B. Csákvári

An electron diffraction study of the molecular structure of hexamethyldisiloxane

Abstract An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 A and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation ( C 2 v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.

Metastability in EuBa2(Cu1-xSnx)3O7-y studied by 119Sn and 151Eu Mössbauer spectroscopy.

The metallic oxide compound system EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ (x = 0.05, 0.10, 0.20) was prepared for investigation with /sup 151/Eu and /sup 119/Sn Moessbauer spectroscopy. The best fits of the /sup 119/Sn Moessbauer lines reveal at least two different Sn/sup IV/ sites. The /sup 151/Eu spectra are characteristic for Eu/sup III/ state in these compounds. We have found a considerable difference between the shapes of /sup 119/Sn Moessbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior.

Metastability in EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ studied by /sup 119/Sn and /sup 151/Eu Moessbauer spectroscopy

The metallic oxide compound system EuBa/sub 2/(Cu/sub 1-//sub x/Sn/sub x/)/sub 3/O/sub 7-y/ (x = 0.05, 0.10, 0.20) was prepared for investigation with /sup 151/Eu and /sup 119/Sn Moessbauer spectroscopy. The best fits of the /sup 119/Sn Moessbauer lines reveal at least two different Sn/sup IV/ sites. The /sup 151/Eu spectra are characteristic for Eu/sup III/ state in these compounds. We have found a considerable difference between the shapes of /sup 119/Sn Moessbauer spectra recorded at room temperature and those recorded at the temperature of liquid nitrogen in the case of the sample having the highest tin content. Nevertheless, there is no evidence that this difference has a direct connection with the superconducting behavior.

N-Substituted trimethylsilylcarbamates as silylating and/or methoxime derivatizing reagents for gas chromatographic analysis

N-Substituted trimethylsilylcarbamates were tested as derivatizing reagents for gas chromatographic analysis. N,N-Dimethyltrimethylsilylcarbamate was found to be a suitable agent for direct trimethylsilylation of the salts of alkaloids without previous liberation of the bases. N-Methoxy-N,O-bistrimethylsilylcarbamate was found to react simultaneously and quantitatively with the hydroxy and keto groups of steroids, resulting in the corresponding methoxime trimethylsilyl derivatives.

Trimethylsilylated N-alkyl-substituted carbamates: III. Structure of trimethylsilyl-N,N-diisopropyl carbamate in solid phase and in solution☆

Abstract The conformers of trimethylsilyl- N , N -diisopropyl carbamate were studied by 1 H NMR spectroscopy. The barriers to rotation by isopropyl groups and to the rotation around the amide bond, and the population of the conformers were determined from the coalescence temperature of the methyne signal. Single crystal X-ray diffraction studies revealed that the crystal had formed from preferred conformer in solution. The mirror symmetric conformer is stabilized by an intramolecular CH ⋯ O hydrogen bond and by an intramolecular O ⋯ Si donor-acceptor interaction.

119Sn and151Eu Mössbauer study of EuBa2(Cu1-xSnx)3O7-y perovskites

Eu and Sn Mössbauer spectroscopy was used to study EuBa2(Cu1-xSnx)3O7-y metallic oxides. The spectra are characteristic for EuIII and SnIV states in all cases. The existence of at least two different Sn states was determined by decomposition of the Sn Mössbauer spectra. These Sn sites can be associated with nonequivalent Cu sites replaced by Sn atoms in the perovskite type lattice. Anomalous changes were observed in the isomer shifts and area fractions in the spectra measured at room temperature and at 77 K. It can be interpreted assuming low temperature phase transformation and phonon softening. Time dependent changes were found in the Sn spectra recorded at 77 K in the case of highest Sn concentration. The observed changes are consistent with a transitional stage of the low temperature phase transformation.

Synthesis and reactions of silylcarbamates with bulky substituents

Abstract New silylcarbamates containing bulky substituents were synthesized in order to study their reactivity with nucleophilic agents as lactones and chloroformates. We suggest a mechanism for these reactions.

Solvolysis of trimethylsilyl-N-aryl carbamates

The kinetics of the solvolysis reaction of various trimethylsilyl esters of N-aryl carbamic acid with isopropanol was studied. UV spectrophotometry was used to monitor the reaction and the pseudo first order rate constants were determined. The experiments were carried out in dioxane solutions containing lithium chloride. A reaction mechanism is suggested on the basis of the dependence of the reaction rate on ionic strength, the parameters of activation, the reaction constant (ϱ) and the isotope effect.

Solvolysis kinetics of trimethylsilyl esters of aromatic N-carboxylic acids

Abstract Some new trimethylsilyl esters of cyclic and/or aromatic N -carboxylic acids were prepared. Their solvolysis reactions with isopropanol were studied by gas chromatography and by UV spectrophotometry respectively. The pseudo-first-order rate constants, the salt effects and the activation parameters were determined.

Comparative Mössbauer study of EuBa2 (Cu1-x 57Fe x )3O7-d and EuBa2Cu1-x 57Fe x )4O8 superconductors

Structural differences between the 1-2-3 type and 1-2-4 type high temperature superconductors were studied by comparing their151Eu and57Fe Mössbauer spectra. The similarities between the corresponding Mössbauer parameters of Eu Mössbauer spectra of 1-2-3 and 1-2-4 compounds demonstrate that the Eu state can be considered to be identical in both perovskites. The changes in the Fe Mössbauer parameters of 1-2-4 type superconductor compared to those of 1-2-3 type compound can be associated with structural differences.