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It is proposed that both polar and radical reactions should be considered when discussing radical ion reactivity. The fact that only the polar reactivity has dominated previous discussions is pointed out. It is argued that if the configuration mixing model is applicable for the estimation of radical cation-nucleophile (or cation-nucleophile) combination reaction barriers, the theory should also explain the reactivity of the proton toward anionic nucleophiles. The model severely overestimates reaction barriers for the latter reactions. The discussion is centered around the following hypothesis: In the absence of severe steric effects, the reactivity of radical ions, especially evident in the reactions with radicals (for example very few radical cations react with dioxygen at measurable rates), is dominated by the degree of coupling between charge and radical centers. Evidence from the literature supporting the hypothesis is presented for a number of different classes of radical ion reactions. Further work to test the validity of the hypothesis is proposed for many of the reaction types. Triarylmethyl radicals and carbenium ions are suggested as models to which the radical and polar reactivities of 9,10-diarylanthracene radical cations may be compared. It is concluded that the radical cation/carbenium ion comparison (for the reaction with acetate ion) would show similar reactivities while the radical cation/free radical comparison (for the reaction with dioxygen) would fail, since no reaction at all would be observed with the radical cation while the free radical reacts rapidly.

The oxidation chemistry of the organometallic hydride complexes ({eta}{sup 5}C{sub 5}H{sub 5})M(CO){sub 3}H (M = Cr, 1; Mo, 2; W, 3), ({eta}{sup 5}-C{sub 5}Me{sub 5})Mo(CO){sub 3}H (4), and ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})H (5) has been investigated in acetonitrile solution by electrochemical and other methods. The thermodynamic acidities of the cation radicals of 1-5 have been estimated by the use of a thermochemical cycle based on the oxidation potentials of 1-5 and their conjugate bases (anions) as well as the solution pK{sub a} values of 1-5. The metal hydride cation radical pK{sub a} estimates fall in the range {minus}10 to +5, which makes these complexes the most acidic hydrides for which pK{sub a} values have been determined. Coulometric measurements show that 1-5 undergo overall two-electron oxidations. Chemical oxidation of 5 gives a 1:1 mixture of dihydride ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})H{sub 2}{sup +} (13) and ({eta}{sup 5}-C{sub 5}H{sub 5})W(CO){sub 2}(PMe{sub 3})(NCMe){sup +} (10), the latter as a mixture of cis and trans isomers in the thermodynamic 95:5 ratio.

Reactions de transfert de proton entre le radical cationique du (10-amino-9-phenyl) anthracene et des pyridines

The linear sweep voltammetry response to competitive radical ion-substrate coupling and radical ion dimerization mechanisms was determined by digital simulation. The simulations were carried out to mimic the conditions under which experimental studies had previously shown that the radical ion-substrate coupling mechanism is the preferred reaction pathway. It was observed that in order for the dependence of the peak potential on substrate concentration (delta Ep/delta log CA) to be in the experimentally observed range (36-40 mV/decade) that the relative rate constants for radical ion substrate coupling and radical ion dimerization (ki/kii) must be greater than about 10. It is pointed out that since the reactants, the transition states and the products differ by only a single electron that these competitive reactions represent an ideal test case for the configuration mixing (CM) model. The CM model predicts an electronic reaction barrier for reaction (i) but not for reaction (ii). The difference in standard free energy changes for reactions (i) and (ii) were estimated to be of the order of 7 kcal mol-1 or greater with (ii) being energetically more favorable than (i). It is concluded that the experimental data for the relative rates of reactions (i) and (ii) do not conform to the CM model predictions in the cases discussed. ArH(.+) + ArH-->+ ArH-ArH.(i) ArH(.+) + APH .+ --> + ArH-ArH+(ii).

Les entropies de formation des radicaux anioniques et cationiques sont determinees a partir de coefficients de temperature des potentiels reversibles de reduction et d'oxydation, dans divers solvants aprotiques. Cas du diphenyl-9,10 anthracene, de l'anthracene, du perylene et du triphenylene

Reduction electrochimique ou par Cp 2 Co des complexes [(η 5 -C 5 H 5 ) 2 M 2 (CO) 3 (μ-CCH 3 )] + , M=Fe, Ru: formation d'un radical neutre et decomposition en complexes pontes par l'ethylidene et le vinylidene

Determination de la relation entre la temperature et les potentiels d'electrode reversibles pour la formation de radicaux ioniques a partir de diphenyl-9,10 anthracene (9,10-DPA), de perylene et de triphenylene en presence de 6 sels differents de tetraalkylammonium. Les entropies de formation des radicaux ioniques dependent de la nature et de la concentration de l'electrolyte dans l'acetonitrile. L'etude la plus detaillee est celle de l'oxydation et de la reduction de 9,10-DPA

Reversible oxidation potentials, rate constants, and activation parameters are reported for the formation and decomposition reactions of all isomers of (methyl-substituted benzene)tricarbonylchromium cation radicals. The reversible potentials were observed to depend both on the number and the positions of the methyl substituents. It was found that the effects on the reversible potentials of adding single methyl substituents to similar structures were additive. The activation energies and entropies for the decomposition reactions were found to be grouped into four different categories depending upon the numbers of adjacent unsubstituted positions on the methylarene ligands. Within each group the deviations from the averages were less than experimental error. The differences in activation parameters are explained in terms of how effective bonding is between the chromium atom and the arene ligands in the cation radicals.

Les equilibres des radicaux anioniques de nitrobenzenes et benzophenones substitues avec l'eau et le methanol dans le DMF sont etudies dans l'intervalle de temperatures 273-298 K

Rate constants were determined for the combination reactions of a series of cation radicals derived from substituted anthracenes with acetate, p-nitrobenzoate, trifluoroacetate, nitrate and perchlorate ions. Rate constants, depending on the identities of the cation radicals and the nucleophiles, ranging from about 200 to 2 × 1010 1 mol−1 s−1 were observed in acetonitrile at 293 K. The key steps in the reaction are (1) reversible complex formation between the anion and the cation radical followed by (2) irreversible bond formation . The preliminary results show that cation radical–anionic nucleophile reactions can be very facile. The cation radicals of 9-nitro- and 9-cyanoanthracene are particularly reactive, giving rise to rate constants close to the diffusion-controlled limit with all anionic nucleophile studied and even react moderately rapidly with perchlorate ion. The reaction with perchlorate ion can be compared to the behavior of stable carbenium ions that coexist with the anion in solution and in crystalline salts.

The pKa and pH dependence of the formation of nitroxide radicals from the following drugs that contain an aliphatic secondary amino group, by oxidation with hydrogen peroxide, have been studied by ESR spectroscopy: Ephedrine, (1R,2S)-1-phenyl-2-methyl-aminopropanol, timolol, (S)-1-(tert-butyl-amino)-3-[(4-morpholino-1,2,3-thiadiazol-3-yl)ox y]-2- propanol, metoprolol, 1-[4-(2-methoxyethyl)phenoxy]-3-[(1-methylethyl)amino]-2-propanol and terodiline, N-tert-butyl-3,3-diphenyl-1-methylpropylamine. Radicals were formed from the non-ionized base only. Therefore, the pKa value of the amine and the pH of the reaction mixture is of crucial importance for the yield of nitroxide radical. At 37 degrees C the pKa values of 1-3 are about 9.2, and of 4 about 9.6, which means that 1.5% of 1-3, and 0.6% of 4, are present in the reactive base form at the physiological pH of 7.4. Horse-radish peroxidase was found both to enhance radical production and to decrease the life-time of the radicals formed in the reaction with hydrogen peroxide.

L'etude est faite par voltammetrie cyclique et par voltammetrie a balayage lineaire, dans l'acetonitrile

Reversible behavior for both electron transfers for oxidation of aromatic compounds to cation radicals and dications was observed in several common electrolytic solvents. Nitriles, nitro compounds and dichloromethane can all be rendered essentially nucleophile free for voltammetric purposes simply by conducting the voltammetric measurements over neutral alumina shortly after the mixture has been stirred. Solvents containing trifluoroacetic acid and the corresponding acid anhydride are not only useful for voltammetry but can also be used to prepare stable solutions of cation radicals and dications. Equilibrium constants for the disproportionation of cation radicals to dications were calculated from the reversible electrode potentials and the effect of changes in the solvent system on the equilibrium constants is discussed.

The reaction between 9-phenylanthracene (PAH) cation radical and acetate ion was studied to probe for an electron-transfer pathway. The decomposition of acetoxy radical to methyl radical and carbon dioxide, accompanied by the regeneration of PAH, is the expected result of an initial electron transfer. Rate-determining attack of acetate ion on PAH{sup {sm bullet}+} followed by rapid product-forming reactions could lead to a secondary {alpha} deuterium kinetic isotope effect. Neither of these results were observed. Attack by acetate ion on PAH{sup {sm bullet}+} in acetonitrile at 293 K was observed to take place at nearly diffusion control.

Biphenylen (I) wird in Methylenchlorid/Trifluoressigsaureanhydrid/Trifluoressigsaure elektrochemisch zum stabilen Kationradikal oxidiert.