Showing papers on "Hydrogen atom abstraction published in 1976"
TL;DR: In this paper, the decomposition of Nb(CH3)5, Ta(CH 3)5 and Ta(Ch2C6H5)5 was investigated and it was shown that α-CH(D) is broken in a slow step of decomposition.
127 citations
TL;DR: In this article, the effect of irradiation of alkyl halides with methanol-O-d was studied in terms of initial homolytic cleavage of the carbon-halogen bond to afford a radical pair, which undergoes competing diffusion and hydrogen abstraction to give reduction products and electron transfer to afford an ion pair.
Abstract: Competing ionic and radical photobehavior has been observed for a number of alkyl halides. The proportion of nucleophilic substitution and reduction products were reported for each halide. Solvent effects of ethylene glycol, triethylamine, and methanol and atmospheric effects of nitrogen, oxygen, and air were studied. The effect of irradiation of the halides with methanol-O-d was reported. The results were discussed in terms of initial homolytic cleavage of the carbon-halogen bond to afford a radical pair, which undergoes competing diffusion and hydrogen abstraction to give reduction products and electron transfer to afford an ion pair. The ion pair, in turn, gives rise to the substitution and elimination products. (DDA)
75 citations
TL;DR: In this paper, potential curves for hydrogen bending and hydrogen abstraction are investigated for both, the HCO and COH form of the system, and the dissociation energy of HCO is calculated to be 18.8 kcal mol −1, in good agreement with more recent experimental measurements; the unpaired electron density is shown to be located in a σ-type orbital with significant amount of hydrogen character.
68 citations
TL;DR: In this paper, a scheme of decomposition involving two paths, each with a two stage formation of methanes, was suggested, and strong evidence exists for slow hydrogen exchange between the eyelopentadienyl rings and the methyl groups before significant production of ethane.
51 citations
TL;DR: The cis-isomeric effect in photochemical hydrogen-atom abstraction from 2-methoxy-4-methyltetrahydropyranyl ethers was investigated in this paper.
Abstract: Hydrogen-atom abstraction from 2-methoxy-4-methyltetrahydropyran (1) by photochemically excited benzophenone in benzene gave methyl 3-methyl-5phenylvalerate and 3-methyl-6-valerolactone. The cis-isomer of 1 was 8.0 times as reactive as the trans-isomer, indicating that abstraction of an axial hydrogen atom is more favorable than abstraction of an equatorial hydrogen. This observation is rationalized in terms of antiperiplanar overlap of the carbon-hydrogen bond being broken with nonbonding orbitals on both oxygens. The two isomers of 1 did not interconvert during irradiation, indicating that the initial hydrogen abstraction is irreversible. An Anomeric Effect in Photochemical Hydrogen Abstraction Reactions of Tetrahydropyranyl Ethers 1
46 citations
TL;DR: In this paper, a model potential to describe the interaction of a diatomic molecule with a solid surface has been derived and applied to the case of H 2 interacting with a tungsten (001) surface.
46 citations
TL;DR: In this article, the use of the rearrangement of hex-5-enyl radical intermediates as a mechanistic probe has been examined in the reductive demercurations of 1-hexene.
Abstract: The use of the rearrangement of hex-5-enyl radical intermediates as a mechanistic probe has been examined in the reductive demercurations of hex-5-enyl-l-mercuric bromide (1). Methylcyclopentane and 1-hexene are the major products from reductions of 1 by sodium borohydride, lithium aluminum hydride, and tri-n-butyltin hydride. The formation of methylcyclopentane and the absence of cyclohexane are consistent with a noncage, free-radical chain mechanism for these reductions. The sodium amalgam reduction of 1 produces only 1-hexene. Hex-5-en-l-ol and a peroxide are formed from sodium borohydride reductions of 1 in the presence of molecular oxygen. Isotope effects for transfer of hydrogen to the intermediate hex-5-enyl radicals have been determined from the 1-hexene/methylcyclopentane product ratio, since rearrangement of the 5-hexenyl radical competes with hydrogen abstraction. The magnitude of these isotope effects (1.8 +- 0.2) is the same for reductive demercurations of 1 by sodium borohydride, lithium aluminum hydride, and tri-n-butyltin hydride; these results provide evidence for a common hydrogen-transfer agent, such as hex-5-enyl-1-mercuric hydride, for all of these metal-hydride reductions. The validity of the competing rearrangement method of determining hydrogen isotope effects has been demonstrated by determining the isotope effect for the tri-n-butyltin hydride reduction of 6-bromo-1-hexene (2.8 +- 0.2) at 40/sup 0/C.
46 citations
TL;DR: In this paper, it was shown that the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain and that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K.
39 citations
TL;DR: In this paper, the authors treated hydrogen abstraction reactions of electronically excited carbonyl compounds as radiationless transitions using the tunnel effect theory, and predicted the rates of reaction are controlled by the Franck-Condon factors of the vibrational stretching motions of the carbonyls group in the excited carboxyclic compounds, and the CH oscillator of the substrate with the CO and OH vibrations in the ground state of the final products.
Abstract: Hydrogen abstraction reactions of electronically excited carbonyl compounds are treated as radiationless transitions using the tunnel-effect theory. The rates of reaction are controlled by the Franck–Condon factors of the vibrational stretching motions of the carbonyl group in the excited carbonyl compounds, and the CH oscillator of the substrate with the CO and OH vibrations in the ground state of the final products. The rates are dependent on electronic energy, vibrational frequencies, the reduced mass of the oscillators, the CH bond strength of the reactive bonds of the substrates and bond distances. In cases where a charge-transfer mechanism is involved, the rates are also a function of the carbonyl compound reduction potential and substrate ionization energy. The model gives rates in good agreement with experiment and explains singlet and triplet reactivity of n, π* and π, π* states, inter- and intra-molecular processes, normal and inverse deuterium isotope effects and the variation of the rates with temperature. Rates are predicted for several systems which have not yet been studied experimentally. Results from the present model are compared with those from other theories of photochemical reactivity.
37 citations
TL;DR: In this paper, the authors used flash photolysis coupled with time resolved detection of O via resonance fluorescence to obtain rate constants for the reaction of O(3-P) with H2S at temperatures from 263 to 495 K and at pressures in the range 10-400 torr.
Abstract: The technique of flash photolysis coupled with time resolved detection of O via resonance fluorescence has been used to obtain rate constants for the reaction of O(3-P) with H2S at temperatures from 263 to 495 K and at pressures in the range 10-400 torr. Under conditions where secondary reactions are avoided, the measured rate constants for the primary step obey the Arrhenius equation k = (7.24 plus or minus 1.07) x 10 to the -12th exp(-3300 plus or minus 100/1.987 T) cu cm/molecules/s. Experiments with D2S show that the reaction exhibits a primary isotope effect, in support of a hydrogen abstraction mechanism.
35 citations
TL;DR: In this article, the absolute rate constants for PH3 were determined for the first time over the temperature range 209 to 495 K. The method employed was flash photolysis of dilute mixtures of PH3 in He combined with time resolved detection of H via Lyman-α resonance fluorescence.
Abstract: Absolute rate constants for the reactions of atomic hydrogen with phosphine (PH3) have been determined for the first time over the temperature range 209 to 495 K. The method employed was flash photolysis of dilute mixtures of PH3 in He combined with time resolved detection of H via Lyman‐α resonance fluorescence. A linear least‐squares treatment of the data leads to the expression k= (4.52±0.39) ×10−11 exp (−1470±50)/(1.987T) cm3 molecule−1 s−1. Evidence is presented in support of a hydrogen abstraction mechanism. Implications of these results for the photochemistry of PH3 in the atmosphere of Jupiter are discussed, especially as this relates to the origin of the Great Red Spot.
TL;DR: In this article, the authors investigated the mechanism of the oxidation of alcohols by peroxydisulphate by trapping the nucleophilic carbon-centered free-radical intermediates by protonated quinoline.
TL;DR: In this article, the thermal reaction between tert-butyl perbenzoate and n-alkanes has been studied under conditions as close as possible to those during actual crosslinking of polyethylene.
Abstract: The thermal reaction between tert-butyl perbenzoate and n-alkanes has been studied under conditions as close as possible to those during actual crosslinking of polyethylene. Product analysis allowed a description of the fate of 90–95% of the peroxide fragments. Approximately 50–60% of the substrate radicals, formed upon hydrogen abstraction, were found to combine and give crosslinks, whereas about 30% showed disproportionation, thus leading to substrate unsaturation. The two competing reactions account for 85–90% of the generated substrate radicals. The model system provides a simple and convenient method for comparing the crosslink efficiencies of peroxides; these data are reported for some commercial peroxides. In order to facilitate an extrapolation of the results in liquid alkanes towards solid polyethylene (PE), the influence of substrate viscosity and temperature on the peroxide decomposition rate, product distribution, and crosslink efficiency was studied. In n-alkanes peroxide decomposition follows first-order kinetics well, with an Arrhenius activation energy of 32.9 ± 0.5 kcal/mole. In addition, the kinetics in the model system are compared with those calculated from Rheometer (torque vs. time) measurements on the curing of unfilled PE and ethylene-propylene-dicyclopentadiene terpolymer (EPDM). Only minor differences in peroxide decomposition rate and overall activation energy were found.
TL;DR: In this article, the radicals induced by X-irradiation at 300 K of single crystals of uracil have been analyzed by E.S.R. spectroscopy.
Abstract: The radicals induced by X-irradiation at 300 K of single crystals of uracil have been analysed by E.S.R. spectroscopy. Immediately after irradiation, three radicals are detected: one hydrogen abstraction and two hydrogen addition radicals. The hydrogen abstraction is from N1 and the hydrogen addition occurs at position C5 and at position O4. Upon subsequent illumination with light of wavelength λ > 400 nm, the C5-addition radical transforms into C6-addition where the added hydrogen atom is localized on carbon C6. As in ohter pyrimidines, the β-proton splitting of the C5-addition radical is much smaller than that of the C6-addition radical (35·5 G instead of 48·0 G).
TL;DR: The dominant free radical trapped in x-irradiated single crystals of 3'-cytidylic acid at 77/sup 0/K was identified as HOC/sub 5'/ (H)C/sub 4'/(H) < and the g-tensor and hyperfine coupling tensors for the cap alpha..-hydrogen and two ..beta…-hydrogens were determined.
Abstract: The dominant free radical trapped in x-irradiated single crystals of 3'-cytidylic acid at 77/sup 0/K was identified as HOC/sub 5'/(H)C/sub 4'/(H) <. For this radical the g-tensor and hyperfine coupling tensors for the ..cap alpha..-hydrogen and two ..beta..-hydrogens were determined. An unusually large isotropic coupling (21 G) to the hydroxyl hydrogen is observed as a result of the O-H bond being constrained so as to give a very small torsion angle with the lone electron p-orbital axis. Crystals grown from D/sub 2/O give a collapsed spectrum where the hydroxyl splitting is reduced by a factor of 1/6.5. On warming to approximately 290/sup 0/K the HOC/sub 5'/(H)C/sub 4'/(H) < radical deprotonates yielding the primary room temperature radical -OC/sub 5'/(H)C/sub 4'/(H) <. A reorientation of approximately 40/sup 0/ is postulated to account for the observed directions of the hyperfine and g-tensor axes determined for the room-temperature radical.
TL;DR: In this article, two types of hot atom energy distribution functions have been considered in an application to the 18F+H2 reaction system: (i) a δ function distribution, and (ii) a steady state Maxwellian distribution characterized by a hot atom temperature TA.
Abstract: The general time‐independent collision theory formulation for the bimolecular rate constant has been adapted for the description of hot atom systems. Two types of hot atom energy distribution functions have been considered in an application to the 18F+H2 reaction system: (i) a δ‐function distribution, and (ii) a steady‐state Maxwellian distribution characterized by a hot atom temperature TA. From the time‐independent solution of the Boltzmann equation together with microscopic reactive cross sections determined from quasiclassical trajectory computations, nonthermal 18F+D2 processes. The results showed little sensitivity to the assumed shape of the hot atom energy distribution or to the magnitude of the barrier height along the reaction coordinate. The intermolecular kinetic isotope effect κH2/κD2 provided a sensitive probe of the average energy of hot reaction, suggesting an average 18F laboratory kinetic energy of 50±10 eV for the 18F+H2 process under nuclear recoil conditions.
TL;DR: In this paper, the reaction between electrochemically generated Fenton's reagent and toluene was studied, and a mechanism involving peroxy and alkoxy radicals as intermediates was proposed.
Abstract: The reaction between electrochemically generated Fenton's reagent and toluene was studied. The products arising from this reaction system were consistent with a primary hydrogen atom abstraction from the methyl group of toluene to give benzyl radicals. Benzaldehyde was obtained with a current yield higher than 60%; benzyl alcohol was also produced in smaller quantities in some experiments. The dependence of the yields on reagent concentrations, temperature and coulombs passed was examined. On the basis of the experimental results a mechanism is proposed involving peroxy and alkoxy radicals as intermediates.
TL;DR: In this paper, the authors examined polyurethanes based on diphenylmethane 4,4′-diisocyanate (MDI) by electronic spectroscopy and by continuous and laser flash photolysis.
Abstract: Polyurethanes based on diphenylmethane 4,4′-diisocyanate (MDI) were examined by electronic spectroscopy and by continuous and laser flash photolysis. Fluorescence and phosphorescence emissions were observed, and although the source of the fluorescence could not be identified, the phosphorescence appears to originate from a benzophenone-type oxidation impurity. On laser flash photolysis, transient absorptions were observed at 375 nm and 580 nm. The latter is assigned to a ketyl radical center formed by hydrogen abstraction from the polymer matrix. Participation of these species in the photochemical reactions leading to deterioration of the MDI-based polyurethane is discussed.
TL;DR: Free radicals formed by the reaction of OH or O/sup -/ radicals with alicyclic compounds containing a peptide group (-CONH-) and structurally related compounds have been studied by the in situ radiolysis-steady state ESR method.
Abstract: Free radicals formed by the reaction of OH or O/sup -/ radicals with alicyclic compounds containing a peptide group (-CONH-) and structurally related compounds have been studied by the in situ radiolysis-steady state ESR method. Eight cyclic peptide radicals resulting from hydrogen abstraction from the C-H bond adjacent to the peptide group have been observed. They are divided into two groups on the basis of the skeletal structure of the radicals: greater than C-NH-CO- and greater than C-CO-NH-. In basic solutions significant changes occur in the ESR parameters of these radicals which can be interpreted in terms of the dissociation of the NH proton in a peptide group. The pK/sub a/ values for the NH proton dissociation have been determined to be in the range of 7.6 to 13.6 for eight cyclic peptide radicals from 5-methyl-2-pyrrolidinone, 2-pyrrolidinone, 2-pyrrolidone-5-carboxylic acid, hydantoin (for both first and second NH proton dissociations), 1-methylhydantoin, 2-thiohydantoin (second dissociation only), succinimide, and 2,5-piperazinedione (first dissociation only) and 10.9 for a related cyclic radical from 2-oxazolidone. These pK/sub a/ values are considerably lower than those for corresponding linear peptide radicals, partly because ..pi..-electron density on the nitrogen atom decreases in cyclic peptide radicals with a more delocalized ..pi..-electronmore » system. Within each group of cyclicpeptide radicals with greater than C-NH-CO- or greater than C-CO-NH- the same trend of changes in ESR parameters was observed upon the dissociation of peptide proton.« less
TL;DR: The triplet excited states of 1,4-disubstituted anthraquinones containing NH2, NHMe, NHAr and OH groups have been studied in benzene using pulse radiolysis and flash photolysis as discussed by the authors.
Abstract: The triplet excited states of 1,4-disubstituted anthraquinones containing NH2, NHMe, NHAr and OH groups have been studied in benzene using pulse radiolysis and flash photolysis. Triplet-triplet absorption spectra, extinction coefficients and triplet energy levels were determined by energy transfer techniques. The ET values lie in the range 100–176 kJ mol–1. Triplet lifetimes (t½∼ 3 µs) are typical of substituted anthraquinone compounds, the rapid decay being attributed to reversible hydrogen abstraction across the intramolecular hydrogen bond.Comparison of the concentrations of biphenyl triplet acting as donor and the loss of singlet dye as acceptor in the energy transfer experiments suggests that these quinones are in an associated state in benzene solutions at concentrations as low as 10–5–10–7 mol dm–3.
TL;DR: The reaction of OH radicals and H atoms with ribose-5-phosphate in deoxygenated aqueous solution at room temperature leads to the following dephosphorylation products (G-values): ribo-pentodialdose 1, 2, 3 and 4.
Abstract: The reaction of OH radicals and H atoms with ribose-5-phosphate (10(-2) M) in deoxygenated aqueous solution at room temperature (dose-rate 2-1 X 10(17) eV/ml-min, dose 5 X 10(18)-15 X 10(18) eV/ml) leads to the following dephosphorylation products (G-values): ribo-pentodialdose 1 (0-2), 2-hydroxy-4-oxoglutaraldehyde 2 (0-06), 5-deoxy-erythro-pentos-4-ulose 3 (0-1) and 3-oxoglutaraldehyde 4 (0-06). In addition, some minor phosphate free products (total G=0-09) are formed. G(inorganic phosphate) =1-3 and G(H2O2)=0-3. On the addition of 10(-3) M (Fe(III) ions, G (1) and G (3) increase to 0-6 and 0-4 respectively. In the presence of 10(-3) M Fe(II), G(1) and G(3) change to 0-4 and 0-8, respectively. The other dephosphorylation products are suppressed by the iron ions. G(1) also increases on the addition of increasing amounts of H2O2. Each product can be assigned a precursor radical formed by hydrogen abstraction from C-5, C-4 or C-3 of the ribose-5-phosphate molecule. Products 1 and 2 are formed by oxydative dephosphorylation of an alpha-phospho radical with preceeding H2O elimination for product 2. Elimination of H3PO4 from a beta-phospho radical leads to product 3; product 4 is formed by elimination of two molecules of H2O from its precursor radical and hydrolytic cleavage of an enol phosphate bond. Deuterium-labelling experiments and the effects of the iron ions and of H2O2 support the mechanisms proposed. The importance of the dephosphorylation mechanisms for the formation of strand breaks in DNA is discussed with special reference to the effects of the radiosensitizers.
TL;DR: Several alkyl substituted oxiranyls derived by hydrogen abstraction from oxiranes are observed in solution by ESR-spectroscopy as mentioned in this paper. But the activation parameters of these processes are given for several cases and are related to radical structure.
Abstract: Several alkyl substituted oxiranyl radicals derived by hydrogen abstraction from oxiranes are observed in solution by ESR.-spectroscopy. The ESR.-spectra demonstrate that oxiranyl radicals have pyramidal configurations at the radical carbon atom and undergo inversion. Alkyl substituted oxiranyls rearrange by ring opening to α-keto alkyl radicals. The rates of inversion decrease and the rates of rearrangement increase with alkyl substitution. The activation parameters of these processes are given for several cases and are related to radical structure. Line broadening effects caused by inversion allow the determination of relative signs of γ-CH3-coupling constants.
TL;DR: Aryl(triarylvinyl)iminyls readily cyclise to quinolines while isopentyl-, butyl-, and propyl-(phenyl) iminyls yield tetralones by intramolecular hydrogen abstraction followed by cyclisation of the ensuing alkyl radicals as discussed by the authors.
Abstract: Aryl(triarylvinyl)iminyls readily cyclise to quinolines while isopentyl-, butyl-, and propyl-(phenyl) iminyls yield tetralones by intramolecular hydrogen abstraction followed by cyclisation of the ensuing alkyl radicals.
TL;DR: In this article, the Arrhenius parameters for the intramolecular hydrogen abstraction from the excited triplet state have been measured for several alkyl ketones, and it was concluded that most of the biradical reactions are controlled by rotation around CC bonds.
TL;DR: In this paper, the photochemistry of unsaturated carbonyl compounds of the 3,4-dihydro-ionone series has been studied, with special attention to the investigation of oxetane formation versus hydrogen abstraction.
Abstract: Compounds of the 3,4-dihydro-ionone series as models for the photochemistry of γ, δ- and δ,ϵ- unsaturated ketones and aldehydes.
The photochemistry of γ, δ- and δ,ϵ-unsaturated carbonyl compounds of the dihydro-ionone series has been studied, with special attention to the investigation of oxetane formation versus hydrogen abstraction.
UV.-irradiation of the dihydro-β-ionone compounds with structure A (1, 7, 14, 18, 24, 29) led to isomeric ethers with structures B (2, 8, 15, 19, 25, 30), C (3, 9, 16, 20, 26, 31) and D (4, 21, 27), isomeric bicyclic alcohols with structure E (5, 10, 17, 22, 28), and photoreduction products with structure F (6, 11, 12, 13). Photolysis of dihydro-γ-ionone (32) gave a complex mixture containing fragmentation product 35, hydrocarbon 36, β-ambrinol (34), oxetane 33, as well as dihydro-β-ionone (1) and three of its photoproducts (2, 3, 5). The dihydro-α-ionone compounds 37 and 40 gave mixtures of fragmentation products and the oxetanes 38 and 41. Irradiation of the side-chain homologues 42 and 45 yielded 43, which photo-cyclizes to 44. In contrast, 3, 4-dihydro-3′,4′-dehydro-β-ionone (46) gave merely the isomeric open-chain triene-ketone 47.
The structures assigned to the ethers 2, 3, 33, 38 and to the alcohols 5, 10, 13 could be confirmed by chemical reactions and mutual interconversions. The structure of the ether 21 had to be established by X-ray analysis, details of which are described.
A novel intramolecular hydrogen transfer is involved in formation of ethers B. The photocyclization A D probably proceeds by addition of the carbonyl-C atom to the double bond (A h), followed by methyl (1 2)-shift (h i). Process A h may also be involved in formation of compounds of type C and E.
TL;DR: In this paper, single crystals of the sulphur-containing DNA-base analogue 6-methylmercaptopurine (6MeMP) and its riboside 6MeMPR have been prepared and irradiated by 4·0 MeV electrons at 77 K. The principal values of the two α-proton hyperfine coupling tensors and the g-tensors were almost the same for both molecules and for 6MeMP were: α1, -28·8, -17·9 and -6·4 G; α2, −28
Abstract: Single crystals of the sulphur-containing DNA-base analogue 6-methylmercaptopurine (6MeMP) and its riboside 6-methylmercaptopurine riboside (6MeMPR) have been prepared and irradiated by 4·0 MeV electrons at 77 K. Electron spin resonance techniques have been used to study the radiation-induced radicals at 77 K. The primary carbon-centred radical, common to both molecules, has been identified as a species formed by hydrogen atom abstraction from the methyl group. The principal values of the two α-proton hyperfine coupling tensors and the g-tensors were almost the same for both molecules and for 6MeMPR were: α1, -28·8, -17·9 and -6·4 G; α2, -28·2, -15·7 and -9·8 G; and g, 2·0063, 2·0024 and 2·0018. These data indicate a spin density of 0·77 on the methyl carbon atom. Molecular orbitals determined from CNDO/2 methods were used in calculations of the directions and magnitude of the g-tensor principal values. Comparison of these calculated values and experimental data suggests that contribution of spin density ...
TL;DR: In this paper, it was shown that the direction of cyclization is controlled by the degree of p-orbital overlap, but not by stability of the developing products, in the transition state.
Abstract: Model compounds of Δ4,5-, Δ5,6-, and Δ6,7-alkenylnitrosamines were synthesized and photolyzed to generate the corresponding alkenylaminium radicals. The Δ5,6-alkenylaminium radicals underwent efficient intramolecular addition leading to the exclusive formation of pyrrolidine derivatives, the yield of which was lower if the steric strain and hindrance were severe in the cyclization transition state. The Δ6,7-alkenylaminium radical cyclized inefficiently to piperidine derivatives and the Δ4,5-alkenylaminium radicals did not appear to cyclize; in both cases other pathways having lower activation energies, such as elimination and hydrogen abstraction, predominated. Evidence that the direction of cyclization is controlled by the degree of p-orbital overlap, but not by stability of the developing products, in the transition state is presented. The stereochemical factors associated with the overlap determined the yields of the cyclization products. This is in excellent agreement with the fact that the transition...
TL;DR: In this paper, it has been demonstrated that various dyes active towards photodegradation of cellulose are also efficient sensitizers of singlet-oxygen, and it is concluded that singletoxygen plays a significant role in the degradation of vat-dyed cotton fabrics.