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971.
Sugiura H Nigorikawa Y Saiki Y Nakamura K Yamaguchi M 《Journal of the American Chemical Society》2004,126(45):14858-14864
Optically active acyclic ethynylhelicene oligomers were synthesized in high yields by a two-directional method involving Sonogashira coupling and deprotection. Their CD spectra in chloroform exhibited large differences between the oligomers with less than seven helicenes and their higher homologues, which indicated the formation of helical structures for the latter and random coil structures for the former. The helical heptamer gradually unfolded to a random coil structure in chloroform at room temperature. The unfolding rate was examined by CD in several aromatic solvents as well, and the rate constant k was found to be highly dependent on the type of aromatic substituent: k differed by seven orders of magnitude between iodobenzene and trifluoromethylbenzene. Several features of the rates are notable: The reaction rates in halobenzenes were in the order of iodobenzene > bromobenzene > chlorobenzene > benzene > fluorobenzene > m-difluorobenzene, those in alkylbenzenes were styrene > phenylacetylene > ethylbenzene > toluene > benzene, and those in heteroatom-substituted arenes were thioanisole > benzonitrile > anisole > ethyl benzoate > benzene > trifluoromethylbenzene. The log k values exhibited good correlation with the absolute hardness, eta, of the arenes, and higher unfolding rates were observed in the soft arenes. Vapor pressure osmometry studies indicated that the helical structure of the heptamer is dimeric in benzene, fluorobenzene, and trifluoromethylbenzene, while the random coil structure of the heptamer is monomeric in chloroform and toluene. When a chloroform solution of the random coil structure was concentrated to a small volume, the helical structure could be regenerated. 相似文献
972.
973.
974.
Hiroki Sumida-Takahashi 《Journal of Number Theory》2004,105(2):235-250
Let p be a prime number and k a finite extension of . It is conjectured that the Iwasawa invariants λp(k) and μp(k) vanish for all p and totally real number fields k. Some methods to verify the conjecture for each real abelian field k are known, in which cyclotomic units and a set of auxiliary prime numbers are used. We give an effective method, based on the previous one, to compute the exact value of the other Iwasawa invariant νp(k) by using Gauss sums and another set of auxiliary prime numbers. As numerical examples, we compute the Iwasawa invariants associated to in the range 1<f<200 and 5?p<10000. 相似文献
975.
Akhtaruzzaman M Tomura M Zaman MB Nishida J Yamashita Y 《The Journal of organic chemistry》2002,67(22):7813-7818
Three novel 4,7-bis(n-pyridylethynyl)-2,1,3-benzothiadiazoles (n = 2, 3, and 4) were synthesized by using the Sonogashira cross-coupling reaction of 4,7-dibromo-2,1,3-benzothiadiazole with the corresponding ethynylpyridines in the presence of a Pd(II) catalyst. The viologen analogues were also prepared by methylation of pyridyl nitrogen atoms. X-ray structure analysis of these compounds revealed the linear molecular structures with unusual columnar crystal structures. Insertion of a benzothiadiazole moiety into the acetylene-pyridine skeleton brings about a large increase in electron affinity and the bispyridyl compounds obtained here show high fluorescence quantum yields. 相似文献
976.
Ono S Ogawa K Yamashita K Yamamoto T Kazuta Y Matsuda A Shuto S 《Chemical & pharmaceutical bulletin》2002,50(7):966-968
(1S,2R)-1-phenyl-2-[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide (2b, PPDC), a new class of potent N-methyl-D-aspartic acid (NMDA) receptor antagonist, was designed based on a new method for restricting the conformation of compounds having a cyclopropane ring. The three-dimensional structures of PPDC obtained by the three different methods of X-ray crystallographic analysis, usual MM2-calculations in vacuum, and MM2 calculations based on the nuclear Overhauser effect (NOE) data in D2O are similar, which are in accord with that hypothesized. These results suggest that this conformational restriction method is particularly effective in designing novel biologically active molecules. 相似文献
977.
High frequency responses of intrinsic Josephson junctions up to 2.5 THz, including the observation of Shapiro steps under various conditions, are reported and discussed in this Letter. The sample was an array of intrinsic Josephson junctions singled out from inside a high T(C) superconducting Bi2Sr2CaCu2O(8+x) single crystal, with a bow-tie antenna integrated to it. The number of junctions in the array was controllable, the junctions were homogeneous, the distribution of applied irradiation among the junctions was even, and the junctions could synchronously respond to high frequency irradiation. 相似文献
978.
Akihiro Furube Tsuyoshi Asahi Hiroshi Masuhara Hiromi Yamashita Masakazu Anpo 《Research on Chemical Intermediates》2001,27(1-2):177-187
On the basis of transient absorption spectral measurements in the whole visible region using femtosecond diffuse reflectance spectroscopy, a hole transfer process from photoexcited TiO2 to an adsorbed molecule SCN- was elucidated. In a TiO2 aqueous suspension with KSCN, absorption rise of dimer radical anion (SCN)?2- was observed indicating hole transfer. Interestingly, there were two rise components with < 1 ps and several 100 ps, which were shorter in lower pH of the solution. Ultrafast hole transfer and subsequent intermolecular geometrical change of a dimmer radical anion are also discussed. 相似文献
979.
Toshiyuki Osakai Naoki Akagi Hiroki Hotta Jie Ding Shigeo Sawada 《Journal of Electroanalytical Chemistry》2000,490(1-2)
The redox reaction between
-ascorbic acid in water and chloranil in nitrobenzene has been studied by means of polarography with an ascending water electrode as well as cyclic voltammetry with a stationary interface. Through accurate measurement of the limiting currents, it has been suggested that the redox reaction should be a two-electron reaction rather than a one-electron reaction described previously. A spectrophotometric technique has also been used to observe that the redox reaction proceeds spontaneously under certain conditions even without electrochemical control. Based on these findings, it has been concluded that the present heterogeneous charge transfer reaction is the ion transfer of chloranil semiquinone radical, which is driven by the homogeneous electron transfer between ascorbic acid and chloranil in the aqueous phase. 相似文献
980.