Showing papers on "Hydrogen atom abstraction published in 1992"
TL;DR: Using self-consistent field molecular-orbital theory, it is shown that the interaction of hydrogen molecules with a Ni{sup +} ion is characteristically different from that with a neutral Ni atom.
Abstract: Using self-consistent field molecular-orbital theory, we show that the interaction of hydrogen molecules with a ${\mathrm{Ni}}^{+}$ ion is characteristically different from that with a neutral Ni atom. While hydrogen chemisorbs dissociatively on the neutral metal atom, it is bound to the cation in its molecular form. The atomic bonding is a consequence of the Pauli exclusion principle whereas the bonding of the molecular hydrogen results from an electrostatic interaction. We predict that a ${\mathrm{Ni}}^{+}$ ion can bind at least six hydrogen molecules.
211 citations
TL;DR: Thioperoxyl radicals, RSOO.
142 citations
TL;DR: The surface-induced dissociation (SID) as discussed by the authors showed that the fluorinated surface is a particularly hard surface, being effective at transferring projectile translational energy into internal energy.
125 citations
TL;DR: In this article, the interaction of atomic hydrogen with halogen-terminated Si(100) surfaces was studied by Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) Mass Spectroscopy.
Abstract: : The interaction of atomic hydrogen with halogen-terminated Si(100) surfaces was studied by Auger electron spectroscopy (AES) and temperature programmed desorption (TPD) mass spectroscopy. Efficient removal of surface halogen has been observed when the halogen-terminated S1(100) surface was exposed to atomic hydrogen at a substrate temperature, 630 K. The reaction rate constants for halogen extraction on the S1(100) surface follow the trend kI greater than kBr greater than kCl. In addition, the halogen extraction kinetics are found to be first order in both the surface coverage of halogen and in the atomic hydrogen flux. Studies of the temperature dependence of the halogen extraction rate show the activation energies for the extraction of Cl and Br are 2.1 and 1.6 kcal mol-1, respectively. The extremely low activation energy for the reaction demonstrates that the H-extraction process follows an Eley-Rideal reaction mechanism where the surface reaction is mainly driven by the high internal energy of incident atomic hydrogen instead of thermal excitation from the S1(100) solid surface.
104 citations
TL;DR: The diastereofacial selectivity in reactions of a series of alkyl-substituted cyclohexyl radicals has been investigated in this article, and it has been found that only substituents bound at the olefinic center being attacked by the radical influence the equatorial-axial selectivity.
Abstract: The diastereofacial selectivity in reactions of a series of alkyl-substituted cyclohexyl radicals has been investigated. In additions of cyclohexyl radicals to alkenes, it has been found that only substituents bound at the olefinic center being attacked by the radical influence the equatorial-axial selectivity. Substituents bound to the radical center or axial substituents p to the radical center lead to increased axial attack. Equatorial /3-substituents or axial ?-substituents increase the amount of equatorial attack. The same trends are observed for halogen and hydrogen abstraction reactions; the amount of axial reaction product is usually somewhat higher than in the addition reactions. The stereoselectivities can be explained with steric and torsional effects very similar to those suggested for nucleophilic addition reactions to cyclohexanones. A MM2 force field has been parameterized to gain further insight into the stereochemistry of the reaction.
91 citations
TL;DR: In this paper, the amount and nature of hydrogen uptake by a Li atom in neutral as well as in ionic form was investigated in both ionic and ionic forms, and it was shown that while the neutral Li atom cannot bind more than one hydrogen atom, the cation can bind a large number of hydrogen atoms in molecular form.
Abstract: Quantum-chemical calculations including correlations have been carried out to study the amount and nature of hydrogen uptake by a Li atom in neutral as well as in ionic form. We have found that while the neutral Li atom cannot bind more than one hydrogen atom, the cation can bind a large number of hydrogen atoms in molecular form. This contrasting behavior originates from the fact that the bonding of hydrogen with neutral Li is covalent while that with the cation is primarily caused by polarization. The bonding of hydrogen with the Li− ion exhibits dual nature. The first H2 molecule binds to the anion dissociatively while the subsequent H2 molecule can only exist in a "physisorbed" state.
78 citations
TL;DR: In this paper, the tropospheric degradation of proposed substitutes for ozone depleting CFCs were obtained by conducting photochemical oxidation studies of HCFCs and HFCs using long path Fourier transform infrared spectroscopy.
Abstract: Data on the tropospheric degradation of proposed substitutes for ozone depleting CFCs were obtained by conducting photochemical oxidation studies of HCFCs and HFCs using long path Fourier transform infrared spectroscopy. The hydrogen abstraction reactions were initiated using Cl radicals rather than OH radicals because of the rather unreactive nature of the compounds. The experimental product yields at T = 25 ± 3°C and 700 Torr of dry air were: CHClF2 (1.11 ± 0.06 C(O)F2); CClFHCF3 (1.00 ± 0.04 CF3C(O)F); CF3CHF2 (1.09 ± 0.05 C(O)F2); CClF2CH3 (0.98 ± 0.03 C(O)F2); CHF2CH3 (1.00 ± 0.05 C(O)F2); CF3CH2F (0.16 ± 0.03 CF3CF(O), and 0.83 ± 0.22 HFC(O)), where all standard deviations are 2σ. For each compound, the critical step in determining the oxidation products was the decomposition of a halogenated alkoxy radical. For HCFC-22 and HCFC-124, the major alkoxy radical decomposition route was Cl elimination. The HFC-125 product data were consistent with CC cleavage of a two carbon alkoxy radical as the major decomposition route whereas both CC cleavage and H abstraction by O2 were significant contributors to the decomposition of the HFC-134a alkoxy radical. Secondary Cl reactions in the HCFC-142b and HFC-152a experiments prevented an unambiguous determination of the decomposition modes; the data are consistent with both CC bond scission and Cl reactions with halogenated aldehydes producing the oxidation product C(O)F2. With the exception of the HFC-134a and HFC-125 data, the proposed mechanisms can account for the major oxidation products. For HFC-134a and HFC-125, a number of product bands could not be identified. The bands are likely due to products from reactions involving the CF3O2 radical. © John Wiley & Sons, Inc.
77 citations
TL;DR: In this article, products and mechanisms of the reaction between the nitrate radical (NO3) and three of the most abundant reduced organic sulphur compounds in the atmosphere (CH3SCH3, CH3SH and CH3SSCH3), have been studied in a 480 L reaction chamber using in situ FT-IR and ion chromatography as analytical techniques.
Abstract: Products and mechanisms of the reaction between the nitrate radical (NO3) and three of the most abundant reduced organic sulphur compounds in the atmosphere (CH3SCH3, CH3SH and CH3SSCH3), have been studied in a 480 L reaction chamber using in situ FT-IR and ion chromatography as analytical techniques. In the three reactions, methanesulphonic acid was found to be the most abundant sulphur containing product. In addition the stable products SO2, H2SO4, CH2O, and CH3ONO2 were identified and quantified and thionitric acid-S-methyl ester (CH3SNO2) was observed in the i.r. spectrum from all of the three reactions. Deuterated dimethylsulphide (CD3SCD3) showed an isotope effect on the reaction Deuterated dimethylsulphide (CD3SCD3) showed an isotope effect on the reaction rate constant (kH/kD) of 3.8±0.6, indicating that hydrogen abstraction is the first step in the NO3+CH3SCH3 reaction, probably after the formation of an inital adduct. Based on the products and intermediates identified, reaction mechanisms are proposed for the three reactions.
74 citations
TL;DR: In this paper, the reactions of α-tocopherol with various radicals and ketone triplets in solution were studied using laser flash photolysis and competitive techniques, and the results showed that the deuterium lunetic isotope effects are frequently very small, reflecting the expected lack of selectivity of fast reactions.
Abstract: Laser flash photolysis and competitive techniques have been employed to study the reactions of α-tocopherol with various radicals and ketone triplets in solution. For example benzophenone triplets abstract hydrogen with rate constants of 5.1×10 9 and 3.7×10 9 M -1 s -1 in benzene and benzene/1.3 M methanol. Similar near-diffusion-controlled values were obtained for several other ketone triplets, as well as tert-butoxyl and 4-methoxybenzoyloxyl radicals. Deuterium lunetic isotope effects are frequently very small, reflecting the expected lack of selectivity of fast reactions
68 citations
TL;DR: In this article, the hydrogen atom abstraction reaction of triplet benzophenone from 2-propanol and 1-4-cyclohexadiene in supercritical CO 2 was studied.
Abstract: Laser flash photolysis studies of the hydrogen atom abstraction reaction of triplet benzophenone from 2-propanol and 1,4-cyclohexadiene in supercritical CO 2 reveal unusual pressure effects on absolute rate constants. Monitoring reactivity close to the critical temperature (T c ) revealed that bimolecular rate constants increase sharply with a decrease in pressure, approaching the critical point. Kinetic investigations along an isotherm further removed from T c and predictive calculations on the pressure effect expected in supercritical CO 2 indicate that enhanced reactivity is due to local substrate clustering
61 citations
TL;DR: In this paper, a unique view of the nascent acid-base reaction between 2naphthol and ammonia along the proton transfer coordinate is provided by analyses of the rotationally resolved S1←S0 electronic spectra of their hydrogen bonded complexes cis and trans-2HNA in the gas phase.
Abstract: A unique view of the nascent acid‐base reaction between 2‐naphthol and ammonia along the proton transfer coordinate is provided by analyses of the rotationally resolved S1←S0 electronic spectra of their hydrogen bonded complexes cis‐ and trans‐2HNA in the gas phase. Both complexes, in both electronic states, have structures in which ammonia, acting as a base, forms an in‐plane hydrogen bond with the hydroxy hydrogen atom of 2‐naphthol. The ground state O–H⋅⋅⋅N heavy atom separations are R=2.77 A in cis‐2HNA and R=2.79 A in trans‐2HNA. Electronic excitation of the significantly more acidic S1 state of 2‐naphthol produces large decreases in R in both complexes. S1 cis‐2HNA has R=2.62 A and S1 trans‐2HNA has R=2.57 A. Comparing these results to the Lippincott–Schroeder potential for the hydrogen bond shows that there is little change in the vibrationally averaged position of the hydroxy hydrogen atom. But decreasing R produces significant decreases in the barrier to proton transfer, in the distance from reac...
TL;DR: In this article, a homolytic mechanism involving oxidation of α-benzylmzalonate (1) to the corresponding malonyl radical, its addition to a triple bond, and intramolecular homolyty aromatic substitution of the vinyl radical adducts, is suggested.
Abstract: Oxidation of diethy α-benzylmzalonate (1) by Mn(III) acetate in acetic acid at 70 o C in the presence of alkynes 2a-j leads to dihydronaphalene derivatives 3a-j in moderate to good yields. A homolytic mechanism, involving oxidation of 1 to the corresponding malonyl radical, its addition to a triple bond, and intramolecular homolytic aromatic substitution of the vinyl radical adducts, is suggest. Propargylic hydrogen abstraction and dimerization at the malonic position for less reactive alkynes are the main side reactions observed
TL;DR: In aqueous medium, at 105-115 °C, SO4-˙(generated from S2O82−) was found to abstract a hydrogen atom from methane and ethane to form the corresponding alkyl radicals which could be trapped efficiently by carbon monoxide, the resultant acyl radicals being ultimately converted into the homologous carboxylic acids as discussed by the authors.
Abstract: In aqueous medium, at 105–115 °C, SO4-˙(generated from S2O82–) was found to abstract a hydrogen atom from methane and ethane to form the corresponding alkyl radicals which could be trapped efficiently by carbon monoxide, the resultant acyl radicals being ultimately converted into the homologous carboxylic acids.
TL;DR: The chemistry of DNA cleavage by esperamicins A 1, C, D, and E (ep A, C-E) has been examined in this article, where high-resolution gel electrophoresis reveals that esp A, a known single-strand cleaver, affords fragmentation products consistent with exclusive 5'-hydrogen abstraction.
Abstract: The chemistry of DNA cleavage by the esperamicins A 1 , C, D, and E (ep A, C-E) has been examined. High-resolution gel electrophoresis reveals that esp A, a known single-strand cleaver, affords fragmentation products consistent with exclusive 5'-hydrogen abstraction. In contrast, ep C-E, analogs that produce significant double-strand cleavage, generate fragmentation products consistent with both 5'- and 4'-hydrogen abstraction
TL;DR: In this article, the process of hydrogen abstraction from methanol and ethanol has been calculated with ab initio quantum chemical methods with extended basis sets (6-311G) and with the inclusion of correlation up to MP4SDQ.
Abstract: The process of hydrogen abstraction from methanol and ethanol has been calculated with ab initio quantum chemical methods with extended basis sets (6-311G) and with the inclusion of correlation up to MP4SDQ. These studies serve as a model of such processes in large molecules of biological importance including the sugar moiety of DNA. A comparison of geometries of ground and transition states optimized at UHF and MP2 levels with the 6-31G basis set shows that the UHF optimized geometries have lower energies at the highest level of theory used (MP4SDQ/6-311G). The transition states occur at a somewhat later stage along the reaction coordinate at the UHF level than at the MP2 level. Energy barriers, along with zero-point energies, were used to calculate the rate constants for H-abstraction from C{sub 1} of methanol and ethanol. Tunneling corrections were applied according to an Eckart treatment of an unsymmetrical unidimensional barrier. The corrected rate constants are in very good agreement with experiment over a wide range of temperatures. The same approach was used to predict the rate constant for the abstraction of the hydrogen from C{sub {beta}} of ethanol, which is not known from experimental measurements. The calculated C-H bond strengths and heatsmore » of reactions are also in good agreement when the correlation energy is scaled according to the MPnSAC approach. The geometric and energetic parameters of the transition states behave according to Hammond`s postulate, i.e., the more exothermic the H-abstraction, the closer is the transition state to the reactants. This relationship suggests that the C-H bond strength is one of the major factors that determine the barrier to H-abstraction. An analysis of the MCSCF wave function constructed from a CAS of three electrons distributed in three orbitals ({sigma}{sub CH}, {sigma}{sub CH}, and the orbital containing the unpaired electron) supports this conclusion. 38 refs., 4 figs., 6 tabs.« less
TL;DR: In this paper, it was shown that acyclic radicals 3a,b exist in preferred conformations I and II and the stereoselectivity of the hydrogen abstraction can be explained by the attack anti to the Bu t group of conformers.
TL;DR: In this article, anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy.
Abstract: Anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy. In addition to several radical pairs, five monoradicals were formed and stabilized at this temperature. It is demonstrated that the dominating radical pair species analyzed previously consists of two radicals, each formed by net hydrogen abstraction from the methyl group. The five monoradicals were identified to be the O4-protonated thymine anion, the thymine-7-yl radical formed by net hydrogen addition to C6, and the 1-yl radical formed by net hydrogen abstraction from N1. Upon thermal annealing, the protonated anion decays before 60 K apparently with no successor, and the 6-yl radical. Possible mechanisms for the formation of these radicals are discussed. These involve processes initiated by primary ionization followed by charge neutralization by protonation/deprotonation or by geminate recombination leading to the formation of hydrogen atoms by homolytic bond scission from molecules in (super) excited states. 23 refs., 5 figs., 3 tabs.
TL;DR: The reaction of cis-Pt(1-C 10 H 6 NMe 2 -8-C,N) 2 with Me 2 SnX 2 (X=Cl, Br) in C 6 H 6 or a mixture of CH 2 Cl 6 and t-BuOH affords the novel Pt(IV) hydride complexes PtX(H) by a process involving intramolecular H abstraction from an SnMe grouping as mentioned in this paper.
Abstract: The reaction of cis-Pt(1-C 10 H 6 NMe 2 -8-C,N) 2 with Me 2 SnX 2 (X=Cl, Br) in C 6 H 6 or a mixture of CH 2 Cl 6 and t-BuOH affords the novel Pt(IV) hydride complexes PtX(H)(1-C 10 H 6 NMe 2 -8-C,N) 2 (X =Cl, Br) by a process involving intramolecular H abstraction from an SnMe grouping.
TL;DR: In this article, free-radical reactions of α,β-epoxy ketones with tributyltin hydride have been studied under both photochemical and thermal conditions.
Abstract: Free-radical reactions of α,β-epoxy ketones with tributyltin hydride have been studied. Thes substances were selectively converted to β-hydroxy ketones under both photochemical and thermal conditions. The photoreaction is initiated by hydrogen abstraction of an epoxy ketone triplet from tributyltin hydride, while azoisobutyronitrile is used as an initiator for the thermal reaction
TL;DR: In this paper, the second-order rate constants of benzyl alcohol oxidation and the potential values of the ruthenium(IV)/ ruthensium(II1) redox couples of the oxorUThenium complexes or the Hammett u values of substituents of the para-substituted triphenylphosphine ligands were observed.
Abstract: Oxoruthenium complexes which contain para-substituted triphenylphosphine ligands cleanly oxidize para-substituted benzyl alcohols to the corresponding benzaldehydes. Linear free energy correlations between the logarithms of the second-order rate constants of benzyl alcohol oxidation and either the potential values of the ruthenium(IV)/ ruthenium(II1) redox couples of the oxoruthenium complexes or the Hammett u values of the substituents of the para-substituted triphenylphosphine ligands were observed. However, there were no good correlations for plots of log (kx/kH) versus the Hammett substituent constants u, u+, or u- for the oxidation of the para-substituted benzyl alcohols by [R~(bpy)~(O)PPh~](Clo~)~. Notably, the plot of log (kx/k~) - u* versus 6, where u* is the free radical stabilization constant, gave an excellent correlation with p = -0.57 and R2 = 0.99 for the oxidation of the parasubstituted benzyl alcohols by [Ru(bpy)2(O)PPh,] (C10& in methylene chloride. From these and other results, we propose a reaction pathway for the oxidation of benzyl alcohol by oxoruthenium(1V) complexes which involves a partial hydrogen atom abstraction from the benzylic carbon in the rate-determining step.
TL;DR: A mechanism-based model for prediction of acute nitrile toxicity was developed using octanol-water partition coefficients (log P) and estimated rates of alpha-hydrogen atom abstraction as variables and the pharmacokinetics of substrate disposition is discussed in the context of multiple metabolic pathways.
Abstract: A mechanism-based model for prediction of acute nitrile toxicity was developed using octanol-water partition coefficients (log P) and estimated rates of alpha-hydrogen atom abstraction as variables. Relative rates of hydrogen atom abstraction were derived from differences in heats of formation for ground-state and radical geometries and radical ionization potentials. Calculated energies of activation for all potential sites of oxidation for a given nitrile were used to estimate partitioning of metabolites among multiple oxidative pathways. logP and the resulting corrected rate constants for alpha-carbon oxidation were effective variables in an acute toxicity model of structurally diverse nitriles. The pharmacokinetics of substrate disposition is discussed in the context of multiple metabolic pathways. Structure-toxicity relationships are also discussed.
TL;DR: Chiral enolates 5 and 10, generated by radical addition and subsequent reduction, show diastereofacial selectivity during protonation as mentioned in this paper, which is comparable to radical stereoselection.
Abstract: Chiral enolates 5 and 10, generated by radical addition and subsequent reduction, show diastereofacial selectivity during protonation. In the presence of substituted amines, diastereoselectivity is enhanced and becomes comparable to radical stereoselection. Diastereoselectivities up to 99:1 and yields up to 90% are reached.
TL;DR: In this paper, the interception of the biradicals produced by photolysis of α-keto esters was examined in the context of developing new radical cyclization methods.
Abstract: In the context of developing new radical cyclization methods, we examined the interception of the biradicals produced by the photolysis of α-keto esters.
TL;DR: A series of 4-substituted cubyl radicals was generated by bromine atom abstraction from 1-bromo-4-substantuted cubanes as discussed by the authors.
Abstract: A series of 4-substituted cubyl radicals was generated by bromine atom abstraction from 1-bromo-4-substituted cubanes. EPR observations showed that the cubyl radicals abstracted secondary hydrogen atoms from the ethyl groups of triethylsilane and decayed mainly by second-order combination reactions. tert-Butoxyl radicals abstracted hydrogen atoms from cubane at least 26 times more rapidly than from cyclopropane at -90 o C. Electron-withdrawing substituents on cubane greatly reduced this rate. tert-Butoxyl radicals selectively abstracted the cage hydrogens rather than primary methyl hydrogens from methylcubane
TL;DR: In this article, two kinds of spin-adducts are detected and identified by ESR spectroscopy: a tertiary carbon radical and a secondary carbon radical which ere formed by eddition of the spin traps (ND or BNB) to two polymer radicals.
Abstract: Radical intermediates formed by UV irradiation of LDPE in the presence of benzophenone (BP) as a photoinitiator have been trapped by using 2,3,5,6-tetramethylnitrosobenzene (ND) end 2,4,6-tri-tert-butylnitrosobenzene (BNB) as spin-trapping reagents. Two kinds of spin-adducts are detected and identified by ESR spectroscopy: a tertiary carbon radical and a secondary carbon radical which ere formed by eddition of the spin traps (ND or BNB) to two polymer radicals. These intermediates are produced by hydrogen abstraction of the BP excited state from branch points and methylene groups of the LDPE chain
01 Oct 1992
TL;DR: In this paper, a rate constant of 2.3 + 0.3 (+/-0.3) bar was derived for the reaction of phenyl readicals with acetylene under the assumption that reaction R 2a is the dominant product pathway.
Abstract: Hydrogen atom abstraction from phenylacetylene (C6H5-C2H) like its reaction with hydrogen atoms have been studied at elevated temperatures behind reflected shocks. - The unimolecular decomposition of very low initial concentrations (3 - 22 ppm) of phenylacetylene was investigated in the temperature range 1600 to 1900 K by monitoring the temporal H-atom production. - For C6H5-C2H + H, the thermal decomposition of very low concentrations (1 - 3 ppm) of C2H5I served as H-atom source. Atomic resonance absorption spectrometry (ARAS) was used to record simultaneously H-atom and I-atom profiles. The experiments covered the temperature range 1190 to 1530 K. - For both series of experiments, the total pressure was about 2.3 (+/-0.3) bar.
For the unimolecular reaction R1: C6H5-C2H C6H4-C2H + H a rate constant expression of:
was found.
For the bimolecular reaction R2: C6H5-C2H + H products a rate constant of:
was deduced.
From the available thermochemical data two product channels have to be discussed:
Detailed evaluation of existing thermodynamic data enables the calculation of an equilibrium constant for reaction R 2a. From this and under the assumption that reaction R 2a is the dominant product pathway at elevated temperatures, a rate constant expression of
is deduced for the reaction of phenyl readicals with acetylene, which is an important process in sooting flames:
This rate constant agrees within a factor of 2 with the data from recent studies executed by Fahr et al. [1a, 1b].
TL;DR: In this article, N-benzyl α, β-unsaturated γ-oxoamides undergo under irradiation intramolecular hydrogen abstraction and consecutive cyclisation leading to spiranic β-lactams; Extension of the reaction to one spiranic oxapenam is also reported.
TL;DR: In this paper, the intramolecular reaction involving exchange of a hydrogen atom between molecular hydrogen and hydride ligands is studied in the model system cis-[Fe(PH 3 ) 4 H(H 2 )] + by means of the molecular orbital ab initio method.
Abstract: The intramolecular reaction involving exchange of a hydrogen atom between molecular hydrogen and hydride ligands is studied in the model system cis-[Fe(PH 3 ) 4 H(H 2 )] + by means of the molecular orbital ab initio method. Several possible mechanisms are considered for the reaction and compared according to their energetic barriers. The favored mechanism is the so-called open direct transfer, which consists of a single step transfer of the hydrogen atom between the two ligands
TL;DR: In this paper, the initial radical reactions in photoinitiated cross-linking of n-paraffins (lignocerane and dodecane) and tertiary (isooctane and 3-ethylhexane) hydrocarbons as LDPE model compounds were studied using 2,4,6-tert-butylnitrosobenzene (BNB) as a spin trap.
Abstract: The initial radical reactions in the photoinitiated cross-linking of n-paraffins (lignocerane and dodecane) and tertiary (isooctane and 3-ethylhexane) and unsaturated (1-hexene, cis- and tans-3-hexene) hydrocarbons as LDPE model compounds in the presence of benzophenone (BP) as photoinitiator have been studied using 2,4,6-tert-butylnitrosobenzene (BNB) as a spin trap. Several kinds of initial radicals were identified. These photoinitiated radicals, which combine to form cross-links, are of tertiary, secondary and primary carbon types formed by hydrogen abstraction from the model compounds by the excited triplet state of BP