Temperature-dependent UV absorption of biphenyl based on intra-molecular rotation investigated within a combined experimental and TD-DFT approach

<p>The temperature dependence of the UV absorption spectra of biphenyl in solution is herein reported. The UV spectral shapes vary with an increase in temperature, where a decrease in intensity (hypochromism) and blue shifts are observed. The predicted UV spectra in the Time Dependent-Density Functional Theory  (TD-DFT) framework within the B97D3 functional indicate a lowering of the biphenyl conjugation band and a blue shift of the spectra upon increasing the biphenyl twist dihedral angle.</p

Figure 4

<p>(A) Particles on the cell membrane. Energy levels (in kiloelectron volts) are indicated on the x axis. The emission lines for selenium are at 1.37 keV (peak SeLa), 11.22 keV (peak SeKa), and 12.49 keV (peak SeKb). (B) Particles in the culture medium. Energy levels (in kiloelectron volts) are indicated on the x axis. The emission lines for selenium are at 1.37 keV (peak SeLa), 11.22 keV (peak SeKa), and 12.49 keV (peak SeKb).</p

Transmission electron micrograph of <i>Rhodopseudomonas palustris</i> strain N.

<p>A, cells cultured without selenite; B–C, cells grown in the presence of selenite are showing electron-dense particles formed by the strain N; D, particles free in the medium, a: particles.</p

Culture of <i>Rhodopseudomonas palustris</i> strain N in medium containing selenite.

<p>A, at the start of cultivation; B, at the 2^nd day of cultivation; C, at the 8^th day of cultivation. Control: strain cultured in a medium without selenite; Treatment: strain cultured in medium containing selenite.</p

Time course of growth and selenite reduction by <i>Rhodopseudomonas palustris</i> strain N.

<p>Symbols: ▪, cell protein; ⧫, selenite concentration.</p

Transmission electron micrograph of the cells cultured without selenite (A) and exposed to 1.0 m mol/L selenite (B).

<p>Arrows indicate the paticles.</p

Novel Chemical Synthesis and Characterization of CeTi₂O₆ Brannerite

Cerium titanate CeTi₂O₆ was prepared by a new soft chemistry route in aqueous solution. A suite of characterization techniques, including X-ray diffraction, thermal analysis, vibrational spectroscopy, and scanning and transmission electron spectroscopy, were employed to investigate the brannerite structure formation and its bulk properties. The synthesized powder formed the brannerite crystal structure upon calcination at temperatures as low as 800 °C. Samples sintered at 1350 °C possess a high level of crystallinity. X-ray absorption near-edge structure results indicate the presence of six-coordinated Ce⁴⁺ in the brannerite samples

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO_4–<i>x</i>: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO₄ undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO_4–<i>x</i>, in space group <i>R</i>3̅<i>m</i> with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO_4–<i>x</i>, in space group <i>P</i>1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO_4–<i>x</i>. Cooling δ-SrUO_4–<i>x</i> toward room temperature results in the reformation of the rhombohedral phase α-SrUO_4–<i>x</i> with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO_4–<i>x</i> to enable the transformation to δ-SrUO_4–<i>x</i> but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO_4–<i>x</i> at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector <b>k</b> = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO_4–<i>x</i> can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO_4–<i>x</i> structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations

Oligo[2]catenane That Is Robust at Both the Microscopic and Macroscopic Scales

Polycatenanes are extremely attractive topological architectures on account of their high degrees of conformational freedom and multiple motion patterns of the mechanically interlocked macrocycles. However, exploitation of these peculiar structural and dynamic characteristics to develop robust catenane materials is still a challenging goal. Herein, we synthesize an oligo[2]catenane that showcases mechanically robust properties at both the microscopic and macroscopic scales. The key feature of the structural design is controlling the force-bearing points on the metal-coordinated core of the [2]catenane moiety that is able to maximize the energy dissipation of the oligo[2]catenane via dissociation of metal-coordination bonds and then activation of sequential intramolecular motions of circumrotation, translation, and elongation under an external force. As such, at the microscopic level, the single-molecule force spectroscopy measurement exhibits that the force to rupture dynamic bonds in the oligo[2]catenane reaches a record high of 588 ± 233 pN. At the macroscopic level, our oligo[2]catenane manifests itself as the toughest catenane material ever reported (15.2 vs 2.43 MJ/m3). These fundamental findings not only deepen the understanding of the structure-property relationship of poly[2]catenanes with a full set of dynamic features but also provide a guiding principle to fabricate high-performance mechanically interlocked catenane materials

Gradual Structural Evolution from Pyrochlore to Defect-Fluorite in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇: Average vs Local Structure

We have studied the long-range average and local structures in Y₂Sn_2–<i>x</i>Zr_<i>x</i>O₇ (<i>x</i> = 0–2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defect-fluorite at <i>x</i> ∼ 1.0–1.2, X-ray absorption near-edge structure (XANES) results at the Zr L₃- and Y L₂-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defect-fluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by <i>ab initio</i> atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y₂Zr₂O₇ defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively