Showing papers on "Hydrogen atom abstraction published in 1980"
TL;DR: In this article, the authors used time resolved laser flash spectroscopy to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile and obtained new estimates for the triplet extinction coefficient, unimolecular and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams.
Abstract: The technique of time resolved laser flash spectroscopy has been used to study the photophysical and photochemical properties of the lowest triplet of benzophenone in water and acetonitrile. New estimates have been obtained for the triplet extinction coefficient, unimolecular and bimolecular rate constant of decay in both solvents, as well as a determination of the quantum yield of ketyl formation in pure water and the rate constants of hydrogen abstraction from various lactams in both solvents. Evidence for hydrogen abstraction from water by a triplet of benzophenone is found. Correlation between rate constants of hydrogen abstraction from lactams by the lowest benzophenone triplet and their ionization potential is discussed. Reversibility of these hydrogen abstractions is considered.
188 citations
TL;DR: In this article, the authors showed that molecular hydrogen can participate directly in free radical reactions under coal liquefaction conditions, and that the rate of the reaction is directly proportional to the hydrogen pressure.
153 citations
TL;DR: A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair as mentioned in this paper.
Abstract: Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbine appears to pivot in place while the C α -H bond weakens. A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair. Bulky substituents on the metal and carbine protect the metal center from intermolecular reactions and control the extent of carbine pivoting. While a secondary interaction weakens the C-H α bond and attracts the a-hydrogen to the metal, full transfer of hydride to the metal is a forbidden reaction, at least for a five-coordinate, 14-electron complex. The metal-hydrogen bonding interaction guides the hydride to a neighbouring alkyl group, facilitating an α-elimination mode characteristic of the reactions of these compounds. The complexed carbene centers are unusually electron-rich, nucleophilic, by comparison with 18-electron d 6 stabilized carbene complexes. This is a consequence of an extremely effective Ta-C overlap.
116 citations
TL;DR: Spectral studies indicated that the immediate precursor of the CO is probably the cytochrome P-450-Fe 2+ dichlorocarbene complex, and a reaction pathway for the formation of CO has been proposed including the possible radical, carbanion and carbene intermediates.
105 citations
TL;DR: In this article, the performance of Co(II)-Schiff base complexes toward selective oxygenation of organic molecules related to biological systems has been reviewed, and preliminary kinetic studies on the regioselective peroxyquinolato Co(III) complex formation show that the reaction involves a rate-determining hydrogen abstraction from the phenol substrate by a superoxo Co(3) complex initially formed, followed by rapid reduction of the resulting phenoxy radical with Co(2) species leading to the formation of a phenolato co-type complex, to which
98 citations
TL;DR: In this article, the authors investigated the role of acid/ether-forming sites and oxidation/acrolein forming sites in the equilibration of C-1 and C-3 of allyl alcohol, and established hydrogen abstraction as the rate-determining step with k H k D ≅ 2.5 (320 °C, oxidation) and 2.1 (430 °C, ammoxidation).
95 citations
TL;DR: In this article, the role of chemisorbed oxygen in inducing the formation of hydroxyl species on a Cu(111)-O surface has been confirmed by selective reduction of a mixed oxy-hydroxide adlayer by CO.
94 citations
TL;DR: In this paper, a simple and internally consistent quantitative model for hydrogen-abstraction reactions in low-temperature solids, which implicitly incorporates zero point energy effects which allow for finite reaction rates at T = 0 K, is derived and applied to new measurements of H-ABstraction rate constants by methyl radicals in methanol and ethanol glasses a t T = 13-99 K.
Abstract: A simple and internally consistent quantitative model for hydrogen-abstraction reactions in low-temperature solids, which implicitly incorporates zero point energy effects which allow for finite reaction rates at T = 0 K, is derived and applied to new measurements of H-abstraction rate constants by methyl radicals in methanol and ethanol glasses a t T = 13-99 K and in acetonitrile and methyl isocyanide crystals a t 69-128 K. Nonlinear least-squares fits of the model to the experimental data yield effective one-dimensional barriers to reaction whose heights are virtually independent of the analytic form used for the potential energy barrier, and are somewhat larger than the activation energies measured for the corresponding reactions in the gas phase. This model predicts that values of the isotopic rate constant ratio k H / k D will be larger than I O 1 * at T = 0 K. Quantum-mechanical tunneling has long been believed to make a dominant contribution to the rates of many chemical reactions at low temperatures.5,6 However, it is only relatively recently that convincing experimental evidence of this behavior has begun to appear. In most ~ t u d i e s , ~ ' ~ the evidence that the reactions considered proceeded mainly by tunneling consisted of (1) pronounced curvature in Arrhenius plots, (2) anomalously small activation energies, and (3) anomalously large10,'4 0002-7863/80/1502-2325$01.00/0
90 citations
TL;DR: In this paper, the temperature dependence of the F+HCl, HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373°K was studied.
Abstract: A laser photolysis/infrared fluorescence technique has been used to study the temperature dependence of the F+HCl, F+HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373 °K. Multiphoton dissociation of SF6 with a TEA CO2 laser was used to provide a transient concentration of fluorine atoms. Their subsequent reaction with hydrogen donors was monitored by observing the appearance rate of HF(v?1) through its infrared fluorescence. The observed rate constants show a marked nonlinear Arrhenius behavior which is consistent with a model in which reaction takes place both vai direct attack by the fluorine atom at the hydrogen end of HX and via formation of a collision complex FXH which rearranges to FHX and then dissociates to HF and X products.
77 citations
70 citations
TL;DR: In this paper, the barrier height and transition state geometry for the reaction CH4+O(3P) → CH3+OH using POL-CI wave functions with a valence double zeta plus polarization basis set were calculated.
Abstract: The barrier height and transition state geometry have been calculated for the reaction CH4+O(3P) →CH3+OH using POL–CI wave functions with a valence double zeta plus polarization basis set. The saddle point geometry is found to be of C3v symmetry with the CH and OH bonds stretched by ∼0.27 A (25%) and ∼0.21 A (21%), respectively. The CH bond lengths of the CH3 group change only very slightly (<0.01 A) during the reaction, while at the transition state each CH bond is ∼13.1° out of the plane perpendicular to the CHO axis and containing the carbon atom (19.5° for CH4 and 0.0° for CH3). The calculated barrier height is 14.4 kcal/mole using a [3s2p1d/2s1p] basis set. From comparison to similiar calculations for O(3P)+H2→OH+H the basis set error in ΔEb is estimated to be ∼2.4 kcal/mole leading to a predicted barrier height of ∼12.0 kcal/mole. Including zero point corrections leads to estimated activation energies of 10.3 kcal/mole (3A′) and 10.1 kcal/mole (3A″) as compared to experimentally derived activation e...
TL;DR: In this paper, the H/D isotope effect on the catalytic production of hydrogen from water using either Cr/sup 2 +/ or (CH/sub 3/)/sub 2/COH radical as reductants was studied.
Abstract: The H/D isotope effect on the catalytic production of hydrogen from water using either Cr/sup 2 +/ or (CH/sub 3/)/sub 2/COH radical as reductants was studied. The catalysts used were gold sols stabilized by either citrate or poly(vinyl sulfate). Similar isotope effects (in the range of 5 to 6.7) were measured for the various reductants and stabilizers. Comparison of this isotope effect to the one measured for the production of hydrogen on gold cathodes leads to the conclusion that similar mechanisms operate in all three systems. It was also observed that hydrogen abstraction from the relatively stable supports, when adsorbed on the colloid, occurs. This leads to the conclusion the hydrogen atoms adsorbed on the particles can either proceed to produced hydrogen molecules by the catalytic reaction or react via other competing pathways.
TL;DR: In this article, a cross molecular beam experiment was performed to study the O(1D2)+CH4 reaction and the results showed that hydrogen atom elimination reaction greatly exceeds molecular hydrogen elimination.
Abstract: A cross molecular beam experiment was performed to study the O(1D2)+CH4 reaction. The results show that hydrogen atom elimination reaction greatly exceeds molecular hydrogen elimination. (AIP)
TL;DR: The authors showed conclusively that the primary photochemical act involves hydrogen abstraction from benzhydrol by the n,π* uncomplexed triplet state of benzophenone to give a triplet radical pair which does not couple to a measurable extent within the solvent cage.
TL;DR: In this article, a study of several photoprocesses which generate reducing radicals in similar photoelectrochemical cells was reported, which can occur for a number of electron transfers and hydrogen abstractions in what appears to be a fairly general process.
Abstract: A study of several photoprocesses which generate reducing radicals in similar photoelectrochemical cells was reported. Coupling of a light-induced reaction to produce a photocurrent concurrent with hydrogen generation in a second compartment can occur for a number of electron transfers and hydrogen abstractions in what appears to be a fairly general process. Irradiation of the RuL/sub 3//sup +2//Et/sub 3/N: photoanode compartment leads to production of a photocurrent together with generation of hydrogen at the cathode. A rather different type of reaction that also results in formation of two reducing radicals as primary photoproducts if the photoreduction of ketones and H-heteroaromatics by alcohols and other hydrogen atom donors. Irradiation of benzophenone/2-propanol/MV/sup +2/ solutions in the photoanode compartment (intensity 1.4 x 10/sup -8/ einstein/s) leads to a buildup of moderate levels of MV/sup +/ and to a steady photocurrent of 320 ..mu..A. The MV/sup +/ is oxidized at the anode of the photolyzed compartment with concomitant reduction of H/sup +/ in the cathode compartment. There was no decrease in benzophenone concentration over moderate periods of irradiation, and a steady production of hydrogen in the cathode compartment was observed. The photocurrent produced was linear with the square of absorbed light intensity. The quantum more » efficiency at the above-indicated intensity is 22%; quantitative analysis of the hydrogen produced gives good agreement with this value. 1 figure, 1 table. (DP) « less
TL;DR: In this article, the effects of acidity, temperature, self-quenching and H-donor concentration on the luminescent state of the aqua-uranyl (VI) ion have been studied in aqueous acidic nitrate and perchlorate solution.
Abstract: The effects of acidity, temperature, self-quenching and H-donor concentration on the luminescent state of the aqua-uranyl (VI) ion have been studied in aqueous acidic nitrate and perchlorate solution. The detailed results cannot be explained by any single simple mechanism such as radiative, nonradiative or spontaneous collisional quenching, or irreversible hydrogen abstraction from water. Quantitative analysis of the results shows a far more complex mechanism, involving the adiabatic formation of the species UO2H2+ and U2O4H4+, as already proposed by the author. This mechanism is supported by state and m.o. correlations. The abstraction of hydrogen from water is shown to take place by H atom transfer in a uranyl–water complex intermediate, rather than by attack of H+ on the fully occupied πu orbitals of uranium (V) in a well-defined uranyl water complex with strong charge transfer character. A qualitative description of the exciplex U2O4H4+ is shown to be possible in a v.b. formalism and the origin of its radiative properties is discussed on this basis.
TL;DR: In this paper, a synchronised mechanism for the thermal disproportionation of 1,2-dihydronaphthalene was proposed. Butler et al. excluded an electrocyclic mechanism.
Abstract: Bei der thermischen Disproportionierung von 1,2-Dihydronaphthalin (1) verlaufen Wasserstoff-abspaltung und -addition als sterisch unspezifische Radikalreaktionen. Ein Synchronmechanismus wurde ausgeschlossen.
Hydrogen Transfer Reactions, Part 2. Radical Mechanism of Thermal Disproportionation of 1,2-Dihydronaphthalene
In the thermal disproportionation of 1,2-dihydronaphthalene (1) both hydrogen abstraction and addition are stereo-unspecific radical reactions. An electrocyclic mechanism was excluded.
TL;DR: In this article, the yields of HO2 from the H+O3 reaction were investigated and the yields suggest that HO2→HO2O(3P) accounts for about 3% of the net reaction.
Abstract: Laser magnetic resonance (LMR) studies of the yields of HO2 from the H+O3 reaction are reported. The yields suggest that H+O3→HO2O(3P), accounts for about 3% of the net reaction. (AIP)
TL;DR: In this paper, the site of radical attack on polypropylene and polystyrene has been elucidated by using a spin trapping technique, and the abstractions of hydrogen atoms by t -butoxy radicals and phenyl radicals have been studied by using spin trapping techniques.
TL;DR: In this article, it was shown that the reactivity of radical sites in biradicals is identical with that of the same sites in monoradicals, and the data from competitive and time-resolved experiments were in excellent agreement.
Abstract: The biradicals generated in the Norrish type II reaction abstract hydrogen atoms from mercaptans and Bu/sub 3/SnH with rate constants of 9 x 10/sup 6/ and 1.1 x 10/sup 6/ M/sup -1/ s/sup -1/, respectively (for the biradical from ..gamma..-methylvalerophenone). The data from competitive and time-resolved experiments are in excellent agreement. The results suggest that the reactivity of radical sites in biradicals is identical with that of the same sites in monoradicals.
TL;DR: The photocyclization of 5-(2-halogenophenyl)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion of solvent participation hydrogen abstraction reactions as discussed by the authors.
Abstract: The photocyclization of 5-(2-halogenophenyl)-1,3-diphenyl pyrazoles proceeds efficiently with the exclusion of solvent participation hydrogen abstraction reactions. In a given solvent, the quantum ...
TL;DR: The ability of 2,2,6,6-tetramethylpiperidine (I) and its nitroxyl (II) to trap radicals involved in hydrocarbon photo-oxidations has been studied in cumene and 1,3,5-trimethylcyclohexane at 27° using AIBN, hydroperoxide and dialkylperoxide as initiators as discussed by the authors.
TL;DR: In this paper, a series of 46 phenylhydrazones, some novel, containing a variety of structural types has been synthesised. And they have been studied by 1H n.m.r. spectroscopy with respect to potential inter- and intra-molecular hydrogen bonding.
Abstract: A series of 46 phenylhydrazones, some novel, containing a variety of structural types has been synthesised. These have been studied by 1H n.m.r. and i.r. spectroscopy with respect to potential inter- and intra-molecular hydrogen bonding. ortho-Nitro or -carbonyl groups form strong hydrogen bonds to the imino-group which are readily observed by 1H n.m.r. but not by i.r. spectroscopy. ‘Terminal’ carbonyl groups also form strong hydrogen bonds to the imino-group and these can be detected by both i.r. and 1H n.m.r. spectroscopy. When both ortho-nitro- or -carbonyl groups and ‘terminal’ carbonyl groups are present the imino hydrogen atom is doubly hydrogen bonded and this is apparent in the 1H n.m.r. spectrum. The formation of an intramolecular hydrogen bond in the phenylhydrazone prevents the formation of an intermolecular hydrogen bond between the phenylhydrazone and a polar solvent.
TL;DR: Single crystals of the disodium salt of uridine-5'-phosphate were irradiated by 4.0-MeV electrons and studied by electron spin resonance spectroscopy in the temperature range 77 to 310 K, suggesting that the radical reaction I ..-->… IV takes place at approximately 130 K and the reaction II ..-->.. III at 150 K.
Abstract: Single crystals of the disodium salt of uridine-5'-phosphate were irradiated by 4.0-MeV electrons and studied by electron spin resonance spectroscopy in the temperature range 77 to 310 K. At 77 K two resonances were observed. The dominating resonance (I) is ascribed to the electron adduct (anion) radical of the uracil base. The main hyperfine interaction observed is a coupling between the unpaired electron and the proton bonded to C/sub 6/. The other resonance (II) exhibits a large g-value anisotropy and two almost isotropic doublet couplings. This resonance is probably due to a secondary alkoxy-type radical formed by a net loss of hydrogen from O/sub 3/' in the sugar moiety. At 270 K three resonances were observed. One (III) is due to a radical formed by hydrogen abstraction from C/sub 5'/ in the sugar moiety, whereas the other two (IV,V) are probably hydrogen addition radicals at the uracil base. Radical IV is the dihydrouracil-5-yl radical formed by a net hydrogen addition to the C/sub 6/ position, whereas radical V tentatively is ascribed to the 6-yl radical. The light-induced conversion V ..-->.. IV took place at 270 K, whereas the reverse heat-induced process was not observed in the temperature range studied. Preliminary more » annealing studies of crystals irradiated at 77 K suggest that the radical reaction I ..-->.. IV takes place at approximately 130 K and the reaction II ..-->.. III at 150 K. « less
TL;DR: In this article, the photolysis of alkyl metallocene derivatives of vanadium and niobium in hydrocarbon solvents results in cleavage of the carbon-metal σ-bond.
TL;DR: In this article, the rate constants for tetraalkylammonium cations were determined by the spectrophotometric pulse radiolysis method for reactions of hydroxyl(OH), sulfate (SO), phosphate (HPO), and chloride radicals (Cl/sub 2//sup -/) with the first four homologues of tetrasalmonium C-H bonds.
Abstract: The rate constants for reactions of hydroxyl(OH), sulfate (SO/sub 4//sup -/), phosphate (HPO/sub 4//sup -/), and chloride radicals (Cl/sub 2//sup -/) with the first four homologues of tetraalkylammonium cations were determined by the spectrophotometric pulse radiolysis method. The reactivity of tetraalkylammonium cations toward these radicals increases with increasing number of C-H bonds in the molecule. Intercomparison of the rate constants for the different radicals shows a trend characteristic for a mechanism involving hydrogen abstraction, e.g., k/sub OH+R/sub 4/N/sup +/ > k/sub SO/sub 4//sup -/+R/sub 4/N/sup +/ > k/sub HPO/sub 4//sup -/+R/sub 4/N/sup +// > k/sub Cl/sub 2//sup -/+R/sub 4/N/sup +/. The ratios of the reactivities are in good agreement with those reported for other aliphatic compounds which undergo hydrogen abstraction. The reactivities of long-chain tetraalkylammonium cations are the sum of the partial reactivities of C-H bonds at different positions in the molecules, indicating that the reaction of OH radicals with these cations involves the abstraction of H atoms in the rate-determining step. The partial reactivity for hydrogen abstraction from carbon atoms adjacent to the nitrogen center is nearly two orders of magnitude less than for comparable groups in aliphatic hydrocarbons. Influence of the nitrogen center extends even to the ..beta..more » position where partial rate constants are a factor approx. 5 less.« less