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151.
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The acid catalyzed rearrangement of two cyclohexanols of spiroannelated four-membered rings has been studied. In accordance with molecular mechanics calculations, far-reaching reorganizations with formation of unsaturated hexacyclic systems, including a fully cycloalkylated cyclohexene with a bispropellane partial structure, were observed. Attempts to convert this bispropellane to a trispropellane failed. 相似文献
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155.
Lon J. Mathias Peter D. Livant Sharf U. Ahmed 《Journal of polymer science. Part A, Polymer chemistry》1987,25(10):2711-2716
Several new copolymers have been synthesized. Initial polycondensation of mixtures of aryl and arylalkyl carboxylic acids containing aromatic hydroxyl and amine substituents led to random copolyamides. Thermal cyclization of these prepolymers (inherent viscosities of about 0.2 dL/g) gave copolybenzoxazoles with inherent viscosities of about 1.4 dL/g. Thermal stability as measured by TGA was similar for the 1:1 copolymers and the homopolymers containing hydrocinnamic units; i.e., catastrophic decomposition occurred at 450°C. Copolymers with lower ratios of the hydrocinnamic units displayed greatly improved thermal stability while maintaining good molecular weight. 相似文献
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157.
From Infinite Chains according to 1∞[Zr(S2O7)4/2] in Zr(S2O7)2 to the unprecedented [Zr(S2O7)4]4– Anion in Ag4[Zr(S2O7)4]
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The reaction of ZrCl4 with oleum (65 % SO3) in the presence of Ag2SO4 at 250 °C yielded colorless single crystals of Zr(S2O7)2 [orthorhombic, Pccn, Z = 4, a = 709.08(6) pm, b = 1442.2(2) pm, c = 942.23(9) pm, V = 963.5(2) × 106 pm3]. Zr(S2O7)2 shows Zr4+ ions in an eightfold distorted square antiprismatic coordination of oxygen atoms belonging to four chelating disulfate units. Each S2O72– ion is connected to a further Zr4+ ion leading to chains according to 1∞[Zr(S2O7)4/2]. The same reaction at a temperature of 150 °C resulted in the formation of Ag4[Zr(S2O7)4] [monoclinic, C2/c, Z = 4, a = 1829.35(9) pm, b = 704.37(3) pm, c = 1999.1(1) pm, β = 117.844(2)°, V = 2277.6(2) × 106 pm3]. Ag4[Zr(S2O7)4] exhibits the unprecedented [Zr(S2O7)4]4– anion, in which the central Zr4+ cation is coordinated by four chelating disulfate units. Thus, in Ag4[Zr(S2O7)4] the 1∞[[Zr(S2O7)4/2] chains observed in Zr(S2O7)2 are formally cut into pieces by the implementation of Ag+ ions. 相似文献
158.
Acoustic resonances are generated by air moving past a plate with a semi-circular leading edge located in a duct. The sound stimulates the small separation bubble near the leading edge to shed vortices into the boundary layer at the sound frequency. The acoustic energy required to sustain the resonances is generated as these vortices pass the trailing edge of the plate. 相似文献
159.
Karl O. Christe David A. Dixon Mathias Hopfinger Thomas M. Klapötke Burkhard Krumm 《Journal of fluorine chemistry》2010,131(7):791-422
The [((C6H5)3P)2N]+, [(C6H5)4P]+ and [N(CH3)4]+ salts of SeF5−, SeF62− and SeOF3− and CsSeO2F were prepared and characterized. Crystal structures were obtained for [((C6H5)3P)2N][SeF5] and [((C6H5)3P)2N][SeOF3] CH2Cl2. In contrast to oxygen-bridged dimeric TeOF3−, the SeOF3− anion in [((C6H5)3P)2N][SeOF3] CH2Cl2 is monomeric and represents the first experimentally well determined molecular structure of a monomeric trifluoro-chalcogenite anion. Similarly, [((C6H5)3P)2N][SeF5] represents the first example of a structure containing a well-isolated undistorted SeF5− anion. The NMR and the vibrational spectra and their assignments were re-examined and corrected by comparison with high-level theoretical calculations. Whereas the previously published normal coordinate analysis of SeF5− is correct, that for SeOF3− needs major revision. 相似文献
160.
R(f)-PEG (fluoroalkyl double-ended poly(ethylene glycol)) hydrogel is potentially useful as a drug delivery depot due to its advanced properties of sol-gel two-phase coexistence and low surface erosion. In this study, (1)H molecular diffusion nuclear magnetic resonance (NMR) and (19)F spin diffusion NMR were used to probe the drug loading and diffusion properties of the R(f)-PEG hydrogel for small anticancer drugs, 5-fluorouracil (FU) and its hydrophobic analog, 1,3-dimethyl-5-fluorouracil (DMFU). It was found that FU has a larger apparent diffusion coefficient than that of DMFU, and the diffusion of the latter was more hindered. The result of (19)F spin diffusion NMR for the corresponding freeze-dried samples indicates that a larger portion of DMFU resided in the R(f) core/IPDU intermediate-layer region (where IPDU refers to isophorone diurethane, as a linker to interconnect the R(f) group and the PEG chain) than that of FU while the opposite is true in the PEG-water phase. To understand the experimental data, a diffusion model was proposed to include: (1) hindered diffusion of the drug molecules in the R(f) core/IPDU-intermediate-layer region; (2) relatively free diffusion of the drug molecules in the PEG-water phase (or region); and (3) diffusive exchange of the probe molecules between the above two regions. This study also shows that molecular diffusion NMR combined with spin diffusion NMR is useful in studying the drug loading and diffusion properties in hydrogels for the purpose of drug delivery applications. 相似文献