Jana Holubová

Thermodynamic and kinetic study of copper(II) complexes with N-methylene(phenylphosphinic acid) derivatives of cyclen and cyclam

Equilibria of Cu(II) with newly synthetised ligands H4L1 and H4L2 were investigated by the glass electrode potentiometry at 25 degrees of Celsius and I = 0.1 M KNO3. A simple chemical model with metal:ligand molar ratio 1:1 was found in the systems. Presence of main species , CuL1 (log Beta = 20.37(4)) and CuL2 (log Beta = 17.19(2)) was also confirmed by MALDI-TOF/MS. The dissociation kinetics of the complexes was followed by spectrophotometry and mechanism of the dissociation was proposed. Activation parameters (activation enthalpy and entropy) of the dissociation were estimated. For Cu(II)-H4L1 system, the complex dissociation starts after protonization of phosphinic pendant arms and its mechasnism is similar to the decomplexation of [Cu(cyclen)]2+. The complex with H4L2 is kinetically much less stable and the proton transfer from phosphinic pendant arm to the azacycle plays a significant role in the reaction mechanism.

Structural relaxation near the glass transition temperature

Abstract Enthalpy relaxation processes on a model bulk glass, As 2 Se 3 , were studied below the glass transition temperature, T g , as well as at temperatures close to the glass transition overshoot. Extents of relaxation processes below T g were measured by enthalpy of endothermic relaxation peak. Relaxation processes above T g were determined by melting enthalpies of partly crystallized glass during relaxation. We found an exponential temperature dependence of physical aging. Completely annealed glasses reach the equilibrium having excess of enthalpy compared to the equilibrium supercooled liquid at the same temperature. Using the Tool–Narayanaswamy–Moynihan (TNM) model, we found that the fictive temperature, T f , decreased during isothermal annealling toward aging temperature, T a , and at equilibrium is equal to annealing temperature, T a . When the glass comes to equilibrium the distribution of relaxation times decreases. The description of relaxation processes at the temperatures slightly above T g is more complicated. The nuclei are created in the first step. In the second one these nuclei can create thermodynamically non-equilibrium agglomerates, which consequently can decrease their energy by becoming amorphous.

Inclusion compounds of cystostatic active (C5H5)2VCl2 and (CH3C5H4)2VCl2 with α-, β- and γ-cyclodextrines: Synthesis, EPR study and microbiological behavior toward Escherichia coli

The inclusion of vanadocene dichloride (VDC) and 1,1′-dimethyl vanadocene dichloride (MeVDC) into cyclodextrines (α-CD, β-CD and γ-CD) was studied by EPR spectroscopy. It was found that VDC and MeVDC with β-CD and γ-CD form true inclusion compounds, but with α-CD, VDC and MeVDC gave only fine dispersion mixtures. The inclusion was validated by anisotropic EPR spectra of solid samples. In addition, the antimicrobial was validated by anisotropic EPR spectra of solid samples. In addition, the antimicrobial behavior (against E. coli) of each of the complexes was determined. It was established that not only did VDC and MeVDC cause elongation of E. coli, but also the new vanadocene inclusion complexes were effective in this regard.

Non-isothermal structural relaxation and the glass transition temperature

Abstract The glass transition was studied by a new stepwise differential scanning calorimetry (DSC) technique on the model glasses As 2 Se 3 and As 2 S 3 . The glass transition was separated to the two components: a reversible (thermodynamic) one, reflecting temperature changing of the vibrational amplitude, and an irreversible (kinetic) one, bears on structural relaxation. The value of the glass transition temperature, T g , determined from the thermodynamic part of the glass transition was found to be independent both on the heating/cooling rate and the thermal history of glass. The value of T g depends only on the chemical composition of the glass and thus it could be regarded as a material constant. The heating/cooling rate dependence of T g , known from DSC or DTA measurements, is due to kinetic part of the glass transition.

Kinetic Analysis of Non-isothermal DSC Data. Computer-aided test of its applicability

The applicability of the kinetic analysis of data obtained by non-isothermal differential scanning calorimetry (DSC) is discussed. The Johnson-Mehl-Avrami (JMA) model was used for the computer simulation of DSC traces subsequently analysed by common methods of kinetic analysis of non-isothermal data. For the temperature-independent kinetic exponent n of the JMA equation, the kinetic analysis was shown to provide correct results, e.g. a correct kinetic model and apparent activation energy. On the other hand, for the temperature-dependent kinetic exponent, there is a great possibility of erroneous determination of the correct kinetic model and apparent activation energy, especially at higher heating rates. Since the temperature dependence of n cannot be determined on the basis of non-isothermal DSC experiments, conclusions must be drawn with appropriate caution.

Crystallization Kinetics of Glassy As2Se3

Isothermal crystallization of bulk As2Se3 glass was studied in temperature range 270-360°C. Johnson-Mehl-Avrami (JMA) equation describes the crystallization process in the whole temperature range. By means of analysis of JMA equation the temperature dependence of kinetic exponent n was found, its value changes from 3.8 to 1.9 with increasing temperature. The relationship between the value of n and crystal morphology was briefly discussed. Furthermore the value of apparent activation energy E was determined as well as melting enthalpy. Temperature dependence of crystal growth rate was also determined.

StepScan DSC: The useful tool for the study of the glass transition phenomenon

StepScan DSC technique was used for the study of the glass transition phenomenon. This method allows relatively good monitoring of reversing and non-reversing processes, and thus is very useful tool for glass transition studies of wide type of glass-forming materials, including inorganic glasses as well as organic polymers. In this work, experience with the StepScan DSC technique is summarized. Some interesting results of its application are presented, such as determination of glass transition temperature independent on thermal history of glass, discovered relationship between the slope of temperature dependence of Cp in Tg, and Angel’s index of fragility and estimation of viscosity glass transition temperature, Tg,η.

Synthesis, characterization and structure of the Bis(methyl-cyclopentadienyl)vanadium(IV) carboxylates

The 1,1’-dimethylvanadocene dichloride ((C5H4CH3)2VCl2) reacts in aqueous solution with various carboxylic acids giving two different types of complexes. The 1,1’-dimethylvanadocene complexes of monocarboxylic acids (C5H4CH3)2V(OOCR)2 (R=H,CCl3, CF3, C6H5) contain two monodentate carboxylic ligands, whereas oxalic and malonic acids act as chelate compounds of the formula (C5H4CH3)2V(OOC-A-COO) (A=−, CH2). The structure of the (C5H4CH3)2 V(OOCCF3)2 complex was determined by single crystal X-ray diffraction analysis. The isotropic and anisotropic EPR spectra of all the complexes prepared were recorded. The obtained EPR parameter values were found to be in agreement with proposed structures.

An X-ray Crystallographic Study of New Monoclinic Selenium Allotrope δ-Se8

COMMENT At normal pressure six structural forms of solid selenium are known. The three red monoclinic allotropes (α-, βand γ-) consist of Se8 rings and differ in the intermolecular packing of the rings in the crystals. The gray metallic trigonal crystalline selenium (t-Se) is formed by helical polymeric chains with the transconfiguration arrangement of atoms in the 4-member fragment and it is thermodynamically the most stable form of the element. Deformed helical chains occur also in amorphous red Se (α-Se). Finally, glassy black selenium (γ-Se), the ordinary commercial form of the element, comprises an extremely complex structure of rings and polymeric chains. Red monoclinic selenium is characterized by the c/s-configuration in the arrangement of four sequentially bonded atoms in the Se8 molecule. The a and βallotropes can be prepared respectively by the slow and rapid evaporation of CS2 solution of γ-Se /1,2/. The γform was obtained from the reaction of dipiperidiniotetraselenane with CS2 /3/. The new monoclinic allotrope of cyclooctaselenium (6-Se8) differs significantly from previously reported ones especially by cell angle beta = 127.50° (cx-Se8: 90.68°; ß-Se8: 92.95°; y-Se8: 93.61°). The direct comparison between the 5-Se8 and α-allotrope firstly described by Cherin and Unger IM and redetermined by measurement at various temperatures by Maaninen et al. I\ l , is that 5-Se8 can be solved and transformed in P2|lc point group instead of P2|In one, with the major difference in cell volume value (911.1(3) and 909.71(3) for α-allotrope and 929.9(2) for the new one). Also observed bond lengths (average Se-Se 2.333(2)Ä at -123°C) and angles differ from published values (average Se-Se 2.301(18) and 2.311(17) for α-allotrope at -150 and -100°C, respectively) l \ l . A plethora of close contacts between selenium atoms (18 per one Se8 unit with distances from 3.534 to 3.774 A) below the sum of van der Waals radii (1.9 A) is present in the crystal lattice.

Isothermal structural relaxation: temperature and time dependencies of relaxation parameters

Abstract The isothermal structural relaxation has been studied on model glass with well-defined thermal history by differential scanning calorimetry (DSC). The computer modeling of DSC scans was based on Tool–Narayanaswamy–Moynihan model with Kohlrausch–William–Watts relaxation function. The parameter of non-exponentiality, β, and the enthalpy change, ΔH, have been evaluated and their time and temperature dependencies have been discussed. It was found that β is both temperature and time dependent. When the relaxed glass has reached equilibrium the corresponding limit enthalpy change, ΔHeq, was found to be less than difference between glassy state and corresponding liquid state obtained by linear extrapolation of its ΔH vs. T dependence below the glass transition temperature.