The electrochemical oxidation of phenylenediamines

Five p-phenylenediamines were oxidized electrochemically at a glassy carbon rotating ring-disk-electrode assembly. The overall reaction comprises the transfer of two electrons per molecule. At low to intermediate potentials the rate determining step for all the five compounds is the acid dissociation of the semiquinonediimine radical cation formed in a pre-equilibrium involving the transfer of one electron. Only at higher potentials can the radical cation be oxidized in parallel to the neutral semiquinone radical to form the quinonediimines. In contrast, the neutral semiquinonediimine radicals are oxidized at the same or even a lower potential than the parent phenylenediamines. The dissociation rate constants of the radical cations are in the range of 10-300 s⁻¹. The effect of the substituents on the oxidation potentials is as expected by their property to donate electrons to the mesomeric system. Their effect on the dissociation rates, however, seems to rather correlate with the inductive effect and might influence not only the electronic structure of the molecule itself but also the structure of the ions and the solvent around them. The semiquinonediimine radical cations are found to be slightly more acidic than their protonated parent phenylenediamine cations with pK_a values somewhat below 6.     

Sodium niobates and protonic niobates nanowires obtained from hydrothermal synthesis: Electrochemical behavior in aqueous electrolyte

Niobium-based oxides can be used in several applications due to a diverse set of properties. In this work, Na₂Nb₂O₆.H₂O sodium niobate nanowires were obtained by hydrothermal synthesis at low temperature. Dehydrated sodium niobate (Na₂Nb₂O₆) and sodium niobate with perovskite structure (NaNbO₃) were obtained by submitting Na₂Nb₂O₆.H₂O to heat treatment (350 °C and 500 °C, respectively). To obtain protonic niobates, sodium niobates were immersed in nitric acid in order to promote ion exchange reactions. From this procedure, protonic niobates (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ were obtained. The sample NaNbO₃ did not undergo any transformation. Cyclic voltammetry tests carried out in a neutral aqueous solution 1 M Na₂SO₄ showed a wide potential window for both niobates (sodium and protonic). However, the protonic niobate samples (H₃O)₂Nb₂O₆.H₂O and (H₃O)₂Nb₂O₆ presented much higher current density values than the sodium niobates. This result can be related to a structural rearrangement that allowed a significant increase in the intercalation of sodium Na ⁺ ions from the electrolyte into the structure of these protonic niobates, when polarized. Therefore, in this work, it was demonstrated that it is possible to obtain protonic niobates from sodium niobates, as well as, it was verified the distinct electrochemical behavior between these materials.     

Synthesis and properties of polystyrene–clay nanocomposites via in situ intercalative polymerization

Organophilic montmorillonite (MMT) was prepared by ion exchange between Na⁺ ions in the clay and twin benzyldimethyloctadecylammonium bromine cations in an aqueous medium. The organophilic MMT particles were easily dispersed and swollen in styrene monomer. Polystyrene–MMT nanocomposites were prepared by the free‐radical polymerization of styrene containing dispersed clay. The intercalation spacing in the nanocomposites and the degree of dispersion of these composites were investigated with X‐ray diffraction and transmission electron microscopy, respectively. The nanocomposites had higher weight‐average molecular weights, lower glass‐transition temperatures, and better thermal stability (the decomposition temperature was improved by ca. 70°C) than the virgin polystyrene. The rheological behavior of the polystyrene–MMT nanocomposites was also studied. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 201–207, 2005     

Effects of exchanged ammonium cations on structure characteristics and CO2 adsorption capacities of bentonite clay

Different alkali and alkaline earth cation forms of bentonite clay were exchanged with protonated mono-, di- and triethanolamine compounds, to study the effect of the exchanged ammonium cations on the structure characteristics, thermal behavior, surface properties and CO₂ adsorption capacities of bentonite clay. The revolution of the interlayer structure characteristics, thermal properties, the specific surface area and elemental analysis were characterized by XRD, FTIR, TGA, BET and CHNS techniques respectively, while the CO₂ adsorption capacities were gravimetrically measured by using magnetic suspension balance (MSB) equipment. It was found that the intercalation of ammonium cations into the interlayer space of bentonite clay induced a step change in its basal spacing, depending on their molar mass and the interlayer molecular arrangement. The presence of the characteristic IR peaks of amine compounds in the spectra of bentonite clay adsorbents modified by amines was qualitatively supported by the incorporation of ammonium cations in the interlayer space of bentonite, while the presence of C, H and N elements using CHNS technique was quantitatively confirmed by the intercalation process of amine compounds. It was also found that the molar mass of amines has an inverse effect on the amount of the adsorbed water (intensity), its desorption temperature (position) and the specific surface area of the synthesized materials. The CO₂ adsorption capacities on all the studied bentonite clay adsorbents modified by amines were found to increase between 2.68 and 3.15 mmol/g, compared to 0.93 mmol/g for untreated bentonite at the studied temperature and pressure. As expected, bentonite clay modified with di- and triethanolammonium cations showed lower CO₂ adsorption capacities than that treated with monoethanolammonium cations, due to their low specific surface area.     

Electrochemical oxidation of aryl cycloheptatrienes

The electrochemical oxidation of various substituted aryl cycloheptatrienes and cyclohepta‐1,3,5‐triene, the parent compound, in acetonitrile is investigated with the aid of cyclic voltammetry (CV), experiments at the rotating ring disk electrode (RRDE) and controlled potential electrolysis. 1‐(p‐methoxyphenyl)‐cycloheptatriene 2c , 1‐ and 3‐(p‐di‐methylaminophenyl)‐cycloheptatriene 2d , 3d are converted by anodic oxidation into the radical cations the main reaction of which is not the deprotonation. According to the proposed mechanism the electrochemical oxidation proceeds along the radical cation dimerization. The dimer cations ( 1 ₂²⁺) decompose under deprotonation into tropylium ions 4 and starting compound. In the case of 1‐(p‐dimethylaminophenyl)‐cycloheptatriene 2d the deprotonation reaction can compete with the dimerization. The oxidation of the cycloheptatriene compounds is kinetically controlled by the homogeneous chemical steps rather than by the initial electron transfer.     

Dihydroimidazophenanthridinium (DIP)-based DNA binding agents with tuneable structures and biological activity

We have synthesised a library of dihydroimidazophenanthridinium cations (DIPs) with large structural diversity (1-29) using a "one-pot" approach. The DNA binding constants of DIPs range from 2x10(4) to 1.3x10(5) M(-1), and the free energies for binding range from -5.9 to -6.40 kcal mol(-1). Viscosity measurements demonstrated that the binding of the compounds caused DNA lengthening, thus signifying binding by intercalation. The cytotoxicities of the compounds were determined by tetrazolium dye-based microtitration assays and showed a large range of values (0.09-11.7 microM). Preliminary molecular modelling studies of the DNA-DIP interactions suggested that the DIP moieties can interact with DNA by intercalation, and some R groups might facilitate binding by minor-groove binding. The results provide insight into how to design biologically active DNA binding agents that can interact in these ways.     

Synthesis and dielectric properties of polystyrene‐clay nanocomposite materials

Polystyrene‐clay nanocomposite (PsCN) materials were synthesized and their properties of crystallinity, thermal behavior, and dielectric characteristics were investigated. A polymerizable cationic surfactant, [2‐(dimethylamino)ethyl]triphenylphonium bromide, was used for the intercalation of montmorillonite (MMT). The organophilic MMT was prepared by Na⁺‐exchanged MMT and ammonium cations of a cationic surfactant in an aqueous medium. Organophilic styrene monomers were intercalated into the interlayer regions of organophilic clay hosts followed by a free‐radical polymerization. Exfoliation to 2 wt % MMT in the polystyrene (PS) matrix was achieved as revealed by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). Thermal properties by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were also studied. The dielectric properties of PsCNs in the form of film with clay loading from 1.0 to 5.0 wt % were measured under frequencies of 100 Hz–1 MHz at 25–70°C. A decreased dielectric constant and low dielectric loss were observed for PsCN materials. The dielectric response at low frequency that originated from dipole orientation was suppressed due to the intercalation of clay materials. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1368–1373, 2004     

Synthesis,crystal structure,DNA binding properties and antioxidant activities of transition metal complexes with 3-carbaldehyde-chromone semicarbazone

3-Carbaldehyde-chromone semicarbazone (L) and its Cu(II), Zn(II), Ni(II) complexes were synthesized and characterized on the basis of crystal structure and other structural characterization methods. The metal ions and Schiff base ligand can form mononuclear five-coordination complexes with 1:1 metal-to-ligand stoichiometry at the metal ions as centres. The transition metal complexes may be used as potential anticancer drugs, because they bind to calf thymus DNA via an intercalation binding mode with the binding constants at the order of magnitude 10⁵–10⁶ M^? 1, and the metal complexes present stronger DNA binding affinities than the free ligand alone. In addition, the antioxidant activities of the ligand and its metal complexes were investigated through scavenging effects for superoxide anion and hydroxyl radical in vitro, indicating that the compounds show stronger antioxidant activities than some standard antioxidants, such as mannitol and vitamin C.     

CTMAB插层有机膨润土的结构分析

通过X射线衍射(X-ray)、差(DTA)和热重(TG)等手段分析了不同工艺条件下制备的十六烷基三甲基溴化铵(CTMAB)插层有机膨润土(CTMAB-MMT),讨论了有机膨润土的结构和层间发生的有机化反应,确定了本实验的最佳工艺条件,首次提出了季铵盐在插层过程中分解出季铵离子头的观点.初步认为不仅体积较大的有机阳离子可以插入蒙脱石片层间,体积较小的有机阳离子也可以插入其中.     

季铵盐插层剂对高密度聚乙烯性能的影响

采用季铵盐阳离子作为插层剂与钠基蒙脱土层间的无机阳离子进行离子交换,制备系列有机化蒙脱土,利用红外光谱对蒙脱土的结构进行表征,结果表明,插层剂已进入蒙脱土的层间.同时将有机化蒙脱土与HDPE进行熔融插层,制备了纳米聚乙烯复合材料,研究了插层剂对其部分力学性能的影响.     

Electrochemical impedance spectroscopic study of the intercalation of lithium and sodium ions into polyparaphenylene in carbonate-based electrolytes

Electrochemical impedance spectroscopy is used to investigate the electrochemical intercalation of lithium and sodium ions into polyparaphenylene under galvanostatic conditions in carbonate-based electrolyte. The evolution of the charge transfer resistance was studied at various selected potentials both during the reduction and the oxidation processes in order to control the stoichiometry of the intercalated compounds. The reversibility of the intercalation process, the effect of the cycling and the stability of the intercalated materials in the electrolyte as a function of the time were examined. A significant decrease of the charge transfer resistance occurs during the intercalation. That is related to an increase of the conductive state especially for the richest compounds Na_0.46(C₆H₄) and Li_0.50(C₆H₄).     

The electrochemical preparation and properties of ionic alkali metal-and NR4-graphite intercalation compounds in organic electrolytes

The preparation of graphite-alkali metal-solvent or graphite-NR₄-Solvent ternary intercalation compounds by electrochemical reduction of graphite is described. The reduction occurs stagewise and the complete formation of a defined stage can be recognized by a striking change in the potential of the graphite electrode. Under certain conditions the stoichiometry of intercalation compounds (e.g. C₄₈K(DMSO), C₂₄K(DMSO), C₁₈Li(DME) can be calculated from the coulombs consumed by a weighed graphite electrode until a step in potential occurs. The electrochemical intercalation of alkali metals into graphite is reversible. It may be reversed with a coulombic efficiency up to practically 100%, e.g. for C_nK(DMSO). A considerable co-intercalation of alkali metal salts from the electrolyte was not observed. From a preparative point of view, a great advantage over the direct chemical reduction of graphite is that a desired degree of intercalation can be specifically prepared. A coulometric determination of the intercalated cations is possible by way of an electrochemical reoxidation. Potential measurements of electrochemically prepared intercalation compounds with a well-defined degree of intercalation represent a simple method for investigating the thermodynamics of these reactions. Furthermore, by means of dynamic electrochemical methods, such as cyclic voltammetry, information about the kinetics of intercalation reactions is obtainable.     

Interpretation von Moleküldiagrammen freier π-Radikalkationen — Naphthalen,Butadien, Styren, 1-Methylstyren, 1,1′Diphenylethylen

Interpretation of the MO-diagrams of Free π-Radical Cations — Naphthalene, Butadiene, Styrene, 1-Methyl-styrene, 1,1′-Diphenylethylene The reactive and spectroscopic behaviour of the radical cations of the title compounds is discussed on the basis of general structural principles by means of molecular diagrams calculated with the PPP-LHP-SCF-MO-CI-method. The greater reactivity of the monomer radicals compared with the dimers is due to the character of polymethene-like compounds, whereas the lower reactivity of the dimer radicals cations can be ascribed to the more aromatic state of these species. The experimentally determined optical absorption maxima of the radical cations can be interpreted by the same classification.     

Polymer–montmorillonite nanocomposites: Chemical grafting of polyvinyl acetate onto Cloisite 20A

This paper is an account of the experiments on grafting polyvinyl acetate onto organophilic montmorillonite. Cloisite 20A was reacted with vinyltrichlorosilane to replace the edge hydroxyl groups of the clay with a vinyl moiety. Because the reaction liberates HCl, it was performed in the presence of sodium hydrogencarbonate to prevent the exchange of quaternary alkylammonium cations with H⁺ ions. Only the silanol groups on the edge of the clay react with vinyltrichlorosilane. After the reaction, the product maintained the same basal spacing as the precursor. The radical polymerization of the product with vinyl acetate as a vinyl monomer leads to chemical grafting of polyvinyl acetate onto montmorillonite surface. The homopolymer formed during polymerization was separated from the grafted organoclay by Soxhelt extraction. Chemical grafting of the polymer onto Cloisite 20A was confirmed by infrared spectroscopy. The prepared nanocomposite materials and the grafted nano‐particles were studied by XRD, SEM, and TEM. Exfoliated nanocomposite was obtained for 0.5% clay content. Nanocomposites with 1% and 3% clay contents showed significant intercalation and agglomeration occurred at higher clay loadings. The nanocomposites were studied by thermogravimertic analysis (TGA) and dynamic mechanical analysis (DMTA). Improved thermo‐mechanical properties were observed for nanocomposite with 0.5% clay content. POLYM. COMPOS., © 2011 Society of Plastics Engineers.     

X-ray diffraction studies of electrochemical graphite intercalation compounds of ionic liquids

Electrochemical intercalation studies are used to characterize a series of ionic liquids composed of a variety of cationic and anionic species. Electrochemically, the ionic liquids are characterized by cyclic voltammograms and charge–discharge experiments for the intercalation and de-intercalation of the various cationic and anionic species into graphite. X-ray structure analysis is also performed to determine the relationship between the electrochemical behaviour of the ionic liquids, and the formation of intercalated graphitic compounds. Two different types of imidazolium cations are studied, specifically the di- and trisubstituted imidazolium. These cations are paired with the following anions: tetrafluoroborate, hexafluorophosphate, bis(trifluoromethanesulfonyl)imide, bis(perfluoroethanesulfonyl)imide, nitrate and hydrogen sulfate. Results indicate stronger intercalation chemistry for the trisubstituted imidazoliums, correlating with the greater charge–discharge efficiencies found for these types of ionic liquids. Many of the anions exhibit very poor charge–discharge efficiencies, correlating to very poorly formed graphite intercalates. The exception to this is the hydrogen sulfate intercalate, which had low charge–discharge efficiencies but formed a well defined graphite intercalate. Only the imide based anions exhibited both high charge–discharge efficiencies and the formation of a clearly defined graphite intercalate.     

Solid state polymerization and intercalation of aniline in Fe rich montmorillonite

Hybrid compounds formed by aniline as the organic phase and Fe rich montmorillonite have been prepared by conventional solution process and a novel solid state intercalation method reported recently in the literature. The structure of hybrids depends closely on the process of preparation. In fact, contrary to conventional solution intercalation method, solid solid reaction favours the intercalation of anilinium cations together with aniline chloride neutral species. In addition, aniline species intercalated by this method are able to polymerize in the interlayer space of clay after 9 weeks without the use of any initiator of polymerization, however, the structure of hybrid compounds prepared by conventional solution method did not change. The aging time is accompanied by the reduction of structural Fe(III) to Fe(II) for hybrids prepared by solid reaction. The structure and electrical properties of the obtained polyaniline/clay nanocomposites have been studied. It was shown that ac conduction shows a regime of constant dc conductivity at low frequencies and a crossover to a frequency-dependent regime of the type A ω^S at high frequencies.     

The effect of the molecular structure of a cationic azo dye on the photoinduced intercalation of phenol in a montmorillonite

The effect of the molecular structure of a cationic azo dye on the photoinduced intercalation of phenol into the azo dye-montmorillonites was reported. Two types of cationic azo dyes were used; one has (2-hydroxyethyl)dimethylammonium group (AZ(OH)⁺), and the other has trimethylammonium group (AZ(CH₃)⁺). Phenol was intercalated into both cation exchanged azo dye-montmorillonites (Kunipia F) by mechanical mixing without solvent. By the UV irradiation, the basal spacings increased further, and subsequent visible light irradiation led to decrease the basal spacings, indicating the intercalation and the deintercalation of phenol by the UV and visible light, respectively. The amounts of the phenol intercalated both chemically and photochemically varied depending on the azobenzene cations, showing the interactions between the cationic head group and phenol.     

Electrooxidation of bromoalkanes on platinum in liquid hydrogen fluoride. Selective displacement of bromine by fluorine

Carbon—bromine bond cleavage was investigated in HF as a privileged route to the formation of electrophilic carbon ions and subsequent obtention of alkyl fluorides.Nucleophilic attack of fluoride ions on electrochemically generated radical cations, R—Br⁺, or carbenium ions, R⁺, proved selective and quantitative in the case of bromomethane, 1,2-dibromoethane, 1,2-dibromopropane, 1,3-dibromo propane and 1,2,3-tribromopropane.For bromoethane, 1-(or 2-)bromopropane, 1-(or 2-)bromobutane, 1-bromo-2-methyl propane and 1-bromo-3-methyl butane, the observed lack of halogenation selectivity is discussed in terms of radical side reactions.     

Characterization of 2:1 type layered aluminosilicate via intercalation reaction

The stable intercalation compounds were synthesized by ion exchange reaction of the cations (Ca²⁺, Mg²⁺, etc.) existing in the interlayer of the 2:1 type layered natural aluminosilicate with n-decylammonium ion and by successive molecular intercalation reaction of the primary n-alcanol (ROH, R = C₁₀H₂₁, C₁₂ H₂₅, C₁₄H₂₉). The charge densities were calculated from the basal spacings of n-decylammonium derivatives under primary n-alcanol. As the result, the aluminosilicates used in this study have the charge density of 0.25, 0.34 per formula unit and interlayer cation exchange capacity of 69.3, 92.4 meg./l00g, respectively.     

Structure and Reactions of Aliphatic Bridged Bifunctional Radical Ions: Exploring Fine-Tuning in Radiation Chemistry

Bridged bifunctional molecules of general structure X (CH₂)_n Y are useful models for investigations into the effects of intra- and intermolecular interactions in primary radiation-induced processes, with a number of implications for radiation chemistry, radiobiology, and molecular electronics. This short Review presents an outline of recent studies on the structure and reactivity of aliphatic bifunctional radical ions in low-temperature matrices using EPR spectroscopy and quantum-chemical calculations. Both EPR data and DFT calculations for radical cations show that the delocalization of charge and spin density occurs if XY or X and Y have close electronic properties. If the difference in the ionization potentials between X and Y is large, localized radical cations are observed with ground-state properties close to those of the corresponding monofunctional species. Nevertheless, the remote second functional group may affect the photochemistry of such radical cations at a specific bridge length, probably due to intramolecular interactions in a certain conformation. The stabilization of bridged bifunctional radical anions containing two carbonyl groups was found to be very sensitive to the local environment, which may be described in terms of a microsolvation model at the MP2 level of theory. Two independent reactions pathways with excess electrons, yielding different types of localized radical anions, were demonstrated for asymmetrical bifunctional molecules. The obtained results and their implications are discussed in terms of fine-tuning effects in radiation-induced chemical processes in condensed phases.