Further synthetic studies on rifamycin s

Two new highly stereocontrolled syntheses of the optically active form of the aliphatic segment 2 of rifamycin S are described.     

Approximation of continuous time stochastic processes by a local linearization method

This paper investigates the rate of convergence of an alternative approximation method for stochastic differential equations. The rates of convergence of the one-step and multi-step approximation errors are proved to be and in the sense respectively, where is discrete time interval. The rate of convergence of the one-step approximation error is improved as compared with methods assuming the value of Brownian motion to be known only at discrete time. Through numerical experiments, the rate of convergence of the multi-step approximation error is seen to be much faster than in the conventional method.

     

Synthesis of carbon tubule nanocoils using Fe-In-Sn-O fine particles as catalysts

We have developed Fe-In-Sn-O fine particle or powder catalysts for synthesizing carbon nanocoils by catalytic thermal chemical vapor deposition. The coprecipitation technique was used to produce the powder catalysts. By optimizing the composition ratios of Fe, In (between 10 and 33% of Fe), and Sn (less than 3.3% of Fe), carbon nanocoils could be grown in high yield. From the study of optimizing the compositions of In and Sn and the study of crystal structures of the catalysts using X-ray diffraction measurements, it was also found that Sn in the catalysts was required to grow carbon nanocoils and that In plays roles in increasing the yield of carbon nanocoils and controlling the coil diameters. This study will lead to the mass production of carbon nanocoils and therefore widen their applications.     

Theory of chemical bonds in metalloenzymes XX: magneto-structural correlations in the CaMn4O5 cluster in oxygen-evolving complex of photosystem II

ABSTRACT

Magneto-structural correlations in oxygen-evolving complex (OEC) of photosystem II (PSII) have been elucidated on the basis of theoretical and computational results in combination with available electron paramagnetic resonance (EPR) experimental results, and extended x-ray absorption fine structure (EXAFS) and x-ray diffraction (XRD) results. To this end, the computational methods based on broken-symmetry (BS) UB3LYP solutions have been developed to elucidate magnetic interactions in the active manganese catalyst for water oxidation by sunlight. The effective exchange interactions J for the CaMn(III)Mn(IV)₃O₅(H₂O)₃Y(Y = H₂O or OH^?) cluster (1) model of OEC of PSII have been calculated by the generalised approximate spin projection (GAP) method that eliminates the spin contamination errors of the BS UB3LYP solution. Full geometry optimisations followed by the zero-point energy (ZPE) correction have been performed for all the spin configurations of 1 to improve the J values that are compared with accumulated EPR in the S₂ state of Kok cycle and magnetic susceptibility results of Christou model complex Ca₂Mn(IV)₃O₄ (2). Using the calculated J values, exact diagonalisation of the spin Hamiltonian matrix has been carried out to obtain excitation energies and spin densities of the ground and lower excited states of 1. The calculated excitation energies are consistent with the available experimental results. The calculated spin densities (projection factors) are also compatible with those of the EPR results. The calculated spin densities have been used to calculate the isotropic hyperfine (A_iso) constants of ⁵⁵Mn ions revealed by the EPR experiments. Implications of the computational results are discussed in relation to the structural symmetry breaking (SSB) in the S₁, S₂ and S₃ states, spin crossover phenomenon induced by the near-infrared excitation and the right- and left-handed scenarios for the O–O bond formation for water oxidation.     

Temperature,humidity, and dimension dependence of the bending motion of ionomer‐based polymer actuators

This paper focuses on the effects of temperature, humidity, and dimensions on the displacement of ionomer‐based polymer actuators. The amount of displacement and velocities of the actuation strongly increased with increasing humidity and temperature. We attributed this behavior to a change in the Young's modulus (the stiffness) and ion conductivity based on water uptake. To evaluate the dependence of the velocity of the displacement on humidity and temperature, we examined three velocities (i.e. the initial, bending, and backtracking velocities). The observed increase in the bending velocity at higher relative humidity (RH) levels arises from an increase in the water uptake, which enhances ion conductivity and decreases the film stiffness. The ratio of the bending velocity to the backtracking velocity at higher RH decreased because of a drastic increase of the backtracking velocity at higher RH. This result would be explained by an increase in the ion conductivity accompanying a decrease in the stiffness. Furthermore, we compared the difference in the amount of displacement of the actuator using actuators of two widths (2 and 10 mm) at 30, 60, and 90% RH and at 25 °C. The difference in the width of the actuator did not completely affect the displacement. These results are reasonably explained by considering the amount of mobile ions per unit volume of the films of the actuators. Copyright © 2016 John Wiley & Sons, Ltd.     

Area-selective formation of macropore array by anisotropic electrochemical etching on an n-Si(100) surface in aqueous HF solution

A photoassisted anodization process to fabricate arrays of uniform and straight macropores at selected areas of a Si wafer surface was developed. The front- and backside surfaces of n-type Si(100) wafers were coated with a thin Si(3)N(4) layer, and the frontside layer was micro-patterned using photolithography and reactive ion etching to form an array of microscopic openings at selected areas. The inverted pyramid-shape micropits were formed at these openings by anisotropic etching using aqueous KOH solution; these pits act as the initiation sites for the anodization to form macropores. The electrochemical etching was carried out in aqueous HF solution under illumination from the backside of the wafer, on which Au/Cr electric contact was formed following removal of the Si(3)N(4) layer. To improve the uniformity of the formation condition of the macropores at the selected area, holes were area-selectively generated by controlling the illumination condition during the anodization. For this, micropatterns were formed on the Au/Cr layer at the backside surface, which were aligned to those at the frontside surface. The parameters, such as HF concentration, current density, and wafer thickness, i.e., hole diffusion length, were optimized, and the arrays of uniform and high-aspect-ratio macropores were formed at the selected area of the domain at the silicon surface.     

Entrapping of exohedral metallofullerenes in carbon nanotubes: (CsC60)n@SWNT nano-peapods

Exohedral C60-based metallofullerenes, CsC60, have been synthesized and successfully encapsulated into single-wall carbon nanotubes (SWNTs) in high yield by reducing C60 molecules into anions. High-resolution transmission electron microscopy (HRTEM) images and in situ electron energy loss spectroscopy (EELS) indicate that Cs atoms and C60 molecules align within SWNTs as CsC60 exohedral metallofullerenes, and that the formal charge state of encaged CsC60 is expressed as Cs+1C60-1. The present peapods with the exohedral metallofullerenes provide a new insight and the possibility to fine-tune the electronic and transport properties of carbon nanotubes.     

Theory of chemical bonds in metalloenzymes III: Full geometry optimization and vibration analysis of ferredoxin‐type [2Fe–2S] cluster

The nature of chemical bonds in a ferredoxin‐type [2Fe–2S] cluster has been investigated on the basis of natural orbitals and several bond indices developed in Parts I and II of this study. The broken‐symmetry hybrid density functional theory (BS‐HDFT) with spin projection approach has been applied to elucidate the natural orbitals and occupation numbers for a model compound [Fe₂S₂(SCH₃)₄] (1), which is used to calculate the indices. The molecular structure, vibration frequencies, electronic structures, and magnetic properties in both oxidized and reduced forms of 1 have been calculated and compared with the experimental values. The optimized molecular structures after approximate spin projection have been in good agreement with experimental data. The structure changes upon one‐electron reduction have been slight (<0.1 Å) and only limited around one side of the Fe atom. Raman and infrared (IR) spectra have been calculated, and their vibration modes have been assigned using the bridging ³⁴S isotope substitution. Their magnetic properties have been examined in terms of spin Hamiltonians that contain exchange interactions and double exchange interactions. The BS‐HDFT methods have provided the magnetic parameters; i.e., effective exchange integral (J) values and valence delocalization (B) values, which agree with the experimental results. It is found that large charge transfer (CT) from the bridging sulfur to the iron atoms has led to the strong antiferromagnetic interactions between iron atoms. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007