scispace - formally typeset
Search or ask a question

Showing papers on "Radical ion published in 1976"


Journal ArticleDOI
TL;DR: In this paper, the e.s.r. spectra of phenoxyl radicals were used to determine the pKa of the radical cations, which varied from −2.0 (phenol radical cation) to −0.8 (hydroquinone radicalcation).
Abstract: Phenoxyl radicals were generated in solutions containing from 0 to 75 % sulphuric acid. The changes observed in the e.s.r. spectra over this range are shown to be due to progressive formation of phenol radical cations. In favourable cases plots of the e.s.r. parameters against acidity enabled us to determine the pKa of the radical cations, which varied from –2.0 (phenol radical cation) to –0.8 (hydroquinone radical cation). Changes in the coupling constants in going from a phenoxyl radical to the corresponding phenol radical cation are usually relatively small, but the g-values change from 2.004 4 ± 0.000 4 to 2.003 2 ± 0.000 4. The free energy changes involved in competing oxidative processes are discussed and the data support the view that the C—O group in phenoxyl resembles that in ketones.

247 citations



Journal ArticleDOI
15 Oct 1976
TL;DR: In this article, the magnetic field modulation of the pyrene triplet and pyrene anion concentrations formed through quenching of singlet excited pyrene molecules by N,N -diethylaniline in methanol at room temperature was investigated.
Abstract: Measuring transient absorptions on the nanosecond time range, we observed the magnetic field ( H = 40 Oe) modulation of the pyrene triplet and pyrene anion concentrations formed through quenching of singlet excited pyrene molecules by N,N -diethylaniline in methanol at room temperature. We interpret this effect as being due to a magnetic field dependent triplet production in the geminate radical ion pair, induced by hyperfine interaction of the unpaired electrons. The discussion is based on a simple model related to the radical pair model in CIDNP theory. Moreover, the sign of the magnetic field modulation of triplets and radical ions allows to infer on the relative probability for the formation of excited and ground state molecules in ion recombination processes.

111 citations


Journal ArticleDOI
TL;DR: Electron transfer rates can be predicted using the Marcus theory from rates of the self-exchange reactions for four benzoquinone derivatives in aqueous solutions in good agreement with the experimentally measured rate constants.
Abstract: Rate constants for self-exchange of electrons between semiquinone radical ions and their parent quinones were measured for four benzoquinone derivatives in aqueous solutions (containing 5 M 2-propanol and 1 M acetone at pH 7) This was accomplished by measuring the effect of the concentration of the quinone on the ESR line width of the semiquinone radical ion produced by in situ photolysis The quinones studied were benzoquinone, 2,5- and 2,6-dimethylbenzoquinone, and duroquinone The rate constants for the electron exchange for these couples were found to be in the range of 05 to 2 x 10/sup 8/ M/sup -1/s/sup -1/ Electron transfer from one semiquinone to another quinone in this group of benzoquinone derivatives was measured using kinetic spectrophotometry and pulse radiolysis Since one-electron reduction potentials for all of these quinone/semiquinone couples are known from pulse radiolysis determination of the equilibria, electron transfer rates can be predicted using the Marcus theory from rates of the self-exchange reactions The calculated rates thus obtained are in good agreement with the experimentally measured rate constants Reaction radii could be estimated and further correlations between the rates of electron transfer and the free energy changes were checked

74 citations




Journal ArticleDOI
TL;DR: In this paper, the sequential two photon dissociation of cyanobenzene radical cation, C6H5CN+, to produce predominantly C 6H+4 and HCN was investigated.

49 citations


Journal ArticleDOI
TL;DR: In this article, the evolution of the singlet character of correlated ion radical pairs, prepared by high-energy impact in alkane solutions, is monitored by nanosecond-time-resolved solute-recombination fluorescence.
Abstract: The evolution of the singlet character of correlated ion radical pairs, prepared by high-energy impact in alkane solutions, is monitored by nanosecond-time-resolved solute-recombination fluorescence. Results obtained in the presence and absence of magnetic fields ($Bl~4$ kG) show evidence for damped periodic modulations ($\ensuremath{\omega}\ensuremath{\approx}10$ MHz) of fluorescence intensity. They are explained in terms of hyperfine-interaction-induced coherent singlet-triplet mixing, and of field-dependent spin-lattice relaxation.

44 citations


Journal ArticleDOI
01 May 1976-Lipids
TL;DR: In this paper, a radical scavenging reaction of α-tocopherol was studied and two main products, a geminal dimethyl cyclohexadienone and methyl ether, were obtained and these structures were determined by 13C nuclear magnetic resonance spectroscopy.
Abstract: To study a radical scavenging reaction of α-tocopherol, it was reacted with methyl radical in dimethyl sulfoxide. Two main products, a geminal dimethyl cyclohexadienone and methyl ether of α-tocopherol, were obtained and these structures were determined by13C nuclear magnetic resonance spectroscopy. The radical methylation data of α-tocopherol suggested that a delocalized radical species would be an intermediate.

38 citations



01 Jan 1976
TL;DR: When benzene or phenol are adsorbed on smectite containing Fe(III) or Cu(II) on the exchange complex, higher molecular weight products are formed.
Abstract: When benzene or phenol are adsorbed on smectite containing Fe(III) or Cu(II) on the exchange complex, higher molecular weight products are formed. Evidence for these products is provided by mass spectroscopy. In the case of benzene most of the reaction products cannot be attributed to coupled benzene rings, while extensive coupling occurs for phenol. Electron spin resonance spectroscopy provides evidence for radical cation formation of the aromatic molecules. The involvement of these radical cations is thus proposed in the formation of the higher molecular weight species. The relevance of these reactions to the natural systems of soils and sediments is suggested.

Journal ArticleDOI
TL;DR: In this article, the energies of the four ∏-state of tetraacetylene radical cation were determined by He(I) photoelectron spectroscopy, and they were shown to be the same as those of the other four ∆-state radical cations.

Journal ArticleDOI
TL;DR: In this paper, a model for peroxidase catalyzed oxidation of indole-3-acetic acid to carboxaldehyde was presented, which supports the earlier inference (Vidigal et al., 1975) that the excited aldehyde is generated in the enzymic process.
Abstract: — The thiophenyl ester of indole-3-acetic acid and indole-3-acetonitrile produce chemiluminescence in aerated dimethylsulfoxide in the presence of potassium t-butoxide. The emitter is the aromatic aldehyde. In the case of acetonitrile, the other product expected from the cleavage of an intermediate dioxetane, cyanate/isocyanate, has also been identified. Other auxins also chemiluminesce under similar conditions, but the emitters have not been properly identified. These systems are models for the peroxidase catalyzed oxidation of indole-3-acetic acid to indole-3-carboxaldehyde and as such support the earlier inference (Vidigal et al, 1975) that the excited aldehyde is generated in the enzymic process. An additional result is the observation of an exciplex between excited indole-3-carboxaldehyde and the thiophenylester of indole-3-acetic acid. This appears to be the first case of chemical generation of an exciplex by a route other than radical ion reaction, presumably by the dioxetane route.


Journal ArticleDOI
TL;DR: In this paper, a simulation of the photochemical A/D-secocorrin corrin cycloisomerization by redox processes is presented, showing that a close parallelism in their chemical reactivity seems to exist between electronically excited A/Dssecocorrins and corresponding radical cations.
Abstract: Experiments on a simulation of the photochemical A/D-secocorrin corrin cycloisomerization by redox processes. Electrochemical oxidation of nickel(II)-1-methylidene-2,2,7,7,12,12-hexamethyl-15-cyano-1,19-secocorrinate perchlorate Can the act of light excitation in the photochemical A/D-secocorrin corrin cycloisomerization be replaced by redox reactions in the dark? Electrochemical oxidation of nickel(II)-A/D-secocorrinate 4 in acetonitrile containing a trace of water produces the secocorrinoxide–nickel-complex 5 (structure determined by X-ray analysis) in almost quantitative yield. This two-electron oxidation involves a hydrogen shift from the methylene group C(19) in ring D to the methylidene carbon atom at ring A in the radical cation intermediate. Since the same type of hydrogen shift occurs in the photochemical A/D-secocorrin corrin cycloisomerization, a close parallelism in their chemical reactivity seems to exist between electronically excited A/D-secocorrins and corresponding radical cations. Formation of the corrin complex 2 (M = Ni+) could be achieved (so far only in modest yields) by electrochemical one-electron oxidation of 4 in acetonitrile/acetanhydride/acetic acid 8:1:1 followed by one-electron reduction. – The transformation of the oxide nickel complex 5 to the corrinoid complex 10 – a new member in the family of dehydrocorrins – is also recorded.

Journal ArticleDOI
TL;DR: In this paper, the triplet character of the electrogenerated chemiluminescence (ECL) was confirmed by experiments involving energy transfer from the benzophenone triplet to naphthalene.

Journal ArticleDOI
TL;DR: In this article, the authors studied methanol solutions of the radical cation of paraquat, 1,1′-dimethyl-4,4′-bipyridylium dichloride.
Abstract: We have studied methanol solutions of the radical cation of paraquat, 1,1′-dimethyl-4,4′-bipyridylium dichloride. The u.v.–visible spectrum changes with temperature in a very similar way to that for the corresponding morphamquat system. E.s.r. experiments show that, although above –20 °C in methanol solution, there is only one paramagnetic species (the paraquat radical cation, PQ+˙), a second paramagnetic species develops at lower temperatures, in contrast to the behaviour of the morphamquat system. The reaction of methanol–paraquat radical cation solutions with oxygen has been followed by the stopped-flow technique over a range of temperature from +40 to –80°C. At +40 °C the reaction is second-order in radical cation, with a velocity constant 8.0 × 108 12 mol–2s–1. The mechanism of this reaction is discussed. We have also studied the reaction of paraquat radical cation with Cr3+ ions in methanol over the temperature range 0–40 °C. The reaction is first order both in Cr3+ and in PQ+˙ It does not go to completion but reaches an equilibrium position. The thermodynamic constants for this equilibrium have been determined and also the thermodynamic constants of activation for the reaction of Cr3+ with PQ+˙ in methanol at +25 °C.


Journal ArticleDOI
TL;DR: In this article, the second order decay of semireduced methylene blue (MB') formed in the photoreduction of the dye by aryl-and alkylamines was examined by flash photolysis.
Abstract: — The decay of semireduced methylene blue (MB') formed in the photoreduction of the dye by aryl- and alkylamines was examined by flash photolysis. The second order decay of MB in methanol produced using arylamines and 1,4-diazabicyclooctane as photoreducing agents led to complete regeneration of the dye and amine. The rate constants for a series of N.N-dimethylanilines showed a small substituent effect (p= 0.69 ± 0.16) compatible with recombination of MB' with arylamine derived radical cations. A study of the solvent effect on the recombination process revealed the importance of the stability of the radical cation toward the solvent and also indicated that the reaction approaches the rate of diffusion. The photoreduction of the dye by most alkylamines examined resulted in permanent bleaching of methylene blue. The second order decay of MB' produced using tertiary aliphatic amines led exclusively to formation of the leuco-dye with no apparent regeneration of methylene blue; this process was tentatively assigned to a reaction of MB' with α-amino radicals formed from the amine radical cation. It was concluded that the methylene blue-sensitized photooxidation of amines involves a Type I process and possible mechanisms are discussed.

Journal ArticleDOI
TL;DR: The electrogenerated chemiluminescence (ECL) of rubrene resulting from direct electron transfer from a zinc oxide semi- conductor electrode to rubrene radical cation to form the triplet state of Rubrene is reported in this paper.

Journal ArticleDOI
TL;DR: In this article, the electron transfer reactions between the cation radicals derived from thianthren, dibenzo-p-dioxin, and 9, 10-diphenylanthracene with anisole and anthracene in acetonitrile and dichloromethane were investigated.
Abstract: The kinetics of the electron transfer reactions between the cation radicals derived from thianthren, dibenzo-p-dioxin, and 9, 10-diphenylanthracene with anisole and anthracene in acetonitrile and dichloromethane were investigated. The reactions were found to be complex under the conditions employed being either first or second order in cation radical depending upon the radical ion concentration. The pseudo-second-order reactions were inhibited by the presence of unoxidized substrate. In all cases, the reactions were found to be first order in the compound undergoing oxidation. The disproportionation mechanism was ruled out for the second-order segment of all the reactions, except for that between diphenylanthracene cation radical and anisole, on the grounds that the disproportionation equilibrium constants are much too small to account for the reaction rates observed.


Journal ArticleDOI
TL;DR: In this paper, it was shown that the presence of small amounts of other oxidation states as an impurity in an otherwise pure sample can increase the electrical conductivity by 5 or 6 orders of magnitude.
Abstract: There is a large class of organic compounds which form stable radical ions at room temperature, that is they have a single electron in the highest occupied molecular orbital and carry a negative (e.g., tetracyanoquinodimethane) or a positive charge (e.g., tetracene tetrasulphide). They undergo simple one electron oxidation and reduction reactions and in the solid state these different oxidation states have high resistivities, generally > 1010 ohm cm. However, we have shown that the presence of small amounts (<1%) of other oxidation states as an impurity in an otherwise pure sample can increase the electrical conductivity by 5 or 6 orders of magnitude. The accompanying changes in the Seebeck Coefficient and activation energy for conduction arc of the order of 1 mv/°C and 0.5 ev respectively. These results are explained by assuming a hopping model of conduction and by postulating the existence of a redox equilibrium in the solid state.


Journal ArticleDOI
TL;DR: The pathway of the reduction of the Cu(II) ion in peptide complexes by the hydrated electron proceeds from the bimolecular encounter step, in which the electron is attached to residues of the peptide (carbonyl and disulfide groups), followed by an intramolecular process to the Cu (II) ions.
Abstract: The reduction mechanism of two Cu(II)--peptide complexes, Cu(II)-- (gly)$sub 3$ and Cu(II)--glutathione, was investigated using the pulse radiolysis method. The hydrated electron, used as a reducing agent, was found to react with the two complexes by a bimolecular process. In the Cu(II)--(gly)$sub 3$ complex the reaction of the e$sup -$/sub aq/is both with the Cu(II) ion and with the carbonyl group of the peptide bond to form a carbonyl ''electron adduct'' radical. This adduct transfers the electron in a first-order process to the Cu(II). In the Cu(II)--glutathione complex the decay of the electron was followed by an appearance of a transient absorption band centered at 410 nm. It was followed by a slower, small increase of absorption at the same wavelength (lambda/sub max/ = 410). This band is assigned to the adduct of an electron with the disulfide bond of the glutathione (RSSR$sup -$ radical ion). The radical ion is formed either by the direct reaction of e$sup -$/sub aq/ with the disulfide bond and the transfer of an electron from the carbonyl adduct formed by the e$sup -$/sub aq/ or entirely via the electron-transfer reaction from the adduct to the S-S bridge. The RSSR$sup -$ decays via a first-order process at the same rate at which the Cu(II) is reduced. CO$sub 2$$sup -$ reduces the Cu(II) ion in the complexes via a bimolecular reaction. In the Cu(II)--glutathione complex a small part (10 percent) of the reduction of the Cu(II) proceeds via the disulfide bridge. The pathway of the reduction of the Cu(II) ion in peptide complexes by the hydrated electron may therefore be delineated: It proceeds from the bimolecular encounter step, in which the electron is attached to residues of the peptide (carbonyl and disulfide groups), followed by an intramolecular process to the Cu(II) ion. (auth)

Journal ArticleDOI
TL;DR: In this paper, the addition of 1 and 2 molar equivalents of bromine to a series of 10-alkylphenothiazines, 1a-d (methyl, ethyl, n-propyl, and isopropyl, respectively), yields the corresponding 3-bromo- and 3,7-dibromo 10 -alkyl phenothiazine (11a,d and 12a-D), respectively, evidence which supports the typical clectrophilic aromatic substitution mechanism.




Journal ArticleDOI
TL;DR: In this paper, a mechanism in which the reacting species is the radical cation of the heteroaromatic compounds was proposed, where the reaction species was the reaction of 1-Adamantyl and other radicals produced by the AgI-catalysed decarboxylation of carboxylic acids by (NH4)2S2O8.
Abstract: 1-Adamantyl and other alkyl radicals, produced by the AgI-catalysed decarboxylation of carboxylic acids by (NH4)2S2O8, react with acyl-heteroaromatic bases effecting the displacement of the acyl group; a mechanism is proposed in which the reacting species is the radical cation of the heteroaromatic compounds.