Radiation induced crosslinking of poly(vinyl methylether) in aqueous solutions

Radiation induced crosslinking of poly(vinyl methylether) (PVME) has been investigated in aqueous solutions. The spectral and kinetic features of the transients involved in the crosslinking reaction have been studied by pulse radiolysis of dilute PVME solutions. H atoms reacts with PVME, like OH radicals, by abstracting an H atom predominantly from β-position with respect to ---OCH₃ group, but the rate of reaction of H atom is an order of magnitude slower than that of OH reaction. The PVME radicals formed by H attack have been found to decay by usual 2nd-order kinetics unlike PVME radicals produced by OH attack that are reported to decay by a complex time-dependent kinetics that deviates strongly from 2nd-order kinetics. The rate constant of e⁻_aq with PVME at pH 5.5 has been found to be 1.2×10⁸ dm³ mol⁻¹ s⁻¹. From the decay behaviour of the transient species formed by reaction of e⁻_aq with PVME, it has been shown that the transient initially reacts with solvent protons by a fast reaction to yield radical species which subsequently recombine by a slow mode. The dependence of gelation dose and radiation yields of crosslinking (Gx) of PVME on various factors such as polymer concentration, dose rate, pH, presence of oxygen and crosslinking agent has also been studied by steady-state radiolysis using an electron-beam accelerator.     

Long-term γ-radiolysis kinetics of NO3(-) and NO2(-) solutions

Radiolysis kinetics in NO(3)(-) and NO(2)(-) solutions during γ-irradiation were studied at an absorbed dose rate of 2.1 Gy·s(-1) at room temperature. Air- or argon-saturated nitrate or nitrite solutions at pH 6.0 and 10.6 were irradiated, and the aqueous concentrations of molecular water decomposition products, H(2) and H(2)O(2), and the variation in the concentrations of NO(3)(-) and NO(2)(-) were measured as a function of irradiation time. The experimental data were compared with computer simulations using a comprehensive radiolysis kinetic model to aid in interpretation of the experimental results. The effect of nitrate and nitrite, present at concentrations below 10(-3) M, on water radiolysis processes occurs through reactions with the radical species generated by water radiolysis, (?)e(aq)(-), (?)O(2)(-), and (?)OH. The changes in H(2) and H(2)O(2) concentrations observed in the presence of nitrate and nitrite under a variety of conditions can be explained by a reduction in the radical concentrations. The kinetic analysis shows that the main loss pathway for H(2) is the reaction with (?)OH and the main loss pathways for H(2)O(2) are reactions with (?)e(aq)(-) and (?)OH. Nitrate and nitrite compete for the radicals leading to an increase in the concentrations of H(2) and H(2)O(2). Post-irradiation measurements of H(2), H(2)O(2), NO(2)(-) and NO(3)(-) concentrations can be used to calculate the radical concentrations and provide information on the redox conditions of the irradiated aqueous solutions.     

壳聚糖在水溶液中的辐射降解反应

研究了壳聚糖在CH3COOH/NaCl缓冲溶液均相体系下的辐射降解反应,给出了H2O2、异丙醇、pH、样品初始分子量等因素对壳聚糖降解的影响,探讨了实验条件下溶液中不同自由基对壳聚糖降解的作用,并对辐照前后壳聚糖的结构进行了表征.结果表明,酸性条件下,壳聚糖的降解主要由.H和.OH自由基共同作用引起,加入H2O2或者通入N2O都能够略微提高.OH自由基浓度,对壳聚糖的降解有促进作用.加入异丙醇后,由于同时降低了.H和.OH自由基浓度,导致壳聚糖降解缓慢.当溶液的pH接近中性后,对壳聚糖的降解起主要作用的为.OH自由基,加入H2O2或者通入N2O都会增加.OH自由基的浓度,从而明显提高壳聚糖的降解速率.此外,研究发现低分子量的壳聚糖具有较快的降解速率.样品的UV、FTIR分析表明,辐照后除在壳聚糖分子链端生成羰基外,壳聚糖主链结构未见变化,脱乙酰度也没有显著改变,显示出辐射降解是一种有效的控制壳聚糖分子量方法.     

N2O饱和黄芩苷水溶液的辐解机理

本工作研究了γ辐解N2O饱和黄芩苷水溶液的辐解机理, 发现四种主要辐解产物(P1, P2, P3, P4)。紫外、质谱和核磁共振谱确定了它们的化学结构。其中P1, P2为OH自由基与C环作用失去B环形成的, 而P3, P4为OH自由基加成到B环并发生歧化反应的的结果。求得OH自由基与黄芩苷的苷元发生加成反应的速率常数为: 2.6×10^9mol^-^1.dm^3.s^-^1。     

Atmospheric reaction of OH radicals with 1,3-butadiene and 4-hydroxy-2-butenal

The gas-phase reaction of OH radicals with 1,3-butadiene and 4-hydroxy-2-butenal in the presence of NO has been studied in a flow tube operated at 295 +/- 2 K and pressures of 950 mbar of synthetic air or 100 mbar of an O(2)/He mixture. OH radicals were generated using three different experimental approaches, namely, ozonolysis of tetramethylethylene (dark reaction), photolysis of methyl nitrite, or via the reaction of HO(2) with NO (HO(2) from the reaction of H-atoms with O(2)). Products of the reaction of OH radicals with 1,3-butadiene were HCHO (0.64 +/- 0.08), acrolein (0.59 +/- 0.06), 4-hydroxy-2-butenal (0.23 +/- 0.10), furan (0.046 +/- 0.014), and organic nitrates (0.06 +/- 0.02) accounting for more than 90% of the reacted carbon. There was no significant dependence of product yields on experimental conditions which were varied in a wide range. The formation of the 1,4-addition product 4-hydroxy-2-butenal was confirmed unambiguously for the first time. The rate coefficient k(OH + 4-hydroxy-2-butenal) = (5.1 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1) was determined using a relative rate technique (p = 100 mbar, T = 295 +/- 2 K). Products of the reaction of OH radicals with 4-hydroxy-2-butenal were glycolaldehyde (0.40 +/- 0.06), glyoxal (0.17 +/- 0.04), trans-butenedial (0.093 +/- 0.033), and organic nitrates (0.043 +/- 0.015) as well as further carbonylic substances remaining unidentified so far. Corresponding reaction mechanisms describing the formation of the detected products are proposed, and the relevance of these data for atmospheric conditions is discussed.     

H2 and Cl2 production in the radiolysis of calcium and magnesium chlorides and hydroxides

The production of H2 has been examined in the gamma-ray and 5 MeV He ion radiolysis of CaCl2.2H2O, CaCl2.6H2O, Ca(OH)2, MgCl2.2H2O, MgCl2.6H2O, and Mg(OH)2. The highest yield for the formation of H2 is observed in the gamma-radiolysis of MgCl2.2H2O (0.72 molecule/(100 eV), 75 nmol/J), but the yield decreases with dose due to the relative instability of the dihydrate. The H2 yields with the other compounds range from 0.04 to 0.2 molecule/(100 eV) (4.2-21 nmol/J), which is lower than that from pure water or 2 M chloride solutions. There appears to be no relationship between the results for H2 from waters of hydration with that from aqueous salt brines. No particular trend is observed in the radiation chemical yields of H2 with respect to the cation or the degree of hydration. The production of Cl2 is found only in the gamma-radiolysis of CaCl2 with a few weight percent of excess water. No Cl2 was found in the 5 MeV He ion radiolysis of identical systems. None of the other compounds examined here showed detectable amounts of Cl2 formation.     

Products of aqueous vitamin B5 (pantothenic acid) formed by free radical reactions

The radiolysis of aqueous vitamin B5 (pantothenic acid) has been investigated under various experimental conditions. The highest vitamin degradation (G=3.22) was observed in solutions saturated with N₂O, where 90% OH radicals are operating. As final products, the following were established: aldehydes, carboxylic acids and ammonia. Their yield strongly depends on the presence/absence of air as well as on N₂O (used to convert e_aq⁻ into OH) and was determined as a function of absorbed radiation dose. HPLC-analysis showed that in all media, a main product is formed, having the highest yield in aerated solutions.Based on the chemical analysis, it appears that the OH radicals are most involved in the degradation process. A precise sequence of the reaction steps could not be given presently, because of the implication of many simultaneous reactions.     

Thermochemistry of aqueous hydroxyl radical from advances in photoacoustic calorimetry and ab initio continuum solvation theory

Photoacoustic signals from dilute ( approximately 30 mM) solutions of H2O2 were measured over the temperature range from 10 to 45 degrees C to obtain the reaction enthalpy and volume change for H2O2(aq) --> 2 OH(aq) from which we ultimately determined DeltafG degrees , DeltafH degrees and partial molal volume, v degrees , of OH (aq). We find DeltarH = 46.8 +/- 1.4 kcal/mol, which is 4 kcal/mol smaller than the gas-phase bond energy, and DeltaVr = 6.5 +/- 0.4 mL/mol. The v degrees for OH in water is 14.4 +/- 0.4 mL/ml: smaller than the v degrees of water. Using ab intio continuum theory, the hydration free energy is calculated to be -3.9 +/- 0.3 kcal/mol (for standard states in number density concentration units) by a novel approach devised to capture in the definition of the solute cavity the strength and specific interactions of the solute with a water solvent molecule. The shape of the cavity is defined by "rolling" a three-dimensional electron density isocontour of water on the ab initio water-OH minimum interaction surface. The value of the contour is selected to reproduce the volume of OH in water. We obtain for OH(aq): DeltafH degrees = -0.2 +/- 1.4 and DeltafG degrees = 5.8 +/- 0.4 kcal/mol that are in agreement with literature values. The results provide confidence in the pulsed PAC technique for measuring aqueous thermochemistry of radicals and open the way to obtaining thermochemistry for most radicals that can be formed by reaction of OH with aqueous substrates while advancing the field of continuum solvation theory toward ab initio-defined solute cavities.     

Reactions of halogenated hydroperoxides and peroxyl and alkoxyl radicals from isoflurane in aqueous solution

Model systems, based on aqueous solutions containing isoflurane (CHF(2)OCHClCF(3)) as an example, have been studied in the presence and absence of methionine (MetS) to evaluate reactive fates of halogenated hydroperoxides and peroxyl and alkoxyl radicals. Primary peroxyl radicals, CHF(2)OCH(OO*)CF(3), generated upon 1-e-reduction of isoflurane react quantitatively with MetS via an overall two-electron oxidation mechanism to the corresponding sulfoxide (MetSO). This reaction is accompanied by the formation of oxyl radicals CHF(2)OCH(O*)CF(3) that quantitatively rearrange by a 1,2-hydrogen shift to CHF(2)OC*(OH)CF(3). According to quantum-chemical calculations, this reaction is exothermic (DeltaH = -5.1 kcal/mol) in contrast to other potentially possible pathways. These rearranged CHF(2)OC*(OH)CF(3) radicals react further via either of two pathways: (i) direct addition of oxygen or (ii) deprotonation followed by fluoride elimination resulting in CHF(2)OC(O)CF(2)*. Route i yields the corresponding CHF(2)OC(OO*)(OH)CF(3) peroxyl radicals, which eliminate H+/O(2)*-. The resulting ester, CHF(2)OC(O)CF(3), hydrolyzes further, accounting for the formation of HF, trifluoroacetic acid, and formic acid with a contribution of 45% and 80% in air- and oxygen-saturated solutions, respectively. A competitive pathway (ii) involves the reactions of the secondary peroxyl radicals, CHF(2)OC(O)CF(2)OO*. The two more stable of the three above mentioned peroxyl radicals can be distinguished through their reaction with MetS. Although the primary CHF(2)OCH(OO*)CF(3) oxidizes MetS to MetSO in a 2-e step, the majority of the secondarily formed CHF(2)OC(O)CF(2)OO* reacts with MetS via a 1-e transfer mechanism, yielding CHF(2)OC(O)CF(2)OO-, which eventually suffers a total breakup into CHF(2)O- + CO(2) + CF(2)O. Quantum-chemical calculations show that this reaction is highly exothermic (DeltaH = -81 kcal/mol). In air-saturated solution this pathway accounts for about 35% of the overall isoflurane degradation. Minor products (10% each), namely, oxalic acid and carbon monoxide originate from oxyl radicals, CHF(2)OC(O)CF(2)O* and CHF(2)OCH(O*)CF(3). An isoflurane-derived hydroperoxide CHF(2)OCH(OOH)CF(3) in high yield was generated in radiolysis of air-saturated solutions containing isoflurane and formate either via a H-atom abstraction from formate by the isoflurane-derived peroxyl radicals or by their cross-termination reaction with superoxide O(2)*-. CHF(2)OCH(OOH)CF(3), is an unstable intermediate whose multistep hydrolysis is giving H(2)O(2) + 2HF + HC(O)OH + CF(3)CH(OH)(2). In the absence of MetS, about 55% of CHF(2)OCH(OO*)CF(3) undergo termination via the Russell mechanism and 27% are involved in cross-termination with superoxide (O(2)*-) and peroxyl radicals derived from t-BuOH (used to scavenge *OH radicals). The remaining 18% of the primary peroxyl radicals undergo termination via formation of alkoxyl radicals, CHF(2)OCH(O*)CF(3).     

Radicals produced by gamma-irradiation of hyperquenched glassy water containing 2'-deoxyguanosine-5'-monophosphate

Hyperquenched glassy water (HGW) has been suggested as the best model for liquid water, to be used in low-temperature studies of indirect radiation effects on dissolved biomolecules (Bednarek et al. J. Am. Chem. Soc. 1996, 118, 9387). In the present work, these effects are examined by X-band electron spin resonance spectroscopy (ESR) in gamma-irradiated HGW matrix containing 2'-deoxyguanosine-5'-monophosphate. Analysis of the complex ESR spectra indicates that, in addition to OH(*) and HO2(*) radicals generated by water radiolysis, three species are trapped at 77 K:(i) G(C8)H(*) radical, the H-adduct to the double bond at C8; (ii) G(- *) radical anion, the product of electron scavenging by the aromatic ring of the base; and (iii) dR(-H)(*) radicals formed by H abstraction from the sugar moiety, predominantly at the C'5 position. We discuss the yields of the radicals, their thermal stability and transformations, as well as the effect of photobleaching. This study confirms our earlier suggestion that in HGW the H atom addition/abstraction products are created at 77 K in competition with HO2(*) radicals, in a concerted process following ionization of water molecule at L-type defect sites of the H-bonded matrix. The lack of OH(*) reactivity toward the solute suggests that the H-bonded structure in HGW is much more effective in recombining OH(*) radicals than that of aqueous glasses obtained from highly concentrated electrolyte solutions. Furthermore, complementary experiments for the neat matrix have provided evidence that HO2(*) radicals are not the product of H atom reaction with molecular oxygen, possibly generated by ultrasounds used in the process of sample preparation.     

A STUDY ON RADIATION CROSSLINKING OFPOLYDIMETHYLSILOXANE （PDMS） RUBBER LATEX

Polydimethylsiloxane (PDMS) rubber latex with two sorts of sensitizers, trimethylol propane tri-methacrylate (TMPTMA) and diethylene glycol di-acrylate (DEGDA), was irradiated with y-rays and electron beams in various conditions. The radiation crosslinking reaction of PDMS occurs in the inner phase of the latex and is relatively isolated from the water phase. Therefore the oxygen and the radicals produced by the radiolysis reaction of water almost have no effect on the crosslinking reaction of polymer. The experimental data correspond with the Charlesby-Pinner relationship in the main. The gelation doses, degree of crosslinking and degradation as well as G values of crosslinking were calculated. From them, the sensitization coefficients were derived to offer a quantitative measurement of the enhancing effect of sensitizer on the radiation crosslinking.     

CH3NO2+X(X=H,OH, CH3, CH2[^3B1], O[^3P])→CH2NO2+XH吸氢反应过渡态 结 构和反应位垒的DFT计算研究

采用DFT(B3LYP)方法,分别在6-311g(d,p),6-311++g(d,p)和自洽相关基组cc-pVIZ水平上优化了基态硝基甲烷和自由基H,OH,CH3,CH2[^3B1]以及O[^3P]等发生吸氢反应时的过渡态结构,并计算了反应的位垒。研究表明,对同一反应,不同基组下优化得到的过渡态几何结构基本一致;反应位垒数值的大小也基本接近,经校正,硝基甲烷同自由基反应位垒的理论计算值同实验结果基本吻合。     

合成气转化为乙醇的反应机理

本文在助剂型Rh催化剂上采用了以CH_2OD、D_2~(18)O为捕获剂的原位化学捕获反应, 以及以D_2~(18)O为重氧源试剂的原位氧同位素交换反应, 对合成气转化为乙醇的反应机理进行了研究。在原位捕获反应中检测到CH_2DCOOCH_3、CH_3COOCH_3和CH_2DCOOD、CH_3COOD的生成, 表明合成乙醇反应过程中存在中间体乙烯酮和乙酰基, 当CH_3OD/H_2比值足够大时主要捕获到CH_2DCOOCH_3, 说明乙酰基主要由乙烯酮的部分氢化反应生成。原位氧同位素交换反应检测到含~(18)O的乙醇、乙醛、乙酸的生成, 表明乙烯酮等C_2-含氧化合物前驱怵与重氧水发生了氧同位素交换反应。籍此, 无须如Katzer等人那样假设乙烯酮互变异构为位能较商的环氧乙烯而后进行氧同位素交换, 就可以得到Katzer等人在~(13)C~(16)O/~(13)C~(18)O+H_2反应中观察到的产物乙醇的同位素组成结果。本文的实验结果进一步说明我们提出的“CO缔合—卡宾—乙烯酮—乙酰基—乙醇(醛)”机理是合理的。     

Dimethylselenide as a probe for reactions of halogenated alkoxyl radicals in aqueous solution. Degradation of dichloro- and dibromomethane

Using pulse radiolysis and steady-state gamma-radiolysis techniques, it has been established that, in air-saturated aqueous solutions, peroxyl radicals CH 2HalOO (*) (Hal = halogen) derived from CH 2Cl 2 and CH 2Br 2 react with dimethyl selenide (Me 2Se), with k on the order of 7 x 10 (7) M (-1) s (-1), to form HCO 2H, CH 2O, CO 2, and CO as final products. An overall two-electron oxidation process leads directly to dimethyl selenoxide (Me 2SeO), along with oxyl radical CH 2HalO (*). The latter subsequently oxidizes another Me 2Se molecule by a much faster one-electron transfer mechanism, leading to the formation of equal yields of CH 2O and the dimer radical cation (Me 2Se) 2 (*+). In absolute terms, these yields amount to 18% and 28% of the CH 2ClO (*) and CH 2BrO (*) yields, respectively, at 1 mM Me 2Se. In competition, CH 2HalO (*) rearranges into (*)CH(OH)Hal. These C-centered radicals react further via two pathways: (a) Addition of an oxygen molecule leads to the corresponding peroxyl radicals, that is, species prone to decomposition into H (+)/O 2 (*-) and formylhalide, HC(O)Hal, which further degrades mostly to H (+)/Hal (-) and CO. (b) Elimination of HHal yields the formyl radical H-C(*)=O with a rate constant of about 6 x 10 (5) s (-1) for Hal = Cl. In an air-saturated solution, the predominant reaction pathway of the H-C(*)=O radical is addition of oxygen. The formylperoxyl radical HC(O)OO (*) thus formed reacts with Me 2Se via an overall two-electron transfer mechanism, giving additional Me 2SeO and formyloxyl radicals HC(O)O(*). The latter rearrange via a 1,2 H-atom shift into (*)C(O)OH, which reacts with O2 to give CO2 and O2(*)(-). The minor fraction of H-C(*)=O undergoes hydration, with an estimated rate constant of k approximately 2 x 10(5) s(-1). The resulting HC(*)(OH)2 radical, upon reaction with O2, yields HCO 2H and H (+)/O2(*-). Some of the conclusions about the reactions of halogenated alkoxyl radicals are supported by quantum chemical calculations [B3LYP/6-31G(d,p)] taking into account the influence of water as a dielectric continuum [by the self-consistent reaction field polarized continuum model (SCRF=PCM) technique]. Based on detailed product studies, mechanisms are proposed for the free-radical degradation of CH 2Cl 2 and CH 2Br 2 in the presence of oxygen and an electron donor (namely, Me 2Se in this study), and properties of the reactive intermediates are discussed.     

OH-initiated oxidation of monoterpenes: reaction of alpha-pinene

The aim of this paper is to study the reaction products of alpha-pinene, beta-pinene, limonene, 3-carene and sabinene with OH radicals by FT-IR spectroscopy and by HPLC-MS-MS, to evaluate the secondary aerosol formation. All gas phase reaction products were quantified using reference compounds. As source of OH radicals were used H2O2 and CH3ONO. The experiments were performed at low terpene concentration (0.9-2.1 ppm) and at high terpene concentration (4.1-13.2 ppm), using H2O2 and CH3ONO as sources of OH radicals.     

Photoproduct channels from BrCD2CD2OH at 193 nm and the HDO + vinyl products from the CD2CD2OH radical intermediate

We present the results of our product branching studies of the OH + C(2)D(4) reaction, beginning at the CD(2)CD(2)OH radical intermediate of the reaction, which is generated by the photodissociation of the precursor molecule BrCD(2)CD(2)OH at 193 nm. Using a crossed laser-molecular beam scattering apparatus with tunable photoionization detection, and a velocity map imaging apparatus with VUV photoionization, we detect the products of the major primary photodissociation channel (Br and CD(2)CD(2)OH), and of the secondary dissociation of vibrationally excited CD(2)CD(2)OH radicals (OH, C(2)D(4)/CD(2)O, C(2)D(3), CD(2)H, and CD(2)CDOH). We also characterize two additional photodissociation channels, which generate HBr + CD(2)CD(2)O and DBr + CD(2)CDOH, and measure the branching ratio between the C-Br bond fission, HBr elimination, and DBr elimination primary photodissociation channels as 0.99:0.0064:0.0046. The velocity distribution of the signal at m/e = 30 upon 10.5 eV photoionization allows us to identify the signal from the vinyl (C(2)D(3)) product, assigned to a frustrated dissociation toward OH + ethene followed by D-atom abstraction. The relative amount of vinyl and Br atom signal shows the quantum yield of this HDO + C(2)D(3) product channel is reduced by a factor of 0.77 ± 0.33 from that measured for the undeuterated system. However, because the vibrational energy distribution of the deuterated radicals is lower than that of the undeuterated radicals, the observed reduction in the water + vinyl product quantum yield likely reflects the smaller fraction of radicals that dissociate in the deuterated system, not the effect of quantum tunneling. We compare these results to predictions from statistical transition state theory and prior classical trajectory calculations on the OH + ethene potential energy surface that evidenced a roaming channel to produce water + vinyl products and consider how the branching to the water + vinyl channel might be sensitive to the angular momentum of the β-hydroxyethyl radicals.     

Fenton chemistry of 1,3-dimethyluracil

Hydroxyl radicals were generated in the Fenton reaction at pH 4 (Fe(2+) + H(2)O(2) --> Fe(3+) + .OH + OH-, k approximately equal to 60 L mol(-1) s(-1)) and by pulse radiolysis (for the determination of kinetic data). They react rapidly with 1,3-dimethyluracil, 1,3-DMU (k = 6 x 10(9) L mol(-1) s(-1)). With H(2)O(2) in excess and in the absence of O(2), 1,3-DMU consumption is 3.3 mol per mol Fe(2+). 1,3-DMUglycol is the major product (2.95 mol per mol Fe(2+)). Dimers, prominent products of .OH-induced reactions in the absence of Fe(2+)/Fe(3+) (Al-Sheikhly, M.; von Sonntag, C. Z. Naturforsch. 1983, 31b, 1622) are not formed. Addition of .OH to the C(5)-C(6) double bond of 1,3-DMU yields reducing C(6)-yl 1 and oxidizing C(5)-yl radicals 2 in a 4:1 ratio. The yield of reducing radicals was determined with tetranitromethane by following the buildup of nitroform anion. Reaction of 1 with Fe(3+) that builds up during the reaction or with H(2)O(2) gives rise to a short-chain reaction that is terminated by the reaction of Fe(2+) with 2, which re-forms 1,3-DMU. In the presence of O(2), 1.1 mol of 1,3-DMU and 0.6 mol of O(2) are consumed per mol Fe(2+) while 0.16 mol of 1,3-DMU-glycol and 0.17 mol of organic hydroperoxides (besides further unidentified products) are formed. In the presence of O(2), 1 and 2 are rapidly converted into the corresponding peroxyl radicals (k = 9.1 x 10(8) L mol(-1) s(-1)). Their bimolecular decay (2k = 1.1 x 10(9) L mol(-1) s(-1)) yields approximately 22% HO(2)./O(2).(-) in the course of fragmentation reactions involving the C(5)-C(6) bond. Reduction of Fe(3+) by O(2).(-) leads to an increase in .OH production that is partially offset by a consumption of Fe(2+) in its reaction with the peroxyl radicals (formation of organic hydroperoxides, k approximately 3 x 10(5) L mol(-1) s(-1); value derived by computer simulation).     

One-electron oxidation of acetohydroxamic acid: the intermediacy of nitroxyl and peroxynitrite

The pharmacological effects of hydroxamate derivatives have been attributed not only to metal chelation or enzyme inhibition but also to their ability to serve as nitroxyl (HNO/NO(-)) and nitric oxide (NO) donors. However, the mechanism underlying the formation of these reactive nitrogen species is not clear and requires further elucidation. In the present study, one-electron oxidation of acetohydroxamic acid (aceto-HX) by (?)OH, (?)N(3), (?)NO(2), CO(3)(?-), and O(2)(?-) radicals was investigated using pulse radiolysis. It is demonstrated that only (?)OH, (?)N(3), and CO(3)(?-) radicals attack effectively and selectively the deprotonated form of the hydroxamate moiety, yielding the respective transient nitroxide radical. This nitroxide radical is a weak acid (CH(3)C(O)NHO(?), pK(a) = 9.1), which decays via a pH-dependent second-order reaction, 2k(2CH(3)C(O)NO(?-)) = (5.6 ± 0.4) × 10(7) M(-1) s(-1) (I = 0.002 M), 2k(CH(3)C(O)NO(?-) + CH(3)C(O)NHO(?)) = (8.3 ± 0.5) × 10(8) M(-1) s(-1)), and 2k(2CH(3)C(O)NHO(?)) = (8.7 ± 1.3) × 10(7) M(-1) s(-1). The second-order decomposition of the nitroxide yields transient species, one of which decomposes via a first-order reaction whose rate increases linearly upon increasing [CH(3)C(O)NHO(-)] or [OH(-)]. One-electron oxidation of aceto-HX under anoxia does not give rise to nitrite even after exposure to O(2), indicating that NO is not formed during the decomposition of the nitroxide radical. The presence of oxidants such as Tempol or O(2) during CH(3)C(O)NO(?-) decomposition had no effect on the reaction kinetics. Nevertheless, in the presence of Temopl, which does not react with NO but does with HNO, the formation of the hydroxylamine Tempol-H was observed. In the presence of O(2), about 60% of CH(3)C(O)NO(?-) yields ONOO(-), indicating that 30% NO(-) is formed in this system. It is concluded that under pulse radiolysis conditions, the transient nitroxide radicals derived from one-electron oxidation of aceto-HX decompose bimoleculary via a complex mechanism forming nitroxyl rather than NO.     

Effect of α-tocopherol and its sulfur-containing analogs on radiation-induced chemical transformations of hexane and ethanol

A series of sulfur-containing structural analogs of α-tocopherol was synthesized, and their effect on the formation of molecular products of ethanol and hexane radiolysis under aerated and deaerated conditions was studied. The O-H bond dissociation enthalpies (BDEs) were calculated for the compounds containing hydroxyl groups. It was found that α-tocopherol and its analogs have the most significant effect on the radiolysis of ethanol and hexane under air-free conditions when the products are formed in the reactions of carbon-centered radicals. The probability of the reaction of the test compounds with alkyl radicals increases with a decrease in the BDE for the OH groups present in these compounds. The carbonyl group in compounds (IV-VIII) makes them efficient oxidants of α-hydroxyalkyl radicals, thus resulting in a change in the product composition for the ethanol radiolysis in presence of these compounds.