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21.
N-(omega-Bromoalkyl)-amino acid derivatives, readily prepared from natural alpha-amino acids, gave cyclic amino acids with a quaternary stereocenter by treatment with potassium hexamethyldisilazaide in DMF. The chirality of parent amino acids was almost completely preserved during an enolate-formation and cyclization process, giving aza-cyclic amino acids in up to 98% ee in retention of configuration. This method is applicable to the asymmetric synthesis of azetidine, pyrrolidine, piperidine, and azepane derivatives. The asymmetric cyclization seems to proceed via an axially chiral enolate intermediate and not through a concerted SEi process. 相似文献
22.
Switching between three different adsorbed states was achieved using potential control to separately induce electrochemical behaviour associated with changes in binding and orientation of the thiophene and pyridine moieties of 2-(2'-thienyl)pyridine adsorbed to Au(111) in neutral and basic solution. 相似文献
23.
Tobe Y Utsumi N Kawabata K Nagano A Adachi K Araki S Sonoda M Hirose K Naemura K 《Journal of the American Chemical Society》2002,124(19):5350-5364
m-Diethynylbenzene macrocycles (DBMs), buta-1,3-diyne-bridged [4(n)]metacyclophanes, have been synthesized and their self-association behaviors in solution were investigated. Cyclic tetramers, hexamers, and octamers of DBMs having exo-annular octyl, hexadecyl, and 3,6,9-trioxadecyl ester groups were prepared by intermolecular oxidative coupling of dimer units or intramolecular cyclization of the corresponding open-chain oligomers. The aggregation properties were investigated by two methods, the (1)H NMR spectra and the vapor pressure osmometry (VPO). Although some discrepancies were observed between the association constants obtained from the two methods, the qualitative view was consistent with each other. The analysis of self-aggregation by VPO revealed unique aggregation behavior of DBMs in acetone and toluene, which was not elucidated by the NMR method. Namely, the association constants for infinite association are several times larger than the dimerization constant, suggesting that the aggregation is enhanced by the formation of dimers (a nucleation mechanism). In polar solvents, DBMs aggregate more strongly than in chloroform due to the solvophobic interactions between the macrocyclic framework and the solvents. Moreover, DBMs self-associate in aromatic solvents such as toluene and o-xylene more readily than in chloroform. In particular, the hexameric DBM having a large macrocyclic cavity exhibits extremely large association constants in aromatic solvents. By comparing the aggregation properties of DBMs with the corresponding acyclic oligomers, the effect of the macrocyclic structure on the aggregation propensity was clarified. Finally, it turned out that DBMs tend to aggregate more readily than the corresponding phenylacetylene macrocycles, acetylene-bridged [2(n)]metacyclophanes, owing to the withdrawal of the electron density from the aromatic rings by the butadiyne linkages which facilitates pi-pi stacking interactions. 相似文献
24.
Edoardo Apr Emily A. Carter Alessandro Fortunelli 《International journal of quantum chemistry》2005,101(2):230-230
Density Functional Theory (DFT) and direct ab initio molecular dynamics (MD) calculations were applied to the hydrogen molecule trapped in a water cluster composed of 12 water molecules (H2O)12. The static DFT calculation showed that the H2 molecule is trapped in the center of mass of (H2O)12. The vibrational frequency of the H–H stretching mode of the H2 molecule trapped in the water cluster was blueshifted from that in vacuo. On the other hand, the vibrational frequency of H2 in water‐hydrogen 1:1 complex (H2O–H2) was redshifted. A direct ab initio MD calculation of H2(H2O)12 at 50 K indicated that the H2 molecule is rotated freely around the center of mass of the water cluster. The origin of the spectral shift of H2 in water ice is discussed on the basis of the theoretical results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
25.
Hiroto Tachikawa Hiroshi Kawabata Kazumi Matsushige 《Journal of organometallic chemistry》2005,690(12):2895-2904
The structures and electronic states of phenyl-capped terthiophene (denoted by P3T) and the ionic species of P3T have been investigated by means of density functional theory (DFT) and direct MO dynamics calculations. P3T is one of the high-performance molecular devices, which has been utilized as a semi-conductor. The calculations indicated that the neutral P3T has a non-planar structure whose the phenyl rings in both ends of thiophene chain are largely deviated from the molecular plane. The cation and anion radicals, dication and dianion were considered as its ionic states. The structure for cation radical of P3T is close to more planar than that of neutral P3T. The structures for anion radical, dication and dianion take a pure planar structure. The first excitation energy of neutral P3T is calculated to be 2.90 eV at the TD-B3LYP/6-31G(d)//B3LYP/6-311+G(d) level, while the P3T cation and anion radicals have lower excitation energies (1.22 and 1.10 eV, respectively). The direct MO dynamics calculation showed that neutral, cation and anion hold near planar structure at 300 K. On the other hand, oligothiophene (n = 5) and its ionic species are strongly deformed from the planar structure, and thiophene rings in both ends of chain rotate rapidly by thermal activation. The mechanism of the electron conductivity in P3T was discussed on the basis of theoretical results. 相似文献
26.
Yoshihiko Ohishi Tomonori Kawabata Tetsuya Shishido Ken Takaki Qinghong Zhang Ye Wang Katsuomi Takehira 《Journal of molecular catalysis. A, Chemical》2005,230(1-2):49-58
M-MCM-41 catalysts (M: V, Cr, Fe, and Ga) prepared by direct hydrothermal synthesis (DHT) have been tested for dehydrogenation of ethylbenzene with CO2. The synthesized materials were characterized by X-ray diffraction (XRD), N2 adsorption (77 K), and diffuse reflectance UV–vis spectroscopic measurements. Cr-MCM-41 showed the highest activity among M-MCM-41 catalysts tested, resulting in the production of styrene with the conversion of 65% and the selectivity above 90%. The rate of styrene formation increased with increasing Cr loading up to 1.7 wt.%. It is suggested that Cr(VI)O4 in tetrahedral coordination is formed as an active monochromate species and reduced to Cr(III)O6 in octahedral coordination as a less active polychromate species during the reaction. Deactivated catalyst was regenerated by a treatment with gaseous oxygen or CO2, during which redistribution as well as reoxidation of polymeric Cr(III)O6 octahedra to monomeric Cr(VI)O4 tetrahedra was observed. The rate of CO formation increased together with that of styrene formation, while the rate of H2 formation decreased, with increasing partial pressure of CO2. It was confirmed that reverse water-gas shift reaction took place over Cr-MCM-41 by a separate experiment. The rate of CO formation during the dehydrogenation of ethylbenzene with CO2 over Cr-MCM-41 was well accounted for by assuming parallel occurrence of two reactions, i.e., direct oxidative dehydrogenation of ethylbenzene with CO2 and simple dehydrogenation of ethylbenzene thermodynamically assisted by reverse water-gas shift reaction. 相似文献
27.
Yahachi Saito Tadao Koyama Kenichiro Kawabata 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,40(1):421-424
Single-layer (SL) carbon tubes were produced by arc evaporation of graphite rods containing iron-group metals (Fe, Co, Ni, Fe/Co, Co/Ni, Fe/Ni) under He and Ar gas. Transmission electron microscopy (TEM) revealed that these elemental and binary metals, excluding Fe which need a special atmosphere (a mixture of Ar and CH4), showed catalytic activity producing SL tubes under pure inactive gases. Fe/Ni alloy was the most effectual catalysts for producing SL tubes. The highest abundance of SL tubes in raw soot was estimated to be ~ 10% from TEM observation. Smoke particles directly caught on TEM grids near an evaporation source during arcburning were also investigated, and it was suggested that small metal particles were first formed in the gas phase and then SL tubes grew from them. 相似文献
28.
A novel stationary phase for micro ion chromatography has been prepared by coating a porous poly(butyl methacrylate-co-ethylene dimethacrylate-co-2-acrylamido-2-methyl-1-propanesulfonic acid) monolith with quaternary amine-functionalized latex particles via simple electrostatic binding. This stationary phase enabled the separation of saccharides in the mobile phase with a high-pH value consisting of aqueous ammonia solution in anion-exchange mode using evaporative light scattering for detection. Effects of both porous properties of the monolithic stationary phase and chromatographic conditions on the separation ability were studied. Under optimized conditions, an efficient separation of seven saccharides was achieved in less than 10 min. The stationary phase also enables the separation of saccharides obtained by the enzymatic hydrolysis of corn starch. 相似文献
29.
Itaru Yamashita Youhei Kawabata Tadashi Kato Masakatsu Hato Hiroyuki Minamikawa 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,250(1-3):485-490
Small angle X-ray scattering (SAXS) is measured for the lamellar phase in aqueous systems of 1-o-β-3,7-dimethyoctyl-D-glucopyranoside (β-Glc(Ger)), which has recently been prepared by us, 1-o-β-decyl-D-glucopyranoside (β-GlcC10), and 1-o-β-octyl-D-glucopyranoside (β-GlcC8). The repeat distance d obtained from the position of the diffraction peak does not follow the swelling law d = 2δhc/hc, where δhc and hc are the thickness and the volume fraction of the hydrophobic layer, respectively. This may result from the fact that δhc increases and, equivalently, the surface area per surfactant molecule (as) decreases with increasing concentration. So we calculate δhc and as from the observed d value at each concentration using the above swelling law. The half-thickness δhc increases in the order β-GlcC8 < β-Glc(Ger) < β-GlcC10 at a fixed concentration. On the other hand, the data on as for β-GlcC10 and β-GlcC8 lie on the same line and the data for β-Glc(Ger) lies above this line. These results suggest that the cross-sectional area of the geranyl chain is larger than that of the glucose headgroup. Existence of water filled defects in bilayer sheets is also discussed based on the SAXS pattern and the concentration dependence of d. 相似文献
30.
Bluhm ME Kim SS Dertz EA Raymond KN 《Journal of the American Chemical Society》2002,124(11):2436-2437
Most species of bacteria employ siderophores to acquire iron. The chirality of the ferric siderophore complex plays an important role in cell recognition, uptake, and utilization. Corynebactin, isolated from Gram-positive bacteria, is structurally similar to enterobactin, a well known siderophore isolated from Gram-negative bacteria, but contains L-theronine instead of L-serine in the trilactone backbone. Corynebactin also contains a glycine spacer unit in each of the chelating arms. A hybrid analogue (serine-corynebactin) has been synthesized. The chirality and relative conformational stability of the three ferric complexes of enterobactin, corynebactin, and the hybrid has been investigated. In contrast to enterobactin, corynebactin assumes a Lambda configuration. However, the ferric serine-corynebactin hybrid forms a racemic mixture, only slightly favoring the Lambda conformation. 相似文献