Synthesis of nonstoichiometric zirconium carbide whiskers by chemical vapor deposition

Nonstoichiometric zirconium carbide crystals with various compositions were prepared by chemical vapor deposition. Two gaseous mixtures, zirconium tetrachloride and argon, toluene and hydrogen, were introduced to the reaction zone where a graphite substrate was heated between 1200 and 1400°C. The deposition rate was proportional to the partial pressure of toluene. The compositional ratio of n_C/n_Zr in the gaseous mixture from 2.0 to 6.0 was found to be optimum for producing needle-like crystals. Needle-like crystal with smaller size were formed when the ratio of n_C/n_Zr was smaller than 2.0, and less needle-like crystals accompanied with more carbon were also produced when the ratio of n_C/n_Zr was larger than 6.0. The temperature of the substrate suitable for the growth of needle crystals was in the range from 1250 to 1300°C. The lattice constants of the products varied as a function of the ratio of n_C/n_Zr in the gaseous mixtures.     

Influence of growth related thickness fluctuations on the spectral and lateral luminescence intensity distribution in GaAs quantum wells

The effect of low energy exciton sinks on the spectral and spatial luminescence intensity distribution has been investigated in a single quantum well (QW) and a multiple QW structure prepared by growth interrupted molecular beam epitaxy (MBE). Both samples contain growth-related lateral variations of the exciton confinement energy on the order of the exciton diffusion length, which cause a lateral and spectral intensity distribution in cathodoluminescence images.     

Phase diagram of growth mode for the SiGe/Si heterostructure system with misfit dislocations

The strain, surface and interface energies of the SiGe/Si (SiGe grown on Si) heterostructure system with and without misfit dislocations were calculated for the Frank–van der Merwe (FM), Stranski–Krastanov (SK) and Volmer–Weber (VW) growth modes essentially based on the three kinds of fundamental and simple structures. The free energies for each growth mode were derived from these energies, and it was determined as a function of the composition and layer thickness of SiGe on Si. By comparison of the free energies, the phase diagrams of the FM, SK and VW growth modes for the SiGe/Si system were determined. The (1 1 1) and (1 0 0) reconstructed surfaces were selected for this calculation. From the phase diagrams, it was found for the growth of SiGe on Si that the layer-by-layer growth such as the FM mode was easy to be obtained when the Ge composition is small, and the island growth on a wetting layer such as the SK mode was easy to be obtained when the Ge composition is large. The VW mode is energetically stable in the Ge-rich compositional range, but it is difficult for the VW mode to appear in the actual growth of SiGe on Si because the VW region is right above the SK region. The regions of the SK and VW modes for the (1 1 1) heterostructure are larger than those for the (1 0 0) one because the strain energy of the (1 1 1) face is larger than that of the (1 0 0) face. The regions of the SK and VW modes for the heterostructure with misfit dislocations are narrower than those for the one without misfit dislocations because the strain energy is much released by misfit dislocations. The phase diagrams roughly explain the behavior of the FM and SK growth modes of SiGe on Si.     

A theoretical study on the inner-valence photoelectron spectra lying between 21 and 26 eV of the O2 molecule

Ab initio calculations were carried out to study the structures of the photoelectron spectra of the O(2) molecule using complete active space self-consistent field and multireference single and double excitation configuration interaction methods. We focused our attention primarily on the inner-valence ionized states lying below 26 eV. Vibrational levels were calculated using adiabatic potential curves and vibrational wave functions were used to obtain Franck-Condon factors. We show that not only discrete vibrational levels of (3) (2)Pi(u), but also the continuum of nuclear motion in (1) (2)Sigma(u) (-) contribute to the bands of vibrational progression and the broad peak of the continuum between 22.5 and 26 eV. Other states with weak intensity lying between 19 and 26 eV are also discussed.     

Ground state properties of the two-band Anderson-type model in one dimension

We study the ground states of the one-dimensional two-band Anderson type model in both the symmetric and the asymmetric cases. In the symmetric case the analytical expression of the charge-complex distribution function is formally derived, which is then applied to calculate the binding energy of the Kondo state. In the general asymmetric cases the behaviors of localized- and conduction-electron numbers are investigated as functions ofU and other parameters by numerically solving the integral equation. Particularly, for the asymmetric limitU2V ² and _{F a} ( _F the Fermi level, _a the localized level), when a nonintegral localized-electron valence is stabilized implying a valence fluctuation, _F lies in the gap, whereas when it is an integral valence, _F lies in the upper band. The former state is semiconducting and the latter is metallic.     

Sesquiterpenes and alkaloids from Lindera chunii and their inhibitory activities against HIV-1 integrase

Three new eudesmane type sesquiterpenoid lindenanolides E (1), F (2) and G (3), and two new aporphine alkaloid lindechunines A (18) and B (20) were isolated from roots of Lindera chunii MERR., together with seven known sesquiterpenes including a new naturally-occurring lindenanolide H (4) and eight known aporphine alkaloids. The structures of these compounds were determined by spectroscopic means. Of the isolated compounds, hernandonine (14), laurolistine (16), 7-oxohernangerine (17) and lindechunine A (18) showed significant anti-human immunodeficiency virus type 1 (HIV-1) integrase activity with IC(50) values of 16.3, 7.7, 18.2 and 21.1 microM, respectively. The major alkaloids presented in the roots of L. chunii were quantitatively analyzed by an HPLC method.     

Sulfonyl-stabilized oxiranyllithium-based approach to polycyclic ethers. Convergent synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin

Convergent synthesis of the ABCDEF-ring system of yessotoxin and adriatoxin, marine polycyclic ether toxins causative of diarrheic shellfish poisoning, has been accomplished. The A-ring fragment was constructed by coupling of an appropriately functionalized sulfonyl-stabilized oxiranyl anion and a triflate prepared from an erythritol derivative. An iterative protocol of the oxiranyl anion strategy was also applied for the construction of the DEF-ring fragment. The triflate derivatives of the A-ring and the DEF-ring fragments were connected with lithium acetylide. The resulting acetylene derivative was further transformed into the hexacyclic ABCDEF fragment via oxidation of the acetylene unit to 1,2-diketone, double methyl acetal formation, and reductive etherification.     

Conformational isomerism of chlorophenylthioureas and intramolecular N---H … Cl hydrogen bonds: IR spectra of N---H stretching vibrations

The vN---H regions of the IR spectra of thioureas with chlorophenyl (ClPh) groups and those with halophenyl groups were measured in dilute CCl₄ solution. The observed vN---H bands were classified into eight groups according to the wavenumbers and the two substituent groups. The suggested conformational states and the formation of intramolecular N---H … Cl hydrogen bonds in these compounds were discussed in comparison with those of the urea analogs. It was found that these thiourea derivatives are more stable in the cis form than the urea analogs and that thioureas with o-ClPh groups form fewer intramolecular N---H … Cl hydrogen bonds than do the urea analogs.     

Highly enantioselective photocyclization of 1-alkyl-2-pyridones to beta-lactams in inclusion crystals with optically active host compounds

[reaction: see text] Upon photoirradiation of a 1:1 inclusion complex of (-)-3 and 2-alkyl-1-pyridone (1) (R = Et, n-Pr, i-Pr, n-Bu, i-Bu) in the solid state, optically active beta-lactams (2) of 91-99.5% ee were obtained in good yield.     

Studies on sustained-release suppositories. I. Effect of alginic acid addition on rectal absorption of bacampicillin in rabbits

Sustained-release suppositories of bacampicillin (BAPC) were prepared by the use of the adduct which was precipitated from an aqueous solution containing BAPC and alginic acid (Alg). As the suppository base, Witepsol H-15 and macrogol were used. Absorptions of BAPC from the suppositories were prolonged in rabbits, but the bioavailabilities were decreased compared to that after administration of BAPC alone. However, these absorptions were improved enormously by the addition of surface-active agents, that is, an excellent prolonged absorption and high bioavailability were obtained. Interestingly, similar prolonged absorption could be obtained only by mixing Alg with BAPC in a suppository base. Further, this absorption rate was found to be controlled by the amount of Alg addition. The absorption profiles from a suppository containing the mixture differed from that containing the adduct in exhibiting both high plasma level and prolonged absorption. This may be due to simultaneous fast absorption of BAPC itself and formation of the adducts. Thus, it seemed that BAPC preparations containing Alg may be practically useful as a rectal preparation with prolonged action and giving a high plasma level.