Synthesis and characterization of the rare-earth dicyanamides Ln[N(CN)2]3 with Ln=La, Ce, Pr, Nd, Sm, and Eu

The rare-earth dicyanamides Ln[N(CN)₂]₃ (Ln=La, Ce, Pr, Nd, Sm, Eu) were obtained via ion exchange in aqueous medium and subsequent drying: The crystal structures were solved and refined based on X-ray powder diffraction data and they were found to be isotypic: Ln[N(CN)₂]₃; Cmcm (no. 63), Z=4, Ln=La: , , ; Ce: , , ; Pr: , , ; Nd: , , ; Sm: , , ; Eu: , , ). The compounds represent the first dicyanamides with trivalent cations. The Ln³⁺ ions are coordinated by three bridging N atoms and six terminal N atoms of the dicyanamide ions forming a three capped trigonal prism. The structure type is related to that of PuBr₃. The novel compounds Ln[N(CN)₂]₃ have been characterized by IR and Raman spectroscopy (Ln=La) and the thermal behavior has been monitored by differential scanning calorimetry (Ln=Ce, Nd, Eu).     

Synthesis and dissolved hyperpolarized xenon NMR studies of sucrose octaoleate-F104

Sucrose octaoleate-F₁₀₄ has been prepared from sucrose and the acid chloride of R_F-oleic acid-F₁₃ in 86% yield using dry pyridine as solvent. The product has been characterized by MALDI-TOF mass spectrometry and and NMR methods. Some preliminary NMR results are included using hyperpolarized xenon gas dissolved in an aqueous emulsion containing egg yolk phospholipid and the octaester. The in vitro spin lattice relaxation time of dissolved xenon as well as the chemical shift suggest that highly fluorinated sucrose polyesters (HFSPE’s) may be promising molecules for use as part of in vivo delivery systems for hyperpolarized xenon.     

Old friends in a new light: “SnSb” revisited

The binary pnictide ‘SnSb’ has been re-investigated using a combination of X-ray, synchrotron and electron diffraction as well as electron microprobe analysis. Its structure was found to be incommensurately modulated with an underlying rhombohedral parent structure of space group symmetry (No. 166), unit cell parameters , in the hexagonal setting. The incommensurate primary modulation wave vector and the superspace group symmetry is (0, 0, ∼1.311) (No. 166.1). The refinement of the incommensurate structure indicates that the satellite reflections arise from displacive shifts of presumably essentially pure Sn and Sb layers along the hexagonal c-axis, with increasing distance between the Sn-layers and decreasing distance between the Sb layers.     

Novel electrolyte salts based on perfluoroalkyltrifluoroborate anions: 1. Synthesis and characterization

Lithium and tetraethylammonium perfluoroalkyltrifluoroborate salts, Li[R_fBF₃]·H₂O and Et₄N[R_fBF₃] (R_f=C₂F₅, n-C₃F₇ and n-C₄F₉), were prepared from the reactions of perfluoroalkylmagnesium reagents (R_fMgBr) and B(OCH₃)₃, followed by fluorination by aq.KHF₂ and aq.HF solutions and the cation exchange reaction of the resultant K[R_fBF₃]. All the salts prepared were characterized by NMR, NMR, NMR, NMR, IR, MS and elemental analysis.     

Structural homologies in benzylamino-N,N-bis methylphosphonic acid and its layered zirconium derivative

A new layered zirconium diphosphonate fluoride, ZrF(O₃PCH₂)₂NHCH₂C₆H₅ has been prepared and its structure determined ab initio by X-ray powder data and refined with the Rietveld method (orthorhombic, , , , space group Pbca, , Z=8, R_wp=0.080). Both phosphonic groups of each diphosphonate building block are bonded to zirconium atoms on the same side of each layer. Benzyl groups from adjacent layers are interdigitated in the interlayer region, with probable π-π stacking interactions. The structure of the free benzylamino-N,N-bis methylphosphonic acid has been determined by single crystal X-ray data (monoclinic, space group P2₁, , , , β=92.930(3)°, , Z=2, R₁=0.072, wR₂=0.150). As in the zirconium derivative, benzyl groups from adjacent layers are interdigitated and create a regular alternation of polar and non-polar regions.     

Tetragonal low-temperature structure of LiAl

The cubic face-centered structure of LiAl (, at ) transforms into a tetragonal body-centered structure (I4₁/amd, , at ). This first-order phase transition at about during heating is probably the reason for the so-called “ anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.     

Synthesis and crystal structures of La3MgBi5 and LaLiBi2

Two new ternary bismuthides, La₃MgBi₅ and LaLiBi₂, have been prepared by solid-state reactions of the corresponding pure metals in welded niobium tubes at high temperature. Their structures have been established by single-crystal X-ray diffraction studies. La₃MgBi₅ crystallizes in the hexagonal space group P6₃/mcm (No.193) with cell parameters of , , , and Z=2. LaLiBi₂ belongs to tetragonal space group P4/nmm (No.129) with cell parameters of , ,, and Z=2. The structure of La₃MgBi₅ is of the ‘‘anti’’ Hf₅Sn₃Cu type, and features 1D linear Bi⁻ anionic chains and face-sharing [MgBi_6/2]⁷⁻ octahedral chains. The structure of LaLiBi₂ is isotypic with HfCuSi₂, and is composed of 2D Bi⁻ square sheets and 2D LiBi layers with La³⁺ ions as spacers. Band calculations indicate that both compounds are metallic.     

Synthesis and NMR studies of 2- and 3-fluorosubstitued five-membered heterocycles

A set of 2-fluoro- and 3-fluoro-substituted thiophenes, pyrroles and furans has been synthesized by a treatment of the corresponding lithio derivatives with N-fluorodibenzenesulfonamide. For all these compounds, and coupling constants and NMR chemical shifts have been measured. In all cases, a dramatic increase of the couplings has been observed in 2-fluoro- and 3-fluoro-substituted compounds in comparison with those measured for the parent compounds. The same is valid for measured in 3-fluoro derivatives.The DFT calculations performed for 2- and 3-fluoro-substituted compounds reproduce very well the experimental coupling values and show that the Fermi contact contribution is the main factor determining their magnitude. Also the trends observed in the NMR shieldings are well reflected in the calculated DFT data.     

Structural and conductivity studies of CsKSO4Te(OH)6 and Rb1.25K0.75SO4Te(OH)6 materials

The crystal structures of the title compounds were solved using the single-crystal X-ray diffraction technique. At room temperature CsKSO₄Te(OH)₆ was found to crystallize in the monoclinic system with Pn space group and lattice parameters: ; ; ; β=106.53(2)°; ; Z=4 and . The structural refinement has led to a reliability factor of R₁=0.0284 (wR₂=0.064) for 7577 independent reflections. Rb_1.25K_0.75SO₄Te(OH)₆ material possesses a monoclinic structure with space group P2₁/a and cell parameters: ; ; ; β=106.860(10)°; ; Z=4 and . The residuals are R₁=0.0297 and wR₂=0.0776 for 3336 independent reflections. The main interest of these structures is the presence of two different and independent anionic groups (TeO₆⁶⁻ and SO₄²⁻) in the same crystal.Complex impedance measurements (Z^*=Z^′—iZ^″) have been undertaken in the frequency and temperature ranges 20-10⁶ Hz and 400-600 K, respectively. The dielectric relaxation is studied in the complex modulus formalism M^*.     

Heat capacities, order-disorder transitions, and thermodynamic properties of rare-earth orthoferrites and rare-earth iron garnets

Rare-earth orthoferrites, RFeO₃, and rare-earth iron garnets (RIGs) R₃Fe₅O₁₂ (R=rare-earth elements) were prepared by citrate-nitrate gel combustion method and characterized by X-ray diffraction method. Isobaric molar heat capacities of these oxides were determined by using differential scanning calorimetry from 130 to 860 K. Order-disorder transition temperatures were determined from the heat capacity measurements. The Néel temperatures (T_N) due to antiferromagentic to paramagnetic transitions in orthoferrites and the Curie temperatures (T_C) due to ferrimagnetic to paramagnetic transitions in garnets were determined from the heat capacity data. Both T_N and T_C systematically decrease with increasing atomic number of R across the series. Lattice, electronic and magnetic contributions to the total heat capacity were calculated. Debye temperatures as a function of absolute temperature were calculated for these compounds. Thermodynamic functions like , , H^o, G^o, , , , , and have been generated for the compounds RFeO₃(s) and R₃Fe₅O₁₂(s) based on the experimental data obtained in this study and the available data in the literature.     

Monitoring structural transformations in crystals. Part 4. Monitoring structural changes in crystals of pyridine analogs of chalcone during [2+2]-photodimerization and possibilities of the reaction in hydroxy derivatives

We studied the [2+2]-photodimerization in crystals of pyridine analogs and hydroxy derivatives of chalcone using the X-ray structure analysis. The mutual orientation of adjacent molecules in the crystals was analyzed in a quantitative way and the results were compared with data for known photoactive crystals undergoing the [2+2]-photodimerization. In the case of one pyridine analog, we processed the single-crystal-to-single-crystal photodimerization and determined the structure for the mixed crystal containing both the substrate and the product. We also explained a role of hydrogen bonds in the [2+2]-photodimerization in the case of the hydroxy derivatives of chalcone. C₅H₄N-CO-CHCH-C₆H₅: , , , β=91.318(10)°, monoclinic, . The irradiated crystal of the above analog: , , , β=91.870(10)°, monoclinic, . C₆H₅-CO-CHCH-C₅H₄N: , , , β=110.01(3)°, monoclinic, C2/c,Z=8. C₆H₅-CO-CHCH-C₆H₄(o-OH): , , , β=109.73(5)°, monoclinic, . C₆H₅-CO-CHCH-C₆H₄(p-OH): , , , orthorhombic, .     

Synthesis and structure of three manganese oxalates: MnC2O4·2H2O, [C4H8(NH2)2][Mn2(C2O4)3] and Mn2(C2O4)(OH)2

Three manganese oxalates have been hydrothermally synthesized, and their structures determined by single-crystal X-ray diffraction. MnC₂O₄·2H₂O (I) is orthorhombic, P2₁2₁2₁, , , , , Z=4, final R, R_w=0.0832, 0.1017 for 561 observed data (I>3σ(I)). The one-dimensional structure consists of chains of oxalate-bridged manganese centers. [C₄H₈(NH₂)₂][Mn₂(C₂O₄)₃] (II) is triclinic, , , , , α=81.489(2)°, β=81.045(2)°, γ=86.076(2)°, , Z=1, final R, R_w=0.0467, 0.0596 for 1773 observed data (I > 3σ (I)). The three-dimensional framework is constructed from seven coordinate manganese and oxalate anions. The material contains extra-framework diprotonated piperazine cations. Mn₂(C₂O₄)(OH)₂ (III) is monoclinic, P2₁/c, , , , β=91.10(3)°, , Z=1, final R₁, wR2=0.0710, 0.1378 for 268 observed data (I>2σ (I)). The structure is also three dimensional, with layers of MnO₆ octahedra pillared by oxalate anions. The hydroxide group is found bonded to three manganese centers resulting in a four coordinate oxygen.     

NMR spectroscopic study of xenon fluorides in the gas phase and of XeF2 in the solid state

The xenon fluorides, XeF₂, XeF₄, and XeF₆, were characterized by gas-phase and NMR spectroscopy, providing the first gas-phase NMR spectroscopic data for the xenon fluorides. Xenon difluoride was also characterized in the solid state by static and magic angle spinning (MAS) and NMR spectroscopy, providing experimental values for the (4260±10 ppm) and (125±5 ppm) shielding anisotropies in XeF₂. A method for encapsulating reactive fluoride samples for study by MAS NMR spectroscopy is also described.     

Solvothermal synthesis and removal capacity for hydrogen chloride gas of Zn(OH)(NO3) with a rare (10,3)-d net

A basic zinc nitrate, [Zn(OH)(NO₃)]_n with a novel framework reported herein is prepared by solvothermal synthesis, and holds a rare three-dimensional (10,3)-d net called by Wells. It crystallizes in orthorhombic, space group, Pna2₁ (No. 33), , , , Z=4, , , , 3.49<θ<27.51, , R₁=0.029, wR₂=0.0773 for 697 (I>2σ(I)) of 1728 [R_(int)=0.0246] unique reflections and 59 parameters. GOF=1.073. Interestingly this basic zinc nitrate, [Zn(OH)(NO₃)]_n behaves well at removal capacity for hydrogen chloride gas, 20.3 g HCl/100 g, and its framework is not destroyed yet. This adsorption character may be useful for the removal of toxic gases in the environment.     

The disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3-MO-Nb2O5 (M=Mg, Ni) systems

The disordered structures and low temperature dielectric relaxation properties of Bi_1.667Mg_0.70Nb_1.52O₇ (BMN) and Bi_1.67Ni_0.75Nb_1.50O₇ (BNN) misplaced-displacive cubic pyrochlores found in the Bi₂O₃-M^IIO-Nb₂O₅ (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B₂O₆ octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O′A₂ tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (, , ) to 96 h (, , ) while the O′ oxygen was shifted from position 8b (, , ) to Wyckoff position 32e (, , ). The refined displacement magnitudes off the 16d and 8b sites for the A and O′ sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively.     

Effect of initial monomer concentration on the equilibrium swelling and elasticity of hydrogels

The linear swelling ratio α and the effective network chain length N of a series of poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration . PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that α is not a monotonic function of . As is increased, α first decreases up to about and remains constant in a narrow range of , but then it increases continuously. The -dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of below 0.1, N follows the scaling relationship , while at higher concentrations, N varies only slightly with . The increase of α with N obeys the relation , as predicted by the Flory-Rehner theory.