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21.
For a class of Serre fibratations
with a weak formal base X (or with a degenerated
-algebra structure on the integral cohomology H*(X)), obstructions are defined by means of spherical twisting cochains of . In particular, for a given section
on n-skeleton of X, the problem of avoiding the (n+1)th obstruction
to the existence of a section on X
n+1 reduces to solving a system of linear equations with respect to cohomology elements of the groups
Homotopy classification theorems for sections as well as for weak formal maps are given, too. 相似文献
22.
The spherulitic structures and morphologies of poly-(3-hydroxybutyrate) (PHB) crystallized from a so- lution and a thin melt film were investigated in this study. The formation mechanisms of banded spherulites under different crystallization conditions are proposed. It was found that the formation of banded spherulites was caused by the rhythmic crystal growth of the spherulites and lamellar twisting growth for the polymer crystallization from a thin melt film and a solution, respectively. 相似文献
23.
Kazuki Abe Xiaotun Yang Shinichi Yano Takashi Kato Hiroyuki Takeuchi 《Liquid crystals》2013,40(6):839-844
Four series of liquid crystalline (LC) compounds containing benzyl perfluoroalkyl thioether groups (BFT), 4-n-alkoxyphenylbenzyl perfluoroalkyl thioethers (nO-BBFT-FmF), 4-n-alkoxyphenylbenzyl perfluoroisoalkyl thioethers (nO-BBFT-FmIF), 4-n-propylcyclohexylbenzyl perfluoroalkyl thioethers (3-C1BFT-FmF) and 4-ethylcyclohexyl-3'-fluorobenzyl perfluoroalkyl thioethers (2-C1FBFT-FmF), were synthesized and their mesomorphic phase transitions and electro-optical properties investigated. nO-BBFT-FmF and nO-BBFT-FmIF have SmA, SmB and CrE phases, while 3-C1BFT-FmF and 2-C1FBFT-FmF showed a SmB phase. It was found that the appearance of a LC phase and the thermal stability were closely connected with the chemical structures of the rigid core part and the terminal groups. The electrooptical properties were investigated for BFT using ZLI-1132 as nematic solvent. Some of the BFT compounds have a good solubility of 15 wt % in ZLI-1132 and can be used as a component for lowering the refractive anisotropy (Delta n) of the host ZLI-1132 solvent. 相似文献
24.
Anvita Srivastava Prabhat K. Singh Manoj Kumbhakar Dr. Tulsi Mukherjee Dr. Subrata Chattopadyay Dr. Haridas Pal Dr. Sukhendu Nath Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(30):9257-9263
An ultrafast intramolecular bond twisting process is known to be the responsible mechanism for the sensing activity of the extensively used amyloid fibril sensor thioflavin T (ThT). However, it is not yet known which one of the two possible single bonds in ThT is actually involved in the twisting process. To resolve this fundamental issue, two derivatives of ThT have been designed and synthesized and subsequently their photophysical properties have been studied in different solvents. It is understood from the present study that the rotation around the central C? C single bond, and not that around the C? N single bond, is primarily responsible for the sensor activity of ThT. Detailed viscosity‐dependent fluorescence studies revealed that the ThT derivative with restricted C? N bond rotation acts as a better sensor than the derivative with free C? N bond rotation. The better sensory activity is directly correlated with a shorter excited‐state lifetime. Results obtained from the photophysical studies of the ThT derivatives have also been supported by the results obtained from quantum chemical calculations. 相似文献
25.
I. M. Gella N. S. Pivnenko L. A. Kutulya T. G. Drushlyak A. Yu. Kulikov N. B. Novikova 《Russian Chemical Bulletin》2005,54(10):2406-2416
Methylenation of (3R,6R)-2-(4-X-benzylidene)-6-isopropyl-3-methylcyclohexanones (X = F, Cl, Ph) with dimethylsulfoxonium methylide occurs stereoselectively
to give 1(S)-(4-X-phenyl)-5(R)-isopropyl-8(R)-methyl-3(R)-spiro[2.5]octanones, whose stereochemistry was established by 1H NMR spectroscopy. The configuration of the chiral centers in the cyclohexanone fragment and its preferred conformation (methyl
is axial and isopropyl is equatorial) in the products do not change with respect to the starting enones. The mutual trans-arrangement of the carbonyl and aryl groups at the newly formed three-membered ring was established; the aryl group also
occupies the trans-position with respect to the axial methyl group of the cyclohexanone fragment. Using methylenation of the compound with X
= Ph as an example, a mixture of by-products resulting from oxidative hydroxylation at the α-position relative to the carbonyl
group was isolated. The resulting chiral spiro[2.5]octanones induce a helical supramolecular ordering in the nematic mesophase
of 4-pentyl-4′-cyanobiphenyl and exhibit a twisting power only somewhat (by 20–30%) lower than the starting enones.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2331–2341, October, 2005. 相似文献
26.
Huapeng Liu Zhuoqun Lu Baolei Tang Cheng Qu Zuolun Zhang Hongyu Zhang 《Angewandte Chemie (International ed. in English)》2020,59(31):12944-12950
Flexible organic single crystals capable of plastic or elastic deformations have a variety of potential applications. Although the integration of plasticity and elasticity in a crystal is theoretically possible and it may cause rich and complex deformations which are highly demanded for potential applications, the integration is hard to realize in practice. Here, we show that through utilizing different modes of external forces for influencing molecular packing in different crystallographic directions, plastic helical twisting and elastic bending can both be achieved for a crystal, and they can even be realized simultaneously. Detailed crystallographic analyses and contrast experiments disclose the mechanisms behind these two kinds of distinct deformations and their mutual compatibility. Based on the plastically twistable nature of the crystal, a new application field of flexible organic single crystals, namely polarization rotators, is successfully opened up. 相似文献
27.
In this study,a novel series of chiral 1,2-propanediol derivatives with different electron-donating and electron-withdrawing groups were synthesized and characterized by FT-IR and ~1H NMR.The helical twisting properties of all the chiral dopants were investigated by doping the chiral dopants into a nematic liquid crystal host(SLC-1717).The results indicate that the donor-acceptor electron effect have a prominent influence on helical twisting property of the chiral nematic phase induced by the chiral dopants. Introducing electron-withdrawing groups into the terminal ends of chiral 1,2-propanediol can decrease the absolute values of the helical twisting power.In addition,the helix inversion temperatures of the induced chiral nematic phase are variational with the change of terminal groups. 相似文献
28.
L. A. Kutulya A. I. Krivoshei N. S. Pivnenko N. I. Shkolnikova 《Journal of Structural Chemistry》2004,45(3):395-404
The molecular conformations of 2,6-bis-(4-phenyl)benzylidene-3R-methyl-cyclohexanone and its mono-(4-phenyl)benzylidene methylcyclohexanone isomers were investigated by molecular simulation using the semiempirical AM1 and PM3 methods and by analyzing the experimental spin-spin coupling constants in the PMR spectra. Mesomorphism and the twisting ability of the induced cholesteric mesophases of the title compounds are analyzed using the ratios between conformers with the axial and equatorial methyl groups and other peculiarities of the spatial structure of molecules (differences in anisometricity and in the degree of flattening of the cinnamoyl fragment). The equatorial orientation of the methyl group in the dominant conformations generally favors the formation of mesophases. The twisting ability is higher for chiral compounds with an axial methyl substituent and with the chiral center directly bonded to the enone and arylidene groups.Original Russian Text Copyright © 2004 by L. A. Kutulya, A. I. Krivoshei, N. S. Pivnenko, and N. I. ShkolnikovaTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 419–429, May–June, 2004. 相似文献
29.
Jinyuan Zhou Gengzhi Sun Zhaoyao Zhan Jianing An Yongzhe Zhang Yani Zhang Lianxi Zheng 《Journal of Raman spectroscopy : JRS》2012,43(9):1221-1226
Polarization behaviors of carbon nanotube (CNT) fibers with different twisting were reported. Scanning electron microscope and polarized Raman spectroscopy were used to investigate the prepared samples. Results indicate that surface twisting angle affects greatly the polarization angle and I///I⊥ ratio of twisted CNT fibers. Raman depth profile measurements imply that the twisted fibers consist of non‐uniform CNT alignments. A simplified two‐CNT‐alignment geometric model was proposed to illuminate the experimental observations. The results suggest that polarized Raman depth profile measurement would be a very useful approach for determining the distribution of CNT alignments in CNT fibers. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
30.
Florin Panaite 《代数通讯》2013,41(5):1937-1952
In a previous article we proved a result of the type “invariance under twisting” for Brzeziński's crossed products. In this article we prove a converse of this result, obtaining thus a characterization of what we call equivalent crossed products. As an application, we characterize cross product bialgebras (in the sense of Bespalov and Drabant) that are equivalent (in a certain sense) to a given cross product bialgebra in which one of the factors is a bialgebra and whose coalgebra structure is a tensor product coalgebra. 相似文献