全文获取类型
收费全文 | 2457篇 |
免费 | 115篇 |
国内免费 | 7篇 |
专业分类
化学 | 1922篇 |
晶体学 | 32篇 |
力学 | 16篇 |
数学 | 124篇 |
物理学 | 485篇 |
出版年
2023年 | 12篇 |
2022年 | 16篇 |
2021年 | 30篇 |
2020年 | 37篇 |
2019年 | 48篇 |
2018年 | 41篇 |
2017年 | 33篇 |
2016年 | 77篇 |
2015年 | 67篇 |
2014年 | 88篇 |
2013年 | 125篇 |
2012年 | 162篇 |
2011年 | 171篇 |
2010年 | 73篇 |
2009年 | 89篇 |
2008年 | 197篇 |
2007年 | 161篇 |
2006年 | 161篇 |
2005年 | 131篇 |
2004年 | 122篇 |
2003年 | 94篇 |
2002年 | 96篇 |
2001年 | 34篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 21篇 |
1997年 | 22篇 |
1996年 | 31篇 |
1995年 | 25篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 16篇 |
1991年 | 12篇 |
1990年 | 21篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 17篇 |
1986年 | 18篇 |
1985年 | 24篇 |
1984年 | 16篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 9篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 14篇 |
1975年 | 15篇 |
1973年 | 9篇 |
排序方式: 共有2579条查询结果,搜索用时 31 毫秒
61.
Masami Sawada Yasuo Okumura Hitoshi Yamada Yoshio Takai Shigetoshi Takahashi Takahiro Kaneda Keiji Hirose Soichi Misumi 《Journal of mass spectrometry : JMS》1993,28(12):1525-1528
Using fast atom bombardment (FAB) mass spectrometry (MS), cross-chiral relationships were confirmed for the first time for the diasteromeric host-guest complexations between the chiral crown ether host (1) and the chiral organic ammonium ion guest (2) on the basis of the relative peak intensities (RPI). Both host–guest combinations (R, R, R, R) – 1, (R) – 2 and (S, S, S, S) – 1, (S) – 2 obviously provided larger RPI values than the combination of both (R, R, R, R) – 1, (S) – 2 and (S, S, S, S) – 1, (R) – 2 by a factor of 1.6 as an averaged value: 1.87 (n = 4)/1.16 (n = 4) = 1.6. These results are consistent with the expected stabilities of the host-guest complexations by CPK model examinations. Successfully observed cross-chiral examinations strongly suggest a potentially useful FABMS/RPI methodology for rapidly searching newly designed and synthesized crown ether-like host compounds with a higher degree of enantioselectivity. 相似文献
62.
Yoshizumi Ishikawa Hideyasu Honda Yoshiyuki Sugahara 《Journal of Sol-Gel Science and Technology》2000,19(1-3):365-369
The conversion of tetrakis(diethylamino)titanium (Ti(NEt2)4) into titania via either a combination of hydrolysis (Ti(NEt2)4 : THF : H2O = 1 : 10 : x, x = 2, 4, 10) at ambient conditions and calcination (method A) or hydrolysis in a water-tetrahydrofuran (THF) mixture (Ti(NEt2)4 : THF : H2O = 1 : 10 : 100) at reflux (method B) was investigated. Titanium tertiary butoxide (Ti(O
t
Bu)4) was also used as a substitute for Ti(NEt2)4. The hydrolysis via method A resulted in the formation of amorphous solids containing organics. Thermal analyses showed that the hydrolysis products showed mass losses up to 500°C probably due to the presence of diethylamine (Et2NH) formed via the hydrolysis of Ti(NEt2)4 in the hydrolysis products, while a mass loss of the hydrolysis product from Ti(O
t
Bu)4 was completed up to about 200°C. After calcination at 600°C, anatase or a mixture of anatase and rutile was obtained. The crystallization behavior of the hydrolysis products from Ti(NEt2)4 was different from that of the hydrolysis product from Ti(O
t
Bu)4. The hydrolysis via method B gave only an amorphous material from Ti(NEt2)4, while a crystalline titania (anatase and brookite) formed from Ti(O
t
Bu)4. 相似文献
63.
Kasai HF Tsubuki M Matsumoto Y Shirao M Takahashi K Honda T Ueda H 《Chemical & pharmaceutical bulletin》2004,52(3):311-315
Gas chromatographic separations of the stereoisomers of menthol derivatives, important intermediates in the synthesis of physiologically active natural products, were carried out on several substituted beta-cyclodextrin (CD) columns, including per-O-methyl-beta-cyclodextrin (PME-beta-CD), heptakis(2,3-di-O-acetyl-6-tert-butyldimethylsilyl)-beta-CD (DIAC-6-TBDS-beta-CD), and heptakis(2,3-di-O-methyl-6-tert-butyldimethylsilyl)-beta-CD (DIME-6-TBDS-beta-CD) as chiral stationary phases (CSPs). With the DIME-6-TBDS-beta-CD column, a separation of the Z- and E-isomers of methylidenementhol was accomplished; no separation was achieved with the other columns. The stereoisomers of methylidenementhol and the corresponding tert-butyldimethylsilyl (TBS) ether were separated on both the beta-CD and the heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TME-beta-CD) columns by high-performance liquid chromatography (HPLC) with a mobile phase involving acetonitrile and H(2)O. For the separation of the Z- and E-isomers of methylidenementhol, the TME-beta-CD column was superior. In contrast, the beta-CD column was preferable in the case of the corresponding TBS ether. 相似文献
64.
A series of iminodiacetic acid (IDA)-linked polyamides (DpPyPyPy-IDA-PyPyPyDp) were prepared and constitute polyamides joined head-to-head by a functionalizable five-atom linker. It was found that the IDA linker exerts a unique influence over the DNA binding conformation differing from both the beta-alanine (extended) or gamma-aminobutyric acid (hairpin) linkers, resulting in cooperative parallel side-by-side 2:1 binding in an extended conformation most likely with a staggered versus stacked alignment. A generalized variant of a fluorescent intercalator displacement (FID) assay conducted on a series of hairpin deoxyoligonucleotides containing a systematically varied A/T-rich binding-site size was used to distinguish between the binding modes of the IDA-linked polyamides. 相似文献
65.
Masakazu Kobayashi Takuya Honda Atsushi Kanzawa 《Plasma Chemistry and Plasma Processing》1985,5(1):39-54
The effect of gas injection on an atmospheric thermal argon plasma flow in a water-cooled tube was investigated experimentally and numerically. The injection gas is argon, helium, or nitrogen. The static pressure with helium injection increases greatly because of its high thermal conductivity, while little increase occurs for nitrogen injection because of the dissociation. The increasing rate of the static pressure depends on the ratio of the momentum term to the viscosity term. The heat flux to the tube wall with gas injection changes less than that without injection. The numerical results showed variations similar to the experimental ones. 相似文献
66.
Electrochemically driven clathration/declathration of ferrocene and its derivatives by a nanometer-sized coordination cage 总被引:1,自引:0,他引:1
The electrochemical, NMR, and crystallographic studies clearly indicate that the clathration and declathration of ferrocene and its derivatives in a self-assmbled Pd(II)-linked nanometer-sized cage can be controlled by the oxidation state of the guest. Namely, the guests are clathrated when they are reduced but declathrated when they are oxidized. The process is rapid on the NMR time scale but slow on the CV time scale. 相似文献
67.
Hetero-biaryl compounds were prepared via the Suzuki-Miyaura coupling reaction of hetero-aryl moieties containing an unprotected NH2 group and arylboronic acids. D-t-BPF was found to be an efficient ligand for the cross-coupling of NH2-unprotected hetero-aryl chlorides with phenylboronic acid. 相似文献
68.
Nobuko WatanabeYasuhiro Nagashima Takahiro YamazakiMasakatsu Matsumoto 《Tetrahedron》2003,59(26):4811-4819
Tetrabutylammonium fluoride induces the decomposition of 1-tert-butyl-4,4-dimethyl-5-(3-siloxyphenyl)-2,6,7-trioxabicyclo[3.2.0]heptane (4a) in DMSO to form an oxyanion of aromatic ketone (14a) as an emitter with high singlet-chemiexcitation yield comparable with that for a chemically initiated electron exchange luminescence (CIEEL) active dioxetane producing an oxyanion of aromatic ester as an emitter. A 7-siloxynaphthalen-2-yl analog (4b) was found on similar treatment to emit light with the maximum wavelength the longest among CIEEL-active dioxetanes hitherto known. 相似文献
69.
Thirteen pregnane glycosides were isolated from fresh leaves of Marsdenia tomentosa collected in the Fukuyama district. Of these, six were glycosides previously obtained from the same plant collected in the Fukuoka district and one from another Asclepiadaceous plant. The structures of the six new glycosides were determined by spectrometric method. 相似文献
70.
A highly efficient copper-catalyzed coupling reaction of aryl iodides and thiobenzoic acid, using 10 mol % of copper iodide, 20 mol % of 1,10-phenanthroline, and iPr2NEt in toluene, was developed. This methodology is applicable to a variety of aryl iodides. 相似文献