Nucleophilic substitution reactions of aryl dithioacetates with pyridines in acetonitrile

Kinetic studies of the pyridinolysis (XC(5)H(4)N) of aryl dithioacetates (CH(3)C(=S)SC(6)H(4)Z) are carried out in acetonitrile at 60.0 degrees C. A biphasic Br?nsted plot is obtained with a change in slope from a large value (beta(X) congruent with 0.9) to a small value (beta(X) congruent with 0.4) at pK(a) degrees = 5.2, which is attributed to a change in the rate-limiting step from breakdown to formation of a zwitterionic tetrahedral intermediate, T(+/-), in the reaction path as the basicity of the pyridine nucleophile increases. A clear-cut change in the cross-interaction constants rho(XZ) from a large positive value (rho(XZ) = +1.34) to a small negative value (rho(XZ) = -0.15) supports the mechanistic change proposed.     

Effect of o-methyl group on rate, mechanism, and resonance contribution: aminolysis of Y-substituted phenyl X-substituted 2-methylbenzoates

Second-order rate constants have been determined spectrophotometrically for the reactions of 4-nitrophenyl X-substituted 2-methylbenzoates (2a-e) and Y-substituted phenyl 2-methylbenzoates (3a-e) with alicyclic secondary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The o-methyl group in the benzoyl moiety of 2a-e retards the reaction rate but does not influence the reaction mechanism. The Hammett plots for the reactions of 2a-e are nonlinear, while the corresponding Yukawa-Tsuno plots are linear with large r values (1.06-1.70). The linear Yukawa-Tsuno plots suggest that stabilization of the ground-state through resonance interaction between the electron donating substituent X and the carbonyl group is responsible for the nonlinear Hammett plots, while the large r values imply that the ground-state resonance interaction is significant. The reactions of 2a-e resulted in smaller rho(X) values but larger r values than the corresponding reactions of 4-nitrophenyl X-substituted benzoates (1a-e). The small rho(X) value for the reactions of 2a-e (e.g., rho(X) = 0.22) is suggested to be responsible for the large r value (e.g., r = 1.70). The reactions of 3a-e with piperidine are proposed to proceed in a stepwise manner with a change in the rate-determining step on the basis of the curved Br?nsted-type plot obtained. Microscopic rate constants associated with the reactions of 3a-e are also consistent with the proposed mechanism.     

Determination of Dialkyltin Compounds in Poly(Vinyl Chloride) by High-Performance Liquid Chromatography

A high-performance liquid chromatography (HPLC) method has been developed for the analysis of dialkyltin compounds in polyvinyl chloride (PVC) materials. The PVC sample was first dissolved in tetrahydrofuran. Concentrated hydrochloric acid was then added to convert the dissolved dialkyltin stabilizers into the chloride forms, followed by extraction with hexane. The extracted dialkyltin chlorides were preconcentrated and were finally separated by HPLC. Separation was performed using a C₁₈ column and an eluent of aqueous methanol containing 8-hydroxyquinoline (oxine) as the completing agent. Photometric detection of the dialkyltin-oxine complexes was carried out at 380 mm. Under optimum experimental conditions, the detection limits for dimethyltin, dibutyltin and dioctyltin are 1.7 ng, 2.1 ng and 2.9 ng (all as tin), respectively. Residual dialkyltin stabilizers in several commercially available PVC products were successfully analyzed via this method.     

Effects of pH During the Base Catalyzed Reaction of Two-Step Acid/Base Catalyzed Process on the Microstructures and Physical Properties of Poly(dimethylsiloxane) Modified Silica Xerogels

Poly(dimethylsiloxane) (PDMS) modified silica xerogels were prepared by two-step acid/base catalyzed sol-gel process. By keeping the acid amount and hydrolysis period in acidic environment fixed, and adding different amounts of base to the sol afterwards, attempts had been made to study the effect of the different amounts of base catalyst on the micro-structure and physical properties of the prepared ORMOSILs. DTA, SEM, BET, FTIR were performed to characterize the derived specimens. The microstructure and physical properties are greatly influenced by the amount of base added to the sol. With increasing base content the crystallite size and porosity increase and the pore size distribution takes a broad spectrum. Whereas, the glass transition temperature seems to decrease with increasing of base catalyst amount. This is explained in terms of the change in the relative amounts of hydrolysis and condensation reactions due to the addition of different amount of base catalyst, which predominantly influences the condensation reactions. The results are reported in this communication along with possible explanations behind the observations.     

Capillary electrophoresis of baclofen with argon-ion laser-induced fluorescence detection

A capillary electrophoretic method with laser-induced fluorescence detection for baclofen (4-amino-3-p-chlorophenylbutyric acid) has been developed. 6-Carboxyfluorescein succinimidyl ester was used for precolumn derivatization of the non-fluorescent drug. Optimal separation and detection were obtained with an electrophoretic buffer of 50 mM sodium borate (pH 9.5) and an air-cooled argon-ion laser (excitation at 488 nm, emission at 520 nm). Linearity (r > or = 0.99) over three orders of magnitude was generally obtained and the lowest derivatizable concentration limit for baclofen in aqueous solution was 10 nM (2 ng baclofen/ml). Coupled with a simple clean up procedure, the method can be applied to the analysis of baclofen in human plasma at micromolar level. Recovery of spiked baclofen in plasma was 95%. The relative standard deviation values on peak size (0.5 microM level) and migration time were 8.2 and 1.0% (n=7), respectively. The limit of detection of baclofen in plasma was 0.1 microM (21 ng/ml).     

Vibrational spectroscopic studies of sol-gel derived physical and chemical bonded ORMOSILs

Fourier transform infrared (FT-IR) and FT-Raman Spectroscopy have been utilized to characterize the structure of physical and chemical bonded ORMOSILs (organically modified silicates) and to compare with the physical properties, reported earlier. The classic and sono ORMOSILs were synthesized by sol-gel method via traditional- and sono-catalysis routes, respectively. The physical and chemical bonding is established by Poly(ethylene glycol) (PEG) and Poly(dimethoxysilane) (PDMS), respectively. Existence of band at ∼560 cm⁻¹ (FT-IR) and ∼490 cm⁻¹ (FT-Raman) for the sono and classic gels with 5-10 wt% of PEG indicates the presence of structural defect due to the four-membered siloxane rings. Application of long molecular PEG chain and PDMS increases this defect. Modification in the intensity and peak position of siloxane (Si-O-Si) bands is found correlative with the physical properties. The xerogels with 5-10 wt% of PEG possess a large number of residual surface silanol groups than the xerogels with long molecular PEG chains and PDMS and is found related with the ν(OH) band at ∼3470 cm⁻¹. These results are attributed to the way of hydrolysis-condensation reactions and discussed with the help of Percolation and DLVO models.     

Shape‐Induced,Hexagonal, Open Frameworks: Rubidium Ion Complexed Cucurbituril

A honeycomb structure is shown by the one‐dimensional coordination polymer comprising D_6h‐symmetric cucurbituril molecules and rubidium ions (see picture). The cucurbituril molecules stack atop one another and show coordination of their carbonyl groups to the rubidium ions in between. The shape and symmetry of the building blocks encourage the coordination polymer chains to be arranged in such a way as to produce an open‐framework structure with large, linear, hexagonal channels.     

Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is D_b≈6×10⁻¹¹exp(−0.29/(k_BT )) cm²/s. Received: 18 May 1999 / Accepted: 8 September 1999 / Published online: 23 February 2000     

Lower limit and Non-linearity of Calibration in the Determination of Cations by Ultralow Background Conductance Suppressed Ion Chromatography

Non-linear calibration is one major drawback of suppressed conductivity ion chromatography and thus, it has been subjected to intensive investigations over the last two decades. The investigations are mostly confined to anion chromatography using weak base eluents like carbonate because the extent of non-linearity is proportional to dissociation constant, Ka, of the suppressed eluate.     

Determination of Butyltins in Sea Water by Gas Chromatography with Flame Photometric Detection

Use of a gas chromatograph with a flame-photometric detector (GC-FPD) is described to determine butyltin compounds in sea water. The butyltins in an acidified water sample (pH 3.0) are first complexed with tropolone, followed by solid-phase extraction (SPE) with a tropolone-treated C₁₈ cartridge. HCI at a small concentration is then added to the concentrated SPE eluate before GC analysis. This procedure is simple and off-column derivatization of analytes is not required. The organotins, viz. mono-, di- and tributyltin, are separated as their respective chlorides on a capillary column (HP-1) and are detected with a flame-photometric detector and an interference filter at 610 nm. Recoveries of the three butyltin species are quantitative (> 90%). Based on a sea water sample 200-mL, the detection limits for mono-, di- and tributyltin are 6,4 and 3 ng tin L^?1, respectively. This method is applied to analysis of trace butyltins in various samples of sea water.