Characterization and thermal properties of Ni-exchanged montmorillonite with benzimidazole

Thermal analysis (TG, DTG), powder diffraction analysis (XRD) and infrared (IR) spectra were used to study of composition and release of benzimidazole from Ni(II)-exchanged montmorillonite under heating. Diffraction analysis indicated that benzimidazole molecules are intercalated into the interlayer space of montmorillonite. IR spectra and the analytical characteristics have shown that different type of interactions of benzimidazole is connected with different reaction conditions (acid or neutral solution). The release of benzimidazole from Ni(II)-montmorillonite under heating from studied samples proceeds in three distinct steps. The first step can be assigned to the release of water molecules while the last (third) one corresponds to the lattice dehydroxylation. The second step can be assigned to release of chemically bonded benzimidazole.     

Thermal and Infrared Study of Desorption of Pyridine and Pyridine Derivatives from Ni-exchanged Montmorillonite

Thermal analysis (TG, DTG) and infrared absorption spectra were used to study desorption of pyridine (py), 4-methylpyridine (4-Mepy) and 4-ethylpyridine (4-Etpy) from Ni(II)-exchanged montmorillonite (Ni-MMT). It is shown that the bonds between the derivatives of pyridine (R-py) and the montmorillonite exhibit predominantly Lewis acid character. The thermal stability of studied samples and the total amounts of R-py inserted in the porous structure of Ni-MMT increase in the following sequence: py/Ni-MMT<4-Mepy/Ni-MMT<4-Etpy/Ni-MMT, i. e. the influence of the substituent on the pyridine ring is evident. This revised version was published online in August 2006 with corrections to the Cover Date.     

Time-resolved near-infrared and two-dimensional near-infrared correlation spectroscopic studies on polymerization of the silane coupling agent perfluorooctyltrimethoxysilane

 The polymerization behavior of perfluorooctyltrimethoxysilane (PFOS) in ethanol, which is acid-catalyzed by 0.25 M HCl, has been examined using time-resolved near-IR and 2D near-IR correlation spectra. In the time-resolved near-IR spectra, the bands at 5164 and 4825 cm⁻¹ have been assigned to the combination bands of water and ethanol OH groups, respectively. It has been found that the absorbance variation of the two near-IR bands occurs in a two-step process. The absorbance of the 5164 cm⁻¹ band rapidly decreases in the initial step but increases exponentially in the second step, while that of the 4825 cm⁻¹ band rapidly increases both in the initial step and, exponentially, in the second step. These results indicate that the time-dependent absorbance variation of the two near-IR bands reflects the polymerization process of PFOS, in which consumption and release of water molecules and release of methanol in the two-step process occur as a consequence of the acid-catalyzed hydrolysis of methoxy groups and the formation of silanols (SiOH) to form a siloxane bond. It has also been found that this polymerization process is distinctly reflected in the 2D near-IR correlation spectra. Received: 23 November 1999/Received in Revised form: 23 February 2000/Accepted 4 March 2000     

Thermal stability of new biologic active copper(II) complexes with 5,6-dimethylbenzimidazole

Three new coordinative compounds that contain mixed ligands (5,6-dimethylbenzimidazole and acrylato anion) were synthesized and characterized. The features of complexes have been assigned from microanalytical, IR, UV–Vis and EPR spectra as well as thermal analysis. IR data are in accordance with unidentate nature of 5,6-dimethylbenzimidazole while the acrilato ion acts as uni- or bidentate ligand. The electronic spectra display the characteristic pattern of square pyramidal or octahedral stereochemistry, which were confirmed by the EPR spectra. Antibacterial and antifungal activities of the complexes have been determined in vitro, against various Gram-negative and Gram-positive bacteria and fungi. The tested complexes exhibited different spectra of antimicrobial activity and inhibited the microbial ability to colonize the inert surfaces, acting as potential anti-adherence and biofilm-controlling agents. Thermal decomposition evidenced several well-defined steps as dehydration (complex 2), 5,6-dimethylbenzimidazole molecule release (all complexes) and the acrylate decomposition in carbonate (complex 3). The final residue is in all cases copper (II) oxide.     

ESR, electrochemical and cyclodextrin-inclusion studies of triazolopyridyl pyridyl ketones and dipyridyl ketones derivatives

The electron spin resonance (ESR) spectra of free radicals obtained by electrolytic reduction of triazolopyridyl pyridyl ketones and dipyridyl ketones derivatives were measured in dimethylsulfoxide (DMSO). The hyperfine patterns indicate that the spin density delocalization is dependent of the rings presented in the molecule. The electrochemistry of these compounds was characterized using cyclic voltammetry, in DMSO as solvent. When one carbonyl is present in the molecule one step in the reduction mechanism was observed while two carbonyl are present two steps were detected. The first wave was assigned to the generation of the correspondent free radical species, and the second wave was assigned to the dianion derivatives. The phase-solubility measurements indicated an interaction between molecules selected and cyclodextrins in water. These inclusion complexes are 1:1 with betaCD, and HP-betaCD. The values of Ks showed a different kind of complexes depending on which rings are included. AM1 and DFT calculations were performed to obtain the optimized geometries, theoretical hyperfine constants, and spin distributions, respectively. The theoretical results are in complete agreement with the experimental ones.     

Discrete electrostatic charge transfer by the electrophoresis of a charged droplet in a dielectric liquid

We have experimentally investigated the electrostatic charging of a water droplet on an electrified electrode surface to explain the detailed inductive charging processes and use them for the detection of droplet position in a lab-on-a-chip system. The periodic bouncing motion of a droplet between two planar electrodes has been examined by using a high-resolution electrometer and an image analysis method. We have found that this charging process consists of three steps. The first step is inductive charge accumulation on the opposite electrode by the charge of a droplet. This induction process occurs while the droplet approaches the electrode, and it produces an induction current signal at the electrometer. The second step is the discharging of the droplet by the accumulated induced charge at the moment of contact. For this second step, there is no charge-transfer detection at the electrometer. The third step is the charging of the neutralized droplet to a certain charged state while the droplet is in contact with the electrode. The charge transfer of the third step is detected as the pulse-type signal of an electrometer. The second and third steps occur simultaneously and rapidly. We have found that the induction current by the movement of a charged droplet can be accurately used to measure the charge of the droplet and can also be used to monitor the position of a droplet under actuation. The implications of the current findings for understanding and measuring the charging process are discussed.     

Discovery of deep and shallow trap states from step structures of rutile TiO2 vicinal surfaces by second harmonic and sum frequency generation spectroscopy

In this report, local electronic structures of steps and terraces on rutile TiO(2) single crystal faces were studied by second harmonic and sum frequency generation (SHG∕SFG) spectroscopy. We attained selective measurement of the local electronic states of the step bunches formed on the vicinal (17 18 1) and (15 13 0) surfaces using a recently developed step-selective probing technique. The electronic structures of the flat (110)-(1×1) (the terrace face of the vicinal surfaces) and (011)-(2×1) surfaces were also discussed. The SHG∕SFG spectra showed that step structures are mainly responsible for the formation of trap states, since significant resonances from the trap states were observed only from the vicinal surfaces. We detected deep hole trap (DHT) states and shallow electron trap (SET) states selectively from the step bunches on the vicinal surfaces. Detailed analysis of the SHG∕SFG spectra showed that the DHT and SET states are more likely to be induced at the top edges of the step bunches than on their hillsides. Unlike the SET states, the DHT states were observed only at the step bunches parallel to [1?1?1] [equivalent to the step bunches formed on the (17 18 1) surface]. Photocatalytic activity for each TiO(2) sample was also measured through methylene blue photodegradation reactions and was found to follow the sequence: (110) < (17 18 1) < (15 13 0) < (011), indicating that steps along [0?0?1] are more reactive than steps along [1?1?1]. This result implies that the presence of the DHT states observed from the step bunches parallel to [1?1?1] did not effectively contribute to the methylene blue photodegradation reactions.     

Detection and chemical profiling of medicine counterfeits by Raman spectroscopy and chemometrics

Raman spectroscopy combined with chemometrics has recently become a widespread technique for the analysis of pharmaceutical solid forms. The application presented in this paper is the investigation of counterfeit medicines. This increasingly serious issue involves networks that are an integral part of industrialized organized crime. Efficient analytical tools are consequently required to fight against it. Quick and reliable authentication means are needed to allow the deployment of measures from the company and the authorities. For this purpose a method in two steps has been implemented here. The first step enables the identification of pharmaceutical tablets and capsules and the detection of their counterfeits. A nonlinear classification method, the Support Vector Machines (SVM), is computed together with a correlation with the database and the detection of Active Pharmaceutical Ingredient (API) peaks in the suspect product. If a counterfeit is detected, the second step allows its chemical profiling among former counterfeits in a forensic intelligence perspective. For this second step a classification based on Principal Component Analysis (PCA) and correlation distance measurements is applied to the Raman spectra of the counterfeits.     

Hydrolysis of cyclic phosphates by ribonuclease A: a computational study using a simplified ab initio quantum model

The second step in the enzyme-catalyzed hydrolysis of phosphate esters by ribonuclease A (RNase A) was studied using an ab initio quantum-based model of the active site including constrained parts of three critical residues, His-12, His-119, and Lys-41, and a small substrate. The competition between release of the cyclic phosphate intermediate and subsequent hydrolysis following transphosphorylation was explored to determine the electronic factors that contribute to preferential intermediate product release observed experimentally. The structural and energetic results obtained at both the RHF and MP2 levels reveal several contributing factors consistent with experimental observation. Although the intrinsic electronic effects tend to favor hydrolysis slightly with an overall activation free energy of approximately 70 kJ mol(-1), entropic and environmental effects favor release of the cyclic phosphate intermediate over hydrolysis. Exploration of the second, hydrolysis step also revealed interesting similarity with the transphosphorylation step, including the observation of autocatalysis by the substrate. Moreover, both steps of the overall RNase A reaction reveal multiple pathways involving proton transfers to sites of similar proton affinities. The anionic phosphate in both steps can act as a stable proton binding site as protons are moved around the active site throughout the progress of the reaction. These results suggest autocatalysis may be representative of more general behavior in enzymes containing highly charged substrates, especially phosphates.     

A Kinetic Aspect of the Thermal Dehydration of Dilithium Tetraborate Trihydrate

The reaction process of the thermal dehydration of dilithium tetraborate trihydrate, Li₂B₄O₇3H₂O, was reinvestigated from a viewpoint of reaction kinetics. On the basis of the results of thermogravimetry, constant rate thermal analysis, powder X-ray diffractometry, infrared spectroscopy and scanning electron microscopy, it was confirmed that the reaction proceeds via three consecutive kinetic steps characterized by different activation energies. The first and second kinetic steps, accompanied by the destruction of the original crystal structure of the reactant, seem to be assigned to the surface and internal reactions, respectively. During the third kinetic step, the thermal dehydration of hydrated amorphous intermediate, produced at the second kinetic step, and crystallization of the final dehydration product, Li₂B₄O₇, are likely to take place concurrently.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermal Decomposition of BaC2O4·0.5H2O Studied by Stepwise Isothermal Analysis and Non-Isothermal Thermogravimetry

Thermal decomposition of BaC2O4·0.5H2O in air was studied by a combination of stepwise isothermal analysis (SIA) and non-isothermal thermogravimetry. The results from both techniques show that the crystal water is released in one step and that anhydrous barium oxalate is decomposed in one step, while BaCO3 decomposes in three steps to BaO, forming two intermediate compounds with the formulas of BaCO3·(BaO)2 and (BaCO3)0.5·(BaO)2.5. Reaction mechanism analyses using the data from SIA measurements show that the controlling mechanism for all the five decomposition steps in isothermal conditions is a two-dimensional phase-boundary controlled process. Kinetic parameters are obtained for the five decomposition steps from the non-isothermal thermogravimetric data. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kinetics of nonisothermal reduction and carburization of WO3─NiO nano-composite powders by CO─CO2

The kinetics of simultaneous reduction and carburization of WO₃─NiO nanocomposite powders by CO/CO₂ was studied using a nonisothermal thermogravimetric analysis. The experiments were carried out at heating rates between 5 and 15°C/min, showing that the thermochemical processes can be divided into four steps, each dominated by a reaction, as following: NiO → Ni, WO₃ → WO₂, WO₂ → W, and W → WC. The apparent activation energy for each step was obtained based on the Flynn-Wall-Ozawa isoconversional method for the individual steps, and the kinetic model was assessed by fitting master plots of various kinetic models for these steps at different heating rates. The Avrami-Erofeev kinetic model was found to fit to the third and fourth steps and main part of the first, and the geometric contracting model fitted the best for the second step. Changing the heating rate did not affect the master plots of the third step. However, for the first step, increasing the heating rate made the Avrami-Erofeev model the best-fitting correlation and also for the second step the matching model changed at the highest heating rate (15°C/min) from a two-dimensional contracting model (cylindrical particles) to a three-dimensional contracting model (spherical particles).     

Investigations on Preparation,Crystal Structure and Thermal Stability of Bis(O-ethylcarbonodithiolato-S,S′)(1,10-phenanthroline-N~1,N~(10))cobalt(II) Complex:[Co(Et-XA)_2·phen]·H_2O (XA=xanthate)

IntroductionTheincreasingcommercialvalueoftransitionmetalcomplexesofxanthateshasarousedconsiderableinterestintheirchemistry .Whiletheiranalyticalapplicationsarewellknown ,1theyarenowfindingextensiveuseinvulcan izationofrubber ,frothfloatationprocessforconcentrationofsulphideores ,asantioxidants ,lubricants ,2 ,3andhavebeenfoundtopossessfungicidalandinsecticidalactivi ties .4 Recently ,molecularrecognitionbetweenhostandguestmolecules ,inclusionphenomenaandnoncovalentmolecularinteractionarefunda…     

3,5-Diformylboron dipyrromethenes as fluorescent pH sensors

A series of boron dipyrromethene (BODIPY) dyes containing two aldehyde functional groups at the 3 and 5 positions have been synthesized in low-to-decent yields in two steps. In the first step, the meso-aryl dipyrromethanes were treated with POCl(3) in N,N-dimethylformamide to afford 1,9-diformylated dipyrromethanes. In the second step, the diformylated dipyrromethanes were first in situ oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and then reacted with BF(3)·OEt(2) to afford 3,5-diformylboron dipyrromethenes. The X-ray structural analysis indicated that the aldehyde groups are involved in intramolecular hydrogen bonding with fluoride atoms, which may be responsible for the stability of the diformylated BODIPY compounds. The presence of two formyl groups significantly alters the electronic properties, which is clearly evident in downfield shifts in the (1)H and (19)F NMR spectra, bathochromic shifts in the absorption and fluorescence spectra, better quantum yields, and increased lifetimes compared to 3,5-unsubstituted BODIPYs. Furthermore, 3,5-diformylboron dipyrromethenes are highly electron-deficient and undergo facile reductions compared to unsubstituted BODIPYs. These compounds exhibit pH-dependent on/off fluorescence and thus act as fluorescent pH sensors.     

Characterization of the Acid-Base Properties of Humic Substances by Chemometric Analysis of Synchronous Fluorescence and pH Potentiometric Data

Abstract

A procedure for the analysis of the acid-base characteristics of humic substances based on a self-modeling analysis of synchronous fluorescence spectra, collected at varying pH, and on a non-linear least squares adjustment of potentiometic pH data, is described. The data analysis methodology consists of two steps: first, the number of acid-base systems and the corresponding spectra and distribution diagrams are calculated by evolving factor analysis (EFA) with concentration constraints of the spectroscopic data; second, the potentiometric data is analyzed by a standard non-linear least square procedure using as fixed parameters the number of components and their pKas, determined in the first step of the analysis. As an example, for a sample of marine fulvic acids studied at pH between 2 and 11, four acid-base systems were found with average pKas: 3.1, 4.8, 8.0 and 10.0. The concentrations of the corresponding systems were: 2.55(5), 1.95(7), 0.14(4) and 1.8(3) meq/g.     

苯胺聚合反应中重铬酸盐的还原机理

应用红外光谱、X射线衍射、元素分析、焙烧实验等手段证明,重铬酸盐氧化聚合苯胺的产物是聚苯胺/Cr(OH)3复合物.这不同于文献中的一般观念.随反应介质酸性的增强,复合物中Cr(OH)3的含量从38.4％逐渐下降到1.3％.强酸性介质有利于重铬酸盐还原为易溶的Cr3＋,弱酸性介质有利于重铬酸盐还原为难溶的Cr(OH)3.基于该实验事实,提出了重铬酸盐还原过程的两步机理.     

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

Optimization of amine-terminated polyacrylonitrile synthesis and characterization

Amine-terminated PANs were prepared in two steps. The first step includes free radical polymerization of acrylonitrile (AN) using initiator pair of ammonium persulfate and sodium thiosulfate as redox system. In the second step, the amino groups were introduced through the reaction of polyacrylonitrile with excess of different diamines (10-fold) including ethylenediamine (EDA), hexamethylenediamine (HMDA) and octamethylenediamine (OMDA), to yield PAN–EDA, PAN–HMDA and PAN–OMDA, respectively. Optimization of the amine-terminated PANs synthesis was carried out at different temperatures (30–90 °C) and different time intervals (4–24 h). In addition, the introduction of the amino group was followed by the piperidine test and recording of the FT-IR spectra. All polymers were characterized by, ¹H NMR spectra, thermogravimetric analysis (TGA), and FT-IR spectra.     

Structural elucidation studies of erythromycins by electrospray tandem mass spectrometry

Erythromycin A (EryA) was studied by electrospray ionisation tandem mass spectrometry (ESI-MS/MS) with the aim of developing a methodology for the structural elucidation of novel erythromycins developed by biological synthetic methods. Skimmer dissociation along with sequential mass spectrometry studies (up to MS5) have been employed in this study. In the low-resolution MS/MS analysis of the polyketides, there are several fragment ions that are easily assigned to various neutral losses. These have all been confirmed by accurate-mass measurements. There is also a series of peaks due to ring opening and fragmentation that can only be assigned by high-resolution MSn analysis. Further experiments were performed in deuterated media (D2O/CD3OD 50%) which, along with the high-resolution MSn of erythromycin analogues, has enabled us to identify some of the steps in the ring fragmentation, particularly the loss of the polyketide starter acid. This is an essential step for determining structural alterations in the novel polyketides, but further labelling experiments and studies on more erythromycin analogues are required before the complete fragmentation pathway can be confirmed.     

A novel process for synthesizing polystyrene and polyarylate block copolymers utilizing telechelic polystyrene

A novel process for synthesizing polystyrene (PS) and polyarylate (PAr) block copolymers utilizing telechelic polystyrene was proposed. This process was comprised of three steps. In the first step, carboxyl-terminated telechelic polystyrene (COOH PS COOH) was prepared by free radical polymerization with 4,4′-azobis(cyanovalelic acid) (ACVA). In the second step, COOH PS COOH was reacted with bisphenol-A by the use of triphenylphosphine, hexachloroethane, and triethylamine to convert carboxyl groups into phenol groups (OH PS OH). In the third step, to produce the PS PAr block copolymer, OH PS OH was added to a polyarylate synthesizing system where bisphenol-A and the mixture of tere/isophthaloyl dichloride (1 : 1 mole ratio) were polymerized by solution polycondensation. PS PAr block copolymers were successively obtained with relatively high PS copolymerization ratio. The ratio was over 70%, while there was a wide variety in molecular composition and molecular weight. Furthermore, by this process PS PAr block copolymers can be obtained from step 1 through step 3 consecutively without isolating the intermediates. This method has potential for industrial applications. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2839–2847, 1998