Large-scale separation of metallic and semiconducting single-walled carbon nanotubes

In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way.     

Preparation and characterization of silicalite-1 membranes prepared by secondary growth of seeds with different crystal sizes

MFI-type zeolite particles of 0.1–1 μm in diameter were prepared by adjusting tetra-n-propylammonium hydroxide (TPAOH) and water contents in synthesis mixtures. Using those particles as seeds, MFI-type zeolite membranes were prepared on the surface of a porous mullite tube by secondary growth. The membranes were formed as polycrystalline zeolite layers on and inside the porous support, and the membranes were composed of the [h 0 h]-oriented crystallites. The membrane consisting of a-oriented crystallites could be also prepared. However, the a-oriented zeolite layers were not active on the permeation properties of butanes. Rather the size and loaded amount of the seed particles influenced on the permeation properties through the membranes. As a result, the n-C₄H₁₀/i-C₄H₁₀ permselectivity could be increased to 220 by adjusting the size and the loaded amount of particles. These results suggest that the number of loaded particles affects on the permeation properties through the membranes.     

Radical cyclization by ipso substitution of the methoxy group: considerable effect of HMPA on samarium-mediated cyclization

Alkyl radicals generated by treatment of thiocarbamates of conformationally favorable 3-alkyl-3-arylpropan-1-ols with tris(trimethylsilyl)silane and AIBN efficiently undergo intramolecular ipso substitution of the methoxy group, yielding the corresponding cyclized products. In contrast, either conformationally favorable or flexible 1-arylalkan-3- or 4-ones easily cyclize into five- or six-membered condensed rings by treatment with SmI(2) via ketyl radical intermediates. The addition of HMPA as cosolvent dramatically changes the cyclization mode of the SmI(2)-induced reaction, and the para-cyclization products are exclusively formed. This "HMPA effect" can be rationalized by the strong chelating ability of HMPA with the samarium atom.     

Asymmetric synthesis of permethric acid. stereochemistry of chiral copper carbenoid reaction

The most effective optical isomer (1$\text{R}$-$\text{cis}$) of permethric acid ($\text{2}$, R = H), a potent intermediate in the production of synthetic pyrethroid, was enantioselectively prepared.     

Subsecond luminescence intensity fluctuations of single CdSe quantum dots

Photoluminescence (PL) intermittency characteristics are examined for single quantum dots (QDs) in a CdSe QD sample synthesized at a slow rate at 75 degrees C. Although the PL quantum efficiency was relatively low ( approximately 0.25), we noticed that the PL intensity of single CdSe QDs fluctuated on a subsecond time scale with short-lived "on" and "off" states. The subsecond PL intensity fluctuations of CdSe QDs are different from "on" and "off" PL blinking generally observed for QDs fluctuating on a millisecond to minute time scale. We characterized single QDs by identifying polarized excitations, topographic imaging using atomic force microscopy (AFM), and transmission electron microscopy (TEM). From analysis of the PL intensity trajectories from >100 single CdSe QDs, the average intermittency time was 213 ms. From the PL quantum efficiency, slow growth of QDs, intensity trajectory analyses, and previous reports relating surface trap states and PL properties of QDs, we attribute the subsecond PL intensity fluctuations of single CdSe QDs and short-lived "on" and "off" states to a high-density distribution of homogeneous surface trap states.     

Total synthesis of (-)-tuberostemonine

The first total synthesis of the complex pentacyclic Stemona alkaloid tuberostemonine was accomplished in 24 steps and in 1.4% overall yield from a hydroindole intermediate which is readily obtained in three steps from Cbz-l-tyrosine. An innovative synthetic strategy was applied that relays the single stereocenter of the amino acid precursor into nine of the ten stereogenic carbons of the target molecule. Among the highlights of the synthetic methodology are the 3-fold use of ruthenium catalysis, first in an azepine ring-closing metathesis and then in an alkene isomerization followed by a cross-metathesis propenyl-vinyl exchange, as well as the stereoselective attachment of the gamma-butyrolactone moiety to the core tetracycle by use of a lithiated ortho ester.     

Chemoenzymatic synthesis of [3,9-C]-labeled NeuAc and KDN

The chemoenzymatic synthesis of ¹³C-labeled sialic acid (NeuAc) and 3-deoxy-d-glycero-d-galacto-2-nonulosonic acid (KDN) as useful molecular probes for studying the conformation of sialyl or KDN oligosaccharides attached to proteins was performed by using [6-¹³C]-ManNAc, [6-¹³C]-Man and [3-¹³C]-pyruvic acid sodium salt. In the synthesis of the compounds, 5,6-anhydro intermediates were found to easily provide not only 6-¹³C-labeled but also 5-, and 6-modified NeuAc and KDN analogs. Furthermore, it was demonstrated that identical results are obtained by NMR for both [3,9-¹³C]-NeuAc and 1:1 mixtures of [3-¹³C]- and [9-¹³C]-NeuAc.     

Spectrophotometric and thermal analytical studies on the dehydration of copper(II) sulfate and its double salts

Spectrophotometric (diffuse reflection) and TG-DTA data on the dehydration of CuSO₄ · 5H₂O, Na₂Cu(SO₄)₂ · 2H₂O, M₂Cu(SO₄)₂ · 6H₂O(M⁺ = K⁺, Rb⁺, Cs⁺ and NH⁺₄) and Co₂Cu(SO₄)₃ · 18H₂O are given. Although complete dehydration of CuSO₄ · 5H₂O brings about a striking color change from blue to white, it was found that there is only a slight decrease in the v?_max. of its d-d band in the course of this change, and the total light absorption in the visible-UV region increases at the same time. The dehydration of the alkali metal and ammonium double salts, most of which contain [Cu(OH₂)₆]²⁺ aquo ions (in contrast to the [Cu(OH₂)₄]²⁺ in CuSO₄ · 5H₂O), occurs generally more easily than that of CuSO₄ - 5H₂O, and their v?_max. increases slightly in the change, leading to blue or green anhydrous products, although the formation of a white modification was observed with the potassium salt. It was also found that the v?_max. of the Cu²⁺ ion in the dehydrated cobalt(II) double salt is still lower than that in anhydrous CuSO₄, i.e., the ligand field and/or tetragonality around it is decreased by the presence of Co²⁺ ions.     

Total synthesis of (-)-stevastelin B

The total synthesis and an unambiguous structure confirmation of stevastelin B 1, a novel 15-membered cyclic depsipeptide, are described; the fatty acid moiety in 1, prepared stereoselectively from L-quebrachitol was converted into the amino carboxylic acid, whose macrolactamization by Shioiri's procedure effectively constructed the cyclic structure of 1.