Reactions Of Radical Anions Research Articles

Gas-phase reactions of oxide and superoxide radical anions with C6F6

The reactions of O− and O2− with C6F6 and C6F6− with O2 were studied using the selected ion flow tube (SIFT) technique. The reaction rate constants and branching fractions were measured. The reactions of O− and O2− with C6F6 occur with near unit efficiency while that of C6F6− with O2 is about 2% efficient. The reaction of O− produces three ionic products, F−, C5F4−, and C6F5O−. The reaction of O2− with C6F6 produces five ionic decomposition products, C4F4−, F−, C4F2−, C5F4O−, C5F5O−, and the electron transfer product, C6F6−. The reaction of C6F6− with O2 produces products similar to those observed for the reaction O2− with C6F6.

Competitive Electron Transfer and SN2 Reactions of Aromatic Radical Anions with Alkyl Halides and Methanesulfonates.

Competitive Electron Transfer and SN2 Reactions of Aromatic Radical Anions with Alkyl Halides and Methanesulfonates.

Dissociative Electron Transfer, Substitution, and Borderline Mechanisms in Reactions of Ketyl Radical Anions. Differences and Difficulties in Their Reaction Paths

Computational studies on ketyl anion radicals with methyl chloride and on ω-chloroalkanal radical anions, Cl(CH2)nC(H)O-• (n = 2, 3), find competing mechanisms: a dissociative electron transfer (ET) mechanism and a substitution (SUB(C)) mechanism leading to a C-alkylation product. H(CN)CO-•/CH3Cl proceeds unequivocally via the SUB(C) mechanism, and ω-chloroalkanal radical anions proceed by the ET mechanism, but the interpretation of the mechanism for H2CO-•/CH3Cl depends on the coordinate system used to explore the path. The steepest descent path in Z-matrix internal coordinates leads to the ET product at both the ROHF/6-31G* and UHF/6-31G* levels. The mass-weighted path leads to the ET product on the restricted open-shell Hartree−Fock (ROHF) surface but to the SUB(C) product on the unrestricted Hartree−Fock (UHF) surface. A valley−ridge inflection point heading in the direction of ET products was located on the mass-weighted UHF path, indicating that the potential energy surface branches toward ET produ...

Electron Transfer Reactions of Radical Cation and Anion of (R)-(−)-1,1‘-Binaphthyl-2,2‘-diyl Hydrogen Phosphate in Aqueous Solution. A Pulse Radiolysis Study

(R)-(−)-1,1‘-Binaphthyl-2,2‘-diyl hydrogen phosphate (BiNPO4H) forms radical anions (λmax = 460 nm and e460 = 8600 L mol-1 cm-1) on reaction with hydrated electrons. The radical anion undergoes acid-catalyzed protonation with a rate constant of (5.7 ± 0.9) × 109 L mol-1 s-1 to form H-adduct, which has absorption at λmax = 410 nm. A similar adduct but having a slightly different absorption λmax = 405 nm (e405 = 2900 L mol-1 cm-1) is formed on the addition of H atoms to BiNPO4H. Addition of OH• radicals to BiNPO4- also formed an adduct which absorbs at λmax = 405 nm (e405 = 3100 L mol-1 cm-1). Oxidation of BiNPO4- with strong oxidizing radicals such as SO4•-, Tl2+ and Cl2•- led to the formation of the radical cation. It is also formed by dehydroxylation of the OH-adduct in acidic solution. The radical cation has a broad absorption spectrum with peaks at 340, 405, 460, and 630 nm (e630 = 2000 L mol-1 cm-1). It decays with a rate constant of 5 × 104 s-1. Reduction potentials of the radical cation and the anio...

Formation and chemistry of radical anions in the gas phase

The research concerned with the ion/molecule chemistry of radical anions is reviewed together with the thermochemical properties of these ions and the related neutral species. Attention has been given to the formation of radical anions in the gas phase by various pathways, such as (dissociative) electron attachment and bimolecular reactions. A brief discussion of the electron affinity of the neutral species related to a given radical anion has been included, because this parameter is important for the discussion of the finding that an HA˙ radical is often more acidic than the related H2A molecule. In addition, the thermochemical properties form the basis for a discussion of the various processes that may occur in reactions of radical anions with organic molecules. In particular, electron transfer, proton transfer, and hydrogen atom abstraction reactions have been described for selected radical anions. Other processes that have been reviewed include SN2 substitution and an attack on a carbonyl function in an organic molecule. The interplay between these latter two processes has been described for the reactions of some halogen-substituted carbene radical anions with the methyl ester of trifluoroacetic acid and dimethyl carbonate. The structural characterization of radical anions, distonic radical anions in particular, is summarized with an emphasis on methods such as isomer-specific ion/molecule reactions and collision-induced dissociation or charge-reversal processes. © 1998 John Wiley & Sons, Inc., Mass Spectrom Rev 16, 181–200, 1997

Laser photolysis study on reactions of sulfate radical and nitrate radical with chlorate ion in aqueous solutions Formation and reduction potential of ClO3 radical

Electron transfer reactions of sulfate radical anion (SO 4 - ) and nitrate radical (NO 3 ) with chlorate ion (ClO 3 - ) have been studied by laser photolysis. Reaction (1) leads to the formation of chlorine trioxide radical (ClO 3 ) which has an absorption peak at 330 nm. Simulation work gave estimated results including: the forward and reverse rate constants of reaction (1), [k f =(4.0±0.5)×10 6 dm 3 mol -1 s -1 , k r =(5±5)×10 5 dm 3 mol -1 s -1 at ionic strength I=0], the second-order decay rate constant of ClO 3 [2k=(9.0±1.6)×10 8 dm 3 mol -1 s -1 ], and its molar absorption coefficient [e=(1500±250) dm 3 mol -1 cm -1 at 330 nm]. Reaction (2) was also observed and the forward and reverse rate constants were determined to be k f =(9.0±1.2)×10 3 dm 3 mol -1 s -1 and k r =(8.3±1.0)×10 2 dm 3 mol -1 s -1 . The equilibrium constant K of reaction (2) was determined to be 42±6, from the observed absorbance of the radicals at equilibrium. The reduction potential of ClO 3 , E 0 (ClO 3 /ClO 3 - ) was deduced to be 2.35±0.05 V vs. NHE when E 0 (NO 3 /NO 3 - ) is taken as 2.45±0.05 V vs. NHE.

Collision-induced oxygen addition to chloroaromatic compounds

The reactions of chloroaromatic radical anions with oxygen were studied with a triple quadrupole mass spectrometer. Two chlorobenzenes and eight polychlorinated biphenyls were analyzed by gas chromatography-tandem mass spectrometry under negative ion chemical ionization. The molecular radical anions were selected with the first quadrupole and reacted with oxygen in the collision cell. Under these conditions, [M+O - Cl] ions were obtained with intensities similar to those of the transmitted precursor ions. This dechlorination reaction was not affected by a detectable chlorine isotope effect. The intensities of the [M+O - Cl] ions vary with the nature of the chloroaromatic compounds and with the oxygen pressure and collision energy. Charge transfer reactions are also observed, and the relative amount of O 2 (-.) produced is controlled by the relative electron affinity of the organochlorine. At high collision energies, collision-induced fragmentation of the molecular ion competes for the production of Cl(-).

Gas-phase Reactions of Isomeric Carbene and Distonic Radical Anions Derived from Methylthioacetonitrile with Dimethyl Disulfide

The gas-phase ionhnolecule reactions of the CH,SCDCN, CH,SCCN and 'CH,SCDCN ions formed in the reaction of the 0-' ion with CH,SCD,CN have been studied with the use of Fourier-transform ion cyclotron resonance (FTICR). In the reaction of the CH,SCDCN carbanion with the model substrate, dimethyl disulfide, a CH,SD molecule is expelled from the collisioncomplex with the indicated formation of a (CH,S),CCN ion. In the reactions ofthe carbene radical anion, CH,SCCN, loss of a CH,S radical occurs and the formation of an ion with a (CH,S),CCN structure is again indicated. A distinct reactivity is observed for the distonic radical anion, 'CH,SCDCN, which reacts with dimethyl disulfide by the competing losses of CH,SH and CH,S from the collision complex. It is proposed that the loss of CH,SH yields a CH,SCHSCDCN or CH,SCHS&CN radical anion, whereas the loss of CH,S is proposed to lead to a CH,SCH,SCDCN carbanion. The formation of these species in the reactions of the 'CH,SCDCN ion is discussed as involving an initial radical or nucleophilic attack on one of the sulfur atoms in dimethyl disulfide followed by cleavage of the sulfur-sulfur bond prior to the loss of a CH,SH molecule and/or a CH,S radical. The structure, stability and unimolecular dissociations of distonic ions have been studied extensively,'-' whereas insight into the gas-phase bimolecular chemistry of such ions is somewhat more limited.6,7 The interest in the bimolecular chemistry of distonic ions, however, may be considered to be of general significance, since the formal separation of the radical and charge sites offers the possibility of examining reactions which involve primarily only one of these centers. With respect to distonic radical cations, a number of studies indicate that these species can react differentIy from the isomeric conventional radical cations and/or the related cations. Notably, a distonic radical cation may react by hydrogen atom abstraction or a formal transfer of a functional group originating from the substrate, whereas the conventional radical cations often prefer to react by charge exchange if this process is exothermic.6, For example, the distonic radical cation, 'CH,CH,CH2C0 reacts with CH,SSCH, by a CH,S radical

Competing electron transfer, proton abstraction and nucleophilic substitutions in gas-phase reactions of (radical) anions with chloro- and bromomethanes

The product ion distributions and rates of the gas-phase reactions of two series of (radical) anions with chloro- and bromomethanes (CH3Cl, CH2Cl2, CHCl3, CCl4, CH3Br, CH2Br2, CHBr3 and CBr4) were determined with the use of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The first series consists of anions (HO−, CH3O−, C2H5O−, C3H7O− and CH3S−), for which the corresponding neutral radicals have a relatively high electron affinity (EA > 150 kJ mol−1). The second series consists of (radical) anions (CH2S−·, CH2(DOUBLE BOND)CHCH2−, CH2(DOUBLE BOND)C(CH3)CH2−, C6H4−· and C6H5−), for which the corresponding neutral species have a relatively low electron affinity (EA ≤ 100 kJ mol−1). These (radical) anions react mainly with the halomethanes to afford (i) halide ions, (ii) halomethyl anions with the same number of halogen atoms as in the parent halomethane and (iii) halomethyl anions with one halogen atom less than the parent substrate. The last process involves nucleophilic attack on a halogen atom and is particularly important in the reactions with substrates containing three or four halogen atoms. The halide ions may arise by a number of different pathways, such as SN2 substitution, α-elimination, halogen attack followed by dissociation of the thus formed halomethyl anion and overall dissociative electron transfer. The SN2 process is held responsible for the formation of halide ions in the reactions with monohalomethanes, whereas α-elimination is likely to be of importance only for the reactions with trichloro- and tribromomethanes. Attack on a halogen atom followed by dissociation of the ion generated initially is likely to be important if CCl4 or CBr4 is the substrate. Electron transfer is only a dominant pathway in the reactions of the CH2S−· ion with the halomethanes. The occurrence of electron transfer in the reactions of this ion with CHCl3, CCl4 and CHBr3 is evidenced by the formation of minor amounts of stable halomethane radical anions in addition to the generation of CH2SCl− or CH2SBr− ions and abundant halide ions. The interplay between the various possible reactions is discussed on the basis of thermodynamic considerations and the rates of the overall processes.

Towards the Definition of the Maximum Allowable Tightness of an Electron Transfer Transition State in the Reactions of Radical Anions and Alkyl Halides

A mechanistic continuum with a sharp changeover zone from electron transfer (ET) to substitution (SUB) reactions has been found in the analysis of the reactions of formyl radical anions with methyl halides (see below). The minimum C – C distance at which the transition state for ET collapses to that of SUB could be defined.

Dibromine radical anion reactions with heme enzymes

Reactions of Br2− radical anion with heme enzymes, catalase and horseradish peroxidase, have been studied by pulse radiolysis. It has been found that Br2− does not react with the heme centre of investigated enzymes. Dibromine radical anion reacts with tryptophan residues of catalase without any influence on the activity of catalase. It is suggested that in pulse radiolysis studies, where horseradish peroxidase is at about tenfold excess towards Br2−, the enzyme is modified rather by Br2, than by Br2−.

Self-exchange reactions of radical anions in n-hexane

The formation and reactions of radical anions in n-hexane at 190 K were investigated by pulse radiolysis and time-resolved fluorescence-detected magnetic resonance (FDMR). Electron attachment was found to occur for compounds with gas-phase electron affinities (EA) more positive than −1.1 ± 0.1 eV. The FDMR concentration and time dependence are interpreted as evidence for self-exchange electron-transfer reactions, indicating that formation of dimer radical anions is not prevalent for the range of molecules studied. FDMR detection of radical anions is mainly restricted to electron acceptors with EA less than approximately 0.5 eV.

Electron Transfer Reactions and Mobile Holes in Radiolysis of Squalane. Time-Resolved FDMR Study

Fluorescence detected magnetic resonance (FDMR) is applied to study ion−molecule reactions in squalane (2,6,10,15,19,23-hexamethyltetracosane). Several time-resolved pulsed FDMR experiments are suggested to measure rate constants of scavenging by aromatic solutes. These constants vary from 5 × 108 to 1.6 × 109 mol-1 dm3 s-1. The ion−molecule reactions of radical cations are somewhat slower than reactions of radical anions. The scavenging of the alkane hole is significantly faster than ion−molecule reactions of molecular cations. The mobile hole decays on a time scale of several tens of nanoseconds. These FDMR results are in accord with our recent work on transient absorption spectroscopy in radiolysis of squalane.

Photoradical ageing of polymers

New data referring to quantum efficiency of electronically excited macroradicals and radical anion reactions in various carbon- and heterochain polymers at temperatures below their glass transition or melting points are presented and discussed. The data on the mechanism of the photochemical reactions of peroxy radicals, the effect of various factors (photon energy, free volume, degree of crystallinity) on the direction and efficiency of reactions of electronically excited macroradicals are considered. The important role of photochemical chain reactions in photoageing is demonstrated. It is found that by varying photon energy, temperature, photolysis time and by taking advantage of the competition between thermal and photochemical reactions, it is possible to change the functional composition and molecular mass distribution of polymers in the desired direction.

.sigma.-Radical Anion as a Key Intermediate in the Dissociation Reaction of Aryl Halide Radical Anions

.sigma.-Radical Anion as a Key Intermediate in the Dissociation Reaction of Aryl Halide Radical Anions

Studies on the effect of sulfite on benzo[ a]pyrene-7,8-dihydrodiol activation to reactive intermediates in human polymorphonuclear leukocytes

Sodium sulfite, a hydrolysis product of the environmental pollutant sulfur dioxide increased the activation of (−)- trans-7,8-dihydroxy-7,8-dihydrobenzo[ a]pyrene (BP-7,8-diol) to the (+) anti-enantiomer of trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[ a]pyrene (BPDE) in phorbol myristate acetate (PMA)-stimulated human polymorphonuclear leukocytes (PMNs). This effect was potentiated in the presence of DMSO. No significant effect of sulfite on BP7,8-diol activation was observed in resting leukocytes. As revealed by the 32P-postlabelling technique the dominant adduct in both intracellular DNA and to DNA added to the leukocytes was (+)- anti-BPDE bound to the exocyclic nitrogen of deoxyguanosine. The mechanism underlying the stimulatory effect of sulfite on diol epoxide production and increased DNA-binding probably involves one-electron oxidation of sulfite to a sulfur trioxide radical anion and subsequent reaction with molecular oxygen to form the corresponding peroxyl radical. This step obviously requires PMA-initiated oxidative burst and thus, production of Superoxide radical anions (O 2⨪).

A comparison of the reactions of the oxide radical anion (O −· with simple aromatic compounds at atmospheric and at reduced pressure conditions

The reactions of oxide radical anions (O −·) with benzene and toluene under atmospheric pressure (APCI) and conventional chemical ionization (CI) conditions were compared. Hydrogen radical (H ·) displacement by oxygen, yielding [M − H + O] −, was observed in both the APCI and the CI source. However, the product, [M − 2H] −·, derived from dihydrogen radical ion (H +· 2) transfer which was observed in the CI spectra, was consistently absent under APCI conditions. This behavior is rationalized in terms of the higher pressures and chemical equilibrium associated with the APCI source. In addition to the formation of the a priori expected phenoxide isomers, the reaction of O −· with toluene to yield the [M − H + O] − product generates a benzyloxide anion. Tandem mass spectrometry data from collision-induced dissociation and isotopic labeling with deuterium support a reaction mechanism initiated by α hydrogen abstraction for both the H· and the H +· 2 transfer pathways.

Photoradical Ageing of Polymers

Abstract New data referring to the mechanism and quantum efficiency of electronically excited macroradicals and radical anion reactions in various carbon- and heterochain polymers at temperatures below their glass transition or melting points are presented and discussed. Basic mechanisms of the photochemical reactions, including new data on the mechanism of the photochemical reactions of peroxide radicals, the effect of various factors (photon energy, free volume, molecular organization of the medium) on the direction and efficiency of reactions of electronically excited macroradicals are considered. The important role of photochemical chain reactions in photoageing, including photooxidation of polymers, is demonstrated. It was found that by varying photon energy, temperature, photolysis time and by taking advantage of the competition between thermal and photochemical reactions, it is possible to change the functional composition and molecular mass distribution of polymers in some desired direction.

Radiation inactivation of bovine intestinal alkaline phosphatase

The effects of 60Co γ-radiation on aqueous solutions of alkaline phosphatase have been studied. The primary radicals of water radiolysis, e − aq, OH· and H· all contribute to the observed inactivation with inactivating efficiencies of 0.019, 0.019 and 0.04, respectively; O − 2 also causes inactivation (efficiency = 0.014). The radical anions (SCN) − 2, (Br) − 2 and (I) − 2 cause inactivation at neutral pH and evidence is presented that cysteine and histidine residues are sites for radical anion reaction. Kinetic evidence suggests that inactivation is due to general denaturation of the protein, rather than destruction of the substrate binding site. Fractionation of the irradiated solution using FPLC following by analysis using fluorescence spectroscopy suggests that one process which leads to inactivation is the formation of alkaline phosphatase dimerized via tyrosine residues.

Photosubstitution of Dicyanobenzenes by Allylic Silanes, Germanes, and Stannanes via Photoinduced Electron Transfer

Abstract Photoreactions of 1,2- and 1,4-dicyanobenzenes (o- and p-DCB) with allylic silanes, germanes, and stannanes in acetonitrile were studied under various conditions. In these photoreactions, one of cyano groups of o- and p-DCB was replaced by allylic groups. With trimethyl(3-methyl-2-butenyl)silane, -germane, and -stannane, the photosubstitution occurred at both α- and γ-positions of the allylic moiety in a constant ratio, regardless of the kinds of metals. 1,3-Dicyanobenzene (m-DCB) was less reactive to the photosubstitution reaction. The reactivity of group 14 organometallic compounds increased in the order: Si- < Ge- < Sn-compounds. The photoreactions were sensitized by aromatic hydrocarbons such as phenanthrene and triphenylene, and also accelerated by adding Mg (ClO4)2 and alkylbenzenes. Quantum yields for the formation of the substitution products increased with decreasing oxidation potentials of group 14 organometallic compounds. A key step for the photoreactions is the reaction of radical anions of dicyanobenzenes with allylic radicals that are generated by cleavage of radical cations of group 14 organometallic compounds. The mechanistic features of these photoreactions are discussed.