Critical behavior and magnetocaloric effect in La0.67Ca0.33Mn1−xCrxO3 (x = 0.1, 0.25)

Universal behavior of the magnetocaloric effect along with structural and critical exponent analysis in mixed manganite La_0.67Ca_0.33Mn_1−xCr_xO₃ (x = 0.1, 0.25), [LCMCr_0.1 and LCMCr_0.25] exhibiting second order phase transition are investigated. Structural study using Reitveld refinement of XRD patterns indicates orthorhombic structure with Pnma space group. Modified Arrott plot method has been adopted to study the critical behavior of the compounds at their transition region, which gives values of β = 0.555(6), γ = 1.17(4) and δ = 2.7096(7) at T_C = 232.5 K for LCMCr_0.1 and β = 0.68 (1), γ = 1.09(3) and δ = 2.9362(4) at T_C = 202.5 K for LCMCr_0.25. The values are close to those expected for mean field ferromagnets with long range order. With increase in Cr content, the temperature corresponding to the maximum entropy change as well as the magnetic transition temperature gradually shifts to low temperatures. The maximum magnetic entropy change was found to be 3.5 J/kg K for x = 0.1 and 2.2 J/kg K for x = 0.25 for a field change of 5 T. The field dependence of the magnetic entropy change is also analyzed, which shows the power law dependence namely, ?S_M ∝ Hⁿ, n = 0.9086(5) at T_C = 232.5 K and n = 0.849(7) at T_C = 202.5 K for LCMCr_0.1 and LCMCr_0.25 respectively. Relative cooling power was found to be about 147 J/kg for LCMCr_0.1 and 88 J/kg for LCMCr_0.25. The field dependence of the relative cooling power for both the compounds shows a H^1+1/δ dependence with the δ values in agreement with the mean field model.     

Hydrothermal synthesis of TiO2 anatase nanocrystals using hexaprismatic-shaped oxo-carboxylate complexes

The molecular structural chemistry of titanium oxo-carboxylate complexes is critically reviewed with particular emphasis on the understanding of the various characterised molecular shapes. Among the four crucial parameters thought to have a structural influence on these shapes (nature of the OR alkoxy group, molar ratio C = O/Ti, stoichiometric ratio S = XCOOH/Ti and chemical nature of the carboxylic X moiety), it was concluded that the nature of R group, the C ratio and the steric hindrance of X were of minor importance. From the available data, it was also concluded that a high S ratio seems to favour corner-sharing versus edge-sharing of TiO₆ octahedra and that it was not possible to establish a clear correlation between the pK_a value of the carboxylic acid and the observed molecular shape. During this study, a new titanium oxo-carboxylate complex was obtained by reacting titanium isopropoxide with phenylacetic acid (PAAH, X = PhCH₂ at S = 3/2). This hexaprismatic Ti₆O₆(OPrⁱ)₆(PAA)₆ complex was characterised by single-crystal X-ray diffraction and used as a molecular precursor of TiO₂ anatase nanocrystals after hydrolysis by NMe₄OH and autoclaving at T = 200 °C. The size and size distribution of these nanocrystals were found to decrease as the R = Ti/NMe₄ ratio was increased. Nucleation and growth of anatase nanocrystals was found to be deeply modified by the presence of phenylacetate ions in the solution, but there was no evidence of an influence of the molecular anisotropy of the molecular precursor on the final shape of the nanocrystals. To cite this article: A. Rammal et al., C. R. Chimie 5 (2002) 59–66     

Synthesis,characterization, and structures of arylaluminum reagents and asymmetric arylation of aldehydes catalyzed by a titanium complex of an N-sulfonylated amino alcohol

A series of phenylaluminum reagents AlPh_xEt_3?x(L) (x = 1–3) containing adduct ligand L [Et₂O, THF, OPPh₃, or 4-dimethylaminopyridine (DMAP)] were synthesized and characterized. NMR studies showed that AlPh_xEt_3?x(L) (x = 1 or 2) exists as an equilibrium mixture of 3–4 species in solution. Solid-state structures of the phenylaluminum reagents reveal a distorted tetrahedral geometry. Asymmetric additions of phenylaluminum to 2-chlorobenzaldehyde were examined employing a titanium(IV) complex [TiL¹(OPrⁱ)₂]₂ 10 (H₂L¹ = (1R,2S)-2-(p-tolylsulfonylamino)-1,3-diphenyl-1-propanol) as a catalyst precursor. It was found that the adduct ligand L had a strong influence on the reactivity and the enantioselectivity in asymmetric phenyl additions to aldehydes. The phenylaluminum reagents with OPPh₃ or DMAP were unreactive toward aldehydes, and AlPh₃(THF) was found to be superior to AlPh₃(OEt₂) or AlPhEt₂(THF). Asymmetric aryl additions of AlAr₃(THF) to aldehydes employing a loading of 5 mol % titanium(IV) complex 10 with a strategy of a slow addition of the aldehydes over 20 min were conducted, and the reactions produced optically active secondary alcohols in high yields with excellent enantioselectivities of up to 94% ee.     

Zirconium complexes containing a diarylamido diphosphine ligand: Structural preferences controlled by ligand electronics and sterics

This work describes the preparation of [PNP]ZrX₃ ([PNP]⁻ = [N(o-C₆H₄PⁱPr₂)₂]⁻; X = Cl, Me, CH₂SiMe₃) whose structural preference is found to be a function of the electronic and steric nature of the monodentate ligand X⁻. The reaction of ZrCl₄(THF)₂ with [PNP]Li in toluene at room temperature generates [PNP]ZrCl₃ as a red solid in 60% yield. Alkylation of [PNP]ZrCl₃ with three equivalents of Grignard reagents in diethyl ether at −35 °C produces cleanly [PNP]ZrR₃ (R = Me, CH₂SiMe₃) as yellow crystalline materials. An X-ray diffraction study of [PNP]ZrCl₃ showed it to be a chloride-bridged binuclear species {[PNP]ZrCl₂(μ−Cl)}₂ in which both zirconium atoms are 7-coordinate whereas that of [PNP]ZrMe₃ revealed a mononuclear, 6-coordinate core structure. Interestingly, with the incorporation of more sterically demanding alkyls, [PNP]Zr(CH₂SiMe₃)₃ is a 5-coordinate compound wherein the amido phosphine ligand is κ²-N,P bound to zirconium. The solution structures of these molecules were also assessed by variable-temperature NMR spectroscopy.     

Effect of electron beam irradiation on the conduction phenomena of unplasticized PVC/PVA copolymer

The effect of electron beam irradiation on the conduction phenomenon of unplasticized PVC/PVA copolymer has been investigated. The current–voltage (J–V) characteristics in the voltage range 0.1–60 V were measured for films irradiated with different doses; 150, 550 and 1100 kGy. The temperature dependence of the J–V characteristics in the temperature range 303–343 K was obtained. The results indicated that the conduction as a function of the applied voltage depends on the presence of localized state or the trapping levels positioned at a specific energy E_t below the conduction band. Therefore, the charge carrier's concentration in the conduction band, trapping parameter θ, electron mobility μ_0, effective electron drift mobility μ_e as well as Fermi level energy E_f and trapping energy E_t were estimated as a function of dose.     

On the explosive properties of 1H-benzotriazole and 1H-1,2,3-triazole

For 1H-benzotriazole, no explosive properties are observable, but the relative high exothermic decomposition energy of 1590 J/g should be kept in mind. Nevertheless, an endothermic melting barrier at 100 °C ensures safe handling at lower temperatures. For 1H-1,2,3-triazole, the exothermic decomposition energy is as high as 2600 J/g, but explosive properties are also not detectable. Therefore, both reagents are hazardous with regard to the exothermic decomposition potential and can be handled safely with precautions.     

Structural transformation and oxidation of Sr2MnO3.5+x determined by in-situ neutron powder diffraction

The oxidation of the n = 1 Ruddlesden-Popper phase, Sr₂MnO_3.5+x, where 0 ≤ x ≤ 0.5 has been investigated using a combination of in-situ diffraction techniques. In agreement with previous reports the room temperature structure of Sr₂MnO_3.5+x was determined to be monoclinic crystallising in space group P2₁/c. On heating in air the material undergoes rapid oxidation at a relatively modest temperature, ∼275 °C. The oxidation process is coincident with a significant change in the structure, with the material now adopting a tetragonal I4/mmm structure. In the oxygen deficient phase where x > 0 the Mn coordination is square pyramidal, with a sixth partially occupied oxygen position giving rise to octahedral coordination. Oxidation of Sr₂MnO_3.5+x results in the filling of the partially occupied O4 positions and a resulting increase in symmetry, with the Mn coordination now adopting solely a distorted octahedral environment.     

A new method for making acyl fluorides using BrF3

While many carboxylic acids could be converted directly to acyl fluorides by using BrF₃, the reaction with acyl chlorides was found to be of a more general nature and yields better results. Surprisingly, reacting t-butyl esters with bromine trifluoride also resulted in acyl fluorides in reasonable yields. The reactions were completed in a few seconds at 0 °C.     

Ferromagnetic to spin glass cross over in (La,Tb)2/3Ca1/3MnO3

In the series La_2/3?xTb_xCa_1/3MnO₃, it is known that the compositions are ferromagnetic for smaller values of x and show spin glass characteristics at larger values of x. Our studies on the magnetic properties of various compositions in the La_2/3?xTb_xCa_1/3MnO₃ series show that the cross over from ferromagnetic to spin glass region takes place above x ≈ 1/8. Also, a low temperature anomaly at 30 K, observed in the ac susceptibility curves, disappears for compositions above this critical value of x. A mixed phase region coexists in the narrow compositional range 0.1 ≤ x ≤ 0.125, indicating that the ferromagnetic to spin glass cross over is not abrupt.     

Berberine isolated from Mahonia nepalensis as an eco-friendly and thermally stable corrosion inhibitor for mild steel in acid medium

BackgroundThe environmental and economic benefits have been the driving force in search of efficient corrosion inhibitors for iron/steel used in industrial acidic medium. This study reports on berberine isolated from methanol extract of high-altitude (1347 m) shrub Mahonia nepalensis as a highly efficient and thermally stable corrosion inhibitor for mild steel (MS) in 1.0 M H₂SO₄ simulating acid pickling condition.MethodsThe weight-loss and electrochemical methods revealed the fast adsorption of berberine.Significant Findings: It achieved above 91% inhibition efficiency (IE) in 0.25 h and reached 94% in 6 h for 1000 ppm berberine. The IE increased with concentration and temperature, giving an IE of 97.2% at 328 K, which makes it a promising candidate for industrial application. It behaved as a mixed type of inhibitor as revealed by open circuit potential and polarization curves. The results indicated suppression of the corrosion by effectively forming an adsorbed berberine layer on the MS surface. Adsorption of the berberine followed a Langmuir adsorption isotherm. The thermodynamic parameters such as activation energy (43.19 kJ/mol), free energy (−35.05 kJ/mol), enthalpy (40.55 kJ/mol), and entropy (−97.83 J/molK) of adsorption supported both physical and chemical interactions of berberine with MS surface. The obtained results also revealed that the adsorption process was endothermic and spontaneous in nature.     

Stereoselective generation of cis-2-lithio-3-CF3-oxirane via CF3-substituted β-oxido carbenoids. Highly stereoselective synthesis of CF3-substituted tri- and tetrasubstituted oxiranes and tetrasubstituted alkenes

Treatment of 2-substituted 3,3-dichloro-1,1,1-trifluoropropan-2-ols with organolithium reagents R²Li in THF at −98°C stereoselectively produces 2,3-disubstituted 2-lithio-3-trifluoromethyloxiranes with Li and CF₃cis. The reagents react with electrophiles El-X or organoboranes R³BR₂ to give CF₃-containing tri- and tetrasubstituted oxiranes or tetrasubstituted alkenes, respectively, with high diastereoselectivities.     

Multi-component pharmacokinetics assessment of Artemisia annua L. in rats based on LC-ESI-MS/MS quantification combined with molecular docking

Artemisia annua L. (A. annua) has been used as herbal medicine in China for thousands of years for clearing deficiency heat, treating malaria and removing jaundice. A rapid, sensitive and specific liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC–ESI–MS/MS) method was developed, validated, and successfully used for simultaneous quantification of the active components in rat plasma after oral administration of A. annua extract. Molecular docking of each component with drug metabolizing enzymes was carried out to explore the effect of each component on CYP-mediated drug metabolism. Two coumarins (scopolin (SPL) and scopoletin (SPLT)), three flavonoids (rutin (RUT), chrysosplenol D (CHD), casticin (CAS)) and three sesquiterpenes (arteannuin B (ARN), dihydroartemisinic acid (DARM) and artemisinic acid (ARM)) were detected in rat plasma after oral administration. CHD and CAS were rapidly absorbed into rat blood with the T_max values of 0.11 ± 0.04 h and 0.13 ± 0.05 h, respectively. Their half-lives (t_1/2 2.68 ± 3.62 h and 0.33 ± 0.07 h) were shorter. SPLT were also rapidly absorbed into the blood (T_max 0.15 ± 0.03 h), but exhibited a longer half-life (t_1/2 6.53 ± 1.84 h), indicating that it could be effective in vivo for a longer period of time. The peak time of SPL, RUT, DARM and ARM ranged from 1 ～ 4 h, demonstrating that they could maintain considerable concentrations for a longer time. ARN showed strong enterohepatic circulation in rats, leading to slower onset time and longer effect. A few components including SPLT, CHD, CAS and ARN could be metabolized into their corresponding II phase metabolites combining with glucuronic acid or sulfuric acid. RUT could decompose its glycosyl to generate genin. The molecular docking results indicated that those flavonoids and coumarins of A. annua interacting with CYPs mainly through hydrogen bonding and π-π stacking had better CYP450 enzyme binding ability than the sesquiterpenoids, which were easier to induce drug interactions. This study presented an integrated strategy for investigating the pharmacokinetic behaviors of eight components in A. annua and laid the foundation for revealing the mechanism of action of A. annua in the organism.     

Energy decomposition analysis of the metal-oxime bond in [M{RC(NOH)C(NO)R}2] (M = Ni(II), Pd(II), Pt(II), R = CH3, H, F, Cl, Br, Ph, CF3)

Quantum chemical calculations using gradient-corrected DFT at the BP86/TZ2P+ level were carried out for the metal-dioxime complexes [M{RC(NOH)C(NO)R}₂]with M = Ni, Pd, Pt, R = CH₃, H, F, Cl, Br, Ph, CF₃. The nature of the metal-ligand bond was investigated with an energy decomposition analysis (EDA). The complexes with electron donating substituents R = H, CH₃ have the strongest metal-ligand interaction energies ΔE_int, as well as the largest bond dissociation energies. The analysis of the bonding situation revealed that the metal ← ligand σ donation is much stronger than the metal → ligand π backdonation. The breakdown of the orbital interactions into the contributions of orbitals with different symmetry indicates that the donation from the in-plane lone-pair donor-orbitals of nitrogen into the d_xy AO of the metal provides about one half of the stabilization which comes from ΔE_orb. Inspection of the EDA data indicates that the electrostatic term ΔE_elstat is more important for the trend of the metal-oxime interactions in [M{RC(NOH)C(NO)R}₂] than the orbital term ΔE_orb.     

Stabilization of the γ form of the rare earth sesquisulfides at lower temperature by doping with Calcium

The preparation of several samples forming a solid solution that can be formulated as Ca_(3/2)yR_2−y_{0.25−(1/2)y}S₃ (R = Ce, Sm, Gd) (0 ≤ y ≤ 0.30) is reported, together with their structural characterization, mainly through transmission electron microscopy. The introduction of Ca²⁺ into the rare earth metal sesquisulfide matrix stabilizes the γ form phase at 900 °C. This effect can be related to the non-stoichiometric nature of this phase, R_3−x_xS₄, because the introduction of Ca²⁺ requires the elimination of cation vacancies from the structure: 2R³⁺ + → 3Ca²⁺ (R = rare earth metal;  = cation vacancies). However, a NaCl-type solid solution is formed for R = Eu, formulated as Eu_1−yCa_yS. Well-ordered crystals are found in every sample, as it is revealed by transmission electron microscopy images and diffraction patterns. The color properties of the samples have been evaluated with reflectance spectra in the visible range and with L*–a*–b* coordinates.     

Enhanced Thermal Decomposition Properties and Catalytic Mechanism of Ammonium Perchlorate over CuO/MoS2 Composite

In this report, CuO/MoS₂ composites were successfully prepared by the hydrothermal method where nano‐sized CuO was uniformly distributed on the surface of hierarchical MoS₂ substrates (CuO/MoS₂ composites). Their physicochemical properties and catalytic performance in ammonium perchlorate (AP) decomposition were investigated and characterized by XRD, SEM, TEM, BET, XPS, TG/DSC and combustion measurement. The results showed that it could decrease AP decomposition temperature at high decomposition stage from 416.5 °C to 323.5 °C and increase the heat release from 378 J/g (pure AP) to 1340 J/g (AP with catalysts), which was better than pure CuO nanoparticles (345.5 °C and 1046 J/g). Meanwhile, it showed excellent performance in combustion reaction either in N₂ or air atmosphere. The results obtained by photocurrent spectra, photoluminescence spectra and time‐resolved fluorescence emission spectra indicated that loading CuO mediated the generation rate and combination rate of electrons and holes, thus tuning the catalytic performance on AP decomposition. This study proved that employing the supports that can synergistically interact with CuO is an efficient strategy to enhance the catalytic performance of CuO.     

Synthesis and characterization of LiCo3/5Mn2/5VO4 ceramics

Dielectric and a.c. conductivity properties of LiCo_3/5Mn_2/5VO₄ ceramic are investigated. This compound is prepared by solution-based chemical method and the formation is checked by X-ray diffraction (XRD) study. XRD analysis at room temperature shows an orthorhombic phase. Frequency dependence of dielectric constant (?_r) at different temperatures shows a dispersive behavior at low frequencies. Temperature dependence of ?_r at different frequencies indicates the transition temperature (T_c) = 235 °C, 245 °C, 257 °C and 265 °C with (?_r)_max. ～3689, 1373, 750 and 386 for 10, 50, 100 and 200 kHz respectively. A.c. conductivity analysis indicates that electrical conduction in the material is a thermally activated process.     

Experimental determination of the (vapor + liquid) equilibrium data of binary mixtures of fatty acids by differential scanning calorimetry

(Vapor + liquid) equilibrium (VLE) data for three binary mixtures of saturated fatty acids were obtained by differential scanning calorimetry (DSC). However, changes in the calorimeter pressure cell and the use of hermetic pans with holes (? = 250 mm) in the lids were necessary to make it possible to apply this analytical technique, obtaining accurate results with smaller samples and shorter operational times.The systems evaluated in this study were: myristic acid (C_14:0) + palmitic acid (C_16:0), myristic acid (C_14:0) + stearic acid (C_18:0), and palmitic acid (C_16:0) + stearic acid (C_18:0), all measured at 50 mm Hg and with mole fractions between 0.0 and 1.0 in relation to the most volatile component of each diagram. The fugacity coefficients for the components in the vapor phase were calculated using the Hayden and O’Connell method [J.G. Hayden, J.P. O’Connell, Ind. Eng. Chem. Process Design Develop. 14 (3) (1975) 209–216] and the activity coefficients for the liquid phase were correlated with the traditional g^E models (NRTL [H. Renon, J.M. Prausnitz, Aiche J. 14 (1968) 135–144], UNIQUAC [D.S. Abrams, J.M. Prausnitz, Aiche J. 21 (1975) 116–128], and Wilson [J.M. Prausnitz, N.L. Linchtenthaler, E.G. Azevedo, Molecular Thermodynamics of Fluid-phase Equilibria, River-Prentice Hall, Upper Saddle, 1999]). The sets of parameters were then compared in order to determine which adjustments best represented the VLE.     

Effect of molecular weight on radiation chemical degradation yield of chain scission of γ-irradiated chitosan in solid state and in aqueous solution

Chitosan A₁, A₂ and A₃ with molecular weight of 471, 207 and 100 kDa respectively, produced from squid pen chitin was degraded by gamma rays in the solid state and in aqueous solution with various doses in air at ambient temperature. Effect of molecular weight on radiation chemical degradation yield of chain scission and degradation rate constants of γ-irradiated chitosan in solid state and in aqueous solution was investigated. The radiation chemical degradation yield G_(s) and degradation rate values were calculated. The molecular weight changes were monitored by capillary viscometry method and the chemical structure changes were followed by UV analysis. The results showed that, the degradation of chitosan was faster in solution, than in solid state. The values of G_(s) in solid state and in aqueous solution were respectively 1.1×10^?8 mol/J and 0.074×10^?7 mol/J for A₁, 4.42×10^?8 mol/J and 0.28×10^?7 mol/J for A₂ and 6.08×10^?8 mol/J and 0.38×10^?7 mol/J for A₃. Degradation rate constants values ranged from 0.41×10^?5 to 2.1×10^?5 kGy^?1 in solid state, whereas in solution they ranged from 13×10^?5 to 68×10^?5 kGy^?1. The chitosan A₃ was more sensitive to radiolysis than A₁ and A₂. The chain scission yield, G_(s) and degradation rate constants seems to be greatly influenced by the initial molecular weight of the chitosan. Structural changes in irradiated chitosan are revealed by the apparition of absorption peaks at 261 and 295 nm, which could be attributed to the formation of carbonyl groups. In both conditions the peak intensity was higher in chitosan A₃ than in A₁ and A₂, the oxidative products decreased with increasing molecular weight of chitosan.     

Crystal structure of Pb2SbS2I3, and re-examination of the crystal chemistry within the group of (Pb/Sn/Sb) chalcogeno-iodides

The crystal structure of Pb₂SbS₂I₃ was solved at room temperature and 100 K. At 293 K it crystallizes in the orthorhombic system, space group Cmcm (No. 63), with unit cell parameters a = 4.3262(9), b = 14.181(3), c = 16.556(3) Å, V = 1017.7(4) Å³, Z = 4. The structure is disordered, and combines a split Pb site (s.o.f. = 0.50) with one mixed (Pb,Sb) site with Pb and Sb in two distinct sub-positions. At 100 K, it is monoclinic, space group P2₁/c, with unit cell parameters a = 7.3629(6), b = 16.466(3), c = 8.5939(7) Å, β = 107.14(2)°, V = 995.6(2) Å³, Z = 4. The structure is now fully ordered, without mixed sites. On the basis of bond valence calculations, new cation distributions are proposed for published structures of the Sn isotypes, Sn₂SbS₂I₃ and Sn₂SbSe₂I₃. A re-examination of the crystal structures of various (Pb/Sn/Sb) chalcogeno-iodides is presented according to modular analysis. All these structures can be described according to three types of 1D modules, (Pb/Sn)I₄, (Sn)₂I₄ and (Pb/Sn/Sb)₄(S/Se)₂I₄. Generally each type of 1D module gives one type of slab, and the final structure corresponds to a specific stacking of two or three among these slabs. A new structural model is proposed for “α-Sn₂SI₂”, which would have the non-stoichiometric composition (Sn_5.42□_0.58)S₂(I_6.87□_0.12), ideally Sn₂₇S₁₀I₃₄, with probably a narrow solid solution field on the SnS–SnI₂ joint.     

NaBH3CN and other systems as substitutes of tin and silicon hydrides in the light or heat-initiated reduction of halosugars: a tunable access to either 2-deoxy sugars or 1,5-anhydro-itols

UV light-promoted reduction of acetobromoglucose by NaBH₃CN in t-BuOH afforded 1,3,4,6-tetra-O-acetyl-2-deoxy-α-d-arabino-hexopyranose in high yield and purity, via a Surzur–Tanner rearrangement, while, with 10 mol % thiophenol added, acetylated 1,5-anhydro-d-glucitol was cleanly obtained. Such tin-free and mild reductions, presumed to proceed via radical pathways, were more efficient with NaBH₃CN compared to NaBH₄ or NaBD₄, and do not occur with acetochloroglucose. Similar reductions to 1,3,4,6-tetra-O-acetyl-2-deoxy-α-d-arabino-hexopyranose were achieved upon heating to 80 °C t-BuOH or CH₃CN solutions of NaBH₃CN and AIBN, but with a lower selectivity due to competing ionic reactions. With other pyranosyl bromides, reductions by NaBH₃CN could be tuned similarly (d-galacto), but some (d-manno, 5-thio-d-xylo) gave mainly or exclusively 1,5-anhydro-itols. Other conditions, or reagents promoting SET process, afforded also reduced products, but with lower rates or selectivities. Primary iodides were reduced readily with NaBH₃CN under UV light.