Kinetics and mechanism of the reaction of para‐chlorophenyl aryl chlorophosphates with anilines in acetonitrile

The kinetics and mechanism of the nucleophilic substitution reactions of p‐chlorophenyl aryl chlorophosphates ( 2 ) with anilines are investigated in acetonitrile at 55°C. Relatively large magnitudes of ρ_X and β_X values are indicative of a large degree of bond making in the TS. Smaller magnitudes of ρ_X (0.20 for X = H) and ρ_XY (?0.30) than those for the corresponding reactions with phenyl aryl chlorophosphates ( 1 ) (ρ_X = 0.54 for X = H and ρ_XY = ?1.31) are interpreted to indicate partial electron loss, or shunt, towards the electron acceptor equatorial ligand (p‐ClC₆H₄O‐) in the bipyramidal pentacoordinated transition state. The inverse secondary kinetic isotope effects (k_H/k_D = 0.64–0.87) involving deuterated aniline (ND₂C₆H₄X) nucleophiles, and small ΔH^? and large negative ΔS^? are obtained. These results are consistent with a concerted nucleophilic substitution mechanism. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 632–637, 2002     

Soft X-ray fluorescence and photoluminescence of Si nanocrystals embedded in SiO2

We have used ion-beam mixing to form Si nano-crystals in SiO₂ and SiO₂/Si multilayers, and applied photoluminescence and soft-X-ray emission spectroscopy to study the nanoparticles. Ion-beam mixing followed by heat treatment at 1100 °C for 2 h forms the Si nanocrystals. The ion-beam-mixed sample shows higher PL intensity than that of a Si-implanted SiO₂ film. Photon and electron-excited Si L_2,3 X-ray emission measurements were carried out to confirm the formation of Si nanocrystal in SiO₂ matrix after ion-beam mixing and heat treatment. It is found that Si L_2,3 X-ray emission spectra of ion-beam-mixed Si monolayers in heat-treated SiO₂ films lead to noticeable changes in the spectroscopic fine structure. Received: 20 November 1999 / Accepted: 17 April 2000 / Published online: 5 October 2000     

Studies of single-step electrodeposition of CuInSe2 thin films with sodium citrate as a complexing agent

The influences of various parameters in a single-step electrodeposition of CuInSe₂ from aqueous solution containing CuCl₂, InCl₃, and SeO₂, with sodium citrate as the complexing agent, are investigated. Co-deposition of CuInSe₂ from a room temperature, aqueous bath of these electrolytes is accomplished by the aid of sodium citrate. In this work the optimum potential for deposition of CuInSe₂ is found to be −0.5 V vs. Ag/AgCl, the deposition time is 800 s, the concentration ratio of CuCl₂, InCl₃, and SeO₂ is 9 mM:22 mM:22 mM in aqueous solution, and the annealing temperature is 225 °C. Under the optimum conditions, crystalline layers of CuInSe₂ having the chalcopyrite structure can be successfully synthesized. Cyclic voltammetry (CV), scanning electron microscope (SEM), X-ray diffractometer (XRD), and energy dispersive X-ray spectrometer (EDX) were used to examine the electrochemistry, morphologies, structures, and compositions of CuInSe₂ thin films deposited on ITO glass.     

Kinetics and Mechanism of the Pyridinolyses of O‐Methyl and O‐Ethyl Phenyl Phosphonochloridothioates in Acetonitrile

Kinetic studies on the reactions of O‐methyl ( 2 ) and O‐ethyl ( 3 ) phenyl phosphonochloridothioates with X‐pyridines have been carried out in acetonitrile at 35.0°C. The pyridinolysis rates of 2 are slightly faster than those of 3 . The substituent effects of X on the pyridinolysis rates of 2 and 3 are similar. The Hammett and Brönsted plots with X in the nucleophiles are biphasic concave upward with a break point at X = 3‐Ph for both substrates. The stepwise mechanism with rate‐limiting leaving group departure from the intermediate is proposed based on the β_X values and biphasic concave upward free energy relationship for both substrates. The biphasic concave upward free energy relationships are rationalized by a frontside nucleophilic attack TSf with more basic pyridines and backside attack TSb with less basic pyridines for both substrates.     

Pt-polyaniline nanocomposite on boron-doped diamond electrode for amperometic biosensor with low detection limit

Boron-doped diamond electrodes covered with a nanostructured Pt nanoparticle-polyaniline composite have been fabricated and employed as sensitive amperometric sensors with low detection limit. A highly conductive boron-doped diamond thin film (BDD) was prepared by chemical vapor deposition, and its morphology was characterized by scanning electron microscopy and transmission electron microscopy. The nanostructured composite layer was grown on the BDD electrode by electrochemical deposition of polyaniline and Pt nanoparticles. Glucose oxidase (GOx) was then adsorptively immobilized on the modified BDD electrode. The biosensor displays a large surface area, high catalytic activity of the Pt nanoparticles, efficient electron mediation through the conducting polymer, and low background current of the electrode. The biosensor exhibits an excellent response to glucose, with a broad linear range from 5.9 μM to 0.51 mM, a sensitivity of 5.5 μA·mM^?1, a correlation coefficient (R) of 0.9947, and a detection limit of 0.10 μM. The apparent Michaelis-Menten constant (K _M ^app ) and the maximum current density of the electrode are 4.1 mM and 0.021 mA, respectively. This suggests that the immobilized GOx possesses a higher affinity for glucose at the lower K _M ^app , and that the enzymatic reaction rate constitutes the rate-limiting step of the response.     

Analysis of alendronate in human urine and plasma by magnetic solid-phase extraction and capillary electrophoresis with fluorescence detection

A simple, rapid and sensitive CE-fluorescence (FL) detection method for the analysis of alendronate (ALEN), a bisphosphonate drug, has been developed. Using a buffer solution of 20 mM sodium phosphate (pH 10.0) and a voltage of 24 kV, separation of ALEN in a 55-cm length (35-cm effective length) capillary was achieved in 5 min. FL detection of ALEN was performed via pre-column derivatization with 2,3-naphthalene dicarbox-yaldehyde (NDA). Linear correlation (r=0.9981, n=6) between FL intensity and analyte concentration was obtained in the range of 7-200 ng/mL ALEN. The developed CE-FL method was applied to the analysis of ALEN in human urine and plasma samples. In order to eliminate the interfering matrix components, SPE using magnetic Fe(3) O(4) @Al(2) O(3) nanoparticles as solid sorbents was employed to clean the biological fluids before CE-FL analysis. The linear ranges of ALEN in urine and plasma were 5-100 ng/mL (r = 0.9982, n = 7) and 5-70 ng/mL (r = 0.9954, n = 7), respectively. The LOD and LOQ in both urine and plasma samples were 1.5 and 5 ng/mL ALEN, respectively. Total analysis time including sample pre-treatment and CE separation was less than 1.5 h.     

Determination of butyltin and octyltin stabilizers in poly(vinyl chloride) products by headspace solid-phase microextraction and gas chromatography with flame-photometric detection

Headspace solid-phase microextraction (HS-SPME) and gas chromatography with flame photometric detection (GC-FPD) have been investigated for determination of butyltin and octyltin stabilizers in poly(vinyl chloride) (PVC) products. The organotin stabilizers were first released from the plastic matrix by dissolving the PVC sample in tetrahydrofuran (THF). The stabilizers were then hydrolyzed to the chloride forms, by treatment with 6 mol L⁻¹ HCl, then derivatized with sodium tetraethylborate (NaBEt₄) in 0.2 mol L⁻¹ sodium acetate buffer (pH 4.5) at 50 °C. HS-SPME was performed with a fused-silica fiber coated with a 100-μm film of polydimethylsiloxane (PDMS). The collected organotin compounds were then desorbed in the GC injector at 280 °C and analyzed by GC-FPD. Linearity (r≥0.994) over a concentration of approximately two orders of magnitude was usually obtained. Limits of quantitation (LOQ) of the four organotin compounds studied, viz., monobutyltin (MBT), dibutyltin (DBT), monooctyltin (MOT), and dioctyltin (DOT), were in the range 0.3–1.0 ng Sn mL⁻¹. Recovery was >90% for butyltins and >80% for octyltins. The method was validated by analyzing two reference standard PVC sheets with known organotin content. The applicability of the method to analysis of organotin stabilizers in commercial PVC products was also demonstrated.     

Improved electrical and thermal properties of nickel silicides by using a NiCo interlayer

We have investigated the effects of a NiCo interlayer on the electrical and thermal properties of nickel silicide as a function of the annealing temperature. For the interlayered samples, 3 nm-thick NiCo(10 at.% Co) films are electron-beam evaporated on Si substrates, on which 27 nm-thick Ni films are deposited without breaking the vacuum. It is shown that all the samples exhibit a distinctive increase in the sheet resistance at temperatures above 900 ^°C. However, the NiCo interlayer sample produces the lowest sheet resistance at 900 ^\circC. X-ray diffraction results show that the Ni only and NiCo interlayer samples produce NiSi and NiSi₂ phases, while NiCo full samples give NiSi and Ni_1−xCo_xSi₂ phases. Scanning electron microscopy results exhibit that for all the samples, the surfaces become degraded with numerous arbitrarily-shaped spots, corresponding to areas uncovered by the silicides. The areal fractions of the silicides for the Ni only, NiCo full, and NiCo interlayer samples are about 57%, 72%, and 81%, respectively. The temperature dependence of the electrical properties of the silicide samples is explained in terms of the formation of resistive phases and the agglomeration of the silicide.     

Surface pattern formation on Cr thin film with ultrafast laser pulse

Laser irradiation of a metal surface with multiple pulses just below the ablation threshold generates a periodic surface pattern known as the laser-induced periodic surface structure. The aim of this study is to characterize these periodic structures by analyzing the electric field distribution at the metal surface. The analysis was conducted using a modified Debye model for the permittivity of the metal and the finite-difference time-domain (FDTD) software package XFDTD. Spatial periodic variation in the electric field distribution formed perpendicularly to the direction of polarization of the laser light, with a period of approximately half the wavelength of the light. This is similar to the period and direction observed experimentally in some studies, and the periodic distribution of the irradiance on the surface would be one of many causes of these structures.     

Effect of platinum on the photoelectrochemical behavior of lead oxide thin film

The effect of Pt on the photoelectrochemical behavior of lead oxide prepared from anodization of Pb-Pt alloy containing various concentrations of Pt has been studied. It is observed that, while Pt reduces the resistivity of the oxide film, it also reduces the photoconversion efficiency of the photoelectrochemical cell prepared from this material. The reason for this low efficiency is discussed. Received: 19 November 1997 / Accepted: 9 March 1998