Superconducting Double Perovskite Bismuth Oxide Prepared by a Low‐Temperature Hydrothermal Reaction

Perovskite‐type structures (ABO₃) have received significant attention because of their crystallographic aspects and physical properties, but there has been no clear evidence of a superconductor with a double‐perovskite‐type structure, whose different elements occupy A and/or B sites in ordered ways. In this report, hydrothermal synthesis at 220 °C produced a new superconductor with an A‐site‐ordered double perovskite structure, (Na_0.25K_0.45)(Ba_1.00)₃(Bi_1.00)₄O₁₂, with a maximum T_c  of about 27 K.     

18π‐Electron Tautomeric Benziphthalocyanine: A Functional Near‐Infrared Dye with Tunable Aromaticity

Dihydroxybenziphthalocyanine 1 , with bulky aryloxy groups, has been synthesized and characterized by X‐ray crystallography, NMR and UV/Vis‐NIR spectroscopy, and theoretical calculations. Macrocycle 1 is the first example of an aromatic benziphthalocyanine with an 18π‐electron structure, and was found to exist as an equilibrium mixture of weakly aromatic and strongly aromatic tautomers. The aromaticity and near‐IR absorption can be controlled by chemical modification at the reactive resorcinol moiety and by variation of the solvent.     

Bisecting a 4-connected graph with three resource sets

Let G=(V,E) be an undirected graph with a node set V and an arc set E. G has k pairwise disjoint subsets T₁,T₂,…,T_k of nodes, called resource sets, where |T_i| is even for each i. The partition problem with k resource sets asks to find a partition V₁ and V₂ of the node set V such that the graphs induced by V₁ and V₂ are both connected and |V₁∩T_i|=|V₂∩T_i|=|T_i|/2 holds for each i=1,2,…,k. The problem of testing whether such a bisection exists is known to be NP-hard even in the case of k=1. On the other hand, it is known that if G is (k+1)-connected for k=1,2, then a bisection exists for any given resource sets, and it has been conjectured that for k?3, a (k+1)-connected graph admits a bisection. In this paper, we show that for k=3, the conjecture does not hold, while if G is 4-connected and has K₄ as its subgraph, then a bisection exists and it can be found in O(|V|³log|V|) time. Moreover, we show that for an arc-version of the problem, the (k+1)-edge-connectivity suffices for k=1,2,3.     

A vibronic state model of the invar problem

A new model is proposed to explain the Invar effect of Fe-Ni alloys and related problems, which an admixing of low and high spin states in vibrating Fe atoms and an itinerant electron ferromagnetism for the admixed state are taken into account.     

Total Synthesis of Siomycin A: Construction of Synthetic Segments

The five practical segments for the total synthesis of siomycin A, that is, the dehydropiperidine segment A ( 5 ), the pentapeptide segment B ( 3 ), the dihydroquinoline segment C ( 6 ), and the β‐phenylselenoalanine dipeptide segments D ( 7 ) and E ( 4 ), were synthesized. Segment A ( 5 ) was constructed by the coupling of the azomethine ylide and the chiral sulfinimine, followed by the stereoselective reduction of the six‐membered imine function. Segment B ( 3 ) was synthesized by the phenylselenylation of the β‐lactone, stereoselective vinylzinc addition to the chiral sulfinimine, and oxazoline–thioamide conversion. Segment C ( 6 ) was prepared by the one‐pot olefination of the tetrahydroquinoline N‐oxide using triflic anhydride and triethylamine, stereoselective reduction of the methyl ketone function, and regioselective Yb(OTf)₃‐catalyzed epoxide opening by the amino group. Segments D ( 7 ) and E ( 4 ) were synthesized by coupling of the properly protected β‐phenylselenoalanines.     

Total Synthesis of Siomycin A: Completion of the Total Synthesis

The total synthesis of siomycin A ( 1 ), a representative compound of the thiostrepton family of peptide antibiotics, was achieved by incorporating the five synthetic segments A ( 2 ), B ( 3 ), C ( 4 ), D ( 5 ), and E ( 6 ). The dehydropiperidine segment A ( 2 ) was esterified with the dihydroquinoline segment C ( 4 ), and the subsequent coupling with the β‐phenylselenoalanine dipeptide segment D ( 5 ) at the segment C portion followed by lactamization between the segments A and D gave segment A‐C‐D ( 27 ). This was amidated with the pentapeptide segment B ( 3 ) at the segment A portion followed by one‐pot cyclization (between segments A and B) and elongation (with the β‐phenylselenoalanine dipeptide segment E ( 6 ) at the segment A portion), thus furnishing siomycin A ( 1 ).     

Direct conformational analysis of a 10 nm long oligothiophene wire

Conformational variations of a 10 nm long oligothiophene wire comprising 24 thiophene rings on Au(111), which are related to the various straight and bent shapes of the long wires, have been directly visualized by scanning tunneling microscopy (STM). The local bending angles within the wire are well characterized as s-cis/s-trans configurations of individual thiophene rings. We find that the partial stabilization of the metastable s-cis conformation results in the wire bending, which should be influenced by solvent and substituents.     

Enantioselective hydrogenation of α-phenylcinnamic acids over cinchonidine-modified Pd/C commercial catalysts

Enantioselective hydrogenation of α-phenylcinnamic acid (PCA) and p,p′-dimethoxyphenylcinnamic acid (DMPCA) was studied over a variety of commercial 5 % Pd/C catalysts to reveal catalyst properties suitable for obtaining high enantioselectivity. The catalysts were characterized by CO adsorption, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). It is confirmed that pretreatment at 353 K under atmospheric pressure of H₂ before modification with cinchonidine is very effective for all the Pd/C catalysts used here to improve the selectivity and reaction rate. It is suggested that the distribution of Pd metal particles is crucial to attain high selectivity (ee% = 79 ± 1 for PCA, 89 ± 2 for DMPCA): a uniform or eggshell-type distribution of Pd is more suitable than an egg-white or egg-yolk-type distribution. It is also suggested that the dispersion of Pd metal particles controls the enantioselectivity over cinchonidine (CD)-modified Pd/C catalysts. XPS techniques are proposed to provide a convenient method to find desirable catalysts. The choice of such Pd/C catalysts could facilitate high-throughput guided study on highly enantioselective hydrogenation of α,β-unsaturated carboxylic acids.     

Large positive magnetoresistance of insulating organic crystals in the non-ohmic region

We report a large positive magnetoresistance ratio in insulating organic crystals theta-(ET)(2)CsZn(SCN)(4) at low temperatures at which they exhibit highly nonlinear current-voltage characteristics. Despite the nonlinearity, the magnetoresistance ratio is independent of the applied voltage. The magnetoresistance ratio depends little on the magnetic field direction and is described by a simple universal function of mu(B)B/k(B)T, where mu(B) is the Bohr magneton. The positive magnetoresistance may be caused by magnetic-field-induced parallel alignment of spins of mobile and localized electrons, and a resulting blockade of electrical conduction due to the Pauli exclusion principle.