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排序方式: 共有502条查询结果,搜索用时 15 毫秒
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W. D. Wu A. Keren L. P. Le G. M. Luke B. J. Sternlieb Y. J. Uemura D. C. Johnston B. K. Cho P. Gehring 《Hyperfine Interactions》1994,86(1):615-621
Muon spin relaxation (SR) studies have been performed in the normal spinel LiTi2O4 and the A-15 superconductor V3Si to measure the magnetic penetration depth . The relaxation rate(T) 1/2 in field-cooled measurements shows a sharp increase belowT
c
followed by saturation at low temperatures in both systems. This feature implies an isotropic energy gap without anomalous zeros, and most likelys-wave pairing. The low temperature penetration depth (T 0) is determined to be 2100Å for LiTi2O4 and 1300Å for V3Si respectively. Assuming a clean limit relation –2 n
s
/m
*, we derive the Fermi temperatureT
F
n
s/
2/3
m
* from the relaxation rate and the Sommerfeld constant asT
F
3/4–1/4. Unlike conventional superconductors, both LiTi2O4 and V3Si have a large ratio ofT
c
/T
F
0.01, only slightly smaller than those ratios in more exotic superconductors.We thank C. Ballard and K. Hoyle for technical assistance. Work at Columbia University is supported by NSF Grant No. DMR-89-13784 and Packard Foundation (YJU). Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. Work at Ames was supported by the Director for Energy Research, Office of Basic Energy Sciences. 相似文献
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16.
Sakae Uemura Haruo Miyoshi Masaya Okano Isao Morishima Toshiro Inubushi 《Journal of organometallic chemistry》1979,165(1):9-19
Treatment of norbornene, norbornadiene, benzonorbornadiene, and chloro- and methoxy-benzonorbornadiene with thallium(III) acetate in methanol affords only the corresponding cis-exo-acetoxythallation adducts in a sharp contrast to oxymercuration of such strained olefins where methoxymercuration prevails. In the cases of substituted benzonorbornadienes the products are obtained as the regioisomeric mixtures, the isomer ratio being determined by 13C NMR. In the cases of 5-norbornene-2,3-dicarboxylic anhydride, 5-norbornene-2-methyl-2,3-dicarboxylic anhydride, and 5-norbornen-2-endo-carboxylic acid, lactonization occurs to give a trans-oxythallation adduct having a lactone ring, no introduction of either methoxy or acetoxy groups being observed. 1H and/or 13C NMR data for several new oxythallation adducts are provided. The alkaline sodium borohydride reduction of adducts in methanol affords mainly the parent olefin together with 10–16% yields of the corresponding exo-alcohol. 相似文献
17.
Kouichi Ohe Tomomi Yokoi Koji Miki Fumiaki Nishino Sakae Uemura 《Journal of the American Chemical Society》2002,124(4):526-527
The reaction of cis vicinal acetylethynylcyclopropanes 1 with a catalytic amount of M(CO)5(THF) (M = Cr or W) in the presence of Et3N at room temperature gave ortho-substituted phenols 7 in good yields as valence isomerized products. In the absence of Et3N the reactions did not work at all. The reaction of a cyclopropane having an ester or an amide instead of an acetyl moiety with M(CO)5(THF) did not take place, whereas an ethynylvinylcyclopropane gave a mixture of 1- and 2-substituted 1,3,5-cycloheptatrienes. These valence isomerization reactions are assumed to proceed via the formation of vinylidene-metal intermediates 2 from terminal alkynyl moieties followed by [3,3]sigmatropy of 2 to give seven-membered carbene complexes 3. 相似文献
18.
Sakae Uemura Shin-Ichi Fukuzawa Akio Toshimitsu 《Journal of organometallic chemistry》1983,250(1):203-215
Treatment of olefinic hydrocarbons with phenyltellurium tribromide or a mixture of diphenylditelluride and bromine in alcohol affords (β-alkoxyalkyl)phenyltellurium dibromides in fair to good yield (alkoxytelluration of olefins). Various aryltellurium trichlorides, diphenylditelluride/CuCl2, and phenyltellurocyanate/CuCl2 can be used for the preparation of (β-alkoxyalkyl)aryltellurium dichlorides. Similar reactions in aqueous tetrahydrofuran or aqueous t-butyl alcohol result in the formation of the corresponding β-hydroxy compound (hydroxytelluration of olefins). The reaction is trans stereospecific in the cases of cis-2-butene and cis- and trans-4-octenes and regiospecific in the cases of all terminal olefins examined (1-hexene, 1-octene, 1-decene, styrene, α-methylstyrene, 2-methyl-1-pentene, and isobutylene), tellurium species attacking the terminal carbon solely. The diorganyltellurium dihalide produced is reduced to the corresponding diorganyltelluride by reducing agents such as N2H4, Na2S, Na2S2O3, and NaHSO4 in aqueous solution. Treatment of the diorganyltellurium dibromide with aqueous NaOH affords either an allylic ether (by telluroxide elimination) or a telluroxide depending on the structure of the telluroxide. 相似文献
19.
The reaction of conjugated dienes such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2,5-dimethyl-2,4-hexadiene, 1,3-cyclopentadiene, and 1,3-cyclohexadiene, with thallium(III)acetate in acetic acid at 10–65° for 0.5–15 hr affords an isomeric mixture of the corresponding diacetoxyalkenes (1,2- and 1,4-addition products) in 10–92% yields. The 1,2-addition products are predominantly formed in all cases examined except the case of 1,3-cyclopentadiene. The reaction is assumed to proceed through acetoxythallation and dethallation steps, the latter step being accompanied and/or followed by an attack of acetoxyl group. An initial attack of thallium moiety is proposed to occur mainly at C-1 and C-2 carbons in the cases of linear terminal dienes and cyclic dienes, respectively. 相似文献
20.
Nishibayashi Y Milton MD Inada Y Yoshikawa M Wakiji I Hidai M Uemura S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1433-1451
The scope and limitations of the ruthenium-catalyzed propargylic substitution reaction of propargylic alcohols with heteroatom-centered nucleophiles are presented. Oxygen-, nitrogen-, and phosphorus-centered nucleophiles such as alcohols, amines, amides, and phosphine oxide are available for this catalytic reaction. Only the thiolate-bridged diruthenium complexes can work as catalysts for this reaction. Results of some stoichiometric and catalytic reactions indicate that the catalytic propargylic substitution reaction proceeds via an allenylidene complex formed in situ, whereby the attack of nucleophiles to the allenylidene C(gamma) atom is a key step. Investigation of the relative rate constants for the reaction of propargylic alcohols with several para-substituted anilines reveals that the attack of anilines on the allenylidene C(gamma) atom is not involved in the rate-determining step and rather the acidity of conjugated anilines of an alkynyl complex, which is formed after the attack of aniline on the C(gamma) atom, is considered to be the most important factor to determine the rate of this catalytic reaction. The key point to promote this catalytic reaction by using the thiolate-bridged diruthenium complexes is considered to be the ease of the ligand exchange step between a vinylidene ligand on the diruthenium complexes and another propargylic alcohol in the catalytic cycle. The reason why only the thiolate-bridged diruthenium complexes promote the ligand exchange step more easily with respect to other monoruthenium complexes in this catalytic reaction should be that one Ru moiety, which is not involved in the allenylidene formation, works as an electron pool or a mobile ligand to another Ru site. The catalytic procedure presented here provides a versatile, direct, and one-step method for propargylic substitution of propargylic alcohols in contrast to the so far well-known stoichiometric and stepwise Nicholas reaction. 相似文献