Stable Radical Anions Research Articles

Dependence of Radical Anion Stability on Crystal Structure

Abstract An INDO calculation using the RHF/CI method indicates that the minimum energy conformation of the [XCH2CONH2]− radical anions where X = F, C1, Br and I occurs if the C-X bond is located in a direction approximately normal to the CON plane. It has been experimentally shown that for these radical anions to be observed in single crystals by EPR methods, the molecular precursor should not exist in the energy maximum conformation for radical anions or else spontaneous elimination of X− occurs upon electron attachment. This suggests that F− elimination from dopants (e.g. AsF5) used in the formation of conducting polymers may be a general phenomenon if the anion is formed from a maximum energy configuration.

Quantum-chemical investigation of the stability of radical-anions and the electron affinity of chlorine-substituted methanes and cyclopropanes

1. The radical-anions of chloromethanes and 1-chloro- and 1,1-dichlorocyclopropanes were calculated by the MINDO/3 method with variation of all the independent degrees of freedom. 2. The accumulation of chlorine atoms at one carbon atom leads to weakening of the C-Cl bond in the chloroalkane and to increase in the adiabatic electron affinity. The last effect predominates and leads to an increase in the strength of the C-Cl bond in the radical-anion with the accumulation of chlorine atoms.

Electrochemical obtention of stable 1,4-dihydroxybenzene dibenzoate lithium radical anion

The electrochemical reduction of 1,4-benzoquinone in dry acetonitrile/anhydrous lithium perchlorate in presence of benzoyl chloride, carried out under controlled cathodic potential, yields a stable radical anion species which is characterised as 1,4-dihydroxybenzene dibenzoate lithium radical anion.

Radical–nucleophilic substitution (SRN1) reactions: electron spin resonance studies of electron capture processes

We have probed two of the steps postulated for the radical–nucleophilic substitution (SRN1) mechanism for α-substituted aliphatic nitro-compounds [Me2C(X)NO2] by detecting the intermediates involved by e.s.r. spectroscopy. These two steps are the electron capture by Me2C(X)NO2 to form radical-anions, and their break-down to yield radicals and anions [equations (1) and (A2) or (B2) in Scheme 1]. A range of radical-anions, [Me2C(X)NO2]–˙ with X = Br, Cl, SCN, NO2, CN, PO3Et2, CO2Et, COMe, SO2Me, SO2Ar, and Me have been unambiguously identified by e.s.r. spectroscopy and shown to be infinitely long lived at low temperature. Our results indicate that one or more of at least three pathways are followed on the reaction of Me2C(X)NO2 with electrons: (a) electron-capture to yield a stable radical-anion, [Me2C(X)NO2]–˙, (b) dissociative electron capture to yield Me2ĊNO2 and X–(for X = Br, Cl, and SCN); (c) dissociative electron-capture to yield Me2ĊX and NO2–(for X = CN, NO2, PO3Et2, and CO2Et). The effect of solvent on these processes, and the structure of the radical-anions and the radicals Me2ĊNO2, Me2ĊPO3Et2, and Me2ĊCN are discussed.

Reduction electrochimique du nitrobenzene, en milieu micellaire aqueux, a divers pH. Influence de la nature des tensio-actifs

The electrochemical reduction of nitrobenzene has been studied in the presence of some anionic, cationic and non-ionic surfactants using a glassy carbon electrode or a dropping mercury electrode (polarography). The results are depending upon the electrode nature, the surfactant polarity and the pH of the solutions. On glassy carbon electrode and with cationic surfactants, the reduction is always a one step process for the totality of the studied pH scale (2–12). With the anionic and non-ionic surfactants, the potential is shifted towards the more negative values for a one step process or for a two step process, according to the pH value. For the polarographical studies, the electrochemical reduction of nitrobenzene is generally retarded by the presence of monomer surfactant molecules adsorbed on the mercury electrode, principally in acid media. In micellar solutions, the CTAB decreases the protonation rate of the nitrobenzene radical anion and facilitates the reduction by a dianion way. The anionic and non-ionic surfactants increase the radical anion stability and slow down the rate of the radical anion reduction.

Electrochemical oxidation of several oxocarbon salts in N, N-dimethylformamide

The oxocarbon salts of croconic acid and its dicyanomethylene derivatives have been shown to undergo two consecutive reversible one-electron transfers in N, N-dimethylformamide to produce stable radical anions and the neutral croconates. Disproportion equilibrium constants were found to be quite small for all the crononate radical anions investigated. Following chemical reactions accompanied the second oxidation process of dicyanomethylene-substituted crononates. Substituent effects were shown to be ring position-independent and are discussed with respect to the unique resonance structure of the crononates.

Electrochemical reduction of sym- and unsym-benzoferroles. The generation of stable radical anion and dianion species

Voltammetric investigations on sym- and unsym-benzoferroles showed both are reduced in two reversible one-electron steps. While sym-benzoferrole undergoes the two reduction processes at the same potential giving a stable dianion, the reduction potentials for unsym-benzoferrole are separated yielding a stable radical anion while the dianion undergoes a relatively fast decomposition. Interpretation of this behavior is made by a comparison with ferrole itself and a correlation of electrochemical, spectroscopic and MO calculations.

Some electrochemical and chemical properties of methoxatin and analogous quinoquinones.

The present study establishes relationships between structure and reactivity for the pyrroloquinoline and phenanthroline quinones. The electrochemical reductions of 1,7- and 1,10-phenanthroline-5,6-quinones, like other quinones, are reversible and occur by 2e- transfer in a single step in aqueous solution and by two 1e(-)-transfer steps in aprotic media. The electron-withdrawing pyridine moieties both increase their potentials and stabilize their aprotic semiquinones. The electrochemistry of the cofactor methoxatin and its trimethylester derivative is similar to the phenanthroline quinones in aqueous solution. However, the electrochemical reductions of methoxatin and its triester in aprotic solutions are characterized by at least three potentials, each accounting for less than 1e-. This has been explained by the proposal of semiquinone complexing with itself and with quinone. Despite an electron-donating pyrrole moiety, methoxatin and its trimethylester have relatively high potentials in aprotic solution. This is presumably due to stabilization of radical anions by the aforementioned complexing or by delocalization with carboxylic acid and ester groups. The reduction potential of methoxatin, in both aqueous and aprotic solvent, suggests that oxidation of methanol should be a thermodynamically favorable process. No evidence for an electrochemically reduced state lower than the quinol was found for any of the compounds. Chemical reactivity is influenced by the orientation of the pyridine nitrogen. The two quinones with a pyridine nitrogen peri to a quinone carbonyl add and oxidize nucleophiles most readily.

EPR spectroscopy and polarography of nitroazoles. 3. Nitropyrazoles

The results of an EPR investigation are presented with polarographs of electrochemical excitation of nitropyrazoles in acetonitrile. It is shown that 3- and 4-nitropyrazoles are excited in two single-electron stages with formation of dianion radicals. Upon excitation, N-alkylnitropyrazoles form stable anion radicals. 1-Nitro- and 1,4-dinitropyrazoles are excited upon splitting off of the NO2 anion. Excitation potentials of the nitropyrazoles and hyperfine interaction constants for the corresponding ion radicals are given.

ESR Spectra of Anion Radicals of 8,8-Dicyanoheptafulvene and Its 4,5-Benzo Derivative

Abstract The hyperfine splitting constants for the 8,8-dicyanoheptafulvene anion radical generated electrolytically at low temperature indicate that the unpaired spin belongs to an a2 orbital of the group C2v. Its 4,5-benzo derivative gives a stable anion radical by reduction with alkali metals. The odd electron belongs to a b2 orbital.

Properties of electrochemically generated primary anion radicals of 2,5-dimethyl-1-nitrophenylpyrroles

The first step of electrochemical reduction of 2,5-dimethyl-1-nitrophenylpyrroles on a platinum electrode in anhydrous acetonitrile consists in an one-electron process producing relatively stable anion radicals. The prediction based on HMO calculations concerning an almost complete localization of the LFMO in the nitrophenyl part of the studied molecules is fully confirmed by means of EPR spectroscopy. Magnitude of the found proton splitting constants is very close to the values found for anion radical of nitrobenzene. The splitting by nucleus of nitrogen atom of pyrrole cycle can be explained exclusively by the mechanism of π-σ spin polarization of σ C-N bond and by the spin population at the carbon atom. The EPR method has also been used for following the kinetics of decomposition of anion radicals of the molecules studied.

Electrochemical reduction of para-substituted 2-acyl-5-phenylfuranes in dimethylformamide

para Substituted chloro, bromo, and nitro derivatives of 2-acyl-5-phenylfurane are reduced polarographically in a one-electron wave to the corresponding anion radicals, which were studied by the EPR method. The reduction of nitro derivatives, studied by the Kalousek switch, is reversible and leads to a stable anion radical with an unpaired electron center on the nitrogen nucleus; the reduction of the halogen derivatives is only partly reversible and leads to unstable ketyl radicals. The bromo derivatives give polarographic maxima typical for concurrent reactions.

Radical ions, 41. Durosemiquinone and its BF Analogue – Detection of 1,4‐Diborine as an Unexpected Elimination Product

AbstractThe photoelectron spectra and the one electron‐reduction behaviour of 1,4‐difluoro‐2,3,5,6‐tetra‐methyl‐1,4‐dibora‐2,5‐cyclohexadiene (1) and of the isoelectronic duroquinone (2) are compared. While duroquinone forms a very stable anion radical, durosemiquinone, the radical anion of the BF analogue was observed only as a short‐lived intermediate. In the presence of potassium metal a secondary anion radical is formed, which can be stabilized considerably by complexation of the K+ cation with dicyclohexyl‐18‐crown‐6. ESR analysis demonstrates that the latter species is the radical anion of 2,3,5,6‐tetramethyl‐1,4‐diborine (3˙⊖), a hitherto unknown benzenoid heterocycle.

Electrochemical behaviour of heterocyclic amine derivatives in media with controlled proton availability: Part III. Reduction mechanism and electron spin resonance study of cyanoquinolines and their N-oxides in DMF

The electroreduction of some cyanoquinolines and their N-oxides in dimethylformamide was studied by a variety of techniques, principally dc and ac phase-selective polarography, potential-sweep voltammetry and ESR spectrometry. Quinolines are reduced in two monoelectronic steps. The first is reversible and leads to a stable anion radical investigated by ESR spectroscopy; the second leads to the 1,4-dihydroquinoline di-anion, which is sufficiently basic to capture protons from any source near the electrode. Quinoline-1-oxides exhibit three reduction steps. The first is a reversible one-electron transfer producing a stable radical. The second electron transfer is followed by a very rapid dehydration producing the corresponding free base, which is further reducible at the same potentials. The third step leads to the 1,4-dihydroquinoline di-anion. The results were compared with those for the previously studied 2-phenyl quinolines and their N-oxide derivatives, with emphasis on the influence of substituents in determining the direction of the electrode process.

Redox reaction of tri(2-deoxy-2- L-ascorbyl)amine. Formation of a persisting free radical species in water

Electrochemical studies showed that tri(2-deoxy-2- L-ascorbyl)amine ( 1 ), a new compound isolated as one of the products of reaction of dehydro- L-ascorbic acid with phenylalanine (with added L-ascorbic acid) in ethanol, is oxidized in aqueous solution in two reversible one-electron transfer steps, on mercury or platinum electrode. The first step occurs through the dianion, and its product is an unusually stable blue anion radical giving a characteristic ESR signal. The product of the second step of oxidation is labile and is slowly converted into the oxidized form of di(2-deoxy-2- L-ascorbyl)amine, presumably by hydrolysis with splitting of L-ascorbic acid.

Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability Part II. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinoline 1-oxides in DMF

The electroreduction of some 4-substituted 2-phenylquinoline 1-oxides in dimethylformamide with and without a protonating agent was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry. In an aprotic medium the 1-oxides studied exhibit three or four subsequent steps. In every case the first is a reversible one-electron transfer producing a stable anion radical characterized by the ESR spectra. Potential sweep voltammetry give evidence of a chemical reaction following the second electron transfer producing the corresponding free bases, which are also electroactive at the same potentials. In a protic medium two subsequent bi-electronic (or only one tetraelectronic step) can be observed, the corresponding free bases and then 1,4-dihydroquinolines (or directly 1,4-dihydroquinolines) being the final reduction products. According to the experimental evidence a multistep scheme is suggested in which the role played by the substituents in determining the electroreduction pattern is considered.

Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability Part I. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinolines in DMF

The electroreduction of some 4-substituted 2-phenylquinolines in dimethylformamide solutions was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry, in the presence and absence of protonating agents. In a protic medium 2-phenylquinolines are reduced via a bielectronic polarographic step to the corresponding 1,4-dihydroquinolines. In an aprotic medium two well-defined reduction steps are observed. The first is a reversible one-electron transfer producing a stable anion radical characterized by ESR spectroscopy. The second is an irreversible one-electron transfer producing the di-anion. A multistep scheme for the reduction process consistent with the experimental results is proposed.

Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability: Part II. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinoline 1-oxides in DMF

The electroreduction of some 4-substituted 2-phenylquinoline 1-oxides in dimethylformamide with and without a protonating agent was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry. In an aprotic medium the 1-oxides studied exhibit three or four subsequent steps. In every case the first is a reversible one-electron transfer producing a stable anion radical characterized by the ESR spectra. Potential sweep voltammetry give evidence of a chemical reaction following the second electron transfer producing the corresponding free bases, which are also electroactive at the same potentials. In a protic medium two subsequent bi-electronic (or only one tetraelectronic step) can be observed, the corresponding free bases and then 1,4-dihydroquinolines (or directly 1,4-dihydroquinolines) being the final reduction products. According to the experimental evidence a multistep scheme is suggested in which the role played by the substituents in determining the electroreduction pattern is considered.

Electrochemical behaviour of heterocyclic amine derivatives in media of controlled proton availability: Part I. Reduction mechanism and electron spin resonance study of 2-phenyl 4-substituted quinolines in DMF

The electroreduction of some 4-substituted 2-phenylquinolines in dimethylformamide solutions was studied by dc and ac phase selective polarography, cyclic voltammetry, controlled-potential coulometry, ESR and UV spectrometry, in the presence and absence of protonating agents. In a protic medium 2-phenylquinolines are reduced via a bielectronic polarographic step to the corresponding 1,4-dihydroquinolines. In an aprotic medium two well-defined reduction steps are observed. The first is a reversible one-electron transfer producing a stable anion radical characterized by ESR spectroscopy. The second is an irreversible one-electron transfer producing the di-anion. A multistep scheme for the reduction process consistent with the experimental results is proposed.

Spectroelectrochemistry of retinal Electrodimerization in the presence of proton donors

Electrochemical reduction of retinal in acetonitrile containing 0.1 M tetra- n-butylammonium perchlorate at the first wave ( E 1/2=−1.38V vs. SCE) is a one-electron process to form a moderately stable radical anion (λ max=515 nm). At this potential the radical anion can be protonated and further reduced to form an anion (λ max=400 nm), which is easily oxidized ( E 1/2=−0.68V). The same anion is formed upon reduction of retinal at the potential of the second wave ( E 1/2=−1.72V). Similar electrochemistry is observed in N,N-dimethylformamide. Addition of proton donors to the solution favors a one-electron electrodimerization, as opposed to a two-electron reduction; and, with water, phenol, or acetic acid, a mixture of dimeric products is obtained. In contrast, malonate esters such as diethyl malonate are unique because they promote dimerization at the carbonyl carbon to produce high yields of retinal pinacol.