La cristallisation du polydioxolanne

Crystallization kinetics have been studied for two samples of polydioxolan (molecular weights 10.000 and 30.000). The crystallization was followed at temperatures between 0 and 21°C in a DSC calorimeter. The results obey Avrami's equation. The Avrami exponent was found to be two indicating a spontaneous and probably heterogeneous mode of nucleation. Over the temperature range studied, the crystallization lead to the growth of two-dimensional spherulites.The morphology of polydioxolan samples was studied by optical microscopy and small-angle light scattering. Samples crystallized in liquid nitrogen are made of small spherulites of the order of 5 μm. Samples crystallized between 0 and 21°C are made of large spherulites, of the order of 1 mm. Samples crystallized at 25 and 35°C show large and “abnormal” spherulites, made of two optical phases corresponding to the centre and the perimeter.No difference was seen between the morphologies of the two samples studied. In both cases, the time of half-crystallization was the same when plotted as a function of the degree of supercooling. Equilibrium melting points of 79 and 85°C were found for the low and high molecular weight samples, respectively.     

Theoretical study of the gas-phase chemiionization reactions La + O and La + O2

The La + O and La + O 2 chemiionization reactions have been investigated with quantum chemical methods. For La + O 2(X (3)Sigma g) and La + O 2(a (1)Delta g), the chemiionization reaction La + O 2 --> LaO 2 (+) + e (-) has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O 2(X (3)Sigma g) reaction conditions, chemielectrons are produced by La + O 2 --> LaO + O, followed by La + O --> LaO (+) + e (-). This is supported by the same chemielectron band, arising from La + O --> LaO (+) + e (-), being observed from both the La + O( (3)P) and La + O 2(X (3)Sigma g) reaction conditions. For La + O 2(a (1)Delta g), a chemielectron band with higher electron kinetic energy than that obtained from La + O 2(X (3)Sigma g) is observed. This is attributed to production of O( (1)D) from the reaction La + O 2(a (1)Delta g) --> LaO + O( (1)D), followed by chemiionization via the reaction La + O( (1)D) --> LaO (+) + e (-). Potential energy curves are computed for a number of states of LaO, LaO* and LaO (+) to establish mechanisms for the observed La + O --> LaO (+) + e (-) chemiionization reactions.     

La2—xSrxCuO4和La2—xNdxCuO4＋y的键共价性计算

使用复杂晶体化学键理论计算了Ｌａ２－ｘＳｒｘＣｕＯ４和Ｌａ２－ｘＮｄｘＣｕＯ４＋ｙ中各键的健共价性,讨论了键性随着掺杂的变化规律,研究表明,对于２１４结构,没有发现明显的化学键性与超导温度的关系,因此２１４结构中有关超导现象产生的机理还有待于进一步研究。     

Etude Polarographique De La Marcellomycine

Abstract

Polarographic study of marcellomycine.

d.c., a.c. and d.p. polarography and cyclic voltammetric techniques were used to investigate the electrochemical properties of marcellomycine. These are found to be quite similar to those of aclacinomycine or carminomycine, the main reversible two-electron wave being attributed tot he reduction of the quinonic structure. The molecule undergoes a degradation process starting in acidic medium and giving rise to the formation of two small irreversible waves at more negative potentials. This process is time, illumination and pH dependent. Important adsorption phenomena interfere in the investigated pH range but are lowered in very acidic medium. Quantitative measurements have been carried out at pH 1 and 6 in a 10^?3 to 10^?5 M concentration range using d.c. polarography. d.p. technique allows determinations in a restricted 10^?5 to 10^?4 M range. A detection limit of 1 · 10^?5 M has been fixed at pH 1.     

Magnetic Dilution of La1.85Sr0.15FeO4 and La1.85Ba0.15FeO4

Russian Journal of General Chemistry -     

Interactions of La(III) with anions in aqueous solutions. A139La NMR study

The interactions of the La(III) cations with three anions (X), nitrate, chloride and perchlorate, in aqueous solutions in the pH range 4.0–6.5, were studied by¹³⁹La NMR spectroscopy. A single model, involving the formation of the contact ion-pair (inner-sphere complex) (LaX)²⁺ was successfully and quantitatively applied to the chemical shift and the transverse relaxation rate data. Both measurements gave values for the thermodynamic equilibrium constants of formation of (LaX)²⁺ (K _th ) in good agreement (average K _th =0.45±0.05; 0.15±0.09; 0.03±0.01, respectively for nitrate, chloride and perchlorate). The complexes are characterized by chemical shifts of –25, 22 and –3.1 ppm and by transverse relaxation rates of 11.2, 5 and 1.65 kHz respectively for nitrate, chloride and perchlorate. The¹³⁹La quadrupolar relaxation rate is not controlled by the reorientational correlation time. This finding is discussed, and it is suggested that the very fast exchange of water molecules in the first coordination sphere of La(III) is responsible for the time fluctuation of the electric field gradient at the¹³⁹La nucleus site.     

La/ZSM-5分子筛热稳定性及镧存在形态研究

借助密度泛函方法,从理论上证实了当催化裂解工艺的催化剂(ZSM-5分子筛)中引入少量镧离子时,其热稳定性得到了显著的提高,主要是由于镧离子与分子筛之间发生了强的相互作用,使得最易发生断裂的键(Al-O键)变得更加牢固,从而有效地延缓了脱铝进程. La(OH)2＋的降解产物中,LaO(OH)的形成几率最大. LaO(OH)和分子筛作用可形成两种稳定的结构Z－ La(OH)2＋和Z-LaO(OH). 其中Z－La(OH)2＋是常温下的稳定构型,而Z-LaO(OH)是温度较高时的稳定构型,此构型中的镧离子可提供L酸作为反应的活性中心,为探索催化反应机理提供了有力的理论依据.     

Structure and Bonding of La2NiBi

La₂NiBi was synthesized by heating a cold pressed pellet of the elements in a sealed and evacuated silica tube at 1070 K. The structure was determined via powder and single crystal X‐ray diffraction. La₂NiBi crystallizes orthorhombically, in the space group Pnma: a = 838.88(6), b = 455.61(11), c = 1210.4(2) pm and V = 0.46261(14) nm³ (wR = 0.1002, 1001 F² values, 26 variables, Z = 4). La₂NiBi represents a higher congener of La₂NiSb and adopts a ternary ordered version of the Bi₃Ni structure type. Similar to La₂NiSb, the nickel atoms form infinite zigzag chains (259 pm Ni–Ni) with trigonal‐prismatic lanthanum coordination. One rectangular face of the lanthanum prism is capped by a bismuth atom (333.08–364.74 pm La–Bi, 281.18 pm Ni–Bi). These zigzag chains run along the b axis. DFT based band structure calculations and DOS representations suggest metallic behavior. This was confirmed via temperature dependent impedance spectroscopic measurements. A Seebeck coefficient of –10 μV · K^–1 in the temperature range up to 873 K substantiates this finding. Thermal analyses show that the compound is stable up to 873 K under inert gas conditions and degrades at higher temperatures. The magnetic measurements show almost no grain boundary nickel impurities characterizing La₂NiBi as a weak Pauli paramagnet.     

Two-dimensional hopping motion of encapsulated La atoms in silylated La(2)@C(80)

The (139)La NMR study of the exohedrally functionalized derivatives of La(2)@C(80) metallofullerene, La(2)@C(80)(Ar(2)Si)(2)CH(2) (: Ar = Mes, Mes = mesityl, : Ar = Dep, Dep = 2,6-diethylphenyl), reveal that the two La atoms hop between two sites along the equator of the C(80) cage.     

镧在细胞膜上键合形态研究

研究了一种简单快速的富膜细胞组分的提取和分离方法，并运用这种方法研究了大豆幼苗根细胞膜上La结合形态，La末与膜蛋白结合，而是与一非膜蛋白组分呈优势结合。     

La_2C~n(n=-1,0,+1)团簇的理论研究

用B3LYP/Lan1 2dz方法优化了La2 Cn(n =- 1 ,0 ,+1 )分子的结构 ,计算了La2 C的电子亲和势和离化能 ,并对计算结果进行了讨论。     

La8Br7Ni4: ribbons of Ni hexagons in condensed La6 trigonal prisms

A ternary lanthanum bromide La 8Br 7Ni 4 was synthesized from La, LaBr 3, and Ni under an Ar atmosphere at 830 degrees C. It crystallizes in space group C2/ m (No. 12) with lattice constants a = 29.528(4) A, b = 4.0249(6), c = 8.708(1) A, and beta = 94.515(2) degrees . The structure features condensed Ni-centered La 6 trigonal prisms. The Ni atoms are bonded to each other to form ribbons of Ni hexagons. Band structure, bonding, and physical properties of the compound have been investigated.     

La Decomposition Thermique de Quelques Thiobenzpiperidides

The thermal decomposition of thiobenzpiperidides and some derivatives containing substituents on the benzene nucleus was studied. By thermal analysis was put in evidence the influence of the nature and position of the substituents on the decomposition process. Kinetic parameters in non-isothermal and quasi-isothermal conditions (CRTA) were calculated. The chromatographic analysis, associated with mass spectrometry, proved that the initiation of the complex decomposition process is the result of the polar bond break. This revised version was published online in July 2006 with corrections to the Cover Date.     

微量元素La对Al—Si合金凝固过程的影响

本文用差热分析法系统地研究了La掺杂对Al-Si共晶合金、亚共晶、超共晶的动态凝固过程的影响。结果表明,La促使α(Al)成核,使体系中初晶α(Al)的析出温度比AJ-Si二元合金中Al初晶的实际析出温度明显提高;La对初晶Si的成核和长大起抑制作用,使超共晶中初晶Si相析出温度较二元合金中的Si初晶的实际析出温度明显降低。     

Formation of the Compound La2BaCuO5 (La4Ba2Cu2O10)

Synthesis of the compound La₂BaCuO₅ (La₄Ba₂Cu₂O_{1 0}), whose X-ray diffraction pattern is identical to that available in the literature for the solid solution La_3.6Ba_2.4Cu_1.8O_9.6, was studied.     

Pentagonal-Dodecahedral La2 Charge Density in [80-Ih]Fullerene: La2@C80

No Abstract