Short-lived Radical Cations Research Articles

Combined Effects of Hemicolligation and Ion Pairing on Reduction Potentials of Biphenyl Radical Cations.

Formal reduction potentials of highly oxidizing and short-lived radical cations of substituted biphenyls generated by pulse radiolysis in 1,2-dichloroethane (DCE) were measured using a redox equilibrium ladder method. The effect of halide ion-radical interactions on reduction potentials of biphenyls was examined by utilizing the ability of DCE to release Cl- in the vicinity of the radical cation. The Hammett correlation of measured potentials across a range of over 700 mV shows saturation at high Hammett sigma values. This effect has been explained by both ion-pairing and hemicolligation interactions between biphenyl radical cations and Cl- and appears to modulate reduction potentials by as much as 400 mV. This finding offers a convenient way to manipulate the energetics of electron transfer involving organic redox species.

Insights into electrogenerated intermediates and rapid screening of electrochemical reactions by surface-modified carbon fiber paper redox spray ionization mass spectrometry

The combination of electrochemistry and mass spectrometry is a powerful analytical tool for studying redox reaction mechanisms and identifying products or intermediates. However, the previously reported devices all require bespoke fabrication and are too complicated to be assembled and used by others. Crucially, the long ion transport distance and small spray volumes make it difficult to capture the short-lived intermediates. We present a practical mass spectrometric method in which surface-modified carbon fiber paper is innovatively applied to detect electrogenerated intermediates. Treating carbon fiber paper with dilute nitric acid removes its surface impurities, enhancing the capability of electro-redox. Electrospray ionization and redox reaction occur simultaneously on the tip of the paper. Transient electro-redox species generate and transfer into gas phase as soon as the appearance of spray. Rapid transport of quantities of electrogenerated ions to the mass spectrometer inlet makes it possible for mass spectrometric identification on the millisecond scale. The short-lived radical cations and iminium ions were successfully captured, reflecting the starting step of the cross-dehydrogenation coupling reaction. The real-time oxidation and online functionalization reactions of tertiary amines were achieved using this device without additional oxidants and electrolytes. In this way we could achieve in-depth mechanistic understanding and rapid screening of serial reactions.

Single entity electrochemistry and the electron transfer kinetics of hydrazine oxidation

The mechanism and kinetics of the electro-catalytic oxidation of hydrazine by graphene oxide platelets randomly decorated with palladium nanoparticles are deduced using single particle impact electrochemical measurements in buffered aqueous solutions across the pH range 2–11. Both hydrazine, N2H4, and protonated hydrazine N2H5+ are shown to be electroactive following Butler-Volmer kinetics, of which the relative contribution is strongly pH-dependent. The negligible interconversion between N2H4 and N2H5+ due to the sufficiently short timescale of the impact voltammetry, allows the analysis of the two electron transfer rates from impact signals thus reflecting the composition of the bulk solution at the pH in question. In this way the rate determining step in the oxidation of each specie is deduced to be a one electron step in which no protons are released and so likely corresponds to the initial formation of a very short-lived radical cation either in solution or adsorbed on the platelet. Overall the work establishes a generic method for the elucidation of the rate determining electron transfer in a multistep process free from any complexity imposed by preceding or following chemical reactions which occur on the timescale of conventional voltammetry.

Capture of Electrochemically Generated Fleeting Carbazole Radical Cations and Elucidation of Carbazole Dimerization Mechanism by Mass Spectrometry

The capture of reactive intermediates is important for the elucidation of reaction mechanisms. We report the first observation of electrochemically generated, short-lived radical cations of carbazole (t1/2 ≈ 97 μs) and two N-substituted carbazole derivatives by mass spectrometry. In addition, online investigation of the reactivity of electrochemically generated carbazole radical cations supports that the carbazole dimerization mechanism involves the reaction of one radical cation with one neutral molecule rather than the previously proposed coupling of two radical cations.

In Situ Mass Spectrometric Screening and Studying of the Fleeting Chain Propagation of Aniline.

A simple and effective approach to studying the mechanism of electrooxidation of aniline (ANI) is reported in this paper. It was accomplished by an innovative electrochemistry (EC)-mass spectrometry (MS) coupling, which can sample directly from a droplet-scale reacting electrolyte for mass spectrometric analysis. With this setup, the polymer chain growth of ANI could be monitored in situ and in real-time. The short-lived radical cations (ANI•+, m/ z 93.06) as well as the soluble dimer ( m/ z 183.09) and oligomers ( m/ z 274.13, 365.18, ...) were successfully captured. Using the EC-MS and tandem mass spectrometry, the dimers produced by head-to-tail (4-aminodiphenylamine), head-to-head (hydrazobenzene), and tail-to-tail (benzidine) coupling of radical cations were found in the same polymerization process. Moreover, the EC-MS method was also applicable for determining the propagation speed of ANI when applying different electrolyte salts and oxidizing potentials.

Picosecond pulse radiolysis of highly concentrated sulfuric acid solutions: evidence for the oxidation reactivity of radical cation H2O(•+).

Aqueous solution of sulfuric acid is used as a suitable system to investigate the reactivity of the short-lived radical cation H2O(•+) which is generated by radiation in water. Ten aqueous solutions containing sulfuric acid with concentration from 1 to 18 mol L(-1) are studied by picosecond pulse radiolysis. The absorbance of the secondary radical SO4(•-) (or HSO4(•)) formed within the 10 ps electron pulse is measured by a pulse-probe method in the visible range. The analysis of the kinetics show that the radicals of sulfuric acid are formed within the picosecond electron pulse via two parallel mechanisms: direct electron detachment by the electron pulse and oxidation by the radical cation of water H2O(•+). In highly concentrated solution when SO4(2-) is in contact with H2O(•+), the electron transfer becomes competitive against proton transfer with another water molecule. Therefore, H2O(•+) may act as an extremely strong oxidant. The maximum radiolytic yield of scavenged H2O(•+) is estimated to be 5.3 ± 0.1 × 10(-7) mol J(-1).

Special Issue on Radiation Chemistry: In Memory of K. D. Asmus

Special Issue on Radiation Chemistry: In Memory of K. D. Asmus

Reactivity of the Strongest Oxidizing Species in Aqueous Solutions: The Short-Lived Radical Cation H2O(•.).

The radical cation H2O(•+) formed under irradiation of liquid water undergoes an ultrafast proton transfer reaction and consequently exhibits an extremely short lifetime. The proton transfer yields an oxidizing OH(•) radical whose reactivity has been extensively studied. By contrast, H2O(•+) reactivity with molecules other than water has not been established experimentally and was subject to controversy. The direct oxidation by H2O(•+) can take place in various situations. In highly concentrated solutions, the radical cation H2O(•+) may also be involved in ultrafast electron transfer reactions. We have applied picosecond pulse radiolysis conducted at the electron accelerator ELYSE on solutions with various H2SO4 concentrations to determine the scavenging yield of H2O(•+). The yield of H2O(•+) at a few tens of femtoseconds is estimated to be around 5.3 × 10(-7) mol J(-1), and its reactivity is quantitatively determined. Moreover, a simple estimation of the reduction potential of this short-lived radical cation shows that it is the most powerful oxidizing species.

Reaction of SO4˙− with an oligomer of poly(sodium styrene sulfonate). Probing the mechanism of damage to fuel cell membranes

Clarification of the mechanism of degradation of model compounds for polymers used in polymer electrolyte fuel cells may identify intermediates that propagate damage; such knowledge can be used to improve the lifetime of fuel cell membranes, a central issue to continued progress in fuel cell technology. In proton-exchange membranes based on poly(styrene sulfonic acid), hydroxycyclohexadienyl radicals are formed after reaction with HO˙ and thought to decay to short-lived radical cations at low pH. To clarify subsequent reactions, we generated radical cations by reaction of SO(4)˙(-) with oligomers of poly(styrene sulfonic acid) (MW ≈ 1100 Da). At 295 K, this reaction proceeds with k = (4.5 ± 0.6) × 10(8) M(-1) s(-1), both at pH 2.4 and 3.4, and yields benzyl radicals with an estimated yield of ≤60% relative to [SO(4)˙(-)]. The radical cation is too short-lived to be observed: based on a benzyl radical yield of 60%, a lower limit of k > 6.8 × 10(5) s(-1) for the intramolecular transformation of the aromatic radical cation of the oligomer to a benzyl radical is deduced. Our results show that formation of the benzyl radical, an important precursor in the breakdown of the polymer, is irreversible.

Fluorescence quenching and laser photolysis of dipyrrolylbenzenes in the presence of chloromethanes

Fluorescence quenching of 1,4-bis(1H-pyrrol-1-yl)benzene, 1-(1H-pyrrol-2-yl)-1-(1-vinyl-1H-pyrrol-1-yl)benzene, and 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene with chloromethanes (methylene chloride, chloroform, and carbon tetrachloride) in solvents with different polarities follows electron-transfer mechanism. The occurrence of an electron-transfer step is confirmed by formation of short-lived pyrrolylbenzene radical cations. An exception is quenching of fluorescence of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene in n-hexane in the presence of CCl4 and CHCl3 and in pure CCl4. In this case, neutral 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene·-Cl radical is formed via recombination of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene radical cation and chloride anion. A relation was found between the nature of the short-lived species detected by laser photolysis and stable product obtained by stationary photolysis.

Efficient Syntheses of C8-Aryl Adducts of Adenine and Guanine Formed by Reaction of Radical Cation Metabolites of Carcinogenic Polycyclic Aromatic Hydrocarbons with DNA

The synthesis of the C(8)-aryl adducts of adenine and guanine formed by reaction of the radical cation metabolites of carcinogenic polycyclic aromatic hydrocarbons (PAHs), such as benzo[a]pyrene (BP) and dibenzo[def,p]chrysene (DBC), with DNA is reported. The synthetic approach involves in the key step direct reaction of a PAH aldehyde with a di- or triamine precursor of a purine. The method is operationally simple, affords good yields of adducts, and is broad in its scope. The C(8)-aryl adducts of adenine and guanine derived from BP (6-BP-8-Ade and 6-BP-8-Gua) and DBC (10-DBC-8-Ade and 10-DBC-8-Gua) were synthesized in good yields by this method. Analogous C(8)-aryl adenine and guanine derivatives of other PAHs (anthracene, benz[a]anthracene, and chrysene) were also readily prepared via this approach. This method of synthesis is superior to the only method that is currently available. It entails direct reaction of short-lived PAH radical cations (generated electrochemically or chemically) with 2'-deoxyribonucleosides or the corresponding purine bases. It provides the adducts in low yields accompanied by complex mixtures of secondary products. An alternative synthesis that involves Pd-catalyzed Suzuki-Miyaura coupling of arylboronic acids with 8-bromopurine nucleosides was also investigated. Although the C(8)-purine adducts of PAHs, such as naphthalene, phenanthrene, pyrene, and chrysene, could be prepared by this method, analogous adducts of carcinogenic PAHs and other structurally related PAHs, e.g., anthracene, benz[a]anthracene, benzo[a]pyrene, and dibenzo[def,p]chrysene, could not be obtained. This difference was shown to be a consequence of the facility of competing hydrolytic deboronation of the corresponding arylboronic acids.

Charge transfer from 2-aminopurine radical cation and radical anion to nucleobases: A pulse radiolysis study

Pulse radiolysis study has been carried out to investigate the properties of the radical cation of 2-aminopurine (2AP) and the probable charge transfer from the radical cation and radical anion of 2AP to natural nucleobases in aqueous medium. The radical cation of 2AP was produced by the reaction of sulfate radical anion ( SO 4 - ). The time resolved absorption spectra obtained by the reaction of SO 4 - with 2AP at neutral pH have two distinct maxima at 380 and 470 nm and is assigned to the formation of a neutral radical of the form 2AP–N 2(–H) ( k 2 = 4.7 × 10 9 dm 3 mol −1 s −1 at pH 7). This neutral radical is formed from the deprotonation reaction of a very short-lived radical cation of 2AP. The transient absorption spectra recorded at pH 10.2 have two distinct maxima at 400 and 480 nm and is assigned to the formation of a nitrogen centered radical (2AP–N(9) ). As the hole transport from 2AP to guanine is a highly probable process, the reaction of SO 4 - is carried out in the presence of guanosine, adenosine and inosine. The spectrum obtained in the presence of guanosine was significantly different from that in the absence and it showed prominent absorption maxima at 380 and 470 nm, and a weak broad maximum centered around 625 nm which match well with the reported spectrum of a neutral guanine radical (G(–H) ). The electron transfer reaction from the radical anion of 2AP to thymine (T), cytidine (Cyd) and uridine (Urd) was also investigated at neutral pH. Among the three pyrimidines, only the transient spectrum in the presence of T gave a significant difference from the spectral features of the electron adduct of 2AP, which showed a prominent absorption maximum at 340 nm and this spectrum is similar to the electron adduct spectrum of T. The preferential reduction of thymine by 2AP − and the oxidation of guanosine by 2AP + clearly follow the oxidation/reduction potentials of the purines and pyrimidines.

Raman Spectroscopy of Short‐Lived Terthiophene Radical Cations Generated by Photochemical and Chemical Oxidation

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2''-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2''-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5''-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2''-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

The paper describes the underlying principles and discusses the most important advantages and limitations of the experimental technique of magnetically affected reaction yield spectroscopy as developed in the authors’ laboratory and guides the reader step by step through a typical experimental sequence using as example the problem of short-lived radical cations of a series of methyl-substituted benzenes in X-irradiated nonpolar solutions. For two of the eight target substances — benzene itself and mesithylene — the paper reports the first unequivocal observation of their radical cations in liquid alkane solution at room temperature and provides a lower estimate of about 10 ns for their relaxation times in low magnetic field.

Lifetime of Veratryl Alcohol Radical Cation Electrogenerated in Acetonitrile

Abstract The electrochemical oxidation of veratryl alcohol at a carbon fiber microdisk electrode with relatively high sweep rates in acetonitrile shows a chemically reversible response. Analysis of the cathodic and anodic current ratio allows one to estimate the lifetime of the short-lived radical cation in the solution.

The photophysics of 1-hydroxypyrene, the acidity of its singlet excited state, and the nature of its photoionization process in polar media.

The photophysics of 1-hydroxypyrene, Py-OH, in various media has been examined by pulsed laser and steady state methods. In alkaline polar media the photophysics of the ionized form, Py-O(-), the pyrenolate ion, is reported. From the data the pK(a) is calculated to be 8.93 for the ground state and 4.5 for the excited state. However, the excited singlet state lifetime of Py-OH is too short to lead to significant dissociation into excited Py-O(-*) and H+, and no equilibrium between the species exists. Excitation of Py-OH leads to the excited singlet, the triplet, electrons (trapped), and a short-lived radical cation. The radical cation rapidly dissociates to give the pyrenyloxyl radical. Excitation of Py-O(-) gives the excited singlet state, the triplet state, solvated electrons and the pyrenyloxyl radical. No evidence of a radical cation of Py-O(-), i.e.,(+.)Py-O(-) is forthcoming, and only the neutral pyrenyloxyl radical is observed. Low temperature experiments in glasses are used to identify the various species produced.

The electrochemical oxidation of Riluzole, a neuroprotective drug: comparison with the reaction with oxygen derived radicals

The electrochemical oxidation of Riluzole, a neuroprotective drug, is investigated. It leads to the formation of an azo dimer through a short lived radical cation. The same dimer is obtained by reaction with dioxygen in the presence of copper or in the presence of superoxide ion. The reaction with electrochemically generated hydroxyl radicals provides two hydroxylated derivatives which have been previously identified as metabolites of Riluzole.

Radical ions and photochemical charge-transfer phenomena. 39. Cycloadditions. 42. Radical cation [3 + 2] cycloadditions of 2H-azirines. Mechanistic studies concerning the intermediate radical cation

The product of the ring opening of 3-aryl-substituted azirines under conditions of photoinduced electron transfer (PET) is shown to be not a nitrile ylide but a short-lived 2-azaallenyl radical cation. This species has been investigated by pulse radiolysis and gamma-radiolysis techniques. The radical cation has an absorption band with lambda(max) at 485 nm. The lifetime of the radical cation is tau = 1.43 mus in n-butyl chloride. The rate constant for the reaction of the radical cation with the imine 9 is 7.8 X 10(9) M-1 s-1. Gamma-Radiolysis in n-butyl chloride leads to the same products, i.e., an N-alkylated imidazole as in the case of PET.

Pulse-radiolytic studies on the reactions of SO -· 4 with uracil dervivatives

Results of pulse-radiolytic studies on the reaction of SO -· 4 in aqueous solution with different uracil derivatives are reported. From 1-methyluracil, 1,3-dimethyluracil and 1,3,6-trimethyluracil upon reaction with SO -· 4 the C(5)-OH adduct radicals are quickly formed in quantitative yields. With 1,3-dimethylthymine strong evidence is presented that additionally a C(5)-CH 2 allyl-type radical is formed. The latter is thought to be the result of a deprotonation from the methyl group at position C(5) of a short-lived radical cation which is preceded by an SO -· 4 adduct radical whose half-life is 2 μs. When the N(1) position of the uracil carries a sugar moiety, further transient absorbing species were observed, the structures of which are not known at present and which cannot be assigned to any of the known intermediates derived from uracil or its derivatives. The results suggest that the presence of a sugar moiety strongly determines the ultimate fate of the base radicals.

ChemInform Abstract: Structure and Dynamics of Olefin Radical Cation Aggregates. Time‐Resolved Fluorescence Detected Magnetic Resonance

The time-resolved EPR spectra and thus the structure and dynamics of transient hydrocarbon radical cations are obtained by the pulse radiolysis-fluorescence detected magnetic resonance (FDMR) technique. Here the authors report the observation of short-lived radical cations from olefins. FDMR-EPR spectra of radical cations from tetramethylethylene and cyclohexadiene are illustrated. The olefin radical cations, FDMR spectra are concentration-dependent, since dimerization with neutral molecules takes place at higher (>10/sup -2/ M) olefin concentration. Rate constants for the dimerization reaction are derived and the effect of solvent viscosity on aggregate formation is demonstrated. By monitoring the further reactions of dimer cations the authors have obtained EPR evidence for previously unobserved higher-order (multimer) radical cation aggregates of olefins. 16 references, 5 figures.