Viscoelastic Parameters of a Side-Chain Nematic Polymer: II—Analysis of NMR Results in Terms of the Anisotropic Dumbbell Model

The two viscoelastic parameters of a siloxane side-chain nematic polymer—twist viscosity coefficient, and static order parameter—deduced from the NMR study of Reference 1, are discussed in terms of the anisotropic dumbbell model of Brochard using structural information obtained by small angle neutron scattering. It is found that this model, originally developed for dilute solutions of flexible polymers in low molecular mass nematics, describes satisfactorily the data, supporting the idea that the backbone polymer chains are not entangled in the nematic melt. An average elastic constant is deduced. This constant is found to increase sharply as the nematic-smectic A phase is approached. The twist elastic constant is calculated using splay and bend elastic constant values of a very similar polymer. Some (possibly only apparent) difficulties associated with the dumbbell model are pointed out.     

Order Parameters of α-Substituted Anthraquinone Dyes in a Nematic Liquid Crystalline Host

A study is presented of the optical order parameters and spectroscopic properties of α-substituted anthraquinone dyes in a nematic liquid crystalline mixture composed of cyanophenylcyclohexane derivatives. Then, the results of this study are discussed in terms of the effects of the dye structure on the order parameters. From the systematic variation in structure, the order parameters are found to be improved when a biphenylamino group is introduced into the α-position of the anthraquinone nucleus. In addition, preliminary data on the solubility and photostability of the dyes are reported.     

Measurement of the Three Elastic Constants and the Shear Viscosity γ1 in a Main-Chain Nematic Polymer

K ₁, K ₂, K ₃ and [mgrave]₁ have been measured for the first time in a typical main-chain polymer belonging to a polyester series. K ₁ is very large compared to conventional nematics, while K ₂ and K ₃ have more conventional values. K ₁ have been measured by two methods, one of them consisting in the measurement of the characteristic rise time of regular striped domains.     

Order Parameters of β-Substituted Anthraquinone Dyes I: 1-Amino-4-Hydroxyanthraquinone Derivatives

A study is presented of the optical order parameters and spectroscopic properties of I-amino-4-hydroxyanthraquinone derivatives containing various substituents in the 2-position in a nematic liquid crystalline mixture composed of cyanophenylcyclohexane derivatives. The results are discussed in terms of the effects of the dye structure on the order parameters. By introducing a-SCH₂-group into the 2-position, magenta dyes have been obtained which show high order parameters and solubilities. In addition, preliminary data on the solubility and photostability of the dyes are reported.     

Synthesis and Crystal Structure of a Copper(II)–Sodium(I) Coordination Polymer

Abstract  

A new coordination polymer, {[Na₂Cu(Ac)₄(H₂O)]·H₂O} _n (HAc = acetic acid), has been synthesized and structurally characterized by IR spectroscopy and X-ray crystallography. The compound crystallizes in the orthorhombic system, space group Pccn with a = 16.9076(17) ?, b = 11.8375(12) ?, c = 15.7567(16) ?, V = 3153.6(6) ?³. Four oxygen atoms from different Ac⁻ groups coordinate to the copper atom in a square-planar arrangement. Each sodium ion is in a distorted octahedral environment, being coordinated by one aqua ligand and five acetate oxygen atoms from two adjacent [Cu(Ac)₄] units. An alternating arrangement of metal ions and bridging acetate ligands results in the formation of two-dimensional sheets which are further linked into a three-dimensional network by the water molecules between the layers through rich hydrogen bonds.     

Studies of Intermolecular Interaction in Binary Nematic Mixtures of p-Methoxybenzylidene—p′-n-Butylaniline and p-Cyano-p′-n-Pentylbiphenyl by Dielectric Relaxations

Anisotropic dielectric relaxations at the nematic state under magnetic field and viscosities at the isotropic state were measured in MBBA-CPB systems with various mole% of CPB. The intermolecular interaction in the systems with different mole% of CPB was estimated by the MaierSaupe theory from the dielectric relaxations and the viscosities. A long range-intermolecular interaction increased anomalously near the two CPB concentrations exhibiting the eutectic points (15 and 75 mole% of CPB). This was explained by some component fluctuation originating in the inversion of solvent-solute relationships at the eutectic mixing ratio.     

Dynamical Properties of a Disordered Crystal: Adamantane—C10D16

Inelastic neutron scattering measurements on C₁₀D₁₆ show the existence of underdamped collective librational modes with temperature-independent frequency widths.

A preliminary dynamical model, with no adjustable parameters, shows that the observed widths are consistent with the existing quasi-static disorder.

Direct measurements of the orientational pair correlation function by means of elastic neutron scattering agree with the prediction of a simple Ising model.

Intensity measurements between room-temperature and the ordering temperature (T _c = 208.6K) indicate that the orientational correlations develop very slowly as T → T _c.     

Sol–gel simulation—I: Scattering response

Scattering of sol–gel structures is investigated computationally. Sol-gels are recreated through an aggregation algorithm incorporating Brownian motion and chemical reactions. Using the fractal character of sol–gels, the concept of recursion is introduced as a tool to perform multi scale computation of the response of sol–gels through the different scales from the molecular level to the macro scale. The concept is illustrated with the prediction of scattering intensity. The relationship between scattering intensity and functionality is investigated, noting that the latter is a function of the Brownian motion and chemical reactivity. Computational simulation tools are developed to predict scattering intensity as a function of density and reactivity, the former represented by the number of particles, or clusters, in the simulation box. Then, the results are correlated to an analytical model that reveals the critical wave number, or critical scale, at which percolation occurs.     

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T_G. The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T_X. Values of T_g and T_X have been tabulated for all alloys investigated and the compositional dependence of T_g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge₂₀Se₄₀Te₄₀ in which a GeSe₂-rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe₂-rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.     

Preparation and Crystal Structural Study of a Novel Coordinated Polymer [HgI<Subscript>2</Subscript>(4-bped)<Subscript>2</Subscript>] (bped = bis(4-pyridyl)ethylene diamine)

Abstract  

A novel coordinated polymer [HgI₂(4-bped)₂] was synthesized through the reaction of bis(4-pyridyl)ethylene diamine and HgI₂ in mixed MeOH–THF (5:2). The crystal structural analysis indicated that the complex crystallized in a monoclinic space group P2₁/c, a = 10.8392(12), b = 19.841(2), c = 8.8279(10), β = 95.452(2), V = 1889.9(4) ?³, Z = 4. The complex possessed a novel three-dimensional supramolecular framework formed by hydrogen bonds among repeating [HgI₂(4-bped)]₂ dimeric units.     

Solid-State Forms of β-Resorcylic Acid: How Exhaustive Should a Polymorph Screen Be?

An extensive experimental screen, coupled with a computational study, revealed seven new solid-state forms of β-resorcylic acid. The known, stable polymorph II° shows a reversible phase transformation to the new, kinetically stable, probably disordered high temperature form I. The study provides a consistent picture of the solid-state of β-resorcylic acid.     

X-ray backscattering (diffraction at a Bragg angle of π/2): A review

The potential of X-ray backscattering in X-ray optics and metrology and in the structural characterization of crystalline objects with different degrees of imperfection are considered.     

Observation of precipitates in a supersaturated Fe?Co alloy by muon spin rotation spectroscopy: a step towards understanding embrittlement of reactor pressure vessel steels

The technique of μ⁺SR spectroscopy was applied to detect precipitate formation due to ageing in the binary Fe-0.8 wt% Cu alloy. As compared to the homogenized specimen, the aged sample gives a μ⁺SR signal with considerably larger relaxation rate in the temperature interval 180 K < T < 350 K, having a maximum near T = 250 K with Λ = 14 μs⁻¹. Since precipitation of copper plays possibly an important role in the embrittlement of neutron irradiated ferritic steels, the signal from an irradiated steel specimen has also been investigated.     

Molecular and crystal structures of mesomorphic aromatic esters: I. Structure of two isomers, p-butoxyphenyl p′-hexyloxybenzoate and p-hexyloxyphenyl p′-butoxybenzoate

Two isomers, p-butoxyphenyl p′-hexyloxybenzoate (1) and p-hexyloxyphenyl p′-butoxybenzoate (2), are studied by X-ray diffraction. Crystals 1 contain two crystallographically independent molecules, which differ in the conformation of one of their side chains. In both compounds, bond lengths and angles have standard values. The geometry of the central fragment Ph-Est-Ph is essentially the same in both isomers. One of the benzene rings is coplanar with the Est group, and the other ring, which is linked with this group through the oxygen atom, is almost perpendicular to the Est plane. In crystal 1, molecules are packed in such a fashion that, in any pair of neighboring molecules, the most closely spaced benzene rings are nearly perpendicular to each other, which corresponds to a weak C-H?π interaction. In the crystal packing of 2, neither perpendicular nor stacking motifs are observed. The supramolecular architecture of this compound is determined by directional specific interactions of the hydrogen bond type between the C-H benzene fragment and the ester oxygen atom of the neighboring molecule.     

First crystal structure of a pentacyclic biomarker: 4,4,6a,12b,14b-Pentamethyl-1,2,3,4,4a,5,6,6a,6b,7, 8,12b,13,14,14a,14b-hexadecahydropicen-3β-ol

The crystal structure of the title compound (1, C₂₇H₄₀O 1/8 H₂O) synthesized via photo-induced electron transfer promoted biomimetic cascade cyclization, is reported. Compound 1 crystallizes in the monoclinic space group C2/c, with cell parameters a = 53.982(4), b = 7.1889(6), c = 11.170(1) Å, = 96.86(1). The structure adopts a fully trans-fused ring junction with all methyl groups axially oriented. The geometrical aspects associated with 1 were discussed and compared with the previously determined X-ray data for structurally analogous compounds.     

Absolute configuration of C27H41NO8Si, the diastereoselective product of the rhodium(II) octanoate catalyzed decomposition of a chiral vinyldiazomethane in the presence of 2-acetylpyrrole: A structural study rendered difficult by disorder of a “heavy” atom

The title compound crystallized in space group P2₁ with a = 12.647(8), b = 14.542(4), c = 17.077(16) Å, = 97.51(6)°, and D _calc = 1.143 mg/m³ for Z = 4. There are two chemically-equivalent C₂₇H₄₁NO₈Si molecules in the crystallographic asymmetric unit. Each contains three chiral centers, one of known absolute configuration based upon the (R)-pantolactone,—CH*—CMe₂—CH₂—O—CO—, moiety. The structural study was greatly complicated by disorder of an —OSiMe₂(t-Bu) group in one molecule.     

Displacements in the cationic motif of nonstoichiometric fluorite phases Ba1 − x R x F2 + x as a result of the formation of {Ba8[R 6F68–69]} clusters: I. The Ba0.78Tm0.22F2.22 crystals

The displacements of Ba²⁺ cations in the cationic motif of Ba_0.78Tm_0.22F_2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba_{1 ? x} R _x F_{2 + x} , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba₈[R ₆F_68–69]} superclusters of structural defects with nanometer linear sizes. The Ba ²⁺ cations are displaced from the fluorite crystallographic positions 4a (space group Fm $ \bar 3 The displacements of Ba²⁺ cations in the cationic motif of Ba_0.78Tm_0.22F_2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba_{1 − x} R _x F_{2 + x} , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba₈[R ₆F_68–69]} superclusters of structural defects with nanometer linear sizes. The Ba ²⁺ cations are displaced from the fluorite crystallographic positions 4a (space group Fm m) to the positions 32f. The static nature of the cation displacements is confirmed by the fact that these displacements are retained at a temperature of 110 K. The correctness of the interpretation of the correlation between the cation displacements and the formation of superclusters of structural defects is supported by the coincidence of the intercationic distances determined in the disordered phase Ba_0.78Tm_0.22F_2.22 with those found in the previously studied ordered phases Ba₄ R ₃F₁₇ (R = Y, Yb). The model with splitting of cationic positions is appropriate for testing in structural investigations of crystals of the fluorite phases M _{1 − x} R _x F_{2 + x} at room temperature. Original Russian Text ? A.M. Golubev, L.P. Otroshchenko, V.N. Molchanov, B.P. Sobolev, 2008, published in Kristallografiya, 2008, Vol. 53, No. 6, pp. 1023–1030.     

Synthesis,X-ray Crystal Structure and Magnetic Study of a μ<Subscript>1,5</Subscript>-dca Bridged Dimeric Copper(II) Complex

Abstract  

The reaction of aqueous solution of copper(II) nitrate trihydrate with methanolic solution of the ligand HL and sodium dicyanamide in aqueous medium results in the formation of a dimeric dicyanamide complex of Cu(II), [Cu₂(L)₂(μ_1,5-dca)₂]·0.5 H₂O (1) [where L = 1-(N-salicylideneamine)-2-(N-ethyl)-aminoethane]. The single crystal X-ray structure reveals that the asymmetric unit of complex 1 consists of two dinuclear units. The complex crystallizes in the monoclinic space group P2₁ with cell parameters, a = 9.8828(19) ?, b = 19.018(4) ?, c = 14.851(3) ?, (°) = 92.979(6) and Z = 4. χ_M T, stays in the 0.94–0.91 cm³ mol⁻¹ K range between 300 and 2 K, which is slightly higher than the spin-only value (χ_M T = 0.75 cm³ mol⁻¹ K) for two uncoupled copper(II) (S = ?) ions assuming g = 2.0, thus indicating that the complex 1 behaves like a simple paramagnet.     

Poly-μ-2,5-dimethylpyrazine-μ-dithiocyanato-N, S,S-dicopper(I): a three-dimensional coordination polymer containing both molecular and anionic rod ligands

Reaction of CuSCN with 2,5-dimethylpyrazine (2,5-me₂pyz) yields a polymeric material, [Cu₂(SCN)₂(2,5-me₂pyz)], which has been crystallographically characterized. The compound crystallizes in the monoclinic space group P2₁/N with a = 5.6850(2) Å, b = 6.92877(3) Å, c = 14.3575(6) Å, = 93.435(1), and Z = 2. It contains [Cu(SCN)] sheets in which each thiocyanate ligand acts in a -N,S,S-bridging mode. The sheets are bridged by 2,5-dimethylpyrazine ligands to generate a three dimensional network. The compound has also been characterized by infrared spectroscopy and thermogravimetric analysis.