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131.
High pressure investigations in CBOOA have provided the first example of a liquid crystal phase transition in which the volume discontinuity is negative upon heating. The nematic-smetic A transition is found to be first order at all pressures. 相似文献
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134.
For the linear chain system Y2[Pt(CN)4]3·21H2O a pressure induced phase transition is observed by emission spectroscopy. At ptrans=(5±0.5) kbar and T=295 K the compound undergoes a first order phase transition, in the course of which the intra-chain Pt-Pt distance R shrinks by . An approximate value had already been found at standard pressure for a temperature induced phase transition (Ttrans=218 K). 相似文献
135.
Brian S. Daniels Xintong Hou Stephanie A. Corio Lindsey M. Weissman Prof. Dr. Vy M. Dong Prof. Dr. Jennifer S. Hirschi Dr. Shaozhen Nie 《Angewandte Chemie (International ed. in English)》2023,62(36):e202306511
We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a CuI-phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation. 相似文献
136.
Goodman JL Petersson EJ Daniels DS Qiu JX Schepartz A 《Journal of the American Chemical Society》2007,129(47):14746-14751
Proteins composed of alpha-amino acids are essential components of the machinery required for life. Stanley Miller's renowned electric discharge experiment provided evidence that an environment of methane, ammonia, water, and hydrogen was sufficient to produce alpha-amino acids. This reaction also generated other potential protein building blocks such as the beta-amino acid beta-glycine (also known as beta-alanine); however, the potential of these species to form complex ordered structures that support functional roles has not been widely investigated. In this report we apply a variety of biophysical techniques, including circular dichroism, differential scanning calorimetry, analytical ultracentrifugation, NMR and X-ray crystallography, to characterize the oligomerization of two 12-mer beta3-peptides, Acid-1Y and Acid-1Y*. Like the previously reported beta3-peptide Zwit-1F, Acid-1Y and Acid-1Y* fold spontaneously into discrete, octameric quaternary structures that we refer to as beta-peptide bundles. Surprisingly, the Acid-1Y octamer is more stable than the analogous Zwit-1F octamer, in terms of both its thermodynamics and kinetics of unfolding. The structure of Acid-1Y, reported here to 2.3 A resolution, provides intriguing hypotheses for the increase in stability. To summarize, in this work we provide additional evidence that nonnatural beta-peptide oligomers can assemble into cooperatively folded structures with potential application in enzyme design, and as medical tools and nanomaterials. Furthermore, these studies suggest that nature's selection of alpha-amino acid precursors was not based solely on their ability to assemble into stable oligomeric structures. 相似文献
137.
We report a novel surface-tension driven instability observed for droplets spreading on a compliant substrate. When a droplet is released on the surface of an agar gel, it forms arms or cracks when the ratio of surface-tension gradient to gel strength is sufficiently large. We explore a range of gel strengths and droplet surface tensions and find that the onset of the instability and the number of arms depend on the ratio of surface tension to gel strength. However, the arm length grows with an apparently universal law L proportional t(3/4). 相似文献
138.
Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200–2730°C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp 2 type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp 2 content of ~70%, rapidly increasing to ~90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon (π?+?σ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density (ρ) is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied σ* states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C–C bond length from core loss EELS spectra with an accuracy of ±0.1?pm. This method suggests an average bond length of 1.44?Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42?Å as both the heteroatom content and proportion of non-sp 2-type hybridized carbon atoms decrease. 相似文献
139.
Sauerbrey S Majhi PK Daniels J Schnakenburg G Brändle GM Scherer K Streubel R 《Inorganic chemistry》2011,50(3):793-799
Metalation reactions were studied of a sterically demanding imidazole derivative, namely, 1-tert-butylimidazole (1), with different metalation reagents and subsequent reaction with diphenylchlorophosphane. The reaction product, 1-tert-butyl-2-diphenylphosphino-imidazole (2), was subjected to oxidation and complexation reactions to yield the corresponding products Ph(2)(Imi)P-E (E = O (3), S (4), Se (5), W(CO)(5) (8)) and in the case of borane-THF the N-BH(3) coordination product 10 was obtained. The analytical data of the new compounds are discussed, including X-ray diffraction studies of 3-5. 相似文献
140.
Alkynyl-substituted phenyldithiafulvenes have been found to act as versatile building blocks for the construction of π-conjugated molecular rods, shape-persistent macrocycles (SPMs), and conducting polymers. Through Cu(I)-catalyzed alkynyl homocoupling, a series of linear-shaped π-extended tetrathiafulvalene analogues (exTTFs) carrying conjugated oligoynes (ranging from diyne to hexayne) as the central π-bridge were readily prepared. The solid-state properties and reactivities of diyne- and tetrayne-centered exTTFs were characterized by X-ray crystallography and differential scanning calorimetry (DSC), while the electronic properties of the oligoyne-exTTFs were elucidated by UV-vis absorption spectroscopy and density functional theory (DFT) calculations. Cyclic voltammetric analysis showed that the terminal phenyldithiafulvene groups of the oligyne-exTTFs could undergo oxidative coupling to form tetrathiafulvalene vinylogue (TTFV)-linked polymer wires. Through a different synthetic route involving oxidative dimerization and Pd/Cu-catalyzed alkynyl homocoupling, the acetylenic phenyldithiafulvene precursors led to shape-persistent macrocycles where the formation of trimeric macrocycles was particularly favored due to the small ring strain incurred. Finally, spectroelectrochemical studies on these oligoyne and TTF hybrid materials disclosed electrochromic and molecular redox-controlled switching properties applicable to molecular electronic and optoelectronic devices. 相似文献