Yu-Chun Chuang

Bond characterization on a Cr-Cr quintuple bond: a combined experimental and theoretical study.

A combined experimental and theoretical charge density study on a quintuply bonded dichromium complex, Cr(2)(dipp)(2) (dipp = (Ar)NC(H)N(Ar) and Ar = 2,6-i-Pr(2)-C(6)H(3)), is performed. Two dipp ligands are bridged between two Cr ions; each Cr atom is coordinated to two N atoms of the ligands in a linear fashion. The Cr atom is in a low oxidation state, Cr(I), and in low coordination number condition, which stabilizes a metal-metal multiple bond, in this case, a quintuple bond. Indeed, it gives an ultrashort Cr-Cr bond distance of 1.7492(1) Å in the complex. The bond characterization of such a quintuple bond is undertaken both experimentally by high-resolution single-crystal X-ray diffraction and theoretically by density functional calculation (DFT). Electron densities are depicted via deformation density and Laplacian distributions. Bond characterizations of the complex are presented in terms of topological properties, Fermi hole function, source function (SF), and natural bonding orbital (NBO) analysis. The electron density at the Cr-Cr bond critical point (BCP) is 1.70 e/Å(3), quite a high value for metal-metal bonding and mainly contributed from the metal ion itself. The quintuple bond is confirmed with one σ, two π, and two δ interactions by NBO analysis and Fermi hole function. The molecular orbitals (MOs) illustrate that five bonding orbitals are predominantly contributed from the 3d orbitals of the Cr(I) ion. The effective bond order from NBO analysis is 4.60. The detail comparison between experiment and theory will be given. Additionally, three closely related complexes are calculated for systematic comparison.

Crystal engineering from a 1D chain to a 3D coordination polymer accompanied by a dramatic change in magnetic properties.

A unique structural transformation in the crystalline state assisted by coordination substitution is induced during a dehydration process. A 1D chain coordination polymer is irreversibly converted to a 3D interpenetrated network accompanied by a change in magnetic properties from a paramagnetic material to a spin crossover system.

Mechanical Performance of Spider Silk Is Robust to Nutrient-Mediated Changes in Protein Composition

Spider major ampullate (MA) silk is sought after as a biomimetic because of its high strength and extensibility. While the secondary structures of MA silk proteins (spidroins) influences silk mechanics, structural variations induced by spinning processes have additional effects. Silk properties may be induced by spiders feeding on diets that vary in certain nutrients, thus providing researchers an opportunity to assess the interplay between spidroin chemistry and spinning processes on the performance of MA silk. Here, we determined the relative influence of spidroin expression and spinning processes on MA silk mechanics when Nephila pilipes were fed solutions with or without protein. We found that spidroin expression differed across treatments but that its influence on mechanics was minimal. Mechanical tests of supercontracted fibers and X-ray diffraction analyses revealed that increased alignment in the amorphous region and to a lesser extent in the crystalline region led to increased fiber strength and extensibility in spiders on protein rich diets.

Bond Characterization of a Unique Thiathiophthene Derivative: Combined Charge Density Study and X-ray Absorption Spectroscopy

Thiathiophthene (TTP), a planar molecule with two fused heterocyclic five-membered rings and an essentially linear S-S-S bond, is a molecule of great interest due to its unique chemical bondings. To elucidate the remarkable bonding nature, a combined experimental and theoretical study on the electron density distribution of 2,5-dimethyl-3,4-trimethylene-6a-TTP (1) is investigated based on a multipole model through high-resolution X-ray diffraction data experimentally and on the density functional calculations (DFT) theoretically. In addition, S K-edge X-ray absorption spectroscopy (XAS) is measured to verify the chemical bonding concerning the sulfur atoms. The molecule can be firmly described as 10π electron with aromatic character among the eight atoms, S3C5, of the two fused five-membered rings plus three-center four-electron σ character along the S-S-S bond. Such bonding description is verified with the calculated XAS spectrum, where the pre-edge absorption for transitions from S 1s to π* and σ* are located. The three-center four-electron S-S-S σ bond makes the terminal S atoms richer in electron density than the central one.

Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and Ni Ions

BaFe xNi1- xO3 with end members of BaNiO3 ( x = 0) and BaFeO3 ( x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba4Fe3NiO12 ( x = 0.75), that adopts the 12R perovskite structure with the space group R3̅ m ( a = 5.66564(7) Å and c = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. The magnetic properties of the compound are also discussed.

Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5

A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ → 0.75Fe3+ + 0.25Fe5+) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R3̅c crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3+ spins align antiferromagnetically and one-fourth of the Fe3+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins.

Charge density studies of 3d metal (Ni/Cu) complexes with a non-innocent ligand

High-resolution X-ray diffraction experiments and atom-specific X-ray absorption experiments are applied to investigate a series of square planar complexes with the non-innocent ligand of maleonitriledithiolate (mnt), [S2C2(CN)2]z-, containing M-S bonds. Four complexes of (PyH)z[M(mnt)2]z-, where M = Ni or Cu, z = 2 or 1 and PyH+ = C5NH6+, were studied in order to clarify whether such one-electron oxidation-reduction, [M(mnt)2]2-/[M(mnt)2]1-, is taking place at the metal or the ligand site. Combining the techniques of metal K-, L-edge and S K-edge X-ray absorption spectroscopy with high-resolution X-ray charge density studies, it is unambiguously demonstrated that the electron redox reaction is ligand based and metal based for Ni and Cu pairs, respectively. The bonding characters in terms of topological properties associated with the bond critical points are compared between the oxidized form [ML]- and the reduced form [ML]2-. In the case of Ni complexes, the formal oxidation state of Ni remains as Ni2+ and each mnt ligand carries a 2- charge in [Ni(mnt)2]2-, but only one of the ligands is formally oxidized in [Ni(mnt)2]1-. In contrast, in the case of Cu complexes, the mnt remains as 2- in both complexes, but the formal oxidation states of the metal are Cu2+ and Cu3+. Bond characterizations and d-orbital populations will be presented. The complementary results of XAS, XRD and DFT calculations will be discussed. The conclusion on the redox reactions in these complexes can be firmly established.

Syngenite and gypsum minerals in aging Equisetum arvense

Equisetum arvense (horsetail grass) is one of the most ancient species known as living fossil plants which has long history and can be traced back to the age of dinosaurs. Equisetum arvense is also considered as one of the most important source of biogenic silica because of its ability of accumulating a great quantity of silica in all parts of the plant. Valentin Valtchev et al [1] reported the in situ zeolitization of a vegetal macrotemplate, induced by the biogenic silica of a fresh dry Equisetum arvense. Recently, Anna Sola‐Rabada et al [2] report the isolation of a wide range of minerals from Equisetum arvense by a thermal treatment. As heat treatment, higher levels of minerals has been reported such as silica, calcium, KCl, calcium sulphate and/or calcium silicates. A fresh and aging Equisetum arvense, which has been stored for two years in ambient condition, have been studied by Synchrotron powder X-ray diffraction (PXRD) method. No clear crystallite was found in fresh sample at room temperature, surprisingly, large amount of Syngenite (K2Ca(SO4)2·2H2O), Gypsum (CaSO4·H2O) and other minerals have appeared in aging sample but no silica crystalline materials was found. The in situ thermal treatment has been applied to investigate the crystals and their phase transitions by focusing the X-ray beam onto single stem. We will report the formation of minerals and their phase transformation in detail. [1]. Valentin Valtchev, et al. (2003). Angew. Chem. Int. Ed. 42, 2782 – 2785. [2]. Anna Sola‐Rabada, et al., (2016) J. Biol. Inorg. Chem. 21, 101–112.

Nutrient and Wind Effects on Dragline Properties: Perspectives from WAXS & SAXS

Spider dragline silk is one of the strongest nature fibers and some of their features are even better than those of the best synthetic fibers. Understanding the mechanisms inducing silk variability may have implications for biomimetics and the synthesis of environmentally responsive materials. Dragline silk contains both elasticity (amorphous) and crystalline regions. Our previous studies had demonstrated that spiders might vary the protein composition and thus physical properties of silks when experiencing food with different nutrient level. In this study we fed Nephila pilipes with high, low and no protein foods and collected their dragline silks for synchrotron Radiation (SR) wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) examinations. The WAXS data showed a significant difference in crystalline fractions of dragline silks produced by N. pilipes experiencing different food treatments. In addition, the orientation of crystallines also varied considerably among silks produced by spiders in three treatment groups. The SAXS data, obtained with the beam incident along and perpendicular to the fiber axis revealed a mesostructure comprising nano crystallites (beta sheets) stack spirally along the spider fibril axis. Such results indicate that spiders experiencing different nutrient stress level might produce dragline silks of different physical properties due to variations in crystalline density, orientation and the meso-phase structures in nano scale. Furthermore, varying environmental wind strength leads to changes in tensile mechanics of spider dragline silk hence produced. Exposing the spider Cyclosa mulmeinensis to controlled stress from constant airflow, we found correlated changes in (i) amino acid composition, (ii) tensile mechanics and (iii) crystallinity, of the dragline silk; which results suggest that protein variation and/or post secretion crystalline variations are associated with the mechanical properties of the spider silks.