Determination of microstructure and glass-transition temperature of acrylonitrile-methyl acrylate copolymers by 13C-NMR spectroscopy

Acrylonitrile-methyl acrylate (A/M) copolymers of different monomer compositions were prepared by bulk polymerization using free radical initiator (benzoyl peroxide). Copolymer compositions were determined by elemental analyses and comonomer reactivity ratios were determined by the nonlinear least squares errors-in-variables methods (EVM). Terminal and penultimate reactivity ratios have been calculated using the observed monomer triad sequence distribution determined from ¹³C{¹H}-NMR spectra. The triad sequence distribution was used to calculate diad concentrations, conditional probability parameters, number-average sequence lengths, and run number in the copolymers. The observed triad sequence concentrations determined from ¹³C{¹H}-NMR spectrum agreed well with those calculated from reactivity ratios. Glass transition temperatures (T_g) of various copolymers determined from DSC gave good agreement with those obtained from NMR. © 1992 John Wiley & Sons, Inc.     

Synthesis of Imidazo[4,5-b]quinoxaline Ribonucleosides as Linear Dimensional Analogs of Antiviral Polyhalogenated Benzimidazole Ribonucleosides

We have recently found that 2,5,6-trichloro-1-(β-D-ribofuranosyl)benzimidazole (TCRB) and the corresponding 2-bromo analog have better in vitro activities against HCMV than the clinically used agents ganciclovir and foscarnet. These benzimidazole nucleosides act by a unique mechanism, however, their biological target has not been completely identified. As an approach to probing the target, we have designed imidazo[4,5-b]quinoxaline nucleosides as linear dimensional analogs of the benzimidazole nucleosides to study the spatial limitation of the binding site in the target enzyme. A convenient route was developed for the synthesis of 2-substituted 6,7-dichloroimidazo[4,5-b]quinoxalines involving a reaction of 2,3,6,7-tetrachloroquinoxaline with ammonia followed by a ring annulation as the key step. This furnished the versatile heterocycle 6,7-dichloroimidazo[4,5-b]quinoxalin-2-one. Ribosylation of 2-substituted imidazo[4,5-b]quinoxalines was influenced by the functional group at the 2-position and the 2-one compound was found to smoothly undergo ribosylation. The 2-one group of the nucleoside was converted into specifically selected 2-substituted compounds. Evaluation of the compounds for activity against two herpesviruses and for cytotoxicity showed they were less active and/or more cytotoxic than TCRB. We conclude therefore, that the binding pocket on the protein target of TCRB will tolerate some electronic and size changes.     

The spectrophotometric determination of molybdenum and uranium with quinalizarin

Quinalizarin reacts very sensitively with molybdenum and uranium to form colored chelates having λ_max at 520 and 630 nm, respectively. The molar ratio for both the chelates is 1:1 (metal:reagent). Optimum conditions including the range for adherence to Beer's law, effect of pH on color intensity, effect of excess of the reagent, sensitvity, and interference of the foreign ions has been reported for the photometric determination of these metal ions using quinalizarin, in 50% ethanolic medium and at 30 °C.     

Effect of cationic gemini surfactants on the hydrolysis of carboxylate and phosphate esters using hydroxamate ions

The kinetics of the hydrolysis of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) by hydroxamate ions mediated by gemini surfactants with quaternary ammonium bromide (16-n-16,2Br⁻, n = 3, 4, 6, 12) and pyridinium chloride (12py-n-py12,2Cl⁻, n = 3, 4) head group have been investigated at 27 °C. The gemini surfactant with the pyridinium head group, 12-py-4-py12,2Cl⁻ (tetramethylene-1,4 bis dodecylpyridinium chloride) shows a large rate acceleration effect than that with an ammonium head group, 16-12-16,2Br⁻, relative to those in water. The apparent pK _a of the hydroxamic acids have been determined in the presence of gemini surfactants. Catalytic system N-phenylbenzohydroxamate/12py-4-py12,2Cl⁻ demonstrated over ~1,590-fold and ~255-fold rate enhancement in the hydrolysis of PNPA and PNPDPP, respectively, for the identical reaction performed in buffer aqueous media at 27 °C. The second order rate constant and binding constants for reactions were determined employing pseudophase model for micellar catalysis.     

Synthesis,spectral, DFT,and antimicrobial studies of tin(II) and lead(II) complexes with semicarbazone and thiosemicarbazones derived from (2-hydroxyphenyl)(pyrrolidin-1-yl)methanone

A new series of metal complexes [M(L)₂] (where M = Sn(II), Pb(II), and HL = semicarbazone, thiosemicarbazone or phenylthiosemicarbazone) have been prepared and characterized by elemental analysis, conductance measurements, molecular weight determinations, UV–visible, infrared, and nuclear magnetic resonance (¹H-, ¹³C-, and ¹¹⁹Sn-NMR) spectral studies. Elemental analysis of the metal complexes suggested 1 : 2 (metal–ligand) stoichiometry. Infrared spectra of the complexes agree with coordination to the metal through the nitrogen of the azomethine (>C=N?) and the oxygen/sulfur of the ketonic/thiolic group. Electronic spectra suggest a distorted tetrahedral geometry for all Schiff base complexes. The bond lengths, bond angles, highest occupied molecular orbital, lowest unoccupied molecular orbital, Mulliken atomic charges, and the lowest energy model structure of the complexes have been determined with DFT calculations. Representative Schiff base and its metal chelates have been screened for their in vitro antibacterial activity against four bacteria, Gram-positive (Bacillus cereus, Staphylococcus aureus) and Gram-negative (Escherichia coli, Klebsiella pneumoniae) and four strains of fungus (Penicillium chrysogenum, Aspergillus niger, Rhizopus nigricans, and Alternaria alternata). The metal chelates possess higher antimicrobial activity than the free ligands.     

An expedient ‘click’ approach for the synthetic evaluation of ester‐triazole‐tethered organosilica conjugates

The present work articulates the synthesis of a new series of organo‐functionalized triethoxysilanes derived from versatile carboxylic acids and 3‐azidopropyltriethoxysilane in excellent yields. A proficient and convenient route implicating the Cu(I)‐catalysed 1,3‐cycloaddition of organic azide with terminal alkynes, labelled as click silylation, has been developed for the generation of ester‐triazole‐linked alkoxysilanyl scaffolds ( 4a – f ). All the synthesized compounds have been thoroughly characterized using elemental analysis and Fourier transform infrared, ¹H NMR and ¹³C NMR spectroscopic techniques. Importantly, the fabricated alkoxysilanes are potentially amenable for an in situ sol–gel condensation reaction with silica nanospheres leading to the incorporation of organic functionality via covalent grafting onto the nanostructured particle system. As a proof of concept, a one‐pot preparation of organic–inorganic hybrid nanoparticles is presented using bis‐silane 4 f . The efficiency and selectivity of the prepared nanocomposite towards metal ions is highlighted using adsorption experiments, and the immobilized nanoparticles present a high sensing efficiency towards Cu²⁺ and Pb²⁺ ions while demonstrating better response than that of the bulk material.