Crystal structures and hydrogen bonding in the morpholinium salts of four phenoxyacetic acid analogues

The anhydrous salts morpholinium (tetrahydro-2-H-1,4-oxazine) phenxyacetate, C4H10NO+ C8H7O3- (I), (4-fluorophenoxy)acetate, C4H10NO+ C8H6FO3- (II) and isomeric morpholinium (3,5-dichlorophenoxy)acetate (3,5-D) (III) and morpholinium (2,4-dichlorophenoxy)acetate (2,4-D), C4H10NO+ C8H5Cl2O3- (IV), have been determined and their hydrogen-bonded structures are described. In the crystals of (I), (III) and (IV), one of the the aminium H atoms is involved in a three-centre asymmetric cation-anion N-H...O,O' R2/1(4) hydrogen-bonding interaction with the two carboxyl O-atom acceptors of the anion. With the structure of (II), the primary N---H...O interaction is linear. In the structures of (I), (II) and (III), the second N-H...O(carboxyl) hydrogen bond generates one-dimensional chain structures extending in all cases along [100]. With (IV), the ion pairs are linked though inversion-related N-H...O hydrogen bonds [graph set R2/4(8)], giving a cyclic heterotetrameric structure.

Hydrogen bonding in peptide helices - Analysis of two independent helices in the crystal structure of a peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe

The crystal structure determination of the heptapeptide Boc-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe reveals two peptide helices in the asymmetric unit, Crystal parameters are: space group P2(1), a = 10.356(2) Angstrom, b = 19.488(5) Angstrom, c = 23.756(6) Angstrom, beta = 102.25(2)degrees), V = 4685.4 Angstrom(3), Z = 4 and R = 5.7% for 7615 reflections [I>3 sigma(I)]. Both molecules adopt largely alpha-helical conformations with variations at the C-terminus, Helix type Is determined by analysing both 4-->1 and 5-->1 hydrogen-bond interactions and comparison with the results of analysis of protein structures. The presence of two 4-->1 hydrogen-bond interactions, besides four 5-->1 interact ions in both the conformations provides an opportunity to characterize bifurcated hydrogen bonds at high resolution, Comparison of the two helical conformations with related peptide structures suggests that distortions at the C-terminus are more facile than at the N-terminus.

Substrate, molecular structure, and solvent effects in 2D self-assembly via hydrogen and halogen bonding

Recently, halogen···halogen interactions have been demonstrated to stabilize two-dimensional supramolecular assemblies at the liquid–solid interface. Here we study the effect of changing the halogen, and report on the 2D supramolecular structures obtained by the adsorption of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) and 2,4,6-tris(4-iodophenyl)-1,3,5-triazine (TIPT) on both highly oriented pyrolytic graphite and the (111) facet of a gold single crystal. These molecular systems were investigated by combining room-temperature scanning tunneling microscopy in ambient conditions with density functional theory, and are compared to results reported in the literature for the similar molecules 1,3,5-tri(4-bromophenyl)benzene (TBPB) and 1,3,5-tri(4-iodophenyl)benzene (TIPB). We find that the substrate exerts a much stronger effect than the nature of the halogen atoms in the molecular building blocks. Our results indicate that the triazine core, which renders TBPT and TIPT stiff and planar, leads to stronger adsorption energies and hence structures that are different from those found for TBPB and TIPB. On the reconstructed Au(111) surface we find that the TBPT network is sensitive to the fcc- and hcp-stacked regions, indicating a significant substrate effect. This makes TBPT the first molecule reported to form a continuous monolayer at room temperature in which molecular packing is altered on the differently reconstructed regions of the Au(111) surface. Solvent-dependent polymorphs with solvent coadsorption were observed for TBPT on HOPG. This is the first example of a multicomponent self-assembled molecular networks involving the rare cyclic, hydrogen-bonded hexamer of carboxylic groups, R66(24) synthon.

Glass forming ability and stability: Ternary Cu bearing Ti, Zr, Hf alloys

Four Cu bearing alloys of nominal composition Zr25Ti25Cu50, Zr34Ti16Cu50, Zr25Hf25Cu50 and Ti25Hf25Cu50 have been rapidly solidified in order to produce ribbons. All the alloys become amorphous after meltspinning. In the Zr34Ti16Cu50 alloy localized precipitation of cF24 Cu5Zr phase can be observed in the amorphous matrix. The alloys show a tendency of phase separation at the initial stages of crystallization. The difference in crystallization behavior of these alloys with Ni bearing ternary alloys can be explained by atomic size, binary heat of mixing and Mendeleev number. It has been observed that both Laves and Anti-Laves phase forming compositions are suitable for glass formation. The structures of the phases, precipitated during rapid solidification and crystallization can be viewed in terms of Bernal deltahedra and Frank-Kasper polyhedra.

Hydrogen and fluorine in crystal engineering: systematics from crystallographic studies of hydrogen bonded tartrate-amine complexes and fluoro-substituted coumarins, styrylcoumarins and butadienes

Three aspects of crystal engineering in molecular crystals are presented to emphasize the role of intermolecular interactions and factors influencing crystal packing. Hydrogen bonded tartrate-amine complexes have been analyzed with the propensity for formation of multidirectional hydrogen bonding as a key design element in the generation of materials for second harmonic generation (SHG). The invariance of the framework in DBT and its possible implications on SHG is outlined. The role of Fluorine in orienting molecules of coumarins, styrylcoumarins and butadienes for photodimerization is described with particular emphasis on its steering capability. Usage of coumarin as an design element for the generation of polymorphs of substituted styrylcoumarins is examined with specific examples.

Metal-free graphitic carbon nitride as mechano-catalyst for hydrogen evolution reaction

Graphitic carbon nitride (g-C3N4), as a promising metal-free catalyst for photo-catalytic and electrochemical water splitting, has recently attracted tremendous research interest. However, the underlying catalytic mechanism for the hydrogen evolution reaction (HER) is not fully understood. By using density functional theory calculations, here we have established that the binding free energy of hydrogen atom (ΔGH∗0) on g-C3N4 is very sensitive to mechanical strain, leading to substantial tuning of the HER performance of g-C3N4 at different coverages. The experimentally-observed high HER activity in N-doped graphene supported g-C3N4 (Zheng et al., 2014) is actually attributed to electron-transfer induced strain. A more practical strategy to induce mechanical strain in g-C3N4 is also proposed by doping a bridge carbon atom in g-C3N4 with an isoelectronic silicon atom. The calculated ΔGH∗0 on the Si-doped g-C3N4 is ideal for HER. Our results indicate that g-C3N4 would be an excellent metal-free mechano-catalyst for HER and this finding is expected to guide future experiments to efficiently split water into hydrogen based on the g-C3N4 materials.

Resolidification behaviour of laser surface-melted metals and alloys

The solidification behaviour is described of two pure metals (Bi and Ni) and two eutectic alloys (A1-Ge and AI-Cu) under nonequilibrium conditions, in particular the microsecond pulsed laser surface melting. The resolidification behaviour of bismuth shows that epitaxial regrowth is the dominant mechanism. For mixed grain size, regrowth of larger grains dominates the microstructure and can result in the development of texture. In the case of nickel, epitaxial growth has been noted. For lower energy pulse-melted pool, grain refinement takes place, indicating nucleation of fresh nickel grains. The A1-Ge eutectic alloy indicates the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density laser irradiation. An equiaxed microstructure containing the same monoclinic phase is obtained at a lower power density laser irradiation. It is shown that the requirement of solution partition acts as a barrier to eutectic regrowth from the substrate. The laser-melted pool of A1-Cu eutectic alloy includes columnar growth of c~-A1 and 0-A12Cu phase followed by the dendritic growth of A12Cu phase with ct-Al forming at the interdendritic space. In addition, a banded microstructure was observed in the resolidified laser-melted pool.

Effect of microstructure on the mechanical behavior of b-modified ti-6al-4v alloys

Small additions of B to Titanium alloys refine the as-cast microstructure significantly and hence improve their mechanical performance. In this work, tensile, fracture and fatigue properties of the as-cast and HIPed Ti-6Al-4V alloy with hypoeutectic wt.% of B additions have been examined, with particular emphasis on identifying the microstructural length scale that controls the mechanical properties of these alloys.

Pressure transitions and electron transport behaviour of crystalline Se-Te alloys

Pressure transitions of Se-Te alloys have been studied over the entire range of compositions. Conductivities have also been measured as a function of temperature and alloy composition. Transition pressures, activation barriers and isothermal conductivities exhibit distinct changes of slope in their variation as a function of composition at about 8 at % of Te. Transition pressures change slope at not, vert, similar 35% Te also. An attempt has been made to explain these observations on the basis of the size effect of Te which, in turn, affects the electron energy dispersions in the band structure.

Subsurface deformation in dry sliding of hypo-eutectic Al-Si alloys

The subsurface deformation during dry sliding of Al-Si alloys is studied by fragmentation of silicon particles. The size of the fragmented particles does not vary with load. The depth of deformation is found to increase with increase in normal load. This experimental observation agrees with load-deformation depth characteristics obtained by a slip line field model.

The nature of non-equilibrium solidification of undercooled Agsingle bondCu alloys in contact with primary copper dendrites

Liquids of silver-copper alloys with near eutectic compositions embedded in a copper matrix were undercooled. The structural and microstructural investigations of these alloys solidified from undercooled temperature indicated the absence of both the eutectic reaction and diffusionless transformation below the equal free energy curve (T0). Instead the liquid maintained local equilibrium with the copper dendrites continuously until it intersected the extended solidus of the silver rich solid solution.

Pressure-Induced transitions in amorphous TlxSe100-x alloys

The electrical resistivity of bulk semiconducting amorphous TlxSe100-x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Aring and c = 4·95 Aring and TlSe having a tetragonal structure with a = 8·0 Aring and c = 7·0 Aring

Zeeman effect of chlorine nuclear quadrupole resonance and hydrogen bonding in 2,6-, 2,5- and 3,5-dichlorophenols

The Zeeman effect of chlorine nuclear quadrupole resonance in polycrystalline samples of 2,6-, 2,5 and 3,5-dichlorophenol has been investigated at room temperature in order to study the effect of hydrogen bonding on the electric field gradient asymmetry parameter n. While the two n.q.r. lines in 3,5-dichlorophenol gave an asymmetry parameter of 10%, those in 2,6- and 2,5-dichlorophenol gave different values of n for the two chlorines. The chlorine atom which is ortho to the OH group and involved in hydrogen bonding (i.e., corresponding to the low frequency line) gave an asymmetry parameter of 0.21 in 2,6-dichlorophenol and 0.17 in 2,5-dichlorophenol while the other chlorine (i.e., corresponding to the high frequency line) gave a lower value of 0.12 in 2,6-dichlorophenol and 0.11 in 2,5-dichlorophenol. These values of n are discussed in terms of hydrogen bonding and bond parameters.

Characterization of dry sliding wear of Al---Si alloys

Controversy exists in the published literature as to the effect of silicon content and pressure on the dry sliding wear of Al---Si alloys. The present paper attempts to clarify the question by reporting a statistical analysis of data obtained from factorially designed experiments conducted on a pinon-disc machine in the pressure range 0.105–1.733 MPa and speed range 0.19–0.94 m s−1. Under these conditions it was found that, in the range 4–24 wt.% Si, wear of binary unmodified alloys does not significantly differ between the alloys. However, it is significantly less than that corresponding to an alloy containing no silicon. The effect of pressure on wear rate was found to be linear and monotonie and, over the narrow range of speeds used, the wear rate was found to be unaffected by speed. The coefficient of friction was found to be insensitive to variations in silicon content, pressure and speed.