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Probably the most prominent features of fullerenes are their highly symmetrical three-dimensional structure and the extended A-system [1–6]. These are strongly reflected in the entire chemistry of this class of carbon allotropes and derivatized compounds derived therefrom. Among those processes which have become especially attractive are electron and energy transfer reactions. They are essentially based on the fullerenes’ capacity to accommodate several electrons per molecule (up to 6), but also on the possibility to oxidize them via removal of an electron, and to excite them into easily detectable singlet and triplet states [7, 8]. A further parameter facilitating reactivity is the pyramidalization of the carbon atoms which renders them excellent targets for electrophilic attack by free radicals [9].

2-(3-Aminopropyl-amino) ethaneperthiol (RSSH, the perthiol analogue of the thiol radioprotector, WR-1065) reacts with the alpha-hydroxy alkyl radical (CH3)2C.OH by donating a hydrogen atom as indicated by the characterization of perthiyl radicals (RSS.; lambda max approximately 374 nm, epsilon 374 approximately 1680 +/- 20 dm3 mol-1 cm-1) by pulse radiolysis. The perthiyl radical abstracts a hydrogen from the alcohol to establish a reversible hydrogen-transfer equilibrium. This equilibrium lies predominantly on the side of radical repair since the rate constants for the forward and reverse reactions at pH 4 are: kappa(RSSH+(CH3)2C.OH) = (2.4 +/- 0.1) x 10(9) dm3 mol-1 s-1 and kappa(RSS.+(CH3)2CHOH) = (3.8 +/- 0.3) x 10(3) dm3 mol-1 s-1 respectively. The pKa (RSSH--RSS(-)+H+) = 6.2 +/- 0.1 was determined from the pH dependence of the rate of perthiol repair. Identical experiments have been performed with WR-1065 allowing a direct comparison of free-radical repair reactivity to be made with the parthiol analogue. At pH approximately 7.4 the reactivities of the thiol and perthiol were similar, both repairing the alcohol radical with a rate constant of approximately (2.4 +/- 0.1) x 10(8) dm3 mol-1 s-1. However, at pH 5 whilst the hydrogen-donation rate of the thiol was 15-20% higher than at pH 7.4, the perthiol reactivity was over an order of magnitude higher. The thermodynamic driving force for the observed enhanced free-radical repair reactivity of RSSH compared to RSH is attributed to the resonance stabilization energy of 8.8 kJ mol-1 within the RSS. radical. These results indicate a possible application of RSSH/RSS- as DNA-targeted antioxidants or chemoprotectors.

Vitamin E is involved in the protection of cellular membranes against lipid peroxidation by scavenging free oxygen centered radicals (1,2), e.g. in the general reaction.

The radiation chemical method of pulse radiolysis is introduced as a suitable tool for the analysis of time-resolved optical changes in radiolytically induced redox and alkylation reactions. A pseudo-first order reaction observable in the radiolysis of C60 in 2-propanol leads to the formation of the radical anion in the reaction of C60 with solvated electrons and dimethylhydroxymethyl radicals. One-electron oxidation in 1, 2-dichloroethane (1, 2-DCE) yielded C60 •+, achieved via interaction of C60 with the radiolytically generated oxidizing (l, 2-DCE)•+ radical cations. The reaction of methyl radicals with C60 resulted in absorption changes which are very different from redox reactions and are ascribed to the methylation of the fullerene. Corresponding experiments in aqueous solutions have been carried out by incorporating C60 into artificial bilayer membranes and larger organic molecules. Several absolute rate constants for the radical induced reactions have been determined.

Absolute rate-constants for the reaction of the nitroxyl free radicals TAN and TMPN with radiation-chemically-formed radicals and ions have been determined. k(TAN + X) (in M(-1) sec(-1)=4-0 X 10(9) (for X = OH-), 2-9 X 10(10) (eaq-), 8-0 X 10(9) (H-), 7-2 X 10(8) (-CH2OH), 4-0 X 10(8) (CH3CHOH), 4-3 X 10(8) ((CH3)2COH) 2-8 X 10(8) (-CH2(CH3)2COH), 5-9 X 10(7) (glucose radical), 4-0 X 10(8) (c-C5H9-), and k(TMPN + X)=3-4 X 10(9) (OH-), 7-8 X 10(9) (eq-), 4-9 X 10(9) (H-), 4-4 X 10(8) (-CH2OH), 4-9 X 10(8) (CH3CHOH), 3-6 X 10(8) ((CH3)2COH), 1-5 X 10(8) (-CH2(CH3)2COH), 4-9 X 10(7) (glucose radical), 4-3 X 10(8) (c-C5H9-). Direct measurements by means of a pulse-radiolysis conductivity technique were based on the formation and destruction of charged species in these reactions within certain pH ranges. It is indicated that the radiosensitizing nitroxyls undergo both redox and addition reactions.

Time-resolved pulse radiolysis studies have been carried out on the R2S+· R2S ⇆ (R2S.˙.SR2)+ equilibrium and associated decay kinetics of sulfur centered radical cations. - Equilibrium constants of K = 2.0 · 105 and 5.4 · 102 mol−1 dm3 at 20°C have been determined for the species with R = CH3 and (CH3)2CH, respectively. Absolute rate constants have been measured for the back reaction of the above equilibrium: k-1 = 1.5 · 104 s−1 and 5.6 · 106 s−1, respectively, at 20°C. Further rate constants refer to the deprotonation of R2S+ by various proton acceptors (phosphate, acetate and water). - ΔH≠ = 57 kJ mol−1 and 17 kJ mol−1 were determined for the dissociation of the sulfur-sulfur 3-electron bond in (Me2S.˙.SMe2)± and (i-Pr2S.˙.S i-Pr2)+, respectively. On this basis the S.˙.S bond energies were estimated to ≲80 kJ mol−1 and ≲40 kJ mol−1 for these two species. The kinetic and thermodynamic results substantiate previous conclusions on the electronic structure and relative stability of S.˙.S bonded radicals and corroborate some theoretical calculations.

Phosphite radicals HPO/sub 3/- and PO/sub 3//sup 2/-, which exist in an acid-base equilibrium with pK = 5.75, are shown to take part in various types of reactions. In the absence of scavengers, they disappear mainly by second-order disproportionation and combination; a first-order contribution to the decay is also indicated. HPO/sub 3/- and PO/sub 3//sup 2/- are good reductants toward electron acceptors such as tetranitromethane. In this reaction phosphate and C(NO/sub 2/)/sub 3/- are formed. Phosphite radicals can, however, also act as good oxidants, e.g., toward thiols and thiolate ions. These reactions lead to the formation of RS. radicals which were identified either directly, as in the case of penicillamine, through the optical absorption of PenS. or more indirectly through equilibration of RS. with RS- to the optically absorbing RSSR-. disulfide radical anion. A homolytic substitution reaction (S/sub H/2) occurs in the reaction of the phosphite radicals with aliphatic disulfides, yielding RS. radicals and phosphate thioester RSPO/sub 3//sup 2/-. Lipoic acid, as an example of a cyclic disulfide, is reduced to the corresponding RSSR-. radical anion and also undergoes the S/sub H/2 reaction with about equal probability. An addition reaction is observed between phosphite radicals and molecular oxygen. The resultingmore » peroxo phosphate radicals establish an acid-base equilibrium HPO/sub 5//sup -/. reversible PO/sub 5//sup 2 -/. + H+ with a pK = 3.4. Absolute rate constants were determined for all reactions discussed.« less

The reduction of carbon tetrachloride by hydrated electrons, H atoms and reducing radicals has been investigated by pulse radiolysis conductivity methods. It has been found that one Cl® ion is formed per reducing species. A chain reaction is observed in the γ-radiolysis of solutions of CCl4 and iso-propanol, diethylether, and tetrahydrofurane. Mechanisms and kinetic data are discussed.

Die absoluten bimolekularen Geschwindigkeitskonstanten der Reaktionen des Tetranitromethans mit hydratisierten Elektronen, Wasserstoffatomen, verschiedenen Alkoholradikalen und mit den Ionen O2−, CH3COCH3− und Cu+ wurden bestimmt. Ferner wird ein Verfahren beschrieben, um Tetranitromethan als „Indikator” fur freie Radikale, die keine starke Lichtabsorption besitzen, zu verwenden. Auf diese Weise wurde die Reaktion OH + CH3OH H2O + CH2OH untersucht. Alle Reaktionen des Tetranitromethans sind sehr schnell und werden im wesentlichen durch die Diffusion der Reaktanden kontrolliert. Die Bildung des aci-Nitroforms bei diesen Reaktionen wird auf die Ubertragung eines Elektrons von dem Reaktionspartner auf das Tetranitromethan mit sofortiger Dissoziation des negativen Molekelions zuruckgefuhrt. Freie neutrale Radikale ubertragen etwa gleich schnell wie negative Radikalionen, wahrend die Ubertragung vom Cu+-Ion langsamer geschieht. Fur die Reaktion des OH-Radikals mit Methanol wurde eine uberraschend hohe Geschwindigkeitskonstante von mehr als 3 · 109 Mol−1 1 sec−1 gefunden; auch dieser Prozes scheint demnach weitgehend durch die Diffusion der Reaktionspartner bestimmt zu sein. The absolute rate constants of the reactions between tetranitromethane and hydrated electrons, hydrogen atoms, various alcohol radicals and the ions O2−, CH3COCH3− and Cu+ have been determined. A procedure is also described to use tetranitromethane as „indicator” for free radicals which have no strong light absorption. The reaction OH + CH3OH H2O + CH2OH has been investigated this way. All reactions of tetranitremethane are very fast and essentially are controlled by the diffusion of the reactands. The formation of aci-nitroform in these reactions is explained by electron transfer from the reaction partner to tetranitromethane and immediate dissociation of the negative molecule ion. Neutral free radicals transfer the electron about as fast as negative radical ions while a slower transfer occurs from the Cu+-ion. A surprisingly high rate constant of more than 3 · 109 mole−1. 1 sec−1 has been found for the reaction of the OH radical with methanol. It therefore seems that this process is also determined by the diffusion of the reactands.

The reactions of chlorinated ethylenes with hydrated electrons and OH radicals have been investigated by using the method of pulse radiolysis. In addition γ-ray experiments were carried out. The reduction of the solutes occurs via a dissoziation electron capture process. The rate constant for the reaction of eaq ⊖ with the more chlorinated compounds is essentially diffusion controlled (k= (1 - 2×1010 l-mole-1 sec-1). Vinylchloride and 1,2-trans-dichloroethylene react more slowly. This can be related to the higher stability of the C-Cl bond in these compounds. Hydroxyl radicals add to the C=C double bond of the chlorinated ethylenes. The rate constant for the reaction with vinylchloride was determined to 7.1 × 109 1 · mole-1 sec-1, and decreases with increasing degree of chlorination of the ethylenes. This effect is explained by the decreasing electron density on the C-atoms and steric hinderance. The hydroxyl radical always adds to the C-atom carrying the smallest number of Cl-atoms. In its reaction with 1,2-dichloro-, trichloro- and tetrachloroethylene a radical is produced with an OH group and a Cl-atom on the same C-atom. It eliminates HCl to form a C=O bond with k>7 × 105 sec-1. The type radical produced in this reaction has an optical absorption in the near UV (ε265 nm = (1-3)×103 1 · mole-1 cm-1). The OH radical addition products of vinylchloride and 1,1-dichloroethylene do not eliminate HCl and have no absorption in the visible and near UV.

A significant property of an oxidized sulfur atom is its high tendency to coordinate with a free electron pair from a second sulfur atom. This leads to the establishment of three-electron bonds between the two sulfurs as formulated for organic sulfides. The electronic structure of the three-electron bond is characterized by two bonding sigma-electrons and one antibonding, i.e. bond weakening sigma/sup */-electron. Identification of most of these three-electron bonded systems has been made by e.s.r. and by optical spectroscopy in aqueous environment. A particularly useful tool has been proven to be the radiation chemical technique of pulse radiolysis which allows a time-resolved observation of short-lived species. Current questions of interest are the stability of these three-electron bonds and possible unique chemical characteristics of this and other types of sulfur-centered radicals. Both subjects are discussed, with particular emphasis on recent advances in the oxidation of organic sulfides. The examples discussed refer mostly to radiation chemical studies with aqueous solutions. In such experiments oxidations can conveniently be initiated by hydroxyl radicals formed upon irradiation of the water, or by secondary radicals derived therefrom.

Es wurden pulsradiolytische und γ-strahlenchemische Versuche zur Reduktion von halogenierten organischen Verbindungen durch hydratisierte Elektronen durchgefuhrt. Dissoziativer Elektroneneinfang setzt aus verschieden chlorierten Methanen, athylenen sowie fluorierten Benzolen Halogenionen frei. Diese wurden mit Hilfe von Leitfahigkeitsmessungen bei der Pulsradiolyse und mit Ionen-sensitiven Elektroden bei den γ-Versuchen quantitativ nachgewiesen. Die Geschwindigkeitskonstanten der Reaktion des eaq− mit den mehrfach halogenierten Verbindungen sind praktisch ausschlieslich durch die Diffusion der Reaktionspartner zueinander bestimmt. Bei den wenig halogenierten Substanzen verlauft die Reaktion langsamer, was mit der groseren Stabilitat der Halogen-Kohlenstoffbindung in Einklang steht. Die primaren Reaktionsprodukte sind Halogenionen und neutrale, kurzlebige Radikale, die keinerlei optische Absorption im Wellenlangenbereich von 250–800 nm aufweisen. Alkohol- und Atherradikale ubertragen ein Elektron auf CCl4. Dies stimmt mit der Tatsache uberein, das die Halbstufenpotentiale fur die Oxidation dieser Radikale negativer sind als dasjenige fur die Reduktion des CCl4. Die theoretisch ebenfalls mogliche Elektronenubertragung vom CO2−-Radikalion auf Tetrachlorkohlenstoff findet nicht statt, da offensichtlich eine zu hohe Aktivierungsenergie fur diese Reaktion vorliegt. The reduction of halogenated organic compounds by hydrated electrons has been investigated by means of pulse radiolysis and γ-irradiation. Halide ions are produced in a dissociative electron capture process by chlorinated methanes and ethylenes, and fluorinated benzenes. A quantitative analysis of the halide ion yield has been carried out by conductivity pulse radiolysis methods, and with ion specific electrodes. The rate constants of the reaction of hydrated electrons with the highly halogenated compounds were found to be essentially diffusion controlled. The slower reaction of eaq− with the less halogenated compounds is explained by the higher stability of the carbon-halogen bond in these molecules. The primary products of the dissociative electron capture process are halide ions and neutral radicals which have no optical absorption in the 250–800 nm range. Carbon tetrachloride is also reduced by an electron transfer reaction from alcohol and ether radicals. This agrees with the fact that the halfwave oxidation potential of these radicals is more negative than the halfwave reduction potential of CCl4. A theoretically possible electron transfer process from CO2− ions to CCl4 has not been observed, presumably due to a high activation energy of this reaction.

Tetranitromethane (TNM) reacts by a dissociative electron capture process with solvated electrons in isopropanol (kTNM + e–s= 2.45 × 1010 M–1 s–1) to produce nitroform anions, C(NO2)–3, and NO2. Iso-propanol radicals and iso-propanolperoxy radicals also reduce TNM via an electron transfer reaction with bimolecular rate constants of 7 × 109 M–1 s–1 and 9 × 108 M–1 s–1, respectively. In neutral and acid (HClO4) solutions C(NO2)–3 ions are neutralized to C(NO2)3H by reaction with solvated protons (k= 3.2 × 108 M–1 s–1) and by a proton transfer reaction from isopropanol molecules (k= 4.5 × 102 M–1 s–1). C(NO2)3H dissociates with k= 3.2 × 103 s–1. The equilibrium constant of C(NO2)3H ⇌ C(NO2)–3+ H+sol in isopropanol was determined to be K= 1.0 × 10–5 M. The yield of scavengable reducing species in irradiated isopropanol increases from G= 3.5 at 6.3 × 10–6 M TNM to G= 6.4 at 3.2 × 10–2 M TNM in argon saturated solutions, and from G= 0.75 to G= 4.2 for the same concentrations of TNM in O2 saturated solutions. G(free solvated electrons) was measured to be 1.2. The formation and disappearance of ions have been observed directly by means of pulse radiolysis conductivity measurements. The specific equivalent conductance of C(NO2)–3 ions and solvated protons was determined to be ΛC(NO2)3–+ Hsol+= 51 Ω–1 cm2 equiv.–1.

The absorption spectra of the radicals HC6H5NO2 and OHC6H5NO2 in aqueous solutions have been obtained. These transients have a strong absorption at 4100 A and their molar extinction coefficients are both 3700 M–1 cm–1. The following rate constants have been measured : k(H+C6H5NO2)=(5.6±0.6)× 109 M–1 sec–1; k(OH+C6H5NO2)=(4.7±0.5)× 109 M–1 sec–1; k(O2+OHC6H5NO2)=(2.3±0.3)× 106 M–1 sec–1; k(OHC6H5NO2+OHC6H5NO2)=(6.0±0.4)× 108 M–1 sec–1. About 25% of the reactions between two nitro-hydroxycyclohexadienyl-radicals lead to nitrophenol-isomers by disproportionation. The majority of these reactions lead to other products (probably dimers by combination) which exist in acid and basic forms, the latter having strong absorptions in the visible range. These products are believed to be derivatives of nitro-cyclohexadiene.