Polymorphic transformations of C26H54 and C28H58 n-paraffins as typical rotator substances

Thermal deformations and polymorphic transformations of two long-chain evennumbered normal paraffins C₂₆H₅₄ and C₂₈H₅₈ were studied by thermal X-ray diffraction (temperature step is tenths of a degree), infrared spectroscopy (temperature step is 1–5°), and differential scanning calorimetry (temperature step is 2° in a sample heating melt cooling mode. The samples are characterized by high homologous purity (99.0%) and belong to so-called “boundary” n-paraffins. The starting C₂₆H₅₄ n-paraffin sample is a triclinic modification at room temperature (Tc _cryst). When quickly cooled, the melt crystallizes as the triclinic Tc _cryst and monoclinic monolayer 1M _cryst forms (two-phase mixture Tc _cryst + 1M _cryst). The starting C₂₈H₅₈ n-paraffin sample is a double-layer monoclinic modification 2M _cryst at room temperature. Crystallization from hexane or slow cooling of a melt leads to a monolayer monoclinic form 1M _cryst. Thermal deformations and temperature ranges of existence of the crystalline forms (Tc _cryst, 1M _cryst, and 2M _cryst), low-temperature rotator crystalline orthorhombic form (Or _rot.1), and high-temperature rotator crystalline hexagonal (H _rot.2) phases of these n-paraffins were evaluated from changes in their diffraction patterns and unit cell parameters. The molecular structure and the conformational composition of these n-paraffins in different states were found from their IR spectra. Differential scanning calorimetry (DSC) was used to determine the phase transition temperature. Thermal X-ray diffraction, IR, and DSC data agree well with one another.     

Structural and vibrational investigation of para-nitraminopyridine N-oxide. A combined experimental and theoretical studies

The X-ray and vibrational spectroscopic analysis of para-nitraminopyridine N-oxide are reported. The crystals of investigated compound belong to P2(1) of the monoclinic system, Z=4, a=3.735 A, b=11.767 A, c=14.679 A and beta=93.27 degrees . Room temperature powder infrared and Raman spectra of the title compound and its deuterated analogue were measured. The molecular structure of p-nitraminopyridine N-oxide has been calculated with the aid of density functional (B3LYP) method with the extended 6-311++G(d,p) basis set. The calculated geometrical parameters of investigated molecule in gas phase were compared with experimental X-ray data. The harmonic frequencies, potential energy distribution (PED) and IR intensities of p-nitroaminopyridine N-oxide and its deuterated analogue were calculated with B3LYP method. The assignment of the experimental spectra has been made on the basis of the calculated PED. The time depend Hartree-Fock (TDHF) method was used for calculations of hyperpolarizability beta coefficient.     

The surface properties of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane after partial solution in nitro esters

The thermophysical properties and surface structure of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (TNCO) particles before and after their contact with liquid nitro esters were studied by differential scanning calorimetry and small-angle X-ray scattering. The thermophysical properties of TNCO changed after contact with nitro esters, and the temperature range of the transition from the β to δ polymorph broadened. This peculiarity was explained in terms of the fracton theory by changes in the heat capacity of particles related to the formation of nanoscale roughness with a fractal (scale-invariant) organization as a result of surface etching by solvents such as nitro esters, which are thermodynamically “bad” with respect to TNCO. The volume properties of particles did not change. The possibility of determining the fractal dimension of the surface of TNCO particles by differential scanning calorimetry was demonstrated. The rate of heating was then used as a “scale ruler” (“yardstick”), and the temperature range of the transition from the β to δ polymorph, as a measured parameter. The dimension of the surface fractal was also determined by an independent method (small-angle X-ray scattering). The results obtained by the two methods were closely similar.     

Magnetocaloric effect in nano- and polycrystalline manganite La0.7Ca0.3MnO3

La_0.7Ca_0.3MnO₃ samples were prepared in nano- and polycrystalline forms by the sol–gel and solid state reaction methods, respectively, and structurally characterized by synchrotron X-ray diffraction. The magnetic properties determined by ac susceptibility and dc magnetization measurements are discussed. The magnetocaloric effect in this nanocrystalline manganite is spread over a broader temperature interval than in the polycrystalline case. The relative cooling power of the poly- and nanocrystalline manganites is used to evaluate a possible application for magnetic cooling below room temperature. PACS 75.30.Sg; 75.47.Lx; 77.80.Bh     

Asymmetric Epoxidation of Olefins with Sodium Percarbonate Catalyzed by Bis-amino-bis-pyridine Manganese Complexes

Asymmetric epoxidation of a series of olefinic substrates with sodium percarbonate oxidant in the presence of homogeneous catalysts based on Mn complexes with bis-amino-bis-pyridine ligands is reported. Sodium percarbonate is a readily available and environmentally benign oxidant that is studied in these reactions for the first time. The epoxidation proceeded with good to high yields (up to 100%) and high enantioselectivities (up to 99% ee) using as low as 0.2 mol. % catalyst loadings. The epoxidation protocol is suitable for various types of substrates, including unfunctionalized alkenes, α,β-unsaturated ketones, esters (cis- and trans-), and amides (cis- and trans-). The reaction mechanism is discussed.     

Phase transition in non-centrosymmetric 2-methyl-5-nitroanilinium dihydrogen phosphate: structural,spectroscopic and optical studies

Structural Chemistry - A reversible phase transition in 2-methyl-5-nitroanilinium dihydrogen phosphate, (H2m5na)H2PO4, has been found and characterized. Lattice parameters of (H2m5na)H2PO4 change...     

The Raman spectroscopic studies of aligned MWCNTs treated under high pressure and high temperature

The study of the aligned multiwalled carbon nanotubes (MWCNTs) for interlinking bonding under high pressures and temperatures have been conducted in the diamond anvil cell. The MWCNT samples were analyzed using the Raman spectroscopy, when treated under the combinations of pressure and temperature ranges of 2-20 GPa and 25-500 °C. The analyses show the formation of interlinking bonding at a pressure above 2.5 GPa when treated under the temperature 500 °C, based on the significant change of the relative intensity between D- and G-bands in the Raman spectra. Comparisons of the data obtained before and after the high pressure and high temperature treatments are reported. The result indicates that the aligned MWCNTs may be easier to form the interlinking bonding compared to randomly oriented MWCNTs.     

Oxyethylated cellulose sulfates

Cellulose derivatives containing poly(oxyethylene oxide) chains in their structural units are prepared by the oxyalkylation of cellulose sulfates in the presence of an alkaline catalyst. The degrees of sulfation of oxyethylated cellulose sulfates vary from 0.8 to 2.0, and the molar substitutions with ethylene glycol residues are in the range from 3 to 10. The NMR study of the products of complete hydrolysis of the compounds of interest shows that oxyethyl groups predominantly add at second and third positions of the anhydrous glucose unit of cellulose. It is found that the oxyethylation of cellulose sulfates decreases the reduced viscosity of their aqueous solutions. The compounds under consideration exhibit inhibitory ability with respect to the blood coagulation, that is, antithrombin activity, which varies in the range 0.5 to 34.4 units/mg.