The temperature dependence of 13C NMR shifts in polar compounds and its role for the determination of conformational equilibria

Intrinsic temperature dependencies of ¹³C NMR shifts in alkanes bearing one polar C-α? X bond are determined with neopentyl and 4-tert-butylcyclohexyl derivatives as conformationally homogeneous model compounds. The increased shiedling for C-α at higher temperatures can be related to a C-α—X bond length increase and, for less polarizable C? X bonds, essentially to a decrease of solvent polarity on raising the temperature. The use of temperature dependent ¹³C shifts in conformationally mixed compounds for the determination of the equilibrium constants, K, is evaluated with n-propyl halides; the computer fit of the unknown conformer shifts and the conformational enthalphy difference, δH° to the time averaged shifts yields δH° values which, although converging rather broadly, are in general agreement with literature data. In compounds with higher conformational barriers, such as methoxy- and bromocyclohexane, low temperature signal integration yields accurate δG° values; inclusion of shifts above coalescence, however, yields unreliable δH° and δS° parameters. This can only partially be remedied by application of temperature shift corrections obtained from parent t-butylcyclohexyl compounds.     

Thermodynamic characteristics of phase conversion of structural isomers for volatile complex of ruthenium (III) trifluoroacetylacetonate

The comprehensive analysis of volatile β-diketonate compound—ruthenium(III) trifruoroacetylacetonate (Ru(tfac)₃)—was carried out. By means of flow method in quasi-equilibrium conditions and static method the temperature dependencies of saturated vapor pressure have been measured over solid and liquid cis- and trans-modifications of Ru(tfac)₃ and isomer mixture. The thermodynamic characteristics of sublimation, evaporation, melting, and phase conversion have been calculated for structural isomers. Also by differential-scanning calorimetry the temperature meanings and the thermodynamic characteristics of melting have been determined for individual isomers of Ru(tfac)₃ and their mixtures. By XRD the structures for cis- and trans-modifications have been determined. Both structures consist of neutral molecules arranged in pseudo layers.     

Thermal reactions by automated mass spectrometric thermal analysis

Combination of differential thermal analysis (DTA) with time-of-flight mass spectrometry and data storage on magnetic tape is used to study thermal decompositions.A modified Du Pont 900 DTA cell is used for reactions at atmospheric pressure with evolved gas sampling into a small time-of-flight spectrometer.For fast reactions a specially developed DTA cell is operated within the mass spectrometer and operates at high vacuum.A combination of both operational modes yields pressure dependencies of chemical decomposition.Examples studied include inorganic compounds and complexes, organometallics and organic chemicals such as fire retardants.The data are presented as intensity/temperature curves for each observed fragment, which not only yields decomposition temperatures but also—by the curve shapes—information on the mode of decomposition.     

Metal atom dynamics in organometallics: Vibrational amplitude determination for bis phosphino ferrocenes including a waxy system

Three structurally related bis phosphino ferrocene complexes have been examined by ⁵⁷Fe Mössbauer effect (ME) spectroscopy in order to evaluate the hyperfine interaction parameters (IS and QS), their temperature dependencies, as well as the dynamics of the metal atom over a temperature range. For two of the compounds (1 and 2), for which single crystal X-ray diffraction data have been reported, a direct comparison can be effected between the U_i,j values derived from the X-ray study and that extracted from the ME data, and are found to be in good agreement. For complex 3, which is a waxy material at room temperature, no X-ray data can be obtained, but the ME data permit an evaluation of the metal atom vibrational amplitudes even in the high (>180 K) temperature regime. In addition, data are presented relating to the anisotropy of the metal atom motion in these ferrocene complexes.     

Liquid chromatographic separation of the thyroid hormones

The liquid Chromatographic separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine with a nonpolar stationary phase was studied as a function of pH, temperature, organic content of the mobile phase, and ionic strength using aqueous phosphate—acetonitrile, aqueous phosphate—methanol, and aqueous phosphate— n-propanol mobile phase systems. It was demonstrated that the quality of the thyroid hormone separations, as determined by normalized peak capacity values, was unchanged with temperature, remained relatively constant with increasing ionic strength, and was affected to the greatest extent by changes in pH and organic modifier content of the mobile phase. Chromatographic behavior of the compounds studied as a function of these variables was found to be consistent with existing Chromatographic theory and/or empirical observations. Recommended conditions are aqueous phosphate—methanol mobile phase, pH 2–5 (aqueous portion), and high temperature (60–70°C).     

Polarography of some sulphur-containing compounds : Part XII.Anodic waves of dialkyldithiocarbamates

Dialkyldithiocarbamates give only a one-electron anodic wave. The various adsorption phenomena are probably caused by varying orientation of the mercury compounds on the electrode surface; the irregular i-t curves recorded at —0.04 V and the anomalous behaviour of the anodic current between +0.2 V and —0.4 V can be interpreted as the behaviour of an adsorbed film at the electrode. Analytical measurements are best made in 60% ethanolic 0.1 M sodium hydroxide media at concentrations of about 10^-5 M dialkyldithiocarbamate; a method is given for the analysis of the monoalkyl and dialkyl compounds in mixtures.     

Spectroscopic studies on olefins. VII–propene and 3-methylbutene-1; proton NMR coupling constants as a function of solvent and temperature in relation to molecular geometry

The proton spectra of the compounds propene (1) and 3-methylbutene-1 (2) have been measured in various solvents and, as neat liquids, at variable temperature. The effects of solvent and temperature on 1 are found to be small, but not negligible. The effect of temperature on the coupling constants of 2 are more pronounced, which is explained in terms of an anti-gauche equilibrium. To obtain a consistent ΔE° value from each temperature dependent coupling constant it is necessary to correct for the non-conformational temperature effects (obtained from 1) as well as to correct for changes in the geometry of the entire molecule, which are concurrent with a conformational transition. Such geometry changes have been calculated by a consistent force field technique, which, when coupled with INDO-MO calculated dependencies of coupling constants on valence (and torsional) angles, provided the changes in coupling constants. The observed temperature dependence of the coupling constants of the H₂C?CH— moiety of 2 could be satisfactorily explained in this fashion. The difference in energy between the anti and gauche forms of 2 were determined as ΔE° = 127 ± 14 cal mol^?1.     

Catalytic—kinetic determination of some iodine-containing organic compounds with different catalytic activities by a biamperostatic method

Iodine-containing organic compounds of the general formula X—C₆H₄—I catalyze the Sandell—Kolthoff reaction to different extents; the catalytic activities depend on the other substituent and on the relative positions of the X and iodine substituents. 4-Iodophenol, 2-iodophenol and 4-iodo-N,N-dimethylaniline can be determined in the microgram range.     

Rheological characterization of the die swell phenomenon of rubber compounds

The die swell phenomenon of rubber compounds in capillary experiments with various ratios of length to diameter of capillaries is investigated. This knowledge is important for the design of injection heads for the extrusion of rubber profiles. The die swell of viscoelastic rubber compounds depends on the geometry of the capillary dies, on the melt temperature and on the shear strain rate. One empirical relationship will consider all these dependencies. Usage of this equation and identification of only one new material parameter enables the comparison and assessment of the die swell of different materials, independently of the corresponding geometry of the capillary die used. Furthermore, the influence of melt temperature, molecular structure and extrusion process on the die swell can be identified. The investigation was performed with various rubber compounds as well as rubber blends used in industry, mainly EPDM and carbon black in different compositions.     

PVC/MMT nanocomposites

Poly(vinyl chloride) (PVC) nanocomposites with sodium montmorillonite (Na-MMT) and organically modified MMT (O-MMT) have been prepared by melt processing using mixing and extrusion techniques. The differential scanning calorimetry (DSC) with stochastic temperature modulation (TOPEM?) results show that the glass transition temperature (T _g) of PVC is slightly higher than T _g of PVC/Na-MMT and PVC/O-MMT which would indicate that MMT plays a role of an internal plasticizer that increases the distance between the PVC macrochains. The DSC TOPEM non-reversing heat flow profiles show enthalpy relaxation effects, and the lowest value has been found for pristine PVC—the presence of MMT (both Na+ and ammonium salt modified) may generate a certain orientation level of PVC macrochains during the extrusion process. Specific heat vs temperature dependencies at different frequencies revealed that the best fit to the single profile was found for PVC/Na-MMT nanocomposite, and this observation may be related to internal stability of the composite material as confirmed by analysis of the change in the specific heat (Δc _p).     

Thermoanalytical methods in vulcanizete analysis. Differential scanning calorimetry. Observations on the vulcanization process

Differential scanning calorimetry was used to follow the unaccelerated and accelerated sulfur vulcanization process in natural rubber and polybutadiene compounds. It was established that in both hard and soft rubber vulcanization, the heat of vulcanization (ΔH_v) depends only on the sulfur concentration provided other ingredients (carbon black, zinc oxide, stearic acid) and the elastomer blend ratio remain constant. Organic accelerators alter the temperature dependence of the exotherm but have no effect on ΔH_v over a considerable concentration range.The DSC exotherm was used to determine variations in sulfur/accelerator concentrations in production compounds. Analysis time is short—approx. 5 min—and a routine quality control method is suggested.     

Conformational analysis of organophosphorus compounds by means of IR spectroscopy and dipole moment methods

Some aspects of rotational isomerism about an ordinary P—X bond in compounds

have been discussed on the basis of spectroscopic data. Changes in spectra in the process of phase transition with variation of dielectric permeability of the environment and temperature have been analyzed. A scheme of structure identification by the dipole moment method with determination of conformer concentration from IR-spectra has been proposed. Data from IR-spectra, dipole moments and the Kerr effect have been applied to complicated systems.Gauche- and trans-orientated O—C and PO bonds are characteristic of compounds for which X = O. It has been shown that in the case of rotation of a chloromethyl group at the phosphorus atom trans and gauche arrangements of C—Cl and PO (S) are stable. The position of the conformational equilibrium depends on the nature of the substituents. The important role of electrostatic interaction has been established.     

Addressing the issue of numerical pitfalls characteristic for SAFT EOS models

This study demonstrates the SAFT models can have doubtless advantages in predicting thermodynamic properties comparing to cubic equations of state. However they might be affected by two kinds of numerical pitfalls, namely the erroneous shapes of their isotherms resulting in the multiple phase equilibria predicted for pure compounds, and the undesired consequences of attaching their reduced densities by temperature dependencies. The latter are the negative heat capacities at very high pressures and the intersections of isotherms. Some versions, such as the SAFT of Chapman et al. [17] and SAFT-VR-SW of Gil-Villegas et al. [19] are found to be free of the pitfalls of the 1st kind. However the unsafe temperature dependencies are still essential for their accuracy. The present study proposes a modification of the Carnahan-Starling repulsive term maintaining the reduced densities temperature-dependent while making the covolumes temperature-independent. It is demonstrated that this manipulation might address the problem of numerical pitfalls and preserve the outstanding performance of the models.     

Polarographische untersuchungen cytostatischer chinonderivate

Ethyleneimine- and N-mustard derivatives of p-benzoquinone were differentiated under various polarographic conditions. Several depolarizers with —

substituents yield double steps, which indicate a mesomeric equilibrium dependent upon the temperature. When the half-wave potential, against the N.C.E. (pH 7), is more positive than —0.5 V, the cytostatic effect is increased. This fact may be significant for further syntheses in this group of compounds as well as for the mechanism of their action in the cell.     

Gaschromatographische Untersuchung der Adsorptionseigenschaften von Silicalit und ZSM-5

High silica molecular sieves (silicalite, ZSM-5) were tested as adsorbents for gas chromatographic trace analysis. Therefore the retention behaviour of low-boiling organic compounds (hydrocarbons, halogenated hydrocarbons, amines, alcohols and ethers) on these materials was investigated. The specific retention volumes at different temperatures have been determined and elution orders and peak shapes were studied. The retention data allow a simple calculation of the breakthrough volumes (dynamic adsorption capacity) and the chromatographic characterisation of the adsorbents. Both nitrogen and oxygen containing compounds could not—or at least unreproducibely—be eluated up to 300°. The elution order and the peak shape of compounds with the same number of carbon atoms but different geometric and electronic structure (e.g.n-butenes;n-hexane, cyclohexane, benzene) can be explained by the action of exclusion effects and different diffusion barriers. On the basis of calculated breakthrough volumes we conclude that silicalite should be useful in the preconcentration of both saturated C₃–C₆ hydrocarbons and C₁–C₂ chlorinated hydrocarbons from gaseous streams.     

Synthesis and properties of a new kind of one-dimensional conductor VIII. Synthesis and characterization of trans-bis-1-alkynyl substituted silicon,germanium, tin phthalocyanines and germanium hemiporphyrazines

The preparation of several new trans-bis-1-alkynyl(phthalocyaninato)silicon (VIc—f), -germanium (VIIc—f) and -tin (VIIId, f) compounds as well as trans-bis-1-alkynyl(hemiporphyrazinato)germanium (XIc, e, f) derivatives is described. The products were obtained in high yields by treating the corresponding dichlorides (I, II, IV, X; X = (Cl) with 1-alkynyl Grignard reagents (Vc—f) in tetrahydrofuran (THF) and were characterized spectroscopically. The compounds obtained are considered as models for a new kind of one-dimensional conductor.     

Verification of epitaxial growth and determination of chain positions in Langmuir—Blodgett films of lead stearate on mica

We have determined from X-ray diffraction that lead stearate, which like many hydrocarbon compounds does not form good crystals in bulk, can be grown epitaxially on mica using the Langmuir—Blodgett technique. The strong alternation in intensities of the (11l) peaks cannot be explained by considering the lead ions only, and imply that the hydrocarbon chains do not close-pack into a triangular array but reproduce the in-plane structure of the lead ions.     

N-Alkyl-5,5-diorganodiptychoxazstannolidone,intramolekular pentakoordinierte stannylester der N-methylminodiessigsäure

The reaction of dialkyltin oxides or dialkyltin dialkoxides with N-methyliminodiacetic acid in dimethylformamide/toluene gives the title compounds in 80–90% yields. The structure of these compounds was investigated by NMR, IR and mass spectroscopy. The temperature dependent ¹H and ¹³C NMR spectra are interpreted by a dissociation—inversion mechanism.