Showing papers on "Radical ion published in 1975"

Substitution Reactions Which Proceed via Radical Anion Intermediates

Abstract: A new type of substition process at a saturated carbon atom is described. These reactions, which proceed via a chain sequence in which radical anions and free radicals are intermediates, are noteworthy for providing novel and powerful means of synthesis: they occur readily under mild conditions, they give excellent yields of pure products, and, in contrast to SN2 displacements, they are rather insensitive to steric hindrance. As a consequence, radical anion processes are especially valuable for the preparation of highly branched structures. Many inorganic and organic anions readily enter into these displacements and, indeed, amines are also effective. Systems which undergo substitutions via this electron transfer mechanism include benzylic, cumylic, strictly aliphatic, and heterocyclic molecules. It is of interest that a number of groups which do not behave as leaving groups in SN2 displacements are readily displaced at room temperature from a satureted carbon atom via the radical anion-free radical pathway, e.g., nitro, azide, sulfone, and ether groups.

The Radical Ion S3N2+. Preparation, Electron Spin Resonance Spectrum, and Crystal Structure of Thiodithiazyl Hexafluoroarsenate S3N2+AsF6−

Abstract: The compound S3N2+AsF6− has been prepared by the reaction of S4N4 with AsF5. The crystal structure has been determined from three-dimensional X-ray counter data. Crystals are monoclinic, space grou...

Electrogenerated chemiluminescence. XXII. Generation of exciplexes in the radical ion reaction

Abstract: Exciplex emission from systems containing tri-p-tolylamine (TPTA) as a donor molecule and various acceptor molecules was observed in both acetonitrile (ACN) and tetrahydrofuran (THF) solutions during electrogenerated chemiluminescence (ecl) studies. Systems containing naphthalene as an acceptor and five donor molecules also showed exciplex emission in A C N solutions. Triplet quenching experiments, magnetic field effect measurements, and temperature studies demonstrated that the exciplex is formed directly in the radical ion reaction. The energy of the exciplex emission showed a linear correlation with the cyclic voltammetric peak potentials for reduction of the acceptor molecules and for oxidation of the donor molecules. Studies of the ecl of the dibenzoylmethane (DBM)-TPTA system in six different electrochemical solvents, where only exciplex emission is observed, demonstrated the importance of solvent dielectric constant on exciplex emission intensity. The role of exciplexes in the general ecl scheme is also discussed. Exciplexes are excited molecular complexes which are dissociated in the ground state.’ They are usually produced by reaction of a photoexcited singlet species (‘A*) with a ground state molecule, D; exciplex emission is usually structureless and red shifted from that of IA* by about 6000 ( 1) ( 2) Previous studies of radical-ion annihilation chemiluminescence reactions and electrogenerated chemiluminescence ( e ~ l ) ~ , ~ in low dielectric constant solvents, such as tetrahydrofuran (THF), have demonstrated the formation of exciplexes by the direct reactions of radical ions ( 3) and it has been suggested that an exciplex may be an intermediate in other cl or ecl reactions, even when exciplex cm-1.1,2 lA* + D -+ ‘(&D+)* l(A-D+)* A + D + hu As+ D * a l(A‘D+)* emission is not observed. It has generally been felt, however, that emission from exciplexes will not be observed in polar solvents, such as acetonitrile (ACN), where dissociation of the exciplex into the radical ions (the reverse of reaction 3) becomes more important.’ We describe here ecl experiments with a number of acceptor and donor molecules in both T H F and ACN which demonstrate the formation of exciplexes and show that the energy of the exciplex emission correlates with the redox potentials of the reactant molecules. Preliminary experiments on the effect of magnetic field, solvent, and temperature variations on the ecl behavior in these systems are also discussed. Experimental Section Chemicals. Tri-p-tolylamine (TPTA) and tri-p-anisylamine (TPAA), kindly provided by Professor R. N. Adams (University of Journal of the American Chemical Society J 97: l l J May 28, 1975

Pulse radiolysis studies of electron transfer reaction in molecules of biological interest. I. The reduction of a disulfide bridge by peptide radicals.

Abstract: The reduction of the disulfide bridge in the lipoate anion by peptide radicals and the formate radical ion (CO$sub 2$$sup -$) has been studied using the pulse radiolysis method. Peptide radicals were produced by the reaction of the hydrated electrons with the peptide. It was found that e/sub aq/$sup -$ reacts with diglycine, chloro-acetyl glycine and chloroacetyl diglycine to form a deaminated peptide radical (.CH$sub 2$CONHCH$sub 2$COR). The reaction of e/sub aq/$sup -$ with triglycine at neutral pH forms two radicals: (a) the deaminated radical which is formed when the e/sub aq/$sup -$ reacts with the carbonyl group in an $alpha$ position to the amino group, and (b) an ''electron adduct'' on the carbonyl group of the peptide bond which is formed when the e/sub aq/$sup -$ peptide reaction occurs at other positions. The ''electron adduct'' on the carbonyl radical can be formed by the reaction of the e/sub aq/$sup -$ with acetyl amide. The rate constant for reduction of the lipoate ion disulfide bridge by the deaminated radical was evaluated to be 5 x 10$sup 6$ M$sup - 1$sec$sup -$1, while that for ''electron adduct'' radical and the formate radical is approximately 5 x 10$sup 8$ M$sup -1$ sec$sup -1$. In view of these results a multistep mechanism is suggested for the e/sub aq/$sup -$ reduction of S-S bridge proteins. CO$sub 2$$sup -$ reacts directly with the disulfide bridge. (auth)

Formation of a sulphur-sulphur bridged radical cation during the oxidation of 1,4-dithian by hydroxyl radicals

Abstract: A transient radical cation with a three-electron bond between the two sulphur atoms in the 1,4-positions is formed upon oxidation of 1,4-dithian by OH radicals in aqueous solutions at low solute concentration.

Bipyridyl radical cations. Part I. Electron spin resonance study of the dimerisation equilibrium of morphamquat radical cation in methanol

Abstract: We have studied the e.s.r. spectra of methanol solutions of the morphamquat {bis-1,1′-[(2,6-dimethylmorpholin-4-yl)carbonylmethyl]bipyridyl} radical cation (I) from +40 to –80°. The value of the splitting constants obtained are A(pyridyl N) 0.400, A[pyridyl H(3)] 0.142, A[pyridyl H(2)] 0.048, A(NCH2) 0.245, and A(morpholine N) 0.024 mT. It is found that as the temperature decreases the concentration of the radical cation decreases until at –80° there is no e.s.r. spectrum. This process is reversible. Concentration experiments show that the morphamquat radical cation is in equilibrium with a dimeric diamagnetic species. The ΔH° value for the equilibrium is found to be –45.05 ± 0.3 kJ mol–1. The ΔG° and ΔS° values at 25 °C are –10.6 kJ mol–1 and –115.6 J mol–1 K–1 respectively.

Electron transfer between tetraalkylhydrazine radical cations and tetraalkylhydrazines

Abstract: The radical cations of both tetramethyl and tetraethyl sym-hexahydrotetrazine have the odd electron localized in one hydrazine unit, and intramolecular electron transfer between the methylene bridged hydrazine units is slow on the ESR time scale. The intermolecular electron transfer rate is also unusually slow between 3.3.7.7-tetramethyl1,4-diazabicyclo[3.3.0]octane and its radical cation, because less than 0.7 g of line broadening is observed when the ESR spectrum of the radical cation is recorded i n a solution over 0.8 M in the neutral hydrazine. W e recently reported' that the radical cation of tetraazatricyclododecane (1) gives an ESR spectrum showing splittings for four equivalent nitrogens and two sets of eight

Radiation chemistry of ethers. Part VI. Photolysis at 254 nm of the diethyl ether–oxygen charge transfer complex

Abstract: Oxygen-saturated diethyl ether exhibits a charge transfer absorption band (λmax. 215 nm) with considerable absorption at 254 nm. Photolysis was carried out with a low pressure mercury arc. Photolysis of products was avoided by keeping conversion low, and chain reactions were minimized by working at –30°. The major products(quantum yields) were: H2O2(0·24), 1-ethoxyethyl hydroperoxide (0·04), ethyl acetate (0·26), acetaldehyde (0·18), ethanol (0·18), ethyl formate (0·04), methanol (0·035), formaldehyde (0·005), and ethyl vinyl ether (0·013). A satisfactory material balance was obtained. The products formed are consistent with a primary photoprocess involving transfer of an electron to the O2 to give O2–. Further, deprotonation of the ether radical cation in the presence of O2 leads to the 1-ethoxyethylperoxyl radical. With the exception of 1-ethoxyethyl hydroperoxide all major products are believed to arise from disproportionation reactions of these two radicals.

Free radical formation in the antioxidant ethoxyquin

Abstract: Electron spin resonance (ESR) studies on the antioxidant 6-ethoxy-1, 2-dihydro-2,2,4-trimethylquinoline (ethoxyquin) showed that the compound exists partly in the free radical form. The radical which was found both in solutions of ethoxyquin and in the compound itself, exhibits an ESR signal consisting of six lines which form a triplet of doublets. The hyperfine structure can be ascribed to an interaction of the unpaired electron with a nitrogen and a proton. The ESR data were: g = 2.0050, AN = 10.8 gauss and AH = 3.7 gauss. The responsible radical is presumably formed by a dissociation of a hydrogen atom from the secondary amine. The radical concentration was found to be increased significantly by the presence of u.v. light. The ESR data were supported by some mass spectral data.

Hydrogen isotope effect on radical addition reaction of sodium hydrogensulfite to allyl alcohol

Abstract: The radical addition reaction of sodium hydrogensulfite to allyl alcohol in D2O solution was investigated. The hydrogen isotope effect was found to be 4.11. This effect is attributed to the hydrogen atom abstraction of HOCH2CHCH2SO3− radical ion from hydrogensulfite ion.

Surface and intercalate chemistry of layered silicates. Part V. Infrared, ultraviolet, and visible spectroscopic studies of benzidine–mont-morillonite and related systems

Abstract: Clay minerals of the montmorillonite family provide a matrix on which radical cations of a variety of amines may be formed. The origin of the different colours associated with the benzidine (bzn)–. NNN′N′-tetramethylbenzidine (tbzn)–, and trans-4.4′-diaminostilbene (dsn)-montmorillonite complexes has been further elucidated by u.v. and visible reflectance and i.r. spectroscopy. It is established that both uni- and di-positive bzn radical cations and protonated bzn moieties can exist at the internal and external clay surfaces. This is in contrast to the situation which prevails for tbzn where only the unipositive radical cation and protonated moieties appear to be formed. The interaction of montmorillonite with dsn dihydrochloride is similar to that of bzn but the nature of the product is modified by the precence of protons in cation-exchange sites.

Application of the Dewar–Zimmerman rules to the reactions between radical cations and nucleophiles

Abstract: The reaction between a 4n+1 aromatic radical cation and a nucleophile, the interacting orbital of which must be used suprafacially in the transition state, should be ‘forbidden’ by the Dewar–Zimmerman rules.

Kinetics on the sulfonation reaction of 1-dodecene with sodium hydrogensulfite

Abstract: The kinetics on the radiation-induced addition of hydrogensulfite to 1-dodecene was investigated in the presence of a surfactant, 1-dodecanesulfonate, in an aqueous solution of t-butyl alcohol The termination was found to be the reaction of 1-dodecanesulfonate radical with sulfite radical ion This result was deduced to be due to the micellar effect

The Role of Donor-Acceptor Complexes in the Initiation of Ionic Polymerization

Abstract: Work carried out in the past few years aimed at elucidating the mechanism of initiation of vinyl polymerization when a donor and an acceptor molecule, one or both of which may be vinyl monomers, is summarized. The emphasis of our investigation has been on polymerizable ether donors and strong electron acceptors which do not undergo polymerization, or the acceptor vinylidene cyanide. Alkyl vinyl ethers were polymerized in the presence of tetracyanoquinodimethane (TCNQ) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in polar solvents. Observation of the ESR spectrum of the DDQ radical anion and the isolation of a 1:1 addition product of DDQ and alkyl vinyl ether when the two are mixed in a 1:1 ratio and quenched in alcohol support an initiation mechanism involving a coupling reaction of the donor monomer (radical cation) and the acceptor initiator (radical anion). The reaction of vinylidene cyanide (VC) with the vinyl ethers p-dioxene, dihydropyran, ethyl vinyl ether, isopropyl vinyl ether, and ...

Sulphur–nitrogen anions: formation from azide ion and elemental sulphur and their role in the synthesis of cyclic sulphur imides

Abstract: The reaction of azide ion with cyclo-octasulphur in tris(dimethylamino)phosphine oxide produces the [S4N]– ion, whereas lithium nitride yields the trisulphur radical ion, S3–·, predominantly. The azide reaction provides a convenient and direct synthesis of S7NH. PO(NMe2)3 Is superior to NN-dimethylformamide (dmf) as a solvent for the S2Cl2–NH3 route to cyclic sulphur imides. Equilibria of the type [S7N]–⇌[S4N]–+⅜S8⇌½[S6N2]2–+½S8 are invoked to explain the formation and relatively low yields of S6(NH)2 isomers in these syntheses.

ESR.-Studies of Nonalternant Radicals Structurally Related to Phenalenyl

Abstract: The radical anion and the radical cation of azuleno[1,2,3-cd]phenalene (III) have been investigated by ESR. spectroscopy, along with the radical anion of 2-phenylazulene (IV). Also studied has been the neutral radical obtained by one-electron reduction of cyclohepta[cd]phenalenium-cation (VI⊕). Assignment of the proton coupling constants for the radical ions III. ·⊖, III ·⊕ and IV·⊕, and the radical VI · is supported by comparison with the ESR. spectra of specifically deuteriated derivatives III-d5 ·⊖, III-d5 ·⊕, IV-d2 ·⊖ and VI-d1′. The experimental results are in full accord with qualitative topological arguments and predictions of HMO models. Whereas the radical anion III ·⊖ exhibits α-spin distribution similar to that of IV ·⊖the corresponding radical cation III ·⊕ and the neutral radical VI · are related in this respect to phenalenyl (V·). It is noteworthy that oxidation of III by conc. H2SO4 yields a paramagnetic species (IIIa ·⊕) which has a similar – but not an identical – structure as the radical cation III ·⊕ produced from III with AlCl3 in CH3NO2.