Use of homogeneous competition for OH radicals in electrochemical studies of H abstraction from organic molecules

The influence on the N₂O photocurrent of homogeneous competition for OH radicals between two organic solutes which form (as the result of hydrogen abstraction) radicals, one of which is reduced by the electrode, the other being oxidised, is considered theoretically. Such competition can be employed to investigate the kinetics of hydrogen abstraction in a case in which uncompetitive homogeneous destruction of OH radicals by a solute has no effect on the photocurrent. The influence of incomplete oxidation of alcohol radicals when a light pulse of short duration is employed is discussed, together with complications caused by adsorption of the organic solute. Competition for OH radicals between phenol and methanol points to a rate constant for H abstraction from phenol of ca. 2.8 × 10⁹ l mol^?1 s^?1 and to rapid heterogeneous reduction of most of the C₆H₄OH radicals throughout the accessible potential range.     

Reactivity of monoenic and conjugated diene systems present in unsaturated olefin terpolymers—I. Reactions with t-butoxy radicals (CH3)3 CO•

The relative rate constants for the hydrogen abstraction and the double bond addition reactions of t-butoxy radicals (CH₃)₃ CO^? with the model compounds 5-ethylidennorbornane (I), dihydrodicyclopentene (II), isopropylidendicyclopentene (III) and methylcyclopentadienylnorbornylmethane (IV) have been determined by using as a reference reaction the hydrogen abstraction for iso-octane. With (I), (III) and (IV) the predominant process is the hydrogen abstraction, whilst for (II) both mechanisms are important. The results have been applied for the elucidation of some aspects of the initiating mechanism of peroxide-induced cross-linking of EPDM and EPTM terpolymers containing (I)-(IV) pendants.     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

Ion flotation studies of the chlorocomplexes of some platinum group metals

The anionic chlorocomplexes of Au(III), Pt(IV), Pd(II), Ir(IV), Ir(III) and Rh(III) can be floated from aqueous solutions with cationic surfactants of the type RNR'₃Br. The flotation behavior of each metal is reported with respect to variations of hydrochloric acid and sodium chloride concentrations, the R and R' chain lengths, initial surfactant concentrations and initial metal ion concentrations. The flotation behavior of the metals is compared to the anion-exchange selectivity coefficients and a flotation selectivity sequence of Au(III) > Pt(IV), Ir(IV), Pd(II) > Ir(III) > Rh(III) is generally observed. Nearly 100% of Au(III), Pt(IV), Ir(IV) and Pd(II) can be recovered from dilute solutions using the ion flotation procedures.     

ESR study of radical processes in irradiated DNA: The effect of the complexation with cis-dichloro diamino Pt(II)

The room temperature esr spectra of irradiated neat duplex hydrated, dehydrated and thermally denaturated DNA are interpreted in terms of the radicals formed by protonation of the thymine and cytosine anion radicals, by deprotonation of the guanine cation radicals, by addition of OH radicals at C8 of guanine and by loss of a methyl hydrogen from thymine. The complexation of DNA with cis-dichlorodiamino Pt(II) causes a drastic alteration of the radical distribution interpreted in terms of electron scavenging by the Pt(II) complex, a partial inhibition of the radical-cation chemistry of guanine, and enhancement of the reactivity of thymine toward hydrogen abstractions.     

Effect of type of peroxide on cross-linking of poly(l-lactide)

Poly(l-lactide) (PLLA) was cross-linked with various types of peroxides under constant mole ratios of peroxide-derived radicals to PLLA during reactive extrusion. Peroxides were classified into three groups according to their decomposition rates (Group I: fast, Group II: moderate and Group III: slow) and comparisons were performed within each group. Cross-linking behavior was readily understood in terms of free radical efficiency and hydrogen abstraction ability of radicals. In the case of Groups II and III, the weight-average molecular weight (M_w) of cross-linked PLLA increased with overall hydrogen abstraction ability, because slow decomposition caused uniform cross-linking in molten PLLA. In Group I, M_w and gel fraction were higher than other groups despite Group I's lower hydrogen abstraction ability, leading to the conclusion that peroxide decomposition localized in solid PLLA caused partial cross-linking because of rapid decomposition. Furthermore, the efficiency of peroxide-induced cross-linking was investigated using the Charlesby-Pinner equation.     

Mechanism of Pt(IV) sonochemical reduction in formic acid media and pure water

Sonochemical synthesis of platinum nanoparticles (Pt?NPs) in formic acid solutions and pure water was investigated using a 20?kHz ultrasonic irradiation. The obtained results gave new insights on the underneath Pt(IV) reduction mechanism in formic acid media under argon and in pure water under Ar/CO atmosphere. It was shown that in pure water sonochemical reduction of platinum ions occurs by hydrogen issued from homolytic water molecule split. Pt(IV)?ion reduction appears to be a very slow process under argon atmosphere in pure water due to formation of oxidizing species like OH radicals and H(2)O(2) leading to reoxidation of intermediate Pt(II) ions. Sonochemical reduction is accelerated manifold in the presence of formic acid or Ar/CO gas mixture. Solution and gas-phase analyses reveal that both CO and HCOOH act as OH(.) radical scavenger and reducing agent under ultrasonic irradiation. Their ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surround the cavitation bubble. An innovative synthesis route for monodispersed Pt?NPs in pure water without any templates or capping agents in the presence of Ar/CO gas mixture is then proposed. Obtained Pt?NPs within the range of 2-3?nm exhibited a strong stability towards sedimentation in water. Since Ar/CO atmosphere is the only restriction of the process, this procedure can be applied in various media and is also compatible with a large array of experimental conditions.     

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R¹COR²→>C=O+R¹R²C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R¹CH=CH₂→>C=O+R¹C·HCH₃; >R¹CH=CH₂+R²C·HCH₂R³→R²C·HCH₃+R²CH=CHR³. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2178–2184, November, 1998.     

Deposition of platinum–ruthenium nano-particles on multi-walled carbon nano-tubes studied by gamma-irradiation

Pt/Ru deposited on multi-walled carbon nano-tubes (MWCNTs) was prepared with water/iso-propanol solutions containing Pt(IV) and Ru(III) ions by γ-irradiation. The water/iso-propanol ratio (v/v), additive amount of surfactant, the concentration ratio of Pt(IV)/Ru(III) ions and the total absorbed doses (kGy) were evaluated as synthesis parameters. The sample morphology was characterized by SEM and the Pt/Ru atomic ratio was obtained by EDX. It has been found that multi-walled carbon nano-tubes can be well distributed in the water/iso-propanol solution with additive of surfactant. Pt(IV) and Ru(III) ions can be reduced by both of hydrated electron and radical of iso-propanol produced from hydrogen abstraction reaction. The Pt/Ru atom ratio can be controlled by changing the ratio of Pt(IV)/Ru(III). Small nano-particles of Pt/Ru deposited on MWCNTs can be obtained for possible application of electro-catalysts in the proton exchange membrane fuel cells (PEMFC) under optimum conditions with absorbed doses, amount of surfactant, water/iso-propanol ratio and so on. The reduction of Pt(IV)/Ru(III) ions in the aqueous solution with additive of surfactant was also studied by use of pulse radiolysis and the mechanism involved in the reduction process has been proposed.     

Homolytic reductive alkylation of methylvinyl ketone with ethers by Ti(III) decomposition of t-butyl hydroperoxide

Reductive alkylatin of methylvinylketone has been accomplished by hydrogen abstraction from cyclic ethers with the redox couple: t-butyl hydroperoxide-titanous chloride. A redox radical mechanism is proposed and the selectivity of the hydrogen abstraction by t-butoxy radicals and reduction of α-ketoalkyl radical by titanous ions is discussed.     

Photostabilizing activity of sterically hindered piperidines—II: Radical trapping ability

The abilities of both 2,2,6,6-tetramethylpiperidine (I) and its nitroxyl (II) to trap radicals involved in hydrocarbon photo-oxidations have been studied in cumene and 1,3,5-trimethylcyclohexane at 27° using AIBN, hydroperoxide and dialkylperoxide as initiators: the light was either the band 300–400 nm or 366 nm. Under conditions of photolysis of ROOH (degenerate branching), I is oxidized to II. II is capable of trapping R' radicals, the rate constant being ～50 times lower than that for RO^.₂ formation. RO^.₂ radicals react with neither I nor II. Under the condition of degenerate branching, II is capable of intercepting the radical fragments from decomposing hydroperoxide. The rate constant of this process is ～500 times higher than that for hydrogen abstraction by these fragments. A reaction mechanism is suggested: hydrogen bonded associates formed between an N-containing stabilizer and ROOH play a dominant role. The principal intermediates in this mechanism are represented by >NO^., >NOH and >NOR species.     

Trapping of the OH radical by alpha-tocopherol: a theoretical study

The antioxidant activity of alpha-tocopherol against the damaging hydroxyl radical was analyzed theoretically by hybrid density functional theory, following the direct dynamics method, where the thermal rate constants were calculated using variational transition-state theory with multidimensional tunneling. We found that the OH radical is only slightly or not at all selective, attacking by different mechanisms at several positions of the alpha-tocopherol molecule, giving competitive reactions. The most favorable pathways are the hydrogen abstraction reaction from the phenolic hydrogen and the electrophilic addition onto the aromatic ring. We propose a final rate constant, the sum of the competitive hydrogen abstraction and addition reactions, > or =2.7 x 10(8) M(-1) s(-1) at 298 K, where the hydrogen abstraction reaction represents only 20% of the total OH radical reaction. This result indicates that, molecule by molecule, in an apolar environment, alpha-tocopherol is less effective than coenzyme Q (which presents a rate constant of 6.2 x 10(10) M(-1) s(-1) at 298 K) as a scavenger of OH radicals. It was also found that both mechanisms are not direct but pass through intermediates in the entry channel, with little or no influence on the dynamics of the reactions. The hydrogen abstraction reaction also presents another intermediate in the exit channel, which may have a significant role in preventing the pro-oxidant effects of alpha-tocopherol, although less important than with free radicals other than OH.     

The gamma radiolysis of aqueous solutions of ferriciminodiacetate and ferricglycinate

Radiolytic G-values were determined for [−Fe(III)IDA], Fe(II), (−IDA) and CHO-COOH in deoxygenated aqueous solutions of Fe(III)IDA and Fe(III)gly, and in the presence of scavengers, t-butanol, 2-propanol, methanol, sodium formate and O₂. The metal ion was reduced by e^-_aq, H or secondary radicals HO₂, CO⁻₂, CH₂-OH and (CH₃)₂C-OH, while the OH radical did so indirectly through α-hydrogen abstraction from the ligand followed by intramolecular electron transfer. The rate constant k[OH + Fe(III)IDA], determined by the competition kinetics method at pH 2.0, was 1.7 × 10⁸ M^-1 s^-1.     

Model Calculations of Radiation-Induced Damage in 1-Methyluracil:9-Ethyladenine

Detailed EPR and ENDOR experiments on the cocrystalline complex of 1-methyluracil:9-Ethyladenine (MUEA) have revealed that the major radiation-induced products observed at 10 K on MU are: MUEA1, a radical formed by net hydrogen abstraction from the N1-CH₃ methyl group, MUEA2, the MU radical anion, and MUEA3, the C5 H-addition radical. The following four products were observed on the adenine moiety at 10 K, MUEA4, the N3 protonated adenine anion, MUEA5, the native adenine cation, MUEA6, the amino deprotonated adenine cation, and MUEA7, the C8 H-addition radical formed by net H-addition to C8 of the adenine base. The geometries, energetics, and hyperfine properties of all possible radicals of MU and EA, the native anions and cations, as well as radicals formed via net hydrogen atom abstraction (deprotonated cations) or addition (protonated anions) were investigated theoretically. All systems were optimized using the hybrid Hartree–Fock–density functional theory functional B3LYP, in conjunction with the 6-31G(d,p) basis set of Pople and co-workers. Calculations of the anisotropic hyperfine couplings for all the radicals observed in MUEA are presented and are shown to compare favorably with the experimentally measured hyperfine couplings. The calculated ionizations potentials indicate that EA would be the preferred oxidation site. In MUEA, both the adenine cation and its N4-deprotonated derivative were observed. The calculated electron affinities indicate that MU would be the preferred reduction site. In MUEA radical, MUEA2 is a uracil reduction product, however the protonation state of this radical could not be determined experimentally. Calculations suggest that MUEA2 is actually the C4=O protonated anion.     

Chiral Pt(II)/Pd(II) pincer complexes that show C-H?Cl hydrogen bonding: Synthesis and applications to catalytic asymmetric aldol and silylcyanation reactions

A chiral C₂-symmetric NCN ligand, (5R,7R)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanoquinolin-2-yl)benzene has been synthesized. A direct cyclometalation of this ligand with K₂MCl₄ (M = Pt, Pd) in dry acetic acid offered the corresponding pincer complexes, [(5R,7R)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanoquinolin-2-yl)phenyl]platinum(II) chloride 5a and its palladium(II) analogue 5b. The Pt(II) and Pd(II) complexes 5 were characterized by NMR spectroscopy, and X-ray crystal structure analysis was done for the Pt(II) complex. The NMR data for both the complexes and X-ray crystal structural data for the chloro-Pt(II) complex indicate the existence of intramolecular C-H?Cl hydrogen bonding both in solution and in solid states. Chloride abstraction from 5a by treatment with silver triflate resulted in the corresponding triflate complex 6a, which generates the corresponding cationic aqua complex 7a in the presence of water molecules. The Pt(II) complex 6a/7a was used as asymmetric catalyst in the aldol reaction between methyl isocyanoacetate and aldehydes and also in the silylcyanation of aldehydes.     

Redox reactions of transient species formed during pulse radiolysis of 2-pyridine carboxaldehyde and 2-pyridine methanol in aqueous solutions

Rate constants for the reactions of e _aq ^? , H and OH radicals with 2-pyridine carboxaldehyde and 2-pyridine methanol have been determined by pulse radiolysis technique. Reactions of reducing radicals such as acetone ketyl radicals and CO₂ ^? with these compounds were also evaluated at various pHs. The species produced by the reaction of reducing radicals with these solutes was a strong reductant itself. While pyridinyl were produced in the case of 2-pyridine methanol, one-electron reduction of 2-pyridine carboxaldehyde led to the formation of PyCHOH radical. The one-electron reduction potential of PyCHOH radicals was estimated by establishing an equilibrium with MV⁺ radical cations to be ?0.6V vs NHE. OH radical reaction with 2-pyridine carboxaldehyde gave an OH adduct, while in the case of 2-pyridine methanol, OH radicals reacted partly by H-abstraction from the ?CH₂OH group. SO₄ ^? radical reaction with 2-pyridine carboxaldehyde produced a species which was reducing in nature. The rate constants for the reaction of e _aq ^? and OH radicals are compared with similar values obtained in the case of other 2-pyridine derivatives to see if there is any electron-inductive effect.     

Pulse radiolysis of 3-pyridine methanol and 3-pyridine carboxaldehyde in aqueous solutions

Reactions of e_aq ^-, H-atom and OH radicals with 3-pyridine methanol (3-PM) and 3-pyridine carboxaldehyde (3-PCA) have been studied at various pHs using pulse radiolysis technique. e_aq ^- was found to be highly reactive with both 3-PM and 3-PCA (k approx. 10¹⁰ dm³ mol 1 s^-1). Semi-reduced species formed in both cases were strongly reducing in nature. In the case of 3-PM, electron addition leads to the formation of pyridinyl radicals whereas in the case of 3-PCA, PyCHOH type radicals are formed. At pH 6.8, H-atom reaction with 3-PCA also gives semi-reduced species (PyCHOH), whereas at pH 1, H-atoms add to the ring. (CH₃)₂ ^·COH radicals were found to transfer electron to 3-PCA at all the pH values tested and by making use of changes in the absorption spectra, pK _a values of the semi-reduced species were determined to be 4.5 and 10.6. OH radicals were found to undergo addition reaction with 3-PCA, whereas in the case of 3-PM they reacted by H-abstraction as well as addition reaction. By following the yield of methylviologen radical cation formed by electron transfer reaction, it was estimated that approx. 50% of OH radicals react with 3-PM by H-atom abstraction at pH 6.8, giving reducing radicals, whereas at pH 3.2, where 3-PM is in the protonated form, the same is only about 10%. At pH 13, O^-· radical anions were found to react exclusively by H-atom abstraction. Reaction of SO₄ ^-· radicals with 3-PCA was found to give a species identical to the one formed by one electron reduction of nicotinic acid at acidic pH values.     

Insight into detailed mechanism of the atmospheric reaction of imidogen with hydroxyl: a computational study

The details of reaction mechanism of imidogen (NH) and hydroxyl radicals are explored at the UMP2(FC)/cc–pVDZ and PMP4(FC,SDTQ)/cc–pVQZ//UMP2 + ZPE levels, theoretically. The initial association between NH and OH radicals leads to the formation of the intermediates, NH…OH, HN…HO, cis HNOH, and trans HNOH, through the barrierless and exothermic processes. By starting from the initial intermediates, all possible paths for the formation of H + HNO, H₂ + NO, H₂O + ⁴N, H₂N + ³O, and H + ³HON products are investigated on potential energy surface. The results reveal that H₂O + ⁴N is the main product involved in the mechanism of hydrogen atom abstraction of NH by OH radical through the intermediate NH…OH.     

Ground-state dynamics of α-methylstyrene. I. Thermally induced oligomerization in bulk

α-Methylstyrene (α-Me-St) in bulk undergoes a variety of thermally induced processes which we investigated in detail by product-distribution analysis. The oligomeric products, shown by GC–MS coupling techniques and independent syntheses, consisted of nine dimers and nine trimers with no detectable higher oligomers or polymers. Two isomeric cyclobutanes and one of the open-chain unsaturated dimers were formed by a conventional two-step cycloaddition; a Flory-type diradical was the common intermediate. In contrast to these 2π + 2π products, the majority of the remaining oligomers could not be interpreted on the basis of π-electron interaction between closed shell molecules. Their structures, however, were compatible with a free radical process in which cumyl(MH·) and 1,4-dimethyl-1-phenyl-tetrahydronaphthalene-yl(THN·), which are consecutive products of 4π + 2π and 2π + 2π interactions, respectively, were involved in addition and transfer reactions. Because of the small rate of initiation and the obvious lack of free radical recombination products, a mechanism was suggested in which MH· and THN· radicals react predominantly in a closed sequence of elementary processes and to a rather small extent only by bimolecular free radical termination. Two sorts of stabilization steps, hydrogen transfer to monomer (R_nH· + M → R_n + MH·) and hydrogen abstraction from a hypothetical 4π + 2π intermediate I (R_nH· + I → R_nH₂ + THN·), are attributed to the high rates of unsaturated and saturated oligomer formation.     

Mechanism of PtIV Sonochemical Reduction in Formic Acid Media and Pure Water

Sonochemical synthesis of platinum nanoparticles (Pt NPs) in formic acid solutions and pure water was investigated using a 20 kHz ultrasonic irradiation. The obtained results gave new insights on the underneath Pt^IV reduction mechanism in formic acid media under argon and in pure water under Ar/CO atmosphere. It was shown that in pure water sonochemical reduction of platinum ions occurs by hydrogen issued from homolytic water molecule split. Pt^IV ion reduction appears to be a very slow process under argon atmosphere in pure water due to formation of oxidizing species like OH radicals and H₂O₂ leading to reoxidation of intermediate Pt^II ions. Sonochemical reduction is accelerated manifold in the presence of formic acid or Ar/CO gas mixture. Solution and gas‐phase analyses reveal that both CO and HCOOH act as OH^. radical scavenger and reducing agent under ultrasonic irradiation. Their ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surround the cavitation bubble. An innovative synthesis route for monodispersed Pt NPs in pure water without any templates or capping agents in the presence of Ar/CO gas mixture is then proposed. Obtained Pt NPs within the range of 2–3 nm exhibited a strong stability towards sedimentation in water. Since Ar/CO atmosphere is the only restriction of the process, this procedure can be applied in various media and is also compatible with a large array of experimental conditions.