Identification of the radical anions of C2N4S2 and P2N4S2 rings by in situ EPR spectroelectrochemistry and DFT calculations

The previously unknown radical anions of unsaturated E2N4S2 ring systems (E=RC, R2NC, R2P) can be generated voltammetrically by the one-electron reduction of the neutral species and, despite half-lives on the order of a few seconds, have been unambiguously characterized by electron paramagnetic resonance (EPR) spectroelectrochemistry using a highly sensitive in situ electrolysis cell. Cyclic voltammetric studies using a glassy-carbon working electrode in CH3CN and CH2Cl2 with [nBu4N][PF6] as the supporting electrolyte gave reversible formal potentials for the [E2N4S2]0/- process in the range of -1.25 to -1.77 V and irreversible peak potentials for oxidation in the range of 0.66 to 1.60 V (vs the Fc+/0 couple; Fc=ferrocene). Reduction of the neutral compound undergoes an electrochemically reversible one-electron transfer, followed by the decay of the anion to an unknown species via a first-order (chemical) reaction pathway. The values of the first-order rate constant, kf, for the decay of all the radical anions in CH2Cl2 have been estimated from the decay of the EPR signals for (X-C6H4CN2S)2*-, where X=4-OCH3 (kf=0.04 s(-1)), 4-CH3 (kf=0.02 s(-1)), 4-H (kf=0.08 s(-1)), 4-Cl (kf=0.05 s(-1)), 4-CF3 (kf=0.05 s(-1)), or 3-CF3 (kf=0.07 s(-1)), and for [(CH3)3CCN2S]2*- (kf=0.02 s(-1)), [(CH3)2NCN2S]2*- (kf=0.05 s(-1)), and [(C6H5)2PN2S]2*- (kf=0.7 s(-1)). Values of kf for X=4-H and for [(CH3)2NCN2S]2*- were also determined from the cyclic voltammetric responses (in CH2Cl2) and were both found to be 0.05 s(-1). Possible pathways for the first-order anion decomposition that are consistent with the experimental observations are discussed. Density functional theory calculations at the UB3LYP/6-31G(d) level of theory predict the structures of the radical anions as either planar (D2h) or folded (C2v) species; the calculated hyperfine coupling constants are in excellent agreement with experimental results. Linear correlations were observed between the voltammetrically determined potentials and both the orbital energies and Hammett coefficients for the neutral aryl-substituted rings.     

Recognition of thymine and related nucleosides by a Zn(II)-cyclen complex bearing a ferrocenyl pendant

A cyclen derivative bearing a ferrocenyl arm (L) and a series of its ZnII complexes [ZnL(OH2)][ClO4]2 (C1), [ZnL(OH)][ClO4] (C2), and [ZnL(Cl)][ClO4].CH3CN (C3) (cyclen = 1,4,7,10-tetraazacyclododecane, L = 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane) have been prepared and characterized spectroscopically. An X-ray structure determination confirmed the formation of complex C1 and revealed that the coordinated water participates in hydrogen bonding with the perchlorate counter ions. The pKa value for deprotonation of the water molecule determined by potentiometric titration was found to be 7.36 +/- 0.09 at 25 degrees C and I = 0.1 (KNO3). The possibility of using complex C1 as a potential sensor for thymine derivatives in aqueous solution has been examined. Shifts in the 1H and 13C NMR resonances showed the binding occurred with thymine (T) and two thymine derivatives, thymidine (dT) and thymidine 5'-monophosphate (TMP2-). Significant shifts of the nuC=O and nuC=C vibrations of the thymine derivatives were also observed via IR spectroscopy upon complexation with the receptor. The thymine adduct, [ZnL(thymine anion)][ClO4].2H2O (C4), has been crystallized and characterized. The X-ray structure of C4 confirmed the thymine binding to the receptor, and the short Zn-N(thymine) distance of 1.975(5) A indicated clearly that the ferrocenyl arm does not affect the complexation of the DNA base. In contrast to the large spectral changes, electrochemical studies showed a small shift of the reversible potential of the redox couple Fc+/Fc (Fc = ferrocene) and subtle changes in voltammetry upon the addition of an excess of dT, TMP2-, and guanine (dG) at physiological pH, indicating the level of interaction is similar in both Fc and Fc+ forms.     

pH-dependence of the aqueous electrochemistry of the two-electron reduced alpha-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

The electrochemistry of the Dawson-like sulfite polyoxometalate anion alpha-[Mo18O54(SO3)2]6-, derived from the TEAH6{alpha-[Mo18O54(SO3)2]} salt (TEAH+ is the triethanolammonium cation; pKa=7.8), has been investigated in aqueous media using cyclic and rotated disk voltammetry at glassy carbon electrodes and bulk electrolysis, with a focus on the pH-dependence for oxidation to alpha-[Mo18O54(SO3)2]4-. In buffered media at pH>or=4, the cyclic voltammetric response for alpha-[Mo18O54(SO3)2]6- reveals two partially resolved one-electron oxidation processes corresponding to the sequential generation of alpha-[Mo18O54(SO3)2]5- and alpha-[Mo18O54(SO3)2]4-. At lower pH, using electrolytes containing sulfuric acid, the two waves coalesce but the individual apparent E0' reversible formal potential values for the two processes can be extracted down to pH 2 by assuming that reversible protonation accompanies fast electron transfer. The results for 2相似文献   

200 years of practical electroanalytical chemistry: past, present and future directions illustrated by reference to the on-line, on-stream and off-line determination of trace metals in zinc plant electrolyte by voltammetric and potentiometric techniques

The millennium being celebrated this year coincides with the 200th anniversary of the birth of practical electrochemistry made possible via Volta’s publication of the battery in the year 1800. The analytical chemists at the beginning of the 19th century were very quick to take advantage of this newly reported device and the first qualitative electrochemical determination of copper rapidly followed this pioneering discovery. In the last 200 years, electrochemical analysis, in its various forms, has been undertaken routinely in countless laboratories all over the world. However, in view of the long and distinguished history of the discipline, and some limitations that have been identified at the time of the celebration of the millennium, electrochemical analysis is regarded in some quarters as being a mature and conservative discipline whose importance in the future, when faced with severe competition from newly emerging alternative analytical techniques, is somewhat unclear. In this paper, an overview of past and present developments in electroanalytical chemistry and the possible future status of the technique is presented. In particular, emphasis is given to describing applications relevant to the also very mature field of electrowinning of zinc from plant electrolyte. This overview encompasses the author’s 25 years’ experience in developing polarographic, stripping voltammetric, adsorptive stripping voltammetric and ion-selective electrode (ISE) methods of analysis in on-line, on-stream and off-line modes for the determination of elements such as Cd, Pb, Ge, Sb (oxidation states (III) and (V)), Co, Ni, Zn, Fe, (oxidation states (II) and (III)), Tl, As (total) and Cu in zinc plant electrolyte. Developments that may contribute to an important future for analytical voltammetry are also considered as are limitations that could inhibit the extent of practical use of these electroanalytical techniques in the 21st century.     

Interference from Trace Copper in Electrochemical Investigations Employing Carboxylic Acid Terminated Thiol Modified Gold Electrodes

Unexpectedly, electrochemistry at variable chain length carboxylic acid terminated alkylthiol self‐assembled monolayers (SAMs) on gold electrodes gives rise to a Faradaic process in buffered aqueous electrolyte solution. In particular, the three‐carbon chain length, 3‐mercaptopropionic acid (MPA), exhibits a chemically reversible process with a mid‐point potential of 175 mV vs. Ag/AgCl under conditions of cyclic voltammetry. This process is associated with the presence of trace (parts per billion) amounts of copper(II) ions present in the chemical reagents used to prepare the aqueous electrolyte and also from the gold electrode itself. The carboxylic acid moiety on the SAM concentrates Cu²⁺ ions by coordination and this surface confined layer is then reduced. Methods to minimize the interference of Cu²⁺ ions at carboxylic acid terminated SAM are discussed and caution with respect to the interpretation of protein electrochemistry is recommended when using carboxylic acid functionalized SAMs to provide biocompatible electrochemical transduction surfaces, unless a metal free environment can be obtained.     

Theoretical and experimental evaluation of screen-printed tubular carbon ink disposable sensor well electrodes by dc and Fourier transformed ac voltammetry

Mass-produced, screen-printed, carbon-ink-based microtubular band (well) electrodes, suitable for routine sensing applications, have been fabricated and evaluated with respect to their theoretical and analytical performance. Microscopic examination of the electrode surface reveals they are inherently rough and could easily suffer from high and variable resistance, capacitance and area, unless care is taken to minimise these problems. Simulation models have been applied to analyse cyclic voltammetric responses obtained at the well electrodes. Results of these theoretical calculations further demonstrate the care needed with electrode design and resistance in carbon ink electrodes. Substantial differences in voltammetry when wells are produced by mechanically punching or laser drilling are considered. The application of multi- and single-frequency Fourier Transform ac voltammetry, previously applied to planar carbon ink disc electrodes for quality control purposes, is now demonstrated with respect to the microtubular band electrode geometry. Theoretical and practical limitations are discussed, as is the analytical application to the reversible redox couple in the presence of oxygen in aqueous solution. Dedicated to Keith on his 80th birthday, a good friend and colleague.     

Reading “A variant of the Hales–Jewett theorem” on its anniversary

In a recent paper “A variant of the Hales–Jewett theorem”, M. Beiglböck provides a version of the classic coloring result in which an instance of the variable in a word giving rise to a monochromatic combinatorial line can be moved around in a finite structure of specified type (for example, an arithmetic progression). We give an elementary proof and infinitary extensions.     

Experimental and theoretical investigations of the sulfite-based polyoxometalate cluster redox series: alpha- and beta-[Mo(18)O(54)(SO(3))(2)](4-/5-/6-)

The synthesis, isolation and structural characterization of the sulfite polyoxomolybdate clusters alpha-(D(3h))(C(20)H(44)N)(4){alpha-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN and beta-(D(3d))(C(20)H(44)N)(4){beta-[Mo(18)O(54)(SO(3))(2)]}CH(3)CN is presented. Voltammetric studies in acetonitrile (0.1 M Hx(4)NClO(4), Hx(4)N=tetra-n-hexylammonium) reveal the presence of an extensive series of six one-electron reduction processes for both isomers. Under conditions of bulk electrolysis, the initial [Mo(18)O(54)(SO(3))(2)](4-/5-) and [Mo(18)O(54)(SO(3))(2)](5-/6-) processes produce stable [Mo(18)O(54)(SO(3))(2)](5-) and [Mo(18)O(54)(SO(3))(2)](6-) species, respectively, and the same reduced species may be produced by photochemical reduction. Spectroelectrochemical data imply that retention of structural form results upon reduction, so that both alpha and beta isomers are available at each of the 4-, 5-, and 6-redox levels. However, the alpha isomer is the thermodynamically favored species in both the one- and two-electron-reduced states, with beta-->alpha isomerization being detected in both cases on long time scales (days). EPR spectra also imply that increasing localization of the unpaired electron occurs over the alpha- and beta-[Mo(18)O(54)(SO(3))(2)](5-) frameworks as the temperature approaches 2 K where the EPR spectra show orthorhombic symmetry with different g and hyperfine values for the alpha and beta isomers. Theoretical studies support the observation that it is easier to reduce the alpha cluster than the beta form and also provide insight into the driving force for beta-->alpha isomerization in the reduced state. Data are compared with that obtained for the well studied alpha-[Mo(18)O(54)(SO(4))(2))](4-) sulfate cluster.     

Measurement of the viscosity-density product using multiple reflections of ultrasonic shear horizontal waves

We have developed an on-line computer-controlled sensor, based on ultrasound reflection measurements, to determine the product of the viscosity and density of a liquid or slurry for Newtonian fluids and the shear impedance of the liquid for non-Newtonian fluids. A 14 MHz shear wave transducer is bonded to one side of a 45-90 degrees fused silica wedge and the base is in contract with the liquid. Twenty-eight echoes were observed due to the multiple reflections of an ultrasonic shear horizontal (SH) wave within the wedge. The fast Fourier transform of each echo was obtained for a liquid and for water, which serves as the calibration fluid, and the reflection coefficient at the solid-liquid interface was obtained. Data were obtained for 11 sugar water solutions ranging in concentration from 10% to 66% by weight. The viscosity values are shown to be in good agreement with those obtained independently using a laboratory viscometer. The data acquisition time is 14s and this can be reduced by judicious selection of the echoes for determining the reflection coefficient. The measurement of the density results in a determination of the viscosity for Newtonian fluids or the shear wave velocity for non-Newtonian fluids. The sensor can be deployed for process control in a pipeline, with the base of the wedge as part of the pipeline wall, or immersed in a tank.     

Preferential synthesis of highly conducting Tl(TCNQ) phase II nanorod networks via electrochemically driven TCNQ/Tl(TCNQ) solid-solid phase transformation

Facile synthesis and characterization of the highly conducting, thermodynamically favored, Tl(TCNQ) phase II microrods/nanorods onto conducting (glassy carbon (GC)) and semiconducting (indium tin oxide (ITO)) surfaces have been accomplished via redox-based transformation of 7,7,8,8-tetracynoquinodimethane (TCNQ) microcrystals. This electrochemically irreversible process involves the one-electron reduction of surface-confined solid TCNQ into TCNQ·^? with concomitant incorporation of the Tl⁺ _(aq) cation, from the bulk solution, at the triple-phase boundary, GC or ITO│(TCNQ_(s)/TCNQ·^? _(s))│Tl⁺ _(aq), through a nucleation/growth mechanism. Consistent with the conceptually related M(TCNQ) systems (M⁺ = Li⁺, Na⁺, K⁺, Ag⁺, and Cu⁺), the TCNQ/Tl(TCNQ) interconversion is strongly dependent upon scan rate, Tl⁺ _(aq) electrolyte concentration, and the method of attaching solid TCNQ onto the electrode surface. Spectroscopic (infrared (IR) and Raman), microscopic (scanning electron microscopy (SEM)), and surface science (X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD)) characterization of the electrochemically synthesized material revealed formation of pure Tl(TCNQ) phase II. Importantly, the generic solid-state electrochemical approach used in this study not only offers facile protocol for controllable and preferential synthesis of Tl(TCNQ) phase II but also provides access to fabricate and tune the morphology to yield microrod/nanorod networks.

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Graphical abstract Controlled synthesis of the highly conducting Tl(TCNQ) phase II with either nanowire or rod-like morphologies is achieved via a redox-based solid-solid phase interconversion of TCNQ microcrystals in the presence of a Tl⁺ _(aq) electrolyte.