A novel high-temperature commensurate superstructure in a natural bariopyrochlore: A structural study by means of a multiphase crystal structure refinement

Additional X-ray diffraction effects yielding an eightfold commensurate superstructure of the ideal pyrochlore structure were observed after annealing at 873 K of a thallium-doped bariopyrochlore single crystal. Electron diffraction indicated the coexistence of two cubic phases, the pyrochlore structure and a new F-centred, cubic phase. The superstructure was solved and refined in the space group . The two phases were combined together and refined as independently diffracting to R=0.0628. The resulting unit-cell content is (A,□)₂₀Nb₁₆Ti₂O₅₃(Z=8), with A=Ba, Tl, Ce, Th. For some atomic positions of the superstructure, third- and fourth-order anharmonic ADP's were used to account for the specific density shape having a continuous character as typical for ionic conductors. There are three distinct clusters in the superstructure, leading to a new structure type no longer strictly of pyrochlore-structure type.     

A spectroscopic study of hydrogen bond formation by silatranes and their analogues

The IR absorption shifts of OH and OD stretching vibrations upon interaction of silatranes XSi(OCH(CH₃)CH₂)₃N and their monocyclic analogues of the type R₂(OCHR′CH₂)₂NR$?with phenol and deuteromethanol, respectively, were measured. In the systems involving silatranes these values are higher than in systems with the corresponding ethoxysilanes. The equilibrium constants and thermodynamic parameters of the interaction of the compounds studied with phenol in n-heptane were measured by electronic spectroscopy. The interaction of phenol with the compounds studied shows two lines correlating with the thermodynamic parameters ΔH - f(ΔS). One of the lines is plotted by alkoxysilanes, cyclic and acyclic ethers. The second line corresponds to the compounds having a O-C-C-N group. This enables a conclusion to be drawn that in a non-polar medium the basic centre of silatranes and their monocyclic analogues is different from the oxygen in ethers and alkoxysilanes.     

Synthesis of a liquid crystal paramagnetic copper complex with Aminovinylketone and the structural study of its single crystal and mesophase

We have synthesized a mesogenic copper complex with aminovinylketone. The X-ray structural analysis has shown that the molecules of the complex in a single crystal (a=9.107, b=11.446, c=11.496 , =81.06, =73.20, =80.72°, and Z=1) form sheets with thickness determined by the cross molecular sizes. When the substance is heated, phase transitions are observed: Cr196Cr X128 S₁187 S_A189 N192°C i. We have carried out EPR studies of various phase states of the complex. It is shown that in the mesophase, the diamagnetic component of magnetic anisotropy is larger than the paramagnetic component resulting in the orientation of the long molecular axes along the outer magnetic field.Kazan Physicotechnical Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 49–55, November–December, 1993.Translated by T. Yudanova     

A novel zigzag chain based on polyoxomolybdate decorated by glycine ligand in covalent bond

An unusual compound, K₂[Mo₄O₁₃(NH₃CH₂COO)₂]?·?2H₂O (1), has been synthesized and structurally characterized by the XRD, elemental analysis, IR spectrum, TG analyses and the single crystal X-ray diffraction. The structure of 1 exhibits a zigzag chain based on corner-shared tetramolybdate chelated by two bidentate glycine ligands.     

Nonlinearly additive forces in multivalent ligand binding to a single protein revealed with force spectroscopy

We present evidence of multivalent interactions between a single protein molecule and multiple carbohydrates at a pH where the protein can bind four ligands. The evidence is based not only on measurements of the force required to rupture the bonds formed between concanavalin A (ConA) and alpha-D-mannose but also on an analysis of the polymer-extension force curves to infer the polymer architecture that binds the protein to the cantilever and the ligands to the substrate. We find that although the rupture forces for multiple carbohydrate connections to a single protein are larger than the rupture force for a single connection, they do not scale additively with increasing number. Specifically, the most common rupture forces are approximately 46, 68, and 85 pN at a loading rate of 650 +/- 25 pN/s, which we argue corresponds to 1, 2, and 3 ligands being pulled simultaneously from a single protein as corroborated by an analysis of the linkage architecture. As in our previous work polymer tethers allow us to discriminate between specific and nonspecific binding. We analyze the binding configuration (i.e., serial vs parallel connections) through fitting the polymer stretching data with modified wormlike chain (WLC) models that predict how the effective stiffness of the tethers is affected by multiple connections. This analysis establishes that the forces we measure are due to single proteins interacting with multiple ligands, the first force spectroscopy study that establishes single-molecule multivalent binding unambiguously.     

A de novo approach to the synthesis of glycosylated methymycin analogues with structural and stereochemical diversity

A divergent and highly stereoselective route to 11 glycosylated methymycin analogues has been developed. The key to the success of this method was the iterative use of the Pd-catalyzed glycosylation reaction and postglycosylation transformation. This unique application of Pd-catalyzed glycosylation demonstrates the breath of α/β- and d/l-glycosylation of macrolides that can be efficiently prepared using a de novo asymmetric approach to the carbohydrate portion.     

A structural and spectroscopic study on para-aminohippuric acid with experimental and theoretical approaches

In this work, the molecular conformation, vibrational and electronic analysis of para-aminohippuric acid (pAHA, C(9)H(10)N(2)O(3)) were presented for the ground state using experimental techniques (FT-IR, FT-Raman and UV) and density functional theory (DFT) employing B3LYP exchange correlation with the 6-311++G(d,p) basis set. FT-IR and FT-Raman spectra were recorded in the regions of 400-4000cm(-1) and 50-4000cm(-1), respectively. The UV absorption spectra of the compound that dissolved in ethanol and water solution were recorded in the range of 190-400nm. Potential energy curve was computed by means of scanning NCCO torsion angle. The geometry optimization and the energies associated possible four conformers (C1-C4) were computed. The computational results diagnose the most stable conformer of pAHA as the C1 form. Optimized structure of compound was interpreted and compared with the earlier reported experimental values. The complete assignments of fundamental vibrations were performed on the basis of the total energy distribution (TED) of the vibrational modes, calculated with scaled quantum mechanics (SQM) method. A study on the electronic properties, such as frontier molecular energies, absorption wavelengths and oscillator strengths, were predicted by time-dependent DFT (TD-DFT) approach, while taking solvent effects into account. To investigate non-linear optical properties: polarizability, anisotropy of polarizability and molecular first hyperpolarizability of molecule were computed. Thermodynamic properties (heat capacity, entropy and enthalpy) of the title compound at different temperatures were calculated.     

A theoretical study of a drastic structural change of bis(phthalocyaninato)lanthanide by ligand oxidation: Towards control of ligand field strength and magnetism of single-lanthanide-ionic single molecule magnet

A quantum chemical study based on the density functional theory (DFT) on anionic and cationic bis(phthalocyaninato)lanthanides revealed that removal of two electrons from the anionic complex shortens considerably the separation between phthalocyaninato (Pc) ligands. This suggests that [Pc₂Tb]⁺, which is generated by two-electron oxidation from the [Pc₂Tb]⁻ SMM previously reported, can have significantly larger ligand field splitting than the original anionic form.     

A combined spectroscopic and computational study of a high-spin S = 7/2 diiron complex with a short iron-iron bond

The nature of the iron-iron bond in the mixed-valent diiron tris(diphenylforamidinate) complex Fe(2)(DPhF)(3), which was first reported by Cotton, Murillo et al. (Inorg. Chim. Acta 1994, 219, 7-10), has been examined using additional spectroscopic and theoretical methods. It is shown that the coupling between the two iron centers is strongly ferromagnetic, giving rise to an octet spin ground state. On the basis of M?ssbauer spectroscopy, the two iron centers, formally mixed-valent Fe(II)Fe(I), are completely equivalent with an isomer shift δ = 0.65 mm s(-1) and quadrupole splitting ΔE(Q) = +0.32 mm s(-1). A large, positive zero-field splitting D(7/2) = 8.2 cm(-1) has been determined from magnetic susceptibility measurements. Multiconfigurational quantum studies of the complete molecule Fe(2)(DPhF)(3) found one dominant configuration (σ)(2)(π)(4)(π*)(2)(σ*)(1)(δ)(2)(δ*)(2), which accounts for 73% of the ground-state wave function. By considering all the configurations, an estimated metal-metal bond order of 1.15 has been calculated. Finally, Fe(2)(DPhF)(3) exhibits weak electronic absorptions in the visible and near-infrared regions, which are assigned as d-d transitions from the doubly occupied metal-metal π molecular orbital to half-occupied π*, δ, and δ* orbitals.     

Synthesis and structural studies of binuclear platinum(II) complexes with a novel phosphorus-nitrogen-phosphorus ligand

The new pyridinediphosphinite ligand PONOP (1) was synthesized in one step from 2,6-pyridinedimethanol and diphenylchlorophosphane. Reaction of 1 with PtCl(2)(PhCN)(2) led to the neutral homobimetallic complex [Pt(2)Cl(4)(PONOP)(2)] (2), where the benzonitriles have been substituted by the phosphorus atoms of 1. The X-ray structure of 2 revealed a metallamacrocycle where the pyridines remained free. Addition of 2 equiv of [Cu(MeCN)(4)](BF(4)) to 2 led to CuCl and to the binuclear dicationic complex [Pt(2)Cl(2)(PONOP)(2)](BF(4))(2) (3), where chloro ligands have been substituted by pyridine groups. Conversely, reaction of 3 with chloride anions gave back complex 2. Solid-state (X-ray) and solution (NMR) studies indicated that after the transformation of 2 into 3, the platinum centers were brought much closer and the pyridinediphosphinite ligand was stiffened. The methylene NMR protons of 3 were strongly deshielded, and the corresponding proton-phosphorus coupling constants followed a Karplus-type relationship.     

Mechanochemical synthesis of a novel C(60) dimer connected by a silicon bridge and a single bond

[reaction: see text] A novel C(60) dimer connected by a silicon bridge and a single bond was synthesized by the mechanochemical solid-state reaction employing a high-speed vibration milling technique and fully characterized by the (1)H and (13)C NMR, APCI mass, and UV-vis spectroscopy. The presence of the electronic interaction between the two C(60) cages was demonstrated by the electrochemical method.     

Preparation and structural study of a novel nonlinear molecular material: the -histidinum dihydrogenarsenate orthoarsenic acid crystal

-Histidinium dihydrogenarsenate orthoarsenic acid (LHA) crystal ;

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belongs to P2₁ space group of the monoclinic system, Z=2, a=9.264(2) Å, b=8.929(2) Å, c=8.874(2) Å, β=108.61(3)°. The crystal is isomorphous to

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(LHP) crystal. The H₂AsO₄⁻ anions and H₃AsO₄ molecules are joined into layers parallel to the (100) crystallographic planes by the O–HO type hydrogen bonds. The histidinium cations occupy the space between the layers. The histidinium cation posses the carboxylic COO⁻ group. The positive charges appear in the ring (the proton is attached to the =N(3)– nitrogen) and in the ammonium group. The differential scanning calorimetric (DSC) diagram does not show any phase transition till 100 K. The infrared spectrum fully confirms the X-ray crystal structure. The LHA crystal is a good second harmonic generator (0.46 relative to KDP)     

A Co(III) complex with a tridentate amine-imine-oxime ligand from 1,2,3,4-tetrahydroquinazoline: synthesis,crystal structure,spectroscopic and thermal characterization

A cobalt(III) complex [Co(L)₂]Cl · PPO · H₂O (1) (HL = 1-(2-aminobenzylimino)-1-phenyl-propan-2-one oxime, PPO = 1-phenyl-1,2-propanedione-2-oxime) has been synthesized and characterized by elemental analyses, spectral, thermal, magnetic and molar conductance measurements and single crystal X-ray diffraction. It crystallizes in the monoclinic crystal system, space group P2₁/c. Complex 1 consists of one uncoordinated water, one 1-phenyl-1,2-propanedione-2-oxime molecule, one bis[1-(2-aminobenzylimino)-1-phenyl-propan-2-one oxime]cobalt(III) cation and one uncoordinated chloride. The coordination geometry around Co is slightly distorted octahedral, completed with six nitrogens of two L^? ligands. The oxime moieties have E configurations. In the crystal structure, intramolecular O–H···Cl and N–H···O and intermolecular N–H···O, O–H···Cl and N–H···Cl hydrogen bonds link the molecules into chains parallel to the c axis; hydrogen-bonded PPO molecules fill the spaces between the chains and stabilization of the structure.     

A novel dimeric zinc(II) complex with a tripodal peptide ligand: a structure stabilized by ligand sharing

The crystal structure of a novel dimeric zinc(II) complex, [ZnL(H₂O)]₂(ClO₄)₂·4H₂O (L?=?N-(bis(2-pyridyl)methyl)-2-pyridinecarboxamide), has been determined by X-ray diffraction. In this complex each planar N_py–N_amido–N_py moiety of the ligand coordinates to one zinc ion and the pendant pyridine of one [ZnL] unit completes the coordination sphere of a [ZnL] neighbor. Units of the complex are connected in a two-dimensional network by intermolecular hydrogen bonds. The thermodynamic properties of the ligand with bivalent metal ions Co(II), Ni(II), Cu(II) and Zn(II) were studied by potentiometric titration and the order of the stability constants is in agreement with the Irving–Williams series. The dimeric complex is stabilized through ligand sharing, as confirmed by the crystal structure and thermodynamic properties.     

Mechanochemical synthesis of a novel C60 dimer connected by a germanium bridge and a single bond

A novel C₆₀ dimer connected by a germanium bridge and a single bond was synthesized by the mechanochemical solid-state reaction using a high-speed vibration milling technique. The structural characterization was made by the ¹H and ¹³C NMR, APCI mass, and UV-vis spectroscopy, and the presence of the electronic interaction between the two C₆₀ cages was demonstrated by the electrochemical method.     

Structural and spectroscopic study of a novel erbium titanate pink pigment prepared by sol-gel methodology

Pyrochlore oxides show a large variety of physical and chemical properties depending on the ordering/disordering of the cations and oxygen vacancies. Taking account of these structural features and the luminescent properties of lanthanides, a new family of colored materials is investigated. This paper studies the structural evolution of the erbium titanate system with temperature to establish its influence on the color properties. The success on the development of color is completely related to the sol-gel preparation method, underlining its higher reactivity compared to classical solid-state synthesis. After firing at 700 degrees C, the sol-gel material develops an intense pink coloration whose intensity significantly diminishes at 800 degrees C. X-ray diffraction and Rietveld refinements indicated the presence of nanocrystals with a fluorite-like structure at 700 degrees C, responsible for the intense coloration, which suffers a gradual atomic rearrangement toward an "ideal" pyrochlore phase. These results were corroborated by infrared and Raman measurements. UV-vis spectroscopy showed the influence of the Er(3+)-O bond covalence on the spectral properties. This study opens new perspectives to the development of more ecological colored sol-gel materials based on rare earth elements. Furthermore, the combination of the optical aspects with the classical pyrochlore properties (magnetization, heat capacity, conductivity, etc.) would provide new multifunctional materials for advanced applications.     

Self-assembly of a tridentate Schiff-base ligand with Zn(II) in the presence of lanthanides: Novel crystal structures and spectroscopic properties

Condensation of picolinaldehyde with methyl 4-amino-3-hydroxy-benzoate resulted in the acquisition of a tridentate Schiff-base ligand (HL) which contains a structural moiety typical of octahedrally cored grid-type analogs. Reactions of HL with Zn(NO₃)₂ in the presence of Ln(NO₃)₃ [Ln = Sm(III), Tb(III) and Yb(III)] result in two types of complexes, viz. [Zn(HL)(L)]₂[Ln(NO₃)₅] [Sm(III), 1a and Tb(III), 1b] and [Zn(HL)L]₂[Yb(NO₃)₅]·C₃H₆O (1c). Despite applying two different synthetic protocols, the transition metal ion displayed a greater propensity towards the meridional tridentate pocket, which is reflected by XRD analysis, the ESI-MS technique and further supported by elemental analysis and IR characterization of each compound. In addition, we have compared the luminescence properties of 1a, 1b and 1c with the previously synthesized [Zn(HL)(L)]₂[Zn(NO₃)₄] (1d) to investigate whether a different metal in the outer coordination sphere could somehow tune the compounds’ spectral behavior.     

A digermyne with a Ge-Ge single bond that activates dihydrogen in the solid state

The reduction of the bulky amido-germanium(II) chloride complex, LGeCl (L = N(SiMe(3))(Ar*); Ar* = C(6)H(2)Me{C(H)Ph(2)}(2)-4,2,6), with the magnesium(I) dimer, [{((Mes)Nacnac)Mg}(2)] ((Mes)Nacnac = [(MesNCMe)(2)CH](-); Mes = mesityl), afforded LGeGeL, which represents the first example of a digermyne with a Ge-Ge single bond. Computational studies of the compound have highlighted significant electronic differences between it and multiply bonded digermynes. LGeGeL was shown to cleanly activate H(2) in solution or the solid state, at temperatures as low as -10 °C, to give the mixed valence compound, LGeGe(H)(2)L.     

Role of triple bond in 1,2-diphenylacetylene crystal: A combined experimental and theoretical study

We have performed a combined experimental and theoretical study of the molecular system of 1,2-diphenylacetylene. The occurrence of two different geometries of the molecule in the crystal structure, one being planar and the other tilted by approximately 6 degrees , has been investigated in relation to the nature of the acetylenic linker. The experimental charge density analysis shows that the acetylenic linker exhibits a noncylindrical density reminiscent of the strong conjugation present in the molecule. The pi-orbitals of the acetylenic linker derived from density functional theory (DFT) calculations are found to sustain a variety of conjugation lengths between the phenyl rings, thereby giving flexibility to the molecule to arrange itself in various packing conformations in the crystal. It is interesting that the energy involved for such distortions is only kBT, allowing several polymorphic forms of the crystal structure as reported in the literature. The distortions entertained by the molecule and the corresponding changes in the charge density distribution and energy are all relevant to molecular electronics.