Electrochemistry of triphenyltin acetate at a mercury-film glassy carbon electrode

The electrochemical behaviour of triphenyltin acetate was investigated by cyclic voltammetry, differential-pulse voltammetry and controlled potential electrolysis at a mercury-film glassy carbon electrode. Effects on the electrochemical response of the composition of supporting electrolytes, pH, electrode rotation speed and triphenyltin acetate concentration were determined. The electrochemical reduction of this compound was found to involve a preliminary adsorption process (E_peak ≈ ?0.7 V vs. SCE), the reduction of triphenyltin acetate to the triphenyltin radical (E_peak ≈ ?1.0 V) and reduction of the radical to the triphenyltin anion (E_peak ≈ ?1.4 V). A procedure for the determination of trace amounts of this compound by differential-pulse anodic stripping voltammetry in 50% (v/v) ethanol with 0.1 M acetic acid + 0.1 M ammonia solution was developed and applied to the analysis of a commercial powder formulation and water and fish samples. The limit of detection was 2.5 × 10^?9 M triphenyltin acetate.     

Electrochemical behavior of ellipticine derivatives: Part III. Reduction of 2-methyl-ellipticinium cations and their conjugated bases

In neutral DMF, ellipticinium cations 9 R—E⁺H—2 CH₃ undergo a reversible 1 e addition (standard potential ca. ?1.35 V) to yield a neutral radical which rapidly dimerizes (dimerization rate constant ca. 10⁷ mol^?1 1 s^?1). In basic DMF or in the presence of superoxide anion, the corresponding conjugated bases undergo a reversible 1 e addition accompanied by a fast and reversible proton addition (standard potential at pH_DMF 0 ca. ?0.52 V) to yield the same neutral radical. R being H, OCH₃ or OH, there is no effect of the substituent at the C-9 position on the electrochemical process.     

Electrochemical reduction of uracil in dimethyl sulfoxide: Autoprotonation of the anion radical

The electrochemical reduction of uracil in dimethyl sulfoxide was investigated, using d.c.and a.c. polarography, cyclic voltammetry, and controlled potential electrolysis. Uracil is reduced in a one-electron step (E_1/2=?2.3 V); the apparent number of electrons transferred (n) decreases from one at infinite dilution to one-half at concentrations above 1mM. The concentration dependent n-value is due to proton transfer by the parent compound to the radical anion formed on reduction. Such a proton transfer, which has been observed for 2-hydroxypyrimidine, deactivates part of the uracil, which would otherwise be available for reduction, by formation of the more difficultly reducible conjugate base. The uracil anion forms insoluble mercury salts, producing two oxidation waves (E_1/2 of ?0.1 and ?0.3 V); the latter wave is due to formation of a passivating film on the electrode. Digital simulations indicate that the protonation rate exceeds 10⁵M^?1 s^?1 and that, at low uracil concentration, some of the free radical formed on protonation is further reduced. At concentrations exceeding 1 mM, all of the free radical dimerizes. The effect of added acids and base on the electrochemical behavior is described.     

Generation and reactions of radical cations from the photolysis of aromatic compounds with tetranitromethane in 1,1,1,3,3,3-hexa-fluoropropan-2-ol

The generation and reactions of aromatic radical cations by photolysis of ArH-tetranitromethane in 1,1,1,3,3,3-hexfluoropropan-2-ol (HFP) at room temperature has been investigated. The primary radical cations (ArH^⊙?) were detectable by EPR spectroscopy in most cases where E ^o(Ar^⊙?/ArH) 1.5 V (vs. Ag/AgCl). Secondary radical cations were obtained from pentamethylbenzene and durene (1,2,3,4,5,6,7,8-octamethyl- and 1,2,4,5,6,8-hexamethyl-anthraceneradical cation), from certain naphthalenes (dehydrodimer radical cations, Ar?Ar^⊙?) and from 2,3-dimethylanisole (the radical of a nitroso derivative). The persistence of the radical cations in the presence of trinitromethanide ion is seen as a consequence of the stabilization of the nucleophile by HFP. This assumption was supported by preparative results on 1-methoxynaphthalene and 1,4-dimethylnaphthalene, where the trinitromethanide ion initiated pathway was completely eliminated in HFP, contrary to results earlier obtained in dichloromethane.     

Surface films obtained by electrochemical oxidation of 1,11-bis(3-indolizinin-2-ylquinoxalin-2-on-1-yl)-3,6,9-trioxyundecane

Electrochemical oxidation of 1,11-bis(3-indolizinin-2-ylquinoxalin-2-on-1-yl)-3,6,9-trioxyundecane in 0.1 M Et₄NClO₄ in MeCN on a glassy-carbon electrode leads to surface films of two types depending on the film formation mode. Oxidation at controlled first-peak potentials (E _p ¹ = 0.30 V relative to Fc/Fc⁺) or in the range of potentials ?0.60 → +0.40 → ?0.60 V, ?0.60 → +0.40 → ?1.60 → ?0.60 V in the polycycling mode forms a redox-active film that covers the entire surface of the electrode. The quinoxaline fragments of the film are reduced irreversibly in the supporting solution, while the indolizine fragments are reduced reversibly, forming stable radical cation states; the latter are recorded by ESR in the form of a singlet with a variable width (g = 2.0025, ΔH = 0.15?0.5 mT). The oxidized (radical cation) form of the film is conductive, while the neutral film is not. An insulating film is obtained when oxidation is performed under rigid conditions (?0.60 → +1.70 → ?1.70 → ?0.60 V). Both types of film are colored and have nanosized pores (channels), which can play a discriminant role. It is demonstrated (using the discriminant role of pores) that small molecules may be selectively reduced on film-coated electrodes in the presence of bulkier and more reactive substrates.     

Mushroom Polysaccharides and Lipids Synthesized in Liquid Agitated and Static Cultures. Part II: Study of Volvariella volvacea

Volvariella volvacea strains were studied in relation with their ability to produce biomass, lipids and polysaccharides. Firstly, screening of four strains (AMLR 188, 190, 191 and 192) was performed in agar cultures, where the mycelial growth rate of the strains was measured, and in static liquid cultures, where the production of biomass, the biosynthesis of total cellular lipids and the consumption of glucose were monitored. For all strains, biomass production was significant (13?C15?g?l^?1) and total lipid in dry weight (%, w/w) ranged from 3 to 12?%. Afterwards, a detailed kinetic analysis of mycelial biomass, extra- and intra- cellular polysaccharides (EPS, IPS, respectively) as well as lipid production by a V. volvacea selected strain was conducted in submerged static and agitated cultures. Maximum values of 15?g?l^?1 biomass, ??1.0?g?l^?1 EPS and 5.5?g?l^?1 IPS were recorded. Agitation did not have severe impact on biomass, EPS and IPS production, but it increased total lipid in dry weight quantities. EPS, IPS and lipid in dry weight values decreased with time. Glucose was the major cellular carbohydrate detected. Total fatty acid analysis of cellular lipids was performed for all V. volvacea strains and linoleic acid ^??9,12C18:2 was predominant. Neutral lipids constituted the major fraction of cellular lipids, but their quantity decreased as fermentation proceeded. Phospholipids were the most saturated lipid fraction.     

Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part III. Reduction of some disulfides at the dropping mercury electrode

In the simultaneous cathodic reduction of diphenyl disulfide (Ph?S?S?Ph) or dibenzoyl disulfide (Ph?CO?S?S?CO?Ph) and a series of fluorescent aromatic hydrocarbons A at the dropping mercury electrode in dimethylformamide the emission of A is observed. This electrogenerated chemiluminescence (ECL) originates from the electron transfer between A^? and the radicals Ph?S or Ph?COS, which are formed in a one-electron reductive cleavage of the disulfide bond by A^?. As an intermediate the anion radical Ph?S?S?Ph^? or Ph?CO?S?S?CO?Ph^? is assumed. In the case of Ph?S?S?Ph the ECL intensity is enhanced by proton donors (water or benzoic acid), which increase the cleavage rate of Ph? S?S?Ph^? in an electrophilic attack by the proton. The relatively negative threshold reduction potential of A (?1.4 to ?1.6 V) for the ECL in comparison with the half-wave potential (?0.85 V) supports a mercury-assisted heterogeneous reduction mechanism of Ph?S?S?Ph. The intensity-potential curves and the intensity—time curves at the mercury drop were measured for different concentrations of Ph?S?S?Ph and A and for different mercury pressures. No luminescence could be detected with o,o′-dinitrodiphenyl disulfide and diethyl disulfide.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

Generation of phenylmethylketyl radicals in aqueous solutions and study of their transformation

Acetophenone and its intermediates formed upon the first electron transfer are studied by laser photoemission and traditional electrochemistry. It is shown that the intermediate reduction is affected by competition of the reactions of the formed radical-anions oxidation and their subsequent transformation to secondary products that are rapidly reduced at the electrode. From the comparison of the data obtained by the laser photoemission method and electrochemical measurements a conclusion was drawn that the product is a metastable complex (associate) of the radical-anion with water molecule; its formation rate constant is rather low (～6 × 10³ M^?1 s^?1). It was also concluded that bulk radical reactions dominate in aprotic media at moderate cathodic potentials; the acetophenone radical-anion is reduced at E ≤ ?1.9 V (SCE). This conclusion agrees with the results of the acetophenone preparative electrolysis in DMFA, where marked yield of pinacon was observed at the potentials of limiting current of the 1st reduction wave, while the stage of 2nd electron transfer occurred at E ≤ ?2.3 V (SCE).     

ESR study of the photo-induced decomposition of polypropylene in the presence of N,N,N′,N′-tetramethyl-p-phenylenediamine

The mechanism of uv (λ > 325 nm) photodegradation of polypropylene (PP) containing N,N,N′,N′-tetramethyl-p-phenylenediamine (T₄MPD) has been investigated by means of ESR spectroscopy. The observed spectra after uv irradiation of both isotactic-PP (IPP) and stereoblock-PP (SPP) samples in vacuum at 77 K consisted principally of a broad singlet which was assigned to a T₄MPD cation radical (T₄MPD^+·). On the other hand, the spectrum observed after irradiation of an atactic polypropylene (APP) sample at 77 K in vacuum was resolved into several components which decayed almost up to ca. 263 K to give rise to the broad singlet of T₄MPD^+·. One component was a sharp quartet which was assigned to a methyl radical, ·CH₃·. The other component, a singlet, was attributed to a trapped electron, e_t^?.By comparison of the ESR spectrum of deuterated T₄MPD with that of the normal compound it was found that 60 ～ 70% of the methyl radicals arose from the added T₄MPD due to β-scission, which also formed the N,N,N′-trimethyl-p-phenylenediamine radical, T₃MPD·. The T₃MPD· radical presumably captures an electron at lower temperatures to become a carbanion, T₃MPD^?, which releases the electron to reproduce the T₃MPD· radical at elevated temperatures. This production of the radical T₃MPD· due to the liberation of an electron provides an explanation for the observed increase in intensity of the decay curve in the temperature range from ? 168 K to 185 K. The remaining fraction, 30 ～ 40%, of the total methyl radicals was produced from the PP matrix by an energy transfer from the excited T₄MPD¹ to the PP matrix. The broad singlet which appeared in the temperature range near 195 K was attributed to an acyl radical ～CH₂CH(CH₃)CH₂?O from the observed g-value. By photoillumination of this sample this broad singlet was converted reversibly into the quartet which was assigned to the radical ～CH₂CH(CH₂·)CH₂CHO.     

Effects of paramagnetic lanthanide ions on decay process of photogenerated radical pairs in porphyrin-viologen linked compounds at reversed micellar surfaces

Radical pairs were generated in reversed micelles by laser excitation of zinc tetraphenylporphyrin-viologen linked compounds (ZPnV) with a polymethylene spacer (?(CH₂)_n?; n=4, 6 and 8). The optical transient-absorption and chemically induced dynamic electron polarization (CIDEP) spectra indicated that paramagnetic lanthanide ions invariably accelerated the decay process of the radical pairs at 0.3 T. The typical E/A/E/A spectra for the ZP6V and ZP8V systems were explained as due to S-T₀ mixing and electron-spin relaxation between Zeeman-splitted triplet sublevels. In the case of ZP4V, the paramagnetic Gd³⁺ ion accelerated the radical decay process even at zero-magnetic field, and strong emissive CIDEP spectra evolved with the elapsed time after laser excitation. Appreciable contribution of S-T level crossing in the radical decay process was suggested to account for the novel features of the ZP4V system.     

Direct electrochemistry of myoglobin in a layer-by-layer film on an ionic liquid modified electrode containing CeO2 nanoparticles and hyaluronic acid

We describe an ionic liquid modified electrode (CPE-IL) for sensing hydrogen peroxide (HP) that was modified by the layer-by-layer technique with myoglobin (Mb). In addition, the surface of the electrode was modified with CeO₂ nanoparticles (nano-CeO₂) and hyaluronic acid. UV-vis and FTIR spectroscopy confirmed that Mb retains its native structure in the composite film. Scanning electron microscopy showed that the nano-CeO₂ closely interact with Mb to form an inhomogeneously distributed film. Cyclic voltammetry reveals a pair of quasi-reversible redox peaks of Mb, with the cathodic peak at ?0.357?V and the anodic peak at ?0.269?V. The peak separation (??E _p) and the formal potential (E ^0??) are 88?mV and ?0.313?V (vs. Ag/AgCl), respectively. The Mb immobilized in the modified electrode displays an excellent electrocatalytic activity towards HP in the 0.6 to 78.0???M concentration range. The limit of detection is 50?nM (S/N?=?3), and then the Michaelis-Menten constant is 71.8???M. We believe that such a composite film has potential to further investigate other redox proteins and in the fabrication of third-generation biosensors.

Determination of antimony(III) and (V) in natural water by cathodic stripping voltammetry with in-situ plated bismuth film electrode

A method is described for the sequential determination of Sb(III) and Sb(V) using Osteryoung square wave cathodic stripping voltammetry. It employs an in-situ plated bismuth-film on an edge-plane graphite substrate as the working electrode. Selective electro-deposition of Sb(III)/Sb(V) is accomplished by applying a potential of ?500 mV vs. Ag/AgCl, followed by reduction to stibine at a more negative potential in the stripping step. Stripping was carried out by applying a square wave waveform between ?500 and ?1400 mV to the antimony deposited. The stripping peak current at ?1150 mV is directly proportional to the concentration of Sb( III)/Sb(V). The calibration plots for Sb (III) were linear up to 12.0?µg L^?1 depending on the time of deposition. The calibration plots for Sb (V) were linear up to 7.0?µg L^?1, also depending on the time of deposition. The relative standard deviation in the determination of 0.1?µg L^?1 of Sb(III) is 4.0% (n?=?5), and the limit of detection is as low as 2 ng L^?1. In case of 0.1?µg L^?1 Sb(V), the relative standard deviation is 3.0% (n?=?5) and the detection limit also is 2 ng L^?1. The method was applied to the analysis of river and sea water samples.     

Determination of structure and properties of modified chlorophylls by using fast atom bombardment combined with tandem mass spectrometry

Tandem mass spectrometry (MS/MS) was used to investigate and compare the decompositions of radical cations (M^+.), radical anions (M^-.), [M + H]⁺ ions, and [M + Cat]⁺ ions (Cat = alkali metal ions) of chlorophylls. Included in this study are chlorophyll a, chlorophyll b, bacteriochlorophyll a, chlorophyll a allomers, and the corresponding pheophytins. Fast atom bombardment of chlorophyll a produces abundant M^+. ions, which decompose metastably and upon collisional activation to give fragment ions from losses of the phytyl chain and the β-keto group of ring V. In addition, previously unreported charge-remote fragmentations are useful for identification of branch points on the phytyl chain. Collisional activation of [M + Cat]⁺ ions produces fragment ions that are complementary to those of the M⁺ and are used to examine the intrinsic gas-phase reactivity of metal ions and chlorophylls. Peripheral metal ion attachment in chlorophyll a in the gas phase is suggested to be at C-9, and the β-keto ester group at C-10, of ring V. Examination of decompositions of chlorophyll dimers suggests that in the gas phase the interaction between monomers involves bonding of the Mg atom of one chlorophyll a molecule and the C-9 carbonyl oxygen of the other, which was also suggested for chlorophyll a dimers in solution.     

Green-light-driven thioxanthylium-based organophotoredox catalysts: Organophotoredox promoted radical cation Diels-Alder reaction

Thioxanthylium-based organophotoredox catalysts that operate under irradiation with green light have been developed. These catalysts present high excited-state reduction potentials (E₀′(C¹/C^?)?=?+1.79–1.94?V vs SCE). They are able to efficiently activate dienophiles under green or blue light irradiation afforded the targeted radical cation Diels-Alder cycloadducts in good yields. The present thioxanthylium-based catalysts provide a new green-light-driven photoredox catalysis system.     

Linear-sweep polarographic determination of active chlorine in aqueous solutions containing phenylthiourea

The linear-sweep polarographic determination of active chlorine is based on its reaction with phenylthiourea in acidic phosphate buffer (pH 2.5) containing potassium chloride. The product, C,C-diphenyldithiodiformamidine, is strongly adsorbed and then reduced at a mercury electrode with two peaks at about ?0.35 V and ?0.87 V (vs. SCE). In the presence of 0.05 M potassium chloride, the potential of the first peak shifts positively to ?0.31 V. This peak provides high sensitivity and selectivity for the determination of traces of active chlorine. The linear range is 1×10^?7?2.5×10^?5 M and the detection limit is 5×10^?8 M (3.6 μg l^?1). The method is used for the direct determination of active chlorine in tap water. The mechanism of the reaction was studied by cyclic voltammetry, electrolysis and potentiometric titration. The first peak (?0.35 V) is ascribed to the reduction of a mercury (II) sulfide film produced by reaction of the adsorbed dithio product with mercury. In the presence of 0.05 M chloride, the formation of a mixed HgS·xHg₂Cl2 film shifts the peak to ?0.31 V.     

Influence of cyanide and sulphide ions on the reduction and reoxidation of cobalt(II) in thiocyanate solution at mercury electrodes

The process of reduction and reoxidation of cobalt(II) in thiocyanate solution at hanging mercury drop electrode has been investigated by cyclic voltammetric, chronoamperometric and anodic stripping methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M cobalt(II), the voltammogram on the first cycle at 0.05 V s^?1 gives a cathodic peak at ?1.06 V with hysteresis on reversal, and an anodic wave with a peak potential of ?0.28 V and with two shoulders near ?0.38 and ?0.45 V, respectively. Multicyclic voltammograms under the same conditions give a cathodic peak at ?0.90 V and an anodic peak at ?0.45 V. The reduction and reoxidation of cobalt(II) in thiocyanate solution is accelerated by the reduction products of thiocyanate ion, cyanide and sulphide ions, which are produced during the electroreduction of cobalt(II).A mechanism of reduction and reoxidation of cobalt(II) which involves a chemical reduction of thiocyanate ion by electroreduced metallic cobalt and takes into account cyanide and sulphide ions is proposed. The hysteresis on the cathodic wave is caused by the difference in reduction potentials of cobalt(II)-thiocyanate and-cyanide complexes. Cyclic voltammetric study of cobalt(II) in perchlorate solution containing trace amounts of cyanide and sulphide ions supports these conclusions.     

Relative concentrations and relaxation properties of isomeric alkyl radicals in γ-irradiated n-alkane single crystals

The relative concentrations of alkyl radicals CH₃C?HCH₂R(I) and R'CH₂C?HCH₂R''(II) were measured at low microwave power in some n-alkane single crystals γ-irradiated at 77 K to a dose of 1 Mrad. The relative concentration of radical (II) increased as the number of carbon atoms became larger. The amount of radical (I) was in agreement with a mechanism where all CH bonds in an n-alkane molecule are raptured with the same probability followed by an isomerization of primary alkyl radicals to radical (I). In n-decane for instance this mechanism predicts 45.5% of radical (I) compared to the experimental value of 45.5%. Saturation measurements of radical (I) and radical (II) under slow passage condition showed that the spin-lattice relaxation time T₁ is shorter for radical (I) (ca. 3 × 10^?4s) than for radical (II) (ca. 80 × 10^?4s), while the spin-spin relaxation times T₂ are similar (ca. 2 × 10^?8s). The relatively short relaxation time T₁ in radical (I) is thought to originate from higher mobility of the end of the alkane chain, where the unpaired electron is localized, and also a modulation of the hyperfine coupling from protons in the nearby rotating methyl group. The broad linewidth in irradiated protiated cyrstals was by comparison with results from deuterated matrices concluded to depend on slightly distorted radicals in damaged regions (spurs, short tracks, blobs) and not on electron dipole-dipole interactions. Unresolved γ-proton couplings in radical (I) are thought to cause the spin-flip transitions at high microwave power.     

Lithium electrochromism of atmospheric pressure plasma jet-synthesized NiO x C y thin films

Reversible lithium intercalation and deintercalation behavior of atmospheric pressure plasma jet (APPJ)-synthesized organonickel oxide (NiO _x C _y ) thin films under various substrate distances is testified in an electrolyte (1?M LiClO₄–propylene carbonate solution) at low driving voltages from ?0.5 to 1.5?V. Fast responses of 2?s bleaching at ?0.5?V and 6?s coloration at +1.5?V are accomplished for the nano-porous NiO _x C _y thin films. This study reveals that a rapid synthesis of electrochromic NiO _x C _y thin films in a single process via APPJ by 21?s is investigated. This study presents a noteworthy electrochromic performance in a light modulation with up to 43% of transmittance variation and a coloration efficiency of 36.3?cm²/C at a wavelength of 830?nm after 200?cycles of cyclic voltammetry measurements.     

Anodic behavior of gold in acid thiourea solutions: A cyclic voltammetry and quartz microgravimetry study

It is shown that at potentials E < 0.5 V (NHE) gold undergoes practically no dissolution in thiourea solutions containing no catalytically active species. The dissolution at a perceptible rate (> 100 μA cm^?2) starts at E ≥ 0.65 V, with the primary process being the oxidation of thiourea, which gives rise a current peak at E ? 1.0 V. The thiourea oxidation at E ≥ 1.1 produces the appearance of catalytically active species, which drastically accelerate the gold dissolution process in the potential region extending from a steady-state value to 0.6 V, where the current efficiency for gold approaches 100% and a peak emerges at E ? 0.55 V. The peak’s height is commensurate with the value of the limiting diffusion current associated with the ligand supply. The species in question make no discernible impact on the thiourea oxidation process. Formamidine disulfide, which forms during the anodic oxidation of thiourea or which is added in solution on purpose, exerts no noticeable catalytic influence on the anodic gold dissolution. The catalytically active species is presumably the S^2? ion, product of decomposition and deep oxidation of thiourea and formamidine disulfide. Indeed, adding sulfide ions in solution has a strong catalytic effect on the gold dissolution, whose character is identical to that of the effect exerted by products of anodic oxidation of thiourea at E ≥ 1.1 V μA cm^?2.