全文获取类型
收费全文 | 898篇 |
免费 | 40篇 |
国内免费 | 1篇 |
专业分类
化学 | 683篇 |
晶体学 | 15篇 |
力学 | 13篇 |
数学 | 44篇 |
物理学 | 184篇 |
出版年
2022年 | 7篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 18篇 |
2018年 | 8篇 |
2017年 | 11篇 |
2016年 | 22篇 |
2015年 | 16篇 |
2014年 | 24篇 |
2013年 | 34篇 |
2012年 | 47篇 |
2011年 | 53篇 |
2010年 | 38篇 |
2009年 | 27篇 |
2008年 | 50篇 |
2007年 | 45篇 |
2006年 | 57篇 |
2005年 | 46篇 |
2004年 | 61篇 |
2003年 | 38篇 |
2002年 | 39篇 |
2001年 | 20篇 |
2000年 | 11篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 14篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 14篇 |
1990年 | 3篇 |
1989年 | 8篇 |
1988年 | 3篇 |
1986年 | 8篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1982年 | 11篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 14篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1971年 | 5篇 |
1970年 | 3篇 |
排序方式: 共有939条查询结果,搜索用时 9 毫秒
21.
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
22.
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
23.
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
24.
Summary In order to prepare a large single crystal, tetraoxymethylene recrystallized by distilled water adjusted to pH 8 was sealed
in an ampoule at a reduced pressure of 1 mm Hg and was crystallized in a furnace with the temperature gradient of 6 °C/cm.
The lowering rate of the ampoule was required to be 6 cm/day for preparing the single crystal with 1 cm diameter and to be
3 cm/day for one with 2 cm diameter. The optimum angle at the pointing end of the ampoule, where a seed crystal is formed,
was about 40°. The b-axis of the obtained single crystal was parallel to the direction of the crystal growth i. e. the wall
of the ampoule. It was polymerized byγ-ray under conditions which prevent the growth of the twin crystal. The obtained polyoxymethylene was porous, because the
polymerization yield of 100% could not be attained. However, it was so tough that a specimen for measurements could be cut
down by a diamond cutter. Its fiber axis was parallel to the direction of the wall of the ampoule. It did not contain the
so-called amorphous regions, although it did contain defects. It was characterized with the extremely high orientation of
the polymer chains.
We are greatly indebted to Dr.Y. Miyake of Mitsui Toatsu Co., Ltd. for providing the tetraoxymethylene. Thanks are to due Dr.M. Nishii of Osaka Laboratories, Japan Association for Radiation Research on Polymer for the radiation polymerization, to Mr.T. Tsukihara for the X-ray measurement, and to Mrs.H. Tanaka for the help with the electron microscopy. We are also grateful to Prof.S. Seki for offering the use of differential scanning calorimeter. 相似文献
Zusammenfassung Zur Pr?parierung gro?er Einkristalle wurde rekristallisiertes Tetraoxymethylen in destilliertem Wasser von pH 8 in eine Ampulle bei einem reduzierten Quecksilberdruck von 1 mm eingefüllt und in einem Ofen mit einem Temperaturgradienten von 6 °C/cm kristallisiert. Die Abkühlgeschwindigkeit der Ampulle betrug etwa 6 cm/Tag für die Pr?paration eines Einkristalls mit 1 cm ? und 3 cm/Tag für einen solchen mit 2 cm ?. Der optimale Winkel am spitzen Ende der Ampulle, wo der Keimkristall gebildet wird, betrug etwa 40°. Dieb-Achse des Einkristalls war parallel zur Wachstumsrichtung, d. h. zur Wand der Ampulle. Der Kristall wurde durch R?ntgenstrahlen polymerisiert unter Bedingungen, die das Wachsen eines Zwillingskristalls vermeiden. Das erhaltene Polyoxymethylen war por?s, doch konnte eine Polymerisationsausbeute von 100% nicht erhalten werden. Doch war der Einkristall so z?h, da? für Messungen eine Probe mit dem Diamantschneider abgeschnitten werden konnte. Die Faserachse war parallel zur Wandrichtung der Ampulle. Das kristalline Material enthielt keine sogenannten amorphen Bereiche, allerdings Defekte. Es war durch extrem hohe Orientierungen der Polymerketten charakterisiert.
We are greatly indebted to Dr.Y. Miyake of Mitsui Toatsu Co., Ltd. for providing the tetraoxymethylene. Thanks are to due Dr.M. Nishii of Osaka Laboratories, Japan Association for Radiation Research on Polymer for the radiation polymerization, to Mr.T. Tsukihara for the X-ray measurement, and to Mrs.H. Tanaka for the help with the electron microscopy. We are also grateful to Prof.S. Seki for offering the use of differential scanning calorimeter. 相似文献
25.
Inada Y Nishibayashi Y Hidai M Uemura S 《Journal of the American Chemical Society》2002,124(51):15172-15173
A novel cationic methanethiolate-bridged diruthenium complex [Cp*RuCl(mu2-SMe)2RuCp*(OH2)]OTf (1e) has been disclosed to promote the catalytic propargylic substitution reaction of propargylic alcohols bearing not only terminal alkyne group but also internal alkyne group with thiols. It is noteworthy that neutral thiolate-bridged diruthenium complexes (1a-1c), which were known to promote the propargylic substitution reactions of propargylic alcohols bearing a terminal alkyne group with various heteroatom- and carbon-centered nucleophiles, did not work at all. The catalytic reaction described here provides a general and environmentally friendly preparative method for propargylic sulfides, which are quite useful intermediates in organic synthesis, directly from the corresponding propargylic alcohols and thiols. 相似文献
26.
Dr. Bernd M. Schmidt Dr. Takafumi Osuga Dr. Tomohisa Sawada Dr. Manabu Hoshino Prof. Dr. Makoto Fujita 《Angewandte Chemie (International ed. in English)》2016,55(4):1561-1564
Self‐assembled coordination cages can be employed as a molecular press, where the bowl‐shaped guest corannulene (C20H10) is significantly flattened upon inclusion within the hydrophobic cavity. This is demonstrated by the pairwise inclusion of corannulene with naphthalene diimide as well as by the dimer inclusion of bromocorannulene inside the box‐like host. The compressed corannulene structures are unambiguously revealed by single‐crystal X‐ray analysis. 相似文献
27.
N-substituted anilines react with triethanolamine in the presence of a catalytic amount of a homogeneous ruthenium catalyst to give the corresponding 1-substituted indoles in good yields. 相似文献
28.
29.
Hiroya Takada Yoshiaki Nishibayashi Sanjay Kumar Srivastava Kouichi Ohe Sakae Uemura 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):629-632
Asymmetric intramolecular selenocyclisation of alkenoic acids, alkenols and alkenyl urethanes using chiral ferrocenylselenenyl cations proceeds smoothly to give the corresponding lactones, cyclic ethers and N-heterocyclic compounds, respectively, in moderate yields with very high diastereoselectivities. 相似文献
30.
Hiroya Takada Yoshiaki Nishibayashi Kouichi Ohe Sakae Uemura 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):363-364
Prochiral sulfides reacted with Phl=NTs in the presence of a catalytic amount of Cu(I) salt together with a chiral 4,4′-disubstituted bis(oxazoline) ligand to afford the corresponding chiral sulfimides. 相似文献