Showing papers on "Hydrogen atom abstraction published in 1990"

The photolysis of potassium peroxodisulphate in aqueous solution in the presence of tert-butanol : a simple actinometer for 254 nm radiation

Abstract: Aqueous potassium peroxodisulphate (0.01 mol dm−3) together with tert-butanol (0.1 mol dm−3) was photolysed with 254 nm light at 20 °C. The peroxodisulphate ion is cleaved, giving rise to sulphate radicals SO4−. These attack the tert-butanol under hydrogen abstraction and hydrogen sulphate formation. In deoxygenated solutions, Φ(H+) = 1.4, Φ(SO42−) = 1.4 and Φ((HOC(CH3)2CH2)2) is approximately 0.7; in the presence of oxygen, Φ(H+) = 1.8. The enhancement of Φ(H+) by oxygen comes about through the production of superoxide via one of the bimolecular decay channels of the tert-butanol-derived peroxyl radical. The superoxide radical reduces S2O82− under liberation of further SO4−. The oxygenated peroxodisulphate-tert-butanol system is shown to constitute a straightforward actinometer for 254 nm light. Some data and observations regarding the tert-butanol-free peroxodisulphate system are also presented.

A product study of the gas-phase reaction of methacrolein with the OH radical in the presence of NOx

Abstract: The gas-phase reaction of methacrolein with the OH radical, in the presence of NOx, was investigated at 298 ± 2 K and atmospheric pressure of air. Hydroxyacetone, methylglyoxal, a peroxyacyl nitrate identified as CH2 C(CH3)C(O)OONO2 (peroxymethacryloyl nitrate), formaldehyde, CO, and CO2 were observed to be the major products. The product yield data for these compounds show that OH radical addition to the >C C< bond accounts for ca. 50% of the overall reaction, with the remaining ca. 50% proceeding via H—atom abstraction from the CHO group. The data suggest that the alkoxy radical formed following the addition of OH to the terminal carbon atom, decomposes primarily to give the formyl radical plus hydroxyacetone. A lower limit ratio of 5: 1 has been estimated for OH radical addition to the terminal carbon atom of the double bond relative to addition on the inner carbon atom.

Ab initio study of the reactions between methane and OH, H, and 3O

Abstract: The primary processes in methane combustion include hydrogen abstraction by radicals to form CH{sub 3}. Ab initio molecular orbital calculations have been used to study three important reactions contributing to the formation of the methyl radical: CH{sub 4} + X {yields} CH{sub 3} + HX, where X = OH, H, and {sup 3}O. Optimized geometries and harmonic vibrational frequencies have been calculated for all reactants, transition states, and products at the HF/6-31G** and UMP2/6-31G** levels. Barriers and heats of reaction have been estimated by fourth-order Moeller-Plesset perturbation theory with spin projection (PMP4(SDTQ)) using the 6-311G** basis set.

Theoretical studies on cytochrome P-450 mediated hydroxylation: a predictive model for hydrogen atom abstractions

Abstract: The semiempirical quantum chemical method, AM1, was used to develop a predictive model for cytochrome P-450 hydrogen abstraction reactions Initially, a general rank-order correlation was observed between the calculated stability of a radical and the tendency of a carbon-hydrogen bond to undergo hydrogen atom abstraction by cytochrome P-450 Of several oxygen radicals studied, the p-nitrosophenoxy radical has the most appropriate transition-state symmetry for use as a model for P-450-mediated hydrogen atom abstractions

The antioxidant activities of phenolic antioxidants in free radical peroxidation of phospholipid membranes

Abstract: Autoxidation of dilinoleoylphosphatidylcholine (DLPC) bilayers photoinitiated by benzophenone takes place by a free radical chain mechanism according to product studies of the cis, trans and trans, trans-9- and -13-linoleate hydroperoxides formed and kinetic studies of the reaction order as a function of light intensity. The absolute rate constant for hydrogen abstraction from DLPC bilayers by peroxyl radicals is found to be 36.1 M−1 s−1 at 37 °C. Preliminary measurements of activities of phenolic antioxidants, α-tocopherol (α-T), 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC), 2,5,7,8-tetramethyl-6-hydroxychroman-2-carboxylate (Trolox), and 2,6-di-tert-butyl-4-methylphenol (BHT) by oxygen uptake studies during inhibition periods using photoinitiation gave uncorrected inhibition rate constants, Kinh, for α-T, PMHC, and Trolox several orders of magnitude lower than observed earlier in chlorobenzene. Three series of phenolic antioxidants, (a) polyalkyl-6-hydroxychromans, (b) polyalkyl-4-methoxyphenols, and (...

Transition structures for hydrogen atom transfers to oxygen. Comparisons of intermolecular and intramolecular processes, and open- and closed-shell systems

Abstract: The transition structures for the intramolecular hydrogen atom abstractions of the butoxy radical, the triplet and the radical cation states of butanal, and the radical cation of butanol, as well as that of the thermal retro-ene reaction of butanal, were located with ab initio molecular orbital calculations. These processes are the rate- and/or product-determining steps in common reactions like the Barton reaction, the Norrish type II photochemical reaction, and the McLafferty rearrangement of radical cations in mass spectrometry. The corresponding intermolecular hydrogen abstractions from methane by the methoxy radical, triplet formaldehyde, formaldehyde radical cation, and the methanol radical cation were located for comparison, using UHF theory and correlation theory corrections at the MP2 level. Differences in activation energy are related to the electronic differences between closed-shell, open-shell, and charged open-shell systems

Free-radical carbonylation. Efficient trapping of carbon monoxide by carbon radicals

Abstract: The authors examined the AIBN-induced radical reaction of n-octyl bromide (1) (Scheme 1) with Bu{sub 3}SnH under CO pressure in hope of trapping of carbon monoxide by octyl radical followed by hydrogen abstraction from tin hydride. When the reaction on a 0.5-mmol scale was conducted under CO pressure, using an autoclave with an inserted glass tube (5 mol % of AIBN, benzene (10 mL), 80{degree}C, 3 h), the desired aldehyde 2 was obtained together with n-octane (3), formed via simple reduction of 1. Surprisingly, this radical/CO trapping sequence proceeds even at 15 atm of CO pressure to give 2 in 38% yield (run 2). Generally, higher pressures of CO resulted in the increase of 2. These results demonstrate that the control of the relative concentrations of CO to tin hydride is an important factor to effect the CO trapping leading to aldehyde 2.

Kinetic and ab initio study of the prooxidant effect of vitamin E. Hydrogen abstraction from fatty acid esters and egg yolk lecithin

Abstract: A kinetic and ab initio study of the prooxidant effect of vitamin E derivatives has been carried out. Rates of hydrogen abstraction from various fatty acid esters and egg yolk lecithin by tocopheroxyl radicals have been determined spectrophotometrically. The second-order rate constant for the hydrogen abstraction dramatically increases as the number of π-electron systems activating the hydrogen (C=C double bonds) increases from zero to two. The rate constants per an active hydrogen for the various fatty acid esters that contain hydrogens activated by two π-electron systems are similar to one another. In order to interpret the observed features of the rate constants, ab initio calculations of models for the fatty acid esters have been carried out. From the calculation results, it is shown that the observed features of the rate constant can be explained in terms of the pseudo-π-conjugation between the C=C double bond and the active hydrogen-carbon bond. The bulkier the substituent groups at the 5- and 7-positions of the tocopheroxyl radical, the smaller the rate constant of the hydrogen abstraction

Examination of the Br+HI, Cl+HI, and F+HI hydrogen abstraction reactions by photoelectron spectroscopy of BrHI−, ClHI−, and FHI−

Abstract: The photoelectron spectra of the ions BrHI−, ClHI−, and FHI−, along with their deuterated counterparts, are presented. These spectra provide information on the transition state region of the potential energy surfaces describing the exothermic neutral reactions X+HI→HX+I(X=Br, Cl, F). Vibrational structure is observed in the BrHI− and ClHI− spectra that corresponds to hydrogen atom motion in the dissociating neutral complex. Transitions to electronically excited potential energy surfaces that correlate to HX+I(2P3/2,2P1/2) products are also observed. A one‐dimensional analysis is used to understand the appearance of each spectrum. The BrHI− spectrum is compared to a two‐dimensional simulation performed using time‐dependent wave packet propagation on a model Br+HI potential energy surface.

Excited-state properties of Escherichia coli DNA photolyase in the picosecond to millisecond time scale.

Abstract: Escherichia coli DNA photolyase contains a stable flavin radical that is readily photoreduced in the presence of added electron donors. Picosecond, nanosecond, and conventional flash photolysis technique have been employed to investigate the events leading to photoreduction from 40 ps to tens of milliseconds following flash excitation. Direct light absorption by the flavin radical produces the first excited doublet state which undergoes rapid (within 100 ps) intersystem crossing to yield the lowest excited quartet (n pi*) state. In contrast, light absorption by the folate chromophore produces a new intermediate state via interaction of the folate excited singlet state with the ground-state flavin radical, leading to an enhanced yield of the excited radical doublet state and hence quartet state. Subsequent reaction of the excited quartet state involves hydrogen atom abstraction from a tryptophan residue. Secondary electron transfer from added electron donors occurs to the oxidized tryptophan radical with rate constants ranging from 10(4) (dithiothreitol) to 4 x 10(6) M-1 s-1 (n-propyl gallate). The low value of the latter rate compared to reduction of the tryptophan radical in lysozyme suggests that the reactive tryptophan is highly buried in photolyase. A redox potential diagram has been constructed for the ground and excited states involved. It is concluded that the one-electron reduction potential of the excited quartet state of the flavin radical must be at least 1.23 V more positive than the ground state, in agreement with the value of delta E greater than 1.77 V calculated from spectroscopic data.

Selective hydroxylation of methyl groups by platinum salts in aqueous medium. Direct conversion of ethanol to ethylene glycol

Abstract: Many intriguing examples of the activation of C-H bonds in alkanes by transition-metal complexes, often under remarkably mild conditions, have appeared in the last few years. Most systems studied are incompatible with O{sub 2}, the most desirable co-reagent to obtain a thermodynamically favored, economically viable catalytic reaction. Also, potential products (alcohols, alkenes, etc.) are often more reactive than the starting alkanes, which limits achievable yields. The latter is especially problematical when a hydrogen atom abstraction route is involved, as with P-450 and models thereof. Shilov's Pt{sup II}-Pt{sup IV} system appears attractive in that the species involved will tolerate O{sub 2}, although catalytic oxidation has not yet been achieved; nor is much known about detailed mechanism or selectivity. In an attempt to further delineate the mechanism they have examined oxidation of water-soluble model substrates and report here unexpected findings of selectivity.

Picosecond Laser Photolysis Studies on the Photoreduction of Excited Benzophenone by Diphenylamine in Solutions

Abstract: The mechanism of photoreduction of benzophenone (BP) by diphenylamine (DPA) in isooctane as well as acetonitrile and other polar solvents has been investigated by means of picosecond laser photolysis and time-resolved transient absorption spectral measurements. The results of measurements have demonstrated clearly that the hydrogen abstraction and charge transfer (CT) or ion pair (IP) state formation by electron transfer are competing at encounter between triplet benzophenone (3BP*) and DPA both in nonpolar and polar solvent, and the CT or IP state relaxed with respect to the donor acceptor configurations and solvation does not contribute to the ketyl radical formation. It has been concluded that the very short-lived CT state at encounter between 3BP* and DPA plays a crucial role in the hydrogen abstraction reaction, i.e. the mutual orientation of 3BP* and DPA in this very short-lived CT state at encounter will determine the successive process, the formation of the ketyl radical or relaxed CT or IP state.

Photodimerization of cyclohexene and methane by decatungstate anions: molecular orbital theory

Abstract: A molecular orbital study has been made of the photodimerization of cyclohexene by excited W{sub 10}O{sub 32}{sup 4{minus}}, yielding 3,3{prime}-dicyclohexene to explain the observations of Yamase. The overall process is radical monomer coupling. Radical formation by an adiabatic H transfer from cyclohexene to O{sup {minus}} on the O 2p to W 5d charge transfer photoexcited oxyanion is described. This process is highly activated, just as on metal oxide surfaces, because of the stability which comes when an electron from the CH bond reduces the hole in the oxyanion O 2p band during H abstraction. Calculated H abstraction activation energies for olefinic, {alpha}, and {beta} CH bonds in cyclohexene and for a CH bond in CH{sub 4} are higher, but they are low enough that in the case of CH{sub 4} it can be suggested that dimerization could be looked for in future experiments using photoactivated oxyanions. Based on the calculated electronic structures, it is possible to envision a nonadiabatic electron transfer to O{sup {minus}} during mild thermal collisions between a CH bond and the clusters yielding an organic radical cation followed by proton transfer at a later time. Polar solvents would enhance the probability for this mechanism.

Isotope effect studies on the mechanism of the cytochrome P-450IIA1-catalyzed formation of delta 6-testosterone from testosterone.

Abstract: Testosterone metabolism by cytochrome P-450IIA1 results in four metabolites: 6 alpha-hydroxytestosterone; 7 alpha-hydroxytestosterone; 17 beta-hydroxy-4,6-androstadiene-3-one (delta 6-T); and 17 beta-hydroxy-4,6-androstadiene-3-one-6,7-oxide. The epoxide is formed upon further oxidation of delta 6-T, and its formation is in competition with the dissociation of delta 6-T from the active site. The analysis of the KM and Vmax values, as well as the product ratios for testosterone and three selectively deuterated analogs, strongly suggest that delta 6-testosterone formation occurs primarily by initial hydrogen atom abstraction at the 6 alpha-position followed by abstraction of the 7 alpha-hydrogen atom.

Hydrogen abstraction from lipids by triplet states of derivatized benzophenone photosensitizers.

Abstract: Laser photolysis techniques have been used to characterize the reactivity of triplet state lipoidal benzophenone derivatives toward fatty acids and glycerides in benzene solution. The reactivities of benzophenone-4-heptyl-4'-pentanoic acid (BHPA) toward fatty acid compounds having different configurations of olefinic bonds have been determined. The rates of hydrogen abstraction are found to be lower when compared with similar measurements using benzophenone alone. However, the contribution of physical quenching of the triplet derivative by double bonds also appears to be slightly lower than that found with benzophenone itself. The hydrogen abstraction efficiencies of three other benzophenone derivatives toward linoleic acid in benzene have also been measured. For benzophenone incorporated into a fatty acid molecule, there is a limited relationship between structure and photoreactivity. Finally, these sensitizers have been incorporated into mixed SDS/linoleate micelles to determine the effects of molecular organization on photochemical behavior of the sensitizer and lipid.

Intrinsic isotope effects suggest that the reaction coordinate symmetry for the cytochrome P-450 catalyzed hydroxylation of octane is isozyme independent.

Abstract: The mechanism of the omega-hydroxylation of octane by three catalytically distinct, purified forms of cytochrome P-450, namely, P-450b, P-450c, and P-450LM2, was investigated by using deuterium isotope effects. The deuterium isotope effects associated with the omega-hydroxylation of octane-1,1,1-2H3, octane-1,8-2H2, and octane-1,1,8,8-2H4 by all three isozymes were determined. From these data the intrinsic isotope effects were calculated and separated into their primary and secondary components. The primary intrinsic isotope effect for the reaction ranged from 7.69 to 9.18 while the secondary intrinsic isotope effect ranged from 1.13 to 1.25. Neither the primary nor secondary isotope effect values were statistically different for any of the isozymes investigated. These data are consistent with a symmetrical transition state for a mechanism involving initial hydrogen atom abstraction followed by hydroxyl radical recombination which is essentially independent of the specific isozyme catalyzing the reaction. It is concluded that (1) in general the porphyrin-(FeO)3+ complex behaves as a source of a triplet-like oxygen atom, (2) the regioselectivity for the site of oxidation is dictated by the apoprotein of the specific isozyme of cytochrome P-450 catalyzing the reaction, and (3) the maximum primary intrinsic isotope effect for any cytochrome P-450 catalyzed oxidation of a carbon center is aboutmore » 9, assuming no tunneling effects.« less

The rate constant for the intramolecular isomerization of pentyl radicals

Abstract: The thermal decomposition of n-pentane has been investigated in the temperature range 737 to 923 K. Making various assumptions, the detailed distribution of major products (methane, ethane, ethene, propene, and 1-butene) is used to evaluate the rate constant for the unimolecular isomerization CH 3 CH 2 CH 2 CH 2 CH 2 →CH 3 CHCH 2 CH 2 CH 3 which proceeds via a five-membered, cyclic transition state. Two alternative sets of assumptions are used. Common to both of them are assumptions concerning the thermochemistry and rate constants for decomposition of the C 5 H 11 radicals. Method 1 assumes that all secondary C−H bonds are equally reactive towards hydrogen abstraction in which case, in addition to the value of k 10 , the ratio of the rate constants for abstraction from primary and secondary C−H bonds is evaluated. Values about a factor of two higher than published values for similar molecules are obtained. The alternative, method 2, assumes that the ration of abstraction from the 1- and 2- positions of n-pentane is the same as that published for n-butane, in which case, in addition to the value of k 10 , the ration of the rates of abstraction from the 3− and 2− positions of n-pentane is obtained. The value obtained is 0.401 which is to be compared with the statistically expected (and assumed in method 1) 0.5. Detailed discussions of the values of k 10 obtained leads to the conclusion that method 1 leads to the best value log(k 10 /s −1 )=11.96±0.77−(23.4±2.0)/θ where θ=2.303RT in kcal/mol and error limits are two standard deviations. Combination of this value with values recalculated from published lower temperature data gives log(k 10 /s −1 )=11.08−20.04/θ which, it is concluded, is the best value in the range 438 to 923 K

Kinetics of .alpha.- and .beta.-hydrogen abstraction from ethyl chloride by bromine atoms: estimate of carbon-hydrogen bond dissociation energies and heat of formation of CH3CHCl and CH2CH2Cl radicals

Abstract: The gas-phase photobromination of ethyl chloride has been investigated over an extended temperature range in the presence and absence of ethane as external competitor. The rate constant for α-hydrogen abstraction in CH 3 CH 2 Cl was redetermined relative to C 2 H 6 between 40 and 150°C. The abstraction of β-hydrogen was measured in the internal competition in the range 70-150°C by use of electron capture detection

Electron spin resonance and pulse radiolysis studies on the reaction of hydroxyl radical and sulfate(1-) radical with five-membered heterocyclic compounds in aqueous solution

Abstract: The reactions of several five-membered oxygen and nitrogen heterocyclics with OH • and SO 4 •− radicals have been investigated in aqueous solution using in-situ radiolysis and photolysis ESR, and optical and conductometric pulse radiolysis techniques for detection. OH . reacts with (the saturated) oxazolidone, imidazolidinone, hydantoin, and oxazoline derivatives by hydrogen abstraction, preferentially from the carbon adjacent to the nitrogen atom. With (the unsaturated) oxazoles and isoxazoles, OH • reacts by addition to the carbon at the 5-position of the ring to produce allylic radicals. In basic and acidic solutions of oxazole a ring opening process follows the OH • addition. SO 4 •− reacts with oxazoles and isoxazoles by addition to C5 yielding SO 4 − adducts, whereas with imidazole, pyrazole, and pyrrole derivatives, SO 4 •− gives rise to neutral, conjugated radicals with the unpaired electron delocalized over the entire ring, which are derived from the parent compounds by one-electron oxidation followed by deprotonation