A spectroscopic and magnetic study of complexes of bis(2-benzothiazolyl)methanate and bis(2-benzoxazolyl)methanate with Co(II), Ni(II), Cu(II) and Zn(II)

The co-ordination geometry of the complexes M(bbtm)2 and M(bbom)2 (M: Co, Ni, Cu, Zn; bbtm, bis(2-benzothiazolyl)methanate; bbom, bis(2-benzoxazolyl)methanate) are discussed on the basis of their IR, Raman, resonance Raman, electronic and ESR spectra. Compounds of Ni, Co, Zn with both ligands and Cu(bbom)2 resulted to have a distorted tetrahedral geometry. The distortion towards a square planar geometry is more marked for the M(bbtm)2 series than for the M(bbom)2 one. It has been impossible to suggest a co-ordination geometry for Cu(bbtm)2, that probably has a polymeric structure.     

Benzene absorption in EPR and clustering mechanism

Benzene equilibrium absorption curve by EPR rubber (50 wt % C₃) was measured at 23°C. Benzene activity a₁ was plotted against its volume fraction ?₁. Benzene clustering function G₁₁/v₁ was calculated according to Zimm and the mechanism of formation of benzene clusters is discussed. It was found that clusters at very low benzene partial pressures are monomolecular and randomly distributed: at higher partial pressures, clusters increase in size. Flory-Rehner theory, giving activity α₁ against ?₁, was checked; predictions of cluster formation were made but there were some discrepancies with Zimm's conclusions.     

The Reaction of Pyruvic Acid with Amines and Aminoesters Reexamined. Preliminary communication

Phenylglycine esters react with pyruvic acid to give α-methylsuccinic amides 9 instead of the expected Schiff bases 8 , analogously to p-anisidine but unlike aniline.     

Activated C,H-Acids: N-Alkyl-9-fluorenimines. Preliminary communication

N-Substituted 9-fluorenimines 1 are easily prepared from primary amines or aminoesters and 9-fluorenimine: their conjugate carbanions 2 are formed by the action of alkoxides and are reprotonated to 1 or to the tautomeric 9-alkylidenamino-fluorenes 3 depending upon substituents.     

Synthesis,stereochemistry and conformational properties of diastereomeric cyclic dipeptides containing tetrahydro-1,4-thiazine-3,5-dicarboxylic acid

The synthesis of linear dipeptides containing N-protected L-phenylalanine and (3R-cis)-tetrahydro-1,4-thiazine-3,5-dicarboxylic acid dialkyl diester residues is described. N-Deprotection of these dipeptides by hydrogenolysis on palladium afforded directly a mixture of cis and trans dioxopiperazines. The stereochemistry and the solution conformational properties of the cyclic dipeptides are determined.     

Hybrid Gels as Host Matrices of Perfumed Essences

The variety of SiO₂-gel structures allows the use of sol-gel derived materials as host matrices for prolonged release of chemicals. In this work, the release of perfumed essences is studied in the case of hybrid SiO₂ materials obtained from alkyl-modified silicon alkoxides. Thermogravimetric analysis in isothermal conditions at various temperatures provide data on the kinetic release process. The effects of physical parameters (porosity reduction, vapour pressure) and chemical interactions (organic molecules with matrix) are discussed for the interpretation of the results.     

The reactivity of 2,3-diaminopyridine toward alkyl chloroformates and dicarbonates

The reaction of 2,3-diaminopyridine (DAP) with alkyl chloroformates and dialkyl dicarbonates is affected by the nature of the acylating agent and the acylation conditions. Mono and dicarbamates of DAP were obtained using chloroformates, while the formation of alkoxycarbonyl derivatives of imidazo[4,5-b]pyridin-2-one was observed when the reaction with dicarbonates was performed at room temperature in THF-pyridine. Our procedure for the synthesis of imidazopyridin-2-ones represents an improvement over the previous methods which require rather severe conditions. The revised structure for a monoacetyl derivative of the above bicyclic system is also reported.     

Simultaneous Determination of Two Antibiotics in Tablets by Spectrophotometry and Principal Component Regression (PCR) Analysis. An Advanced Undergraduate Experiment Involving Chemometrics

An advanced analytical chemistry laboratory experiment involving the simultaneous determination of the antibiotics sulfamethoxazole and trimethoprime in commercial tablets is described. It involves the following steps: (1) preparation of nine calibration mixtures and the recording of their absorption spectra in the region 200–320 nm, (2) dissolution of the tablet contents and the recording of spectrophotometrics data, and (3) processing the data with the multivariate calibration technique of principal component regression (PCR). The theory of PCR is discussed, and a Visual Basic program is made available for data processing. The latter program allows students to obtain and save relevant statistical information (root-mean-square deviation, correlation coefficient, and relative error of prediction), as well as calibrate spectral factors and spectral residuals for each test sample. This program helps to illustrate the PCR technique in detail. The reagents used are of low cost and nontoxic; the experiment is simple and provides students with insight into a the real practice of integrating chemistry, instrumentation and computer techniques.     

Variational methods for nonlinear Steklov eigenvalue problems with an indefinite weight function

We consider the problem of finding a harmonic function u in a bounded domain ${\Omega\subset\mathbb{R}^N}$ , N ≥ 2, satisfying a nonlinear boundary condition of the form ${\partial_{\nu}u(x)=\lambda\,\mu(x)h(u(x)), x\in \partial\Omega}$ where μ changes sign and h is an increasing function with superlinear, subcritical growth at infinity. We study the solvability of the problem depending on the parameter λ by using min-max methods.