Showing papers on "Radical ion published in 1992"

Electrochemical Origin of Radical Cations Observed in Electrospray Ionization Mass Spectra

Abstract: A variety of experimental data is presented that implicates electrochemical oxidation of analytes in the electrospray (ES) needle as the mechanism for formation of molecular radical cations observed in the ES ionization mass spectra of alkylsubstituted metalloporphyrins, polycyclic aromatic hydrocarbons (PAH`s), and other compound types. Analyte structural characteristics and solution-phase half-wave oxidation potentials (which correlate with gas-phase ionization energies) can be used to evaluate the likelihood of forming and observing a particular compound as a radical cation. Use of an appropriate solvent is critical in the observation of radical cations generated by the ES process. In addition to dissolving the analyte and providing a stable electrospray, the solvent-(s) must {open_quotes}stabilize{close_quotes} or otherwise protect the radical cation from reactions in solution. Appropriate solvent systems (e.g., methylene chloride/0.1% trifluoroacetic acid) are much the same as used in traditional studies of electrochemically generated radical cations. The ability to produce radical cations in the ES process expands the utility of ES ionization mass spectrometry to include compound classes not normally amenable to the technique (e.g., neutral, nonpolar compounds such as PAH`s) and provides for generation of a different type of molecular species than normally produced in positive-ion ES ionization (i.e., M{sup {sm_bullet}+} versus (Mmore » + H){sup +}, (M + Na){sup +}, etc.).« less

Stable photoinduced charge separation in layered viologen compounds

Abstract: SOLAR energy can be used and stored by the efficient production of long-lived photoinduced charge separation1–3—a state achieved in photosynthetic systems by the formation of a long-lived radical pair4–7. A number of artificial systems have been reported that efficiently undergo photochemical charge transfer8–10; unfortunately, the thermal back electron transfer often proceeds at an appreciable rate, limiting the utility of these systems. Here we report the photochromic behaviour of novel layered zirconium phosphonate/viologen compounds which show very efficient photo-induced charge transfer, and form a charge-separated state which is long-lived and stable in air. Spectroscopic studies indicate that the photoproduct is the dialkyl viologen radical cation, produced in the inter lamellar region of the zirconium phosphonate. We suggest that the remarkable stability of the charge-separated state arises from structural features which allow for stabilization of the radicals by delocalization and shielding from molecular oxygen.

Ab initio molecular orbital calculations of DNA bases and their radical ions in various protonation states: evidence for proton transfer in GC base pair radical anions

Abstract: Ab initio molecular orbital calculations of various protonation states of DNA base and DNA base radical ions were performed to aid our understanding of primary radiation processes in DNA. Each of the structures was optimized at the STO-3G and 3-21 levels of theory, and single point calculations were performed at the 6-31G*//3-21G and 6-31+G(d)//3-21G levels. Each of the protonation states important to proton-transfer reactions in base pair radical ions found in irradiated DNA was considered. Calculations for three protonation states for the cytosine reduced radical were performed and the most stable is found to be the N-3 ring-protonated species. Calculations for the vertical ionization energies of the individual DNA bases yielded the following order T>C>A>G with the same order found for the adiabatic electron affinities of the bases. The best fit to experiment was found with Koopmans` theorem ionization potentials. The energy for proton transfer in the GC and AT base pair radical cation and radical anions were estimated from the energies of the individual species. All proton transfers were found to be energetically unfavorable except for the GC anion radical which is favored by 13 kcal. INDO calculations for ions of stacked four base DNA model system AT/GC predictmore » the site of electron trapping in DNA model system is thymine, and the hole is found on guanine as predicted from ab initio energies for the individual bases. Both ab initio and INDO results for the AT/GC anion predict the site of the anion shifts from thymine to cytosine on the transfer of a proton from guanine to the cytosine anion. These results lead to a revised model for ion radical localization after irradiation of DNA. 54 refs., 7 figs., 4 tabs.« less

Production of fullerene (C60) radical cation by photosensitized electron transfer

Abstract: The radical cation of C{sub 60} is prepared by electron transfer to singlet N-methylacridinium hexafluorophosphate. It has {lambda}{sub max} near 980 nm. The radical cation can also be prepared in higher yield by cosensitization, using high concentrations of biphenyl to form biphenyl radical cation which is the ultimate oxidant for C{sub 60}. 27 refs., 2 figs.

Long range intramolecular electron transfer in azurins

Abstract: The disulfide bridge linking cysteines-3 and -26 in the blue copper aruzin isolated from Alcaligenes faecalis (Alc.faec.) and Pseudomonas fluorescens B (Ps.fluor) reacts with pulse radiolytically produced CO 2 - radical anions to yield the disulfide radical ion, RSSR - . This radical then dacays by an intramolecular electron transfer (et) to the Cu(II) center whith is at a distance of ∼2.6nm. At 25 o C and pH 7.0, the rate constants of the intramolecular electron transfer reactions are 11 ± 2 and 22 ± 3 s -1 in Alc. faec. and Ps. Fluor. azurin, respectively

Infrared spectrum of matrix-isolated naphthalene radical cation

Abstract: Radical cations of naphthalene (N) have been formed by electron bombardment of a vapor-phase mixture of naphthalene, carbon tetrachloride, and argon and trapped in a matrix (Ar) at 12 K. Infrared spectral scans of this matrix compared to ones containing neutral N alone or electron-bombarded CCl 4 reveal new vibrational bands at 1525, 1519, 1401, 1218, 1215, 1023, and 1016 cm -1 . These bands are attributable to the N cation by their positive correlation with the known N cation 2 B 3g (D 2 )←X 2 A u (D 0 ) band system at 675 nm

Models for peroxidase compound I: generation and spectroscopic characterization of new oxoferryl porphyrin .pi. cation radical species

Abstract: Oxoferryl porphyrin π cation radical species have been generated by the oxidative action of m-chloroperoxybenzoic acid on the triflato complexes of (tetrakis(2,6-dichlorophenyl)porphyrinato]- and (tetrakis(2,4,6-trimethoxyphenyl)porphyrinato]-iron(III) in methylene chloride/methanol-d 4 . The new high-valent complexes have been characterized by a variety of spectroscopic techniques.

The photofragmentation of naphthalene and azulene monocations in the energy range 7–22 eV

Abstract: Photoion mass spectrometry was used to study the fragmentation of naphthalene and azulene monocations over the excitation energy range 7–22 eV. Fifteen fragmentation processes in naphthalene and twelve in azulene have been examined in detail. The photoionization mass spectra at 20.58 eV are quasi-identical for the two isomers. This, and the constant value of the difference between the fragment appearance energies (AE) for naphthalene and azulene, equal to the difference in the heats of formations of the neutral parents, suggest that identical products are formed. The unimolecular dissociations fall mainly into (i) a “low energy” group, (AE 18 eV). The reactions in (i) have in common the bicyclic precursor C 10 H + 8 ion 18 which decays via rupture of one ring. The group (ii) reactions involve rupture of both rings to give an open chain precursor, the 1,6-bis-ethinyl-hexatriene radical cation 20 . Thermodynamic and mechanistic arguments are given to propose specific reaction pathways and product structures. Two general schemes rationalize the low-energy and high-energy ionic decompositions.

Structure and reactivity of organic radical cations

Abstract: Photoinduced electron transfer has attracted much attention because of its central role in the chemistry of life and as a method for generating radical ion pairs in solution. Radical cations so generated can be characterized by many spectroscopic techniques, and their reactions can be studied conveniently. Photoinduced electron transfer is compared with alternative methods of radical ion generation; the application of these methods is discussed in conjunction with suitable techniques of observation.

Radical ion probes. 2. Evidence for the reversible ring opening of arylcyclopropylketyl anions. Implications for mechanistic studies

Abstract: Aryl cyclopropyl ketones have frequently been utilized as diagnostic probes for single electron transfer (SET) for a variety of organic transformations. The implicit assumption in these studies is that the formation of rearranged product(s) signals the intermediacy of a ketyl anion. Through a detailed examination of the mechanism, kinetics, and products arising from the decay of several arylcyclopropylketyl anions (generated electrochemically), we have demonstrated that the assumptions made in the use of these substrates as SET probes are tenuous

Antioxidant activity of the pyridoindole stobadine. Pulse radiolytic characterization of one-electron-oxidized stobadine and quenching of singlet molecular oxygen.

Abstract: Antioxidant properties of stobadine, a pyridoindole derivative described to exhibit cardioprotective properties, were characterized. The radical scavenging potential of stobadine was evaluated using pulse radiolysis with optical detection, by which it is shown that one-electron oxidation of stobadine with radicals such as C6H5O., CCl3O2., Br2.-, and HO. (reaction rate constants approximately 5 x 10(8)-10(10) M-1 s-1) leads to the radical cation (absorbance maxima at 280 and 445 nm) which deprotonates from the indolic nitrogen (pKa = 5.0) to give a nitrogen-centered radical (absorbance maxima at 275, 335, and 410 nm), probably bearing a positive charge at the pyrido nitrogen. The radical of stobadine reacts with Trolox (i.e., 6-hydroxy-2,5,7,8-tetramethyl-chromane-2-carboxylic acid) with a rate constant of 1.2 x 10(7) M-1 s-1 at pH 7.0 by one-electron oxidation to yield the phenoxyl-type radical of Trolox. This reaction is reversible (k = 2 x 10(5) M-1 s-1). The redox potential of stobadine at pH 7 is 0.58 V/NHE. Stobadine is also a quencher of singlet molecular oxygen (1O2) with an overall quenching rate constant of 1.3 x 10(8) M-1 s-1, determined with the endoperoxide of 3,3'-(1,4-naphthylene)dipropionate (NDPO2) as 1O2 source and by monitoring 1O2 photoemission with a germanium diode.

Fluorescence of excited charge-transfer complexes and absolute dynamics of radical-ion pairs in acetonitrile

Abstract: An analysis of the dynamics of the radical-ion pairs of a series of 2,6,9,10-tetracyanoanthracene acceptor/alkylbenzene donor systems in acetonitrile is described in this paper. This analysis is carried out by using a combination of time-resolved emission and absorption spectroscopies and measurements of {Phi} {sub ions} from the contact radical-ion pair (CRIP) and the solvent-separated radical-ion pair (SSRIP).

Electrooxidation of Soluble Alpha, Alpha-Coupled Thiophene Oligomers

Abstract: : The electrooxidation of alpha,alpha-coupled thiophene oligomers with terminal trimethylsilyl and P-methyl groups was studied in methylene chloride at room temperature. The oligomers with 4, 5, and 7 thiophene rings undergo two stepwise oxidations to produce the radical cation and dication, respectively, as confirmed by Vis/Nir esr spectra. The redox wave are chemically reversible and are well-separated (ca. 200 mV) in the cyclic voltammagram and suggested that the same electrochemical behavior should be observed with the next higher oligomers or in the n-conjugated segments of polythiophene. Thus the broad featureless voltammagram observed with films of polythiophene is not an inherent property of the polymer segments but may reflect complications from the solid state nature of the film.

Evidence for the product of the viologen comproportionation reaction being a spin-paired radical cation dimer

Abstract: The mechanism of the bipyridylium comproportionation reaction has been determined using the redox states of methyl viologen (MV). The reaction, hitherto presumed to follow the stoichiometry MV2++ MV0→ 2 MV+, has been investigated using in situ electrochemical EPR and UV–VIS spectroscopy and been shown to proceed via an intermediate step forming a spin-paired radical cation dimer (MV)22+. Dimer dissociation occurs as a subsequent reaction stage.

Formation of radical ion pairs in aniline adsorption on zeolites

Abstract: The formation of the aniline radical cation was observed on H-mordenite and H-faujasite in which the distribution of aluminium among different framework and nonframework coordination sites has been characterized by high-resolution solid-state NMR.

Electron transfer photoinduced cleavage of acetals. A mild preparation of alkyl radicals

Abstract: Electron transfer from 2-alkyl- and 2,2-dialkyldioxolanes as well as from open-chain ketals to singlet excited benzene-1,2,4,5-tetracarbonitrile (TCNB) is foolowed by fragmentation of the donors radical cation to yield alkyl radicals and dialkoxy carbocations. The first species are trapped by TCNB to yield alkylbenzentricarbnitriles (substitution of a secon syano group can be obtained sequentially) and in a minor path are reduced to alkanes, while the latter ones react with nucleophiles to give ortho acide derivates

Role of contact and solvent-separated radical ion pairs in the diffusional quenching of trans-stilbene excited singlet state by fumaronitrile

Abstract: Picosecond absorption spectroscopy is used to examine the question of whether contact radical ion pairs (CRIP) or solvent-separated radical ion pairs (SSRIP) are formed upon the quenching, by electron transfer, of the first excited singlet state of trans-stilbene (S{sub 1}) by fumaronitrile (FN). Prior to these experiments, it was generally believed that for exothermic reactions in polar solvents, SSRIP are formed upon quenching by electron transfer. However the present experiments reveal that the quenching of S{sub 1} by FN in acetonitrile leads to the formation of CRIP. To establish the nature of the radical ion pair formed upon electron transfer, the kinetics for the decay of the CRIP and the SSRIP are established. 22 refs., 5 figs., 1 tab.

A substrate-cofactor free radical intermediate in the reaction mechanism of copper amine oxidase.

Abstract: Reduction of copper amine oxidase with substrate led to the appearance of a free radical which can be detected in anaerobiosis by ESR and optical spectroscopy. The origin of this radical was examined through studies of the semiquinones of 6-hydroxydopamine, an analogue of the recently identified cofactor 6-hydroxydopa. The ESR spectrum of the 6-hydroxydopamine radical was too narrow to account for the enzyme radical signal; however, after spontaneous reaction with primary amines the hyperfine splittings and spectral width obtained by modulation broadening became very similar to those observed for the oxidase radical species. This effect was ascribed to covalent binding of a nitrogen atom directly to the aromatic ring structure, suggesting that the amine oxidase radical is an amino-6-hydroxydopa semiquinone. Identical ESR spectra were obtained using the amines putrescine, cadaverine, p-[(dimethylamino)methyl]benzylamine, and ethylenediamine; these oxidase substrates gave identical enzyme radical spectra as well. The interaction between cofactor and substrate was proved unambiguously by the technique of isotopic labeling: addition of [15N2]ethylenediamine instead of the normal 14N-labeled compound changed the ESR spectra of both the enzyme radical and its 6-hydroxydopamine counterpart. The results were confirmed by optical spectroscopy measurements; 6-hydroxydopamine and oxidized 6-hydroxydopamine gave spectra identical to those of reduced and oxidized amine oxidase, respectively. The 6-hydroxydopamine radical showed a sharp peak at 440 nm; upon addition of amines the maximum shifted to 460 nm, as found for the enzyme. It is proposed that copper amine oxidase represents the first example of a mixed substrate-cofactor radical within the family of tyrosine radical enzymes.

Synthetic Perspectives of Photoinduced Electron Transfer Generated Amine Radical Cations

Abstract: "True sensitized" photo-SET reactions from a variety of tertiary amines and singlet excited cyanoarenes [mainly 1,4-dicyanonaphthalene and 9,10-dicyanoanthracene] have been observed. The free amine radical cation, formed via a solvent separated ion pair, undergoes efficient deprotonation-desilylation at the carbon adjacent to nitrogen. The synthetic usefulness of this step is demonstrated in generating reactive intermediates such as nitrones, iminium cations, α -amino radicals and azomethine ylides depending on the substrates and reaction medium. 1. Introduction 2. Generation of Cyclic Nitrones: A Versatile 1,3-Dipole 3. Generation of Regiospecific Iminium Cation 3.1. Synthesis of Oxaazabicyclo[m.n.o]alkanes and Related Compounds 3.2. Stereoselective Synthesis and Utilization of Tetrahydro-1,3-oxazines 3.3. SET Promoted Photo N-Debenzylation and N-Dealkylation 4. Generation and Reactivity of α-Amino Radical (or Equivalent) 5. Generation of Non-Stabilized Azomethine Ylides 6. Conclusion.

Catalytic mechanisms and regulation of lignin peroxidase

Abstract: Lignin peroxidase (LiP) is a fungal haemoprotein similar to the lignin-synthesizing plant peroxidases, but it has a higher oxidation potential and oxidizes dimethoxylated aromatic compounds to radical cations. It catalyses the degradation of lignin models but in vitro the outcome is net lignin polymerization. LiP oxidizes veratryl alcohol to radical cations which are proposed to act by charge transfer to mediate in the oxidation of lignin. Phenolic compounds are, however, preferentially oxidized, but transiently inactivate the enzyme. Analysis of the catalytic cycle of LiP shows that in the presence of veratryl alcohol the steady-state turnover intermediate is Compound II. We propose that veratryl alcohol is oxidized by the enzyme intermediate Compound I to a radical cation which now participates in charge-transfer reactions with either veratryl alcohol or another reductant, when present. Reduction of Compound II to native state may involve a radical product of veratryl alcohol or radical product of charge transfer. Phenoxy radicals, by contrast, cannot engage in charge-transfer reactions and reaction of Compound II with H2O2 ensues to form the peroxidatically inactive intermediate, Compound III. Regulation of LiP activity by phenolic compounds suggests feedback control, since many of the products of lignin degradation are phenolic. Such control would lower the concentration of phenolics relative to oxygen and favour degradative ring-opening reactions.

Insertion of nitrogen oxide and nitrosonium ion into the cyclopropane ring: a new route to 2-isoxazolines and its mechanistic studies

Abstract: The 9,10-dicynoanthracene (DCA)-sensitized photoreaction of 1,2-diarylcylopropanes 1a-d in nitrogen oxide (NO)-saturated CH 3 Cn afforded 3,5-diaryl-2-isoxazolines 2a-d in excellent yiels. The reaction of 1a-d with NOBF 4 or with a mixture of NO and O 2 in CH 3 CN also afforded 2a-d or 2a-b. These reactions proceed via the attack of NO on the radical cation of 1 which is formed by electron transfer from 1 to 1 DCA . or NO +