全文获取类型
收费全文 | 3799篇 |
免费 | 110篇 |
国内免费 | 20篇 |
专业分类
化学 | 2956篇 |
晶体学 | 44篇 |
力学 | 41篇 |
数学 | 249篇 |
物理学 | 639篇 |
出版年
2023年 | 19篇 |
2022年 | 23篇 |
2021年 | 32篇 |
2020年 | 48篇 |
2019年 | 45篇 |
2018年 | 30篇 |
2017年 | 27篇 |
2016年 | 73篇 |
2015年 | 66篇 |
2014年 | 96篇 |
2013年 | 223篇 |
2012年 | 186篇 |
2011年 | 215篇 |
2010年 | 152篇 |
2009年 | 129篇 |
2008年 | 219篇 |
2007年 | 217篇 |
2006年 | 239篇 |
2005年 | 259篇 |
2004年 | 216篇 |
2003年 | 182篇 |
2002年 | 182篇 |
2001年 | 76篇 |
2000年 | 64篇 |
1999年 | 39篇 |
1998年 | 42篇 |
1997年 | 37篇 |
1996年 | 33篇 |
1995年 | 23篇 |
1994年 | 32篇 |
1993年 | 22篇 |
1992年 | 35篇 |
1991年 | 26篇 |
1990年 | 25篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 14篇 |
1986年 | 28篇 |
1985年 | 44篇 |
1984年 | 58篇 |
1983年 | 33篇 |
1982年 | 50篇 |
1981年 | 53篇 |
1980年 | 39篇 |
1979年 | 35篇 |
1978年 | 23篇 |
1977年 | 32篇 |
1976年 | 24篇 |
1975年 | 23篇 |
1974年 | 23篇 |
排序方式: 共有3929条查询结果,搜索用时 15 毫秒
41.
[reaction: see text] Boryl substitution on an olefin activates the olefinic double bond toward addition of an organozinc reagent. Addition of an allylic zinc reagent to an alkenylboronate thus takes place smoothly to afford a variety of gem-zincio/boryl species. Theoretical studies with density functional calculations on the reaction pathway revealed that the reaction proceeds via a zincio-ene reaction rather than a bora-Claisen rearrangement. 相似文献
42.
The high frequency oscillator provides an excellent means of detecting the end-points of titrations performed with ethylenediaminetetraacetic acid or its salts. The determination of various divalent metals is reported, based on both direct and indirect titrations. Because of the great sensitivity of the method it is possible to determine cobalt, nickel, copper (II), zinc, cadmium, lead and manganese (II) in concentration ranges of 1/1000 to 1/6000M. 相似文献
43.
The relative stability of various organosulfur radicals is estimated by competitive elimination technique using tributyltin radical and acetophenone derivatives having two different sulfur substituents at α and α positions. 相似文献
44.
Tomoya Kinumi Kazuo Tsujimoto Mamoru Ohashi Reiko Hara Tomiyuki Hara Koichi Ozaki Miho Saka Yuko Katsuta Akimori Wada Masayoshi Ito 《Photochemistry and photobiology》1993,58(3):409-412
Abstract— 3, 7-Dimethyl-2, 4, 6, 8, 10-dodecapentaenal was synthesized for reconstitution of the retinochrome analog. Its opsin shift was 1000 cm 1 smaller than that of native retinochrome, whose chromophore contains the same number of double bonds. The conformational change from 6-s-trans to 6-s-cis , as figured in a retinal molecule, plays an important role in the formation of the retinochrome analog, based on the estimation of opsin shifts for retinal analogs locked in the 6-s conformation. Thus the conformation of the 6–7 single bond in the native retinochrome was suggested to be 6 -cis . Analysis of the circular dichroic spectra of retinochrome analogs revealed that the 6-s conformation is independent of the appearance of the β-band. The stereoselectivity in the photoisomerization of the retinal analogs by a retinochrome template depends on the hydrophobic binding in the region of the β-ionone ring. 相似文献
45.
Mitsuo Ishikawa Takamasa Fuchikami Makoto Kumada 《Journal of organometallic chemistry》1978,162(2):223-238
Photolysis of phenylpentamethyldisilane, 1,1-diphenyltetramethyldisilane and 1-isopropyl-1-phenyltetramethyldisilane in the presence of a wide variety of olefins has been studied. All reactions investigated, with the exception of those with 1-methylcyclohexene and tetramethylethylene, afforded the corresponding 1/1 addition products. Yields of the adducts depended highly on the structure of the olefins used. In the photolysis of phenylpentamethyldisilane in the presence of an equimolar mixture of 1,1- and 1,2-bis(trimethylsilyl)ethene under competitive reaction conditions, the main product was the one arising from the 1,1-isomer. A possible mechanism for the process of addition of olefin to the photochemically generated silicon-carbon double-bonded intermediate (A) is discussed. 相似文献
46.
Makoto?OhtsukaEmail author Christina?Wedel Kimio?Itagaki 《Monatshefte für Chemie / Chemical Monthly》2005,136(11):1909-1914
Summary. Isothermal sections of the Ni–Mn–Ga ternary phase diagram at 1073 and 1273 K were investigated over a wide range of alloy
compositions. The range of the β-Ni2MnGa phase, its equilibria with the γ-(Mn, Ni), α′-Ni3Ga, and γ-Ni3Ga2 phases, and the liquidus and solidus lines were determined experimentally. The aging effect on the shape memory effect (SME)
of Ni2MnGa sputtered films was also investigated. The two-way SME of the constraint-aged films was confirmed by the temperature
change. 相似文献
47.
[structure: see text] The first total synthesis of gambierol, a marine polycyclic ether toxin, has been achieved. The synthesis features the Pd(PPh3)4/CuCl/LiCl-promoted Stille coupling for the stereoselective construction of the sensitive triene side chain that includes a conjugated (Z,Z)-diene moiety. 相似文献
48.
Makoto Kumada Koji Sumitani Yoshihisa Kiso Kohei Tamao 《Journal of organometallic chemistry》1973,50(1):319-326
The hydrosilylation of olefins catalyzed by nickel(II) chloride complexed with 1,2-bis(dimethylphosphino)-1,2-dicarba-closo-dodecaborane produces terminal and internal adducts in comparable amounts. This unusual feature of the reaction is explained in terms of the electron-accepting nature of the carboranyl group. 相似文献
49.
Mo YJ Jiang DL Uyemura M Aida T Kitagawa T 《Journal of the American Chemical Society》2005,127(28):10020-10027
A series of poly(aryl ether) dendrimer chloroiron(III) porphyrin complexes (LnTPP)Fe(III)Cl (number of aryl layers [n]=3 to 5) were synthesized, and their Boltzmann temperatures under IR irradiation were evaluated from ratios of Stokes to anti-Stokes intensities of resonance Raman bands. While the Boltzmann temperature of neat solvent was unaltered by IR irradiation (LnTPP)Fe(III)Cl (n=3 to 5), all showed a temperature rise that was larger than that of the solvent and greater as the dendrimer framework was larger. Among vibrational modes of the metalloporphyrin core, the temperature rise of an axial Fe-Cl stretching mode at 355 cm-1 was larger than that for a porphyrin in-plane mode at 390 cm-1. Although most of the IR energy is captured by the phenyl nu8 mode at 1597 cm-1 of the dendrimer framework, an anti-Stokes Raman band of the phenyl nu8 mode was not detected, suggesting the extremely fast vibrational relaxation of the phenyl mode. From these observations, it is proposed that the energy of IR photons captured by the aryl dendrimer framework is transferred to the axial Fe-Cl bond of the iron porphyrin core and then relaxed to the porphyrin macrocycle. 相似文献
50.
Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols 总被引:1,自引:0,他引:1
Iwasawa T Tokunaga M Obora Y Tsuji Y 《Journal of the American Chemical Society》2004,126(21):6554-6555
In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 degrees C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields. 相似文献