Analysis of the sensitivity and sample–furnace thermal-lag of a differential thermal analyzer

The heat exchange between the horizontal furnace of a differential thermal analyzer (DTA) and the sample is analyzed with the aim of understanding the parameters governing the thermal signal. The resistance due to radiation and conduction through the gas has been calculated and compared to the experimental values of the thermal-lag between the sample and furnace and apparatus sensitivity. The overall evolution of these parameters with the temperature and their relative values are well understood by considering the temperature differences that arise between the sample and holder. Two RC thermal models are used for describing the apparatus performance at different temperature ranges. Finally, the possibility of improving the signal quality through the control of the leak resistances is stressed.     

AsCl(3): from the crystalline to the liquid state. XRD (176 < T (K) < 250) and WAXS (295 K) studies

This paper presents structural studies on crystalline and liquid AsCl(3), performed using X-ray diffraction (XRD) and wide-angle X-ray scattering (WAXS) in the 176-250 K temperature range and at 295 K for the crystalline and liquid samples, respectively. The XRD results, collected using a single-crystal diffractometer, show that AsCl(3) crystallizes in the orthorhombic system with P2(1)2(1)2(1) space group and the unit cell parameters a = 9.475(3) A, b = 11.331(2) A, and c = 4.2964(8) A at 221 K. This structure is stable in the temperature range 176-243 K. Above the melting point, at 257 K, transition to the liquid state is observed. The WAXS data were recorded up to a maximum scattering vector K(max) = 16 A(-1) and then converted to real space by the sine Fourier transform, yielding to the reduced radial distribution function (RRDF). For a series of models, based on the crystalline AsCl(3) structure, the intensity and RRDF functions have been computed and compared with the experimental data. These simulations indicate that the model consisting of six AsCl(3) molecules, arranged along the y axis, accounts satisfactorily for the experimental observation. The results of the structure analysis in both crystalline and liquid states are discussed in relation to the influence of the As lone electron pair.     

Molecular structure,quantum chemical investigation,and thermal behavior of (DNAZ-CO)<Subscript>2</Subscript>

Bis-(3,3-dinitroazetidinyl)-oxamide ((DNAZ-CO)₂) is an acyl derivative of 3,3-dinitroazetidine (DNAZ). It is prepared and its crystal structure is determined. The crystal is orthorhombic, Fdd2 space group, a = 13.136(14) Å, b = 19.48(3) Å, c = 10.326(14) Å, V = 2642 (6) ų, Z = 8. A density functional theory (DFT) method of the Amsterdam Density Functional (ADF) package is used to calculate the geometry, frequencies, and properties. The optimized geometry, frontier orbital energy, and main atomic orbital percentage are obtained. The thermal behavior is studied under a non-isothermal condition by DSC and TG/DTG methods. The apparent activation energy (E _a) and pre-exponential factor (A) of the exothermic decomposition reaction of (DNAZ-CO)₂ are 164.10 kJmol^?1 and 10^13.38 s^?1 respectively. The critical temperature of thermal explosion is 272.20°C. The values of ΔS ^≠, ΔH ^≠, and ΔG ^≠ of this reaction are 6.44 Jmol^?1·K^?1, 163.76 kJmol^?1 and 160.34 kJmol^?1 respectively.     

Inelastic collisions in molecular nitrogen at low temperature (2 < or = T < or = 50 K)

Theory and experiment are combined in a novel approach aimed at establishing a set of two-body state-to-state rates for elementary processes ij --> lm in low temperature N(2):N(2) collisions involving the rotational states i,j,l,m. First, a set of 148 collision cross sections is calculated as a function of the collision energy at the converged close-coupled level via the MOLSCAT code, using a recent potential energy surface for N(2)-N(2). Then, the corresponding rates for the range of 2 < or = T < or = 50 K are derived from the cross sections. The link between theory and experiment, aimed at assessing the calculated rates, is a master equation which accounts for the time evolution of rotational populations in a reference volume of gas in terms of the collision rates. In the experiment, the evolution of rotational populations is measured by Raman spectroscopy in a tiny reference volume (approximately 2 x 10(-3) mm(3)) of N(2) traveling along the axis of a supersonic jet. The calculated collisional rates are assessed experimentally in the range of 4 < or = T < or = 35 K by means of the master equation, and then are scaled by averaging over a large set of experimental data. The scaled rates account accurately for the evolution of the rotational populations measured in a wide range of conditions. Accuracy of 10% is estimated for the main scaled rates.     

Radiolysis of aqueous solutions of poly(vinyl alcohol) at 77 K

Paramagnetic products of low-temperature X-ray radiolysis of aqueous poly(vinyl alcohol) solutions (2.5 and 5% by weight) were studied by ESR spectroscopy. Experimental spectra were ascribed to a superposition of signals from hydroxyl radicals and –CH₂?^?C(OH)–CH₂? macroradicals (C_α-macroradicals), respectively. No ESR signals corresponding to trapped electrons were observed that was attributed to the peculiarities of microheterogenous structure of the frozen aqueous polymer solutions. Annealing at 115 K resulted in partial conversion of OH radicals to C_α-macroradicals. It was suggested that main part of hydroxyl radicals was stabilized in phase of polycrystalline ice while macroradicals were formed in “mixed” water–polymer phase. The radiation–chemical yields of paramagnetic species stabilized in the systems under study were determined.     

Synthesis,crystal structure,and thermal decomposition kinetics of complex [Nd(BA)<Subscript>3</Subscript>bipy]<Subscript>2</Subscript>

The complex of [Nd(BA)₃bipy]₂ (BA = benzoic acid; bipy = 2,2′-bipyridine) has been synthesized and characterized by elemental analysis, IR spectra, single crystal X-ray diffraction, and TG/DTG techniques. The crystal is monoclinic with space group P2(1)/n. The two–eight coordinated Nd³⁺ ions are linked together by four bridged BA ligands and each Nd³⁺ ion is further bonded to one chelated bidentate BA ligand and one 2,2′-bipyridine molecule. The thermal decomposition process of the title complex was discussed by TG/DTG and IR techniques. The non-isothermal kinetics was investigated by using double equal-double step method. The kinetic equation for the first stage can be expressed as dα/dt = A exp(−E/RT)(1 − α). The thermodynamic parameters (ΔH ^≠, ΔG ^≠, and ΔS ^≠) and kinetic parameters (activation energy E and pre-exponential factor A) were also calculated.     

Unusual low-temperature thermodynamic properties of pellet samples of (DMe-DCNQI)<Subscript>2</Subscript>M (<Emphasis Type="Italic">M</Emphasis>=Li,Ag)

Summary We have performed low-temperature heat capacity measurements on pellet samples of (DMe-DCNQI)₂M (M=Li, Ag) which is known to show spin-Peierls transitions at 51 and 86 K, respectively. A linearly temperature-dependent term in the low-temperature heat capacity has been observed for both the samples: It is attributable to the spin-wave excitations induced by the inhomogeneous pressure effects produced in the pellet forming process. Although the temperature dependences of the magnetic susceptibility in both materials are almost the same, the coefficient of T-linear term of the Ag salt becomes three times larger that that of the Li salt. The peculiar electronic state originating from the competition of the spin-Peierls mechanism and the Coulomb repulsion is suggested.     

[Co(solv)<Subscript>6</Subscript>][B<Subscript>10</Subscript>H<Subscript>10</Subscript>] (solv = DMF and DMSO) for low-temperature synthesis of borides

The synthesis and structure of complexes [Co(solv)₆][B₁₀H₁₀] (solv = DMF and DMSO) have been reported. Both complexes have been prepared in a high yield by the reaction between cobalt(II) salts and closo-decaborates Cat₂[B₁₀H₁₀] in the corresponding solvent. The complexes have been characterized by elemental analysis, IR and UV spectroscopy, X-ray powder diffraction, and X-ray crystallography. The thermal properties of the compounds have been studied in the temperature range 20–600°C under argon. The conditions to form cobalt borides have been determined based on the results of thermal analysis, subsequent annealing of the complexes in various conditions, and analysis of IR spectra of the resulting thermolysis products.     

J(77Se,1H) and J(77Se,13C) couplings of seleno‐carbohydrates obtained by 77Se satellite 1D 13C spectroscopy and 77Se selective HR‐HMBC spectroscopy

Seleno‐carbohydrates are those in which the oxygen of the glycosidic bond or the hydroxyl group is artificially replaced with selenium. This substitution changes ¹H and ¹³C chemical shifts and produces spin coupling constants involving ⁷⁷Se. Coupling constants, such as ^2‐3J(⁷⁷Se, ¹H), are likely to be useful for conformational analyses of glycans because such couplings are never observed in natural glycans. Several papers have discussed the relationship between ^2‐3J(⁷⁷Se, ¹H) and conformation; however, only few reports describe ^1‐3J(⁷⁷Se, ¹³C), which could also be useful. Here, we obtain ⁷⁷Se coupling constants of seleno‐carbohydrates from ⁷⁷Se‐selective HR‐HMBC and ⁷⁷Se satellites in 1D ¹³C spectra and examine their conformations using the Newman projection scheme.     

Synthesis,spectral and thermal studies of zinc(II) and mercury(II) complexes with bidentate Schiff-base ligand (234-MeO-Ba)<Subscript>2</Subscript>En: the crystal structure of Zn(234-MeO-Ba)<Subscript>2</Subscript>En)I<Subscript>2</Subscript>

New Symmetric bidentate Schiff-base ligands N,N′-bis(2,3,4-trimethoxybenzylidene)-1,2-di-aminoethane, (234-MeO-Ba)₂En, and its corresponding zinc(II) and mercury(II) complexes, Zn((234-MeO-Ba)₂En)I₂ (I), Hg((234-MeO-Ba)₂En)Cl₂ (II) have been synthesized and characterized by elemental analyses (CHN), FT-IR and 1H NMR spectroscopy. The thermal behaviors of complexes were study using thermogravimetry in order to evaluate their thermal stability and thermal decomposition pathways. The crystal structure of I was determined from single-crystal X-ray diffraction. The coordination polyhedron about the zinc(II) center in complex I is best described as a distorted tetrahedron.     

Electronic structures,chemical bonding,and defect formation in mixed cyanoferrates M<Subscript>2</Subscript>Cu[Fe(CN)<Subscript>6</Subscript>] (M = Na,K, Rb,and Cs)

The effect of the alkali metal nature on the electronic structures and chemical bonding in mixed cyanoferrates M₂Cu[Fe(CN)₆] (M = Na, K, Rb, and Cs) was studied by ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method (in the spin-polarized implementation) and the extended Hückel molecular orbital (EHMO) method. It was found that the X-ray photoelectron spectra of the ferrimagnetic compounds Na₂Cu[Fe(CN)₆] (I), K₂Cu[Fe(CN)₆] (II), Rb₂Cu[Fe(CN)₆] (III), and Cs₂Cu[Fe(CN)₆] (IV) are similar. The magnetic moments on Cu²⁺ and iron ions remain virtually constant in compounds I–IV (μ(Cu) = 0.9 μ_B, μ(Fe) < ?0.06 μ_B). Analyses of the electron density maps and the bond overlap populations showed that the cubic frameworks of cyanoferrates are built from stable fragments ?-Fe-C≡N-Cu-?. The bond strength in these fragments decreases substantially in the order C-N → Fe-C → Cu-N and only slight in the order IV → III → II → I. The calculated total energies of the cyanoferrates Cs_2?x Cu[Fe(CN)₆], CsHCu[Fe(CN)₆], and NaHCu[Fe(CN)₆] for different concentrations and configurations of defects (cesium vacanices and hydrogen substitution defects) suggest mutual repulsion of defects. This repulsion is responsible for the experimentally observed lowering of the ionic conductivity with an increase in the defect concentration in the mixed cyanoferrates.     

Sol-gel processing of lead-free (Na,K)NbO<Subscript>3</Subscript> ferroelectric films

The sol-gel processing of lead-free (Na,K) NbO₃ ferroelectric films was studied. Sodium ethoxide (NaOC₂H₅) and potassium ethoxide (KOC₂H₅) were prepared by reacting solid Na and K with ethanol (99.7%) in a solvent of 2-methoxyethanol. 0.5-μm-thick (Na,K)NbO₃ thin films with orthorhombic perovskite structure were obtained by pyrolyzing at 400°C and annealing at 800–900°C. The films had relatively dense and uniform microstructure with grain size of about 50 nm, whose ferroelectricity was proved by the P-E hysteresis loop measurement. It was found that excess K was effective to reduce the annealing temperature for the crystallization of sol-gel-derived (Na,K)NbO₃ thin films.     

Radiolysis of Aqueous Solutions of Poly(ethylene oxide) at 77 K

Low-temperature (77 K) γ-radiolysis of aqueous solutions of poly(ethylene oxide) was studied by ESR spectroscopy. Trapped electrons, hydroxyl radicals, and -CH₂-ĊH-O- radicals were identified as principal paramagnetic products of radiolysis. It was shown that an increase in the polymer concentration in solution led to a growth in number of trapped electrons and -CH₂-ĊH-O- macroradicals. The decay of hydroxyl radicals occurring at 110–115 K was accompanied by the formation of -CH₂-ĊH-O- macroradicals and partial recombination of hydroxyl radicals. Action of visible light resulted in virtually quantitative transformation of trapped electrons into -CH₂-ĊH-O- macroradicals. The radiation-chemical yields of the species trapped at 77 K were estimated and the scheme of radiation-induced chemical processes was suggested.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 4, 2005, pp. 243–249.Original Russian Text Copyright © 2005 by Zakurdaeva, Nesterov, Feldman.     

[Co(NH<Subscript>3</Subscript>)<Subscript>6</Subscript>](WO<Subscript>4</Subscript>)Cl: Synthesis,crystal structure,and thermal properties

The crystal structure of [Co(NH₃)₆](WO₄)Cl complex salt is determined by single crystal X-ray. The thermal properties are examined, and the products obtained on heating the salt in different gaseous atmospheres are analyzed by powder X-ray diffraction.     

Polymorphism and thermal decomposition of [Ni(DMSO)<Subscript>4</Subscript>]I<Subscript>2</Subscript>

Tetrakis(dimethyl sulphoxide)nickel(II) bis(iodide) was studied by thermogravimetry (TG) and simultaneous differential thermal analysis (SDTA) and differential scanning calorimetry (DSC). The gaseous products of the decomposition were on-line identified by a quadrupole mass spectrometer (QMS). Thermal decomposition of the title compound proceeds in three main stages. In the first stage, which starts just above ca. 419 K, the compound loses two dimethyl sulphoxide (DMSO) molecules per one formula unit and small amount of iodide ion. In the second stage (464–552 K) the next DMSO ligands and the iodide ion simultaneously are released. In the last stage (552–900 K) NiSO4 is created which next decomposes to NiO and SO₃.