ester anion decay found in aqueous glasses applies to ..gamma..-irradiated neat long-chain esters and triglycerides. Results found in this work are compared to the results of product analysis.« less
TL;DR: Pentadienyl radicals were generated from penta-1,4-diene and cis and trans-1-bromopenta-2,4diene, and were observed in the E,E(1) and E,Z(2) conformation in hydrocarbon solution by e.s.r..
Abstract: Pentadienyl radicals were generated from penta-1,4-diene and cis and trans-1-bromopenta-2,4-diene, and were observed in the E,E(1) and E,Z(2) conformation in hydrocarbon solution by e.s.r. spectroscopy. The E,Z-radicals are converted into the E,E-radicals at T > ca. 170 K, but the E,E-radicals are not converted into the E,Z-radicals in the accessible temperature range. From the estimated barrier to rotation in E,Z-pentadienyl radicals the methane based stabilization energy (E8Me – H) was estimated to be 104 kJ mol–1. Pentadienyl radicals can also be obtained from ring-opening of cyclobut-2-enylmethyl radicals (3). Bromine abstraction from cyclobut-2-enyl methyl bromide by triethylsilyl radicals gave cyclobut-2-enylmethyl radicals at temperatures below ca. 230 K. Above this temperature homolytic fission of the cyclobutene ring occurred and pentadienyl radicals in the E,E-conformation were detected by e.s.r. Initially, E,Z-pentadienyl radicals must be formed, but at the temperature of ring fission these are converted into the E,E-radicals and so are not observed. Hydrogen abstraction from neither 3-methylcyclobutene nor from bicyclo[2.1.0]pentane yields (3): instead 3-methylcyclobutenyl radicals and cyclopent-3-enyl radicals are formed, respectively. E.s.r. parameters are also reported for a range of substituted pentadienyl radicals generated from the corresponding 1,4-dienes. Of these radicals only 3-trimethylsiloxypentadienyl was observed in two conformations.
TL;DR: In this paper, an XPS study of the chemisorption of HCl and H 2 S by Cu(111)-O surfaces at low temperature was shown to involve hydrogen abstraction with the formation of surface hydroxyl species.
TL;DR: A series of alkyl (C4 or greater) heteroaryl iminyls have been generated by oxidation of the corresponding oxime-O-acetic acids with persulphate as discussed by the authors.
Abstract: A series of alkyl (C4 or greater) heteroaryl iminyls have been generated by oxidation of the corresponding oxime-O-acetic acids with persulphate. These react to give fused heteroaryl cyclohexanones.
TL;DR: The reaction of oxygen atoms with ethanol in a fast flow system was studied with a photoionization mass spectrometer and acetaldehyde was found to be acetaldehyde, showing that acetaldehyde is produced by the hydrogen abstraction reaction by O(3P) from the O–H in the alpha‐ethanol radical.
Abstract: The reaction of oxygen atoms with ethanol in a fast flow system was studied with a photoionization mass spectrometer A radical formed from O+C2H5OH was photoionized by the Xe lamp and identified as the alpha‐ethanol radical, CH3CH(OH), ie, CH3CD(OH) (m/e = 46) and CD3CD(OH) (m/e = 49) were observed in reactions of O+CH3CD2OH and C2D5OH, respectively The major primary product in the subsequent reaction of alpha‐ethanol radicals with atomic oxygen was found to be acetaldehyde (79%±8%) and no formation of vinyl alcohol was observed, ie, CH3CDO (m/e = 45) and CD3CDO (m/e = 48) were observed in reactions of O+CH3CD2OH and C2D5OH, respectively: O(3P)+CH3CH(OH)→CH3CHO+OH (4a) The reaction of 18O(3P)+C2D5OH produced only CD3CDO (m/e = 48) This fact shows that acetaldehyde is produced by the hydrogen abstraction reaction by O(3P) from the O–H in the alpha‐ethanol radical The competition experiment among O(3P) and O2 for the alpha‐ethanol radical shows that alpha‐ethanol radicals react (014±004) times mo
TL;DR: In this article, low-energy molecular ions of gas-phase primary and secondary aliphatic amines undergo reciprocal NH/CH exchange of hydrogen atoms prior to fragmentation on the microsecond time-scale; the subsequent decomposition reactions are significantly different from those observed in the mass spectrometer ion source.
TL;DR: In this paper, Hartree-Fock 4-31G calculations were performed on carbon-centred radicals to analyze the influence of substituents on the static and dynamic properties of these systems.
Abstract: Unrestricted Hartree-Fock 4-31G calculations were performed on carbon-centred radicals to analyze the influence of substituents on the static and dynamic properties of these systems. dd-substituted radicals are not planar and little stabilized. cc (or cd)-substitution leads to planar systems which are highly stabilized. The formation of sigma -radicals by hydrogen abstraction from the parent molecule is characterized by a smaller exothermicity and presumably a higher activation energy than the formation of pi -systems. The autoreaction of alkyl radicals substituted by two captor groups is thermodynamically less favoured than their addition to olefins. Capto-dative substitution seems to favour the whole process: 2RH rarr 2R rarr R-R.
Patent•
16 Sep 1981TL;DR: In this paper, a photoinitiator is selected from the group consisting of organic peroxide compounds which have as an identifying and critical feature at least one carbonyl group directly bonded through a carbon-oxygen bond to a peroxy oxygen atom.
Abstract: Energy-curable compositions can be cured in the presence of oxygen by exposure to actinic radiation employing as a photocatalyst system an essentially homogeneous admixture of (1) at least one compound which permits free radical polymerization through bimolecular photochemical reactions of the energy donor or energy transfer type, the hydrogen abstraction type, or by the formation of a donor-acceptor complex with monomers or additives leading to ionic or radical species; and (2) at least one photoinitiator selected from the group consisting of organic peroxide compounds which have as an identifying and critical feature at least one carbonyl group directly bonded through a carbon-oxygen bond to a peroxy oxygen atom.
TL;DR: In the photodecomposition of N-haloamides (ZCONRX) in the presence of olefins, the 1,2-addition chain competes with the hydrogen abstraction chain(s) leading to the parent amide (the quantum yields).
Abstract: In the photodecomposition of N-haloamides (ZCONRX) in the presence of olefins, the 1,2-addition chain competes with the hydrogen abstraction chain(s) leading to the parent amide (the quantum yields...
TL;DR: In this article, the ESR spectra were interpreted by assuming superposition of three signals; they were ascribed to radicals formed by chain scission and hydrogen abstraction, and the amount of chain scissions was followed under various experimental conditions and appeared as a function of the initiator.
Abstract: General investigations of radical processes in starch were carried out. ESR experiments showed that UV irradiation of starch, both in photosensitized and unphotosensitized experiments, results in chain scission and radical generation on the glucosidic ring. It was shown that benzophenone, deoxybenzoin and derivatives (2a–d) are efficient sensitizers. The ESR spectra were interpreted by assuming superposition of three signals; they were ascribed to radicals formed by chain scission and hydrogen abstraction. In addition photodegradation processes of starch in the presence and absence of photosensitizers were investigated. The amount of chain scissions was followed under various experimental conditions and appeared as a function of the initiator. Moreover, oxygen was shown to enhance the yield of chain scissions: this was explained through the formation of peroxide functions at C1-position. UV light induced grafting of a vinyl monomer onto starch was also investigated. It appeared that chemical grafting can easily be achieved through a radical reaction. Moreover, the percentages of grafting and degradation were strongly dependent on the photoinitiator. The whole results gave the possibility to select an efficient initiator of the grafting reaction. In the case of methyl methacrylate, α,α-dimethoxydeoxybenzoin (2a) appears to be the most efficient initiator. However, previous works as well as the present work suggest that the grafting efficiency is strongly dependent on the three components: substrate, monomer, and initiator.
TL;DR: In this paper, the effect of molecular structure on the degradation of polypeptides by ionizing radiation was investigated using homopolymers of the amino acids gly, ala, val, phe and tyr, and the main reactions were scission of the entire side chain leading to its hydrogen adduct, fragmentation of the side chain, and fragmentation leading to a reduction in molecular weight and elimination of a segment of the chain.
TL;DR: In this article, the exact rate constants for the reaction of the conjugate base of metiazinic acid, MZ, with hydrated electrons (k 1.5 × 109 l mol−1 s−1 ) were measured.
Abstract: Absolute rate constants have been measured for the reaction of the anionic conjugate base of metiazinic acid, MZ–, in the pH range 8–11, with hydrated electrons (k 1.5 × 109 l mol–1 s–1), and for the oxidation of MZ– by OH˙, Br2–˙, (SCN)2–˙, CO3–˙, TI(OH)2, N3, ˙˙CH2CHO, (CH3SCH3)2+˙, CH3SSCH3+˙ and lipoate+˙. The k values for these oxidations are found to be in the 109–1010 l mol–1 s–1 range, except for k(˙CH2CHO + MZ–) 2 × 108 l mol–1 s–1. The oxidation of metiazinic acid leads to MZ±˙ radical zwitterions with yields ranging from 50 to 100% with the minimum value referring to the oxidation by ˙CH2CHO. The MZ±˙ radical zwitterion is characterized by two strong optical absorption bands with maxima at 530 (Iµ 11 000 l mol–1 cm–1) and 270 nm (51 500). Other primary oxidation products are indicated, but only a transient absorption around 350 nm (OH˙ adduct and/or hydrogen abstraction product) could qualitatively be identified in the reaction of metiazinic acid with hydroxyl radicals.