全文获取类型
收费全文 | 5147篇 |
免费 | 196篇 |
国内免费 | 13篇 |
专业分类
化学 | 4157篇 |
晶体学 | 43篇 |
力学 | 51篇 |
数学 | 251篇 |
物理学 | 854篇 |
出版年
2023年 | 30篇 |
2022年 | 34篇 |
2021年 | 53篇 |
2020年 | 74篇 |
2019年 | 94篇 |
2018年 | 60篇 |
2017年 | 47篇 |
2016年 | 114篇 |
2015年 | 99篇 |
2014年 | 155篇 |
2013年 | 244篇 |
2012年 | 268篇 |
2011年 | 337篇 |
2010年 | 134篇 |
2009年 | 151篇 |
2008年 | 313篇 |
2007年 | 310篇 |
2006年 | 294篇 |
2005年 | 305篇 |
2004年 | 252篇 |
2003年 | 189篇 |
2002年 | 196篇 |
2001年 | 128篇 |
2000年 | 145篇 |
1999年 | 61篇 |
1998年 | 53篇 |
1997年 | 49篇 |
1996年 | 67篇 |
1995年 | 45篇 |
1994年 | 61篇 |
1993年 | 40篇 |
1992年 | 59篇 |
1991年 | 49篇 |
1990年 | 47篇 |
1989年 | 47篇 |
1988年 | 53篇 |
1987年 | 44篇 |
1986年 | 65篇 |
1985年 | 58篇 |
1984年 | 58篇 |
1983年 | 35篇 |
1982年 | 42篇 |
1981年 | 38篇 |
1980年 | 34篇 |
1979年 | 37篇 |
1978年 | 33篇 |
1977年 | 28篇 |
1976年 | 27篇 |
1975年 | 34篇 |
1974年 | 31篇 |
排序方式: 共有5356条查询结果,搜索用时 31 毫秒
71.
Takahiro Hosokawa Takanori Kono Toru Shinohara Shun-Ichi Murahashi 《Journal of organometallic chemistry》1989,370(1-3):C13-C16
The oxidative cyclization of 2-(3-pentenyl)phenol catalyzed by [(η3-pinene)PdOAc]2 gives optically active (+)-2-vinylchroman (25% e.e.), while (−)-2-(1-hydroxyethyl)chroman (56% e.e.) is formed as a single diastereomer upon treatment with t-BuOOH in the presence of Ti(OiPr)4 and
-(+)-diethyl tartrate. 2-(2-Butenyl)phenol also undergoes the Ti-promoted asymmetric cyclization to give (2S,1′R)-(−)-2-(1-hydroxyethyl)-2,3-dihydrobenzofuran (29% e.e.). 相似文献
72.
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
73.
On acetone-sensitization, 5-bromo-1,3-dimethyluracil ( reacts with Nb-methoxycarbonyltryptamine ( under the assistance of an electron carrier such as 2-methoxynaphthalene to give a coupling product . A mechanism involving a double electron transfer via the triplet state of is proposed. 相似文献
74.
Yoshikatsu Ito Meng Ji-Ben Shin Suzuki Yoshitaka Kusunaga Teruo Matsuura Keiichi Fukuyama 《Tetrahedron letters》1985,26(17):2093-2096
In contrast to simple imidazoles, 1-methy1–2,4,5-triphenylimidazole () produced stable oxetane photoadducts - with good efficiency upon irradiation in the presence of benzophenone derivatives - in acetonitrile solution. Irradiation in the solid-phase was also studied. The oxetanes - readily underwent cycloreversion by acid catalysis or by heating. 相似文献
75.
A nitro-introduced Wang resin-type linker for soluble and insoluble polymer support oligosaccharide synthesis is described. The linker was used for connecting glycosyl donors and polymer supports, and was completely stable under the glycosylation conditions tested. The cleavage of the linker was performed under reductive conditions without affecting the protecting groups to release disaccharides. 相似文献
76.
C. Yonezawa M. Magara H. Sawahata M. Hoshi Y. Ito E. Tachikawa 《Journal of Radioanalytical and Nuclear Chemistry》1995,196(1):171-178
The extraction behavior of uranium(VI), plutonium(IV) and some fission products like zirconium(IV), ruthenium(III) and europium(III) from 3.5M nitric acid with -irradiated organic phase pre-equilibratedn-dodecane solutions of dihexyl derivatives of hexanamide (DHHA), octanamide (DHOA) and decanamide (DHDA) has been investigated as a function of absorbed dose upto 184·104 Gy. The results indicate that the extraction of uranium(VI) decreases gradually with dose upto 72·104 Gy and becomes almost constant thereafter, while, the extraction of plutonium(IV) decreases upto a dose of 20·104 Gy and then increases rapidly up to a dose of 82·104 Gy indicating synergistic effects of radiolytic products formed at higher doses. Extraction of zirconium(IV) increases gradually upto a dose of 72·104 Gy. Europium(III) does not get extracted with any of these amides in the entire dose range (0–184·104 Gy) studied, however, ruthenium shows insignificant increase in extraction with dose. The decrease inD values noticed in the case of plutonium and zirconium after the dose of 72·104 Gy which was attributed to the third phase formation and emulsification. Infrared studies confirm the final products of radiolysis as the respective amines and carboxylic acids. The degraded amide contents have been estimated by quantitative IR spectrophotometric technique. Extraction data obtained for uranium(VI) and plutonium(IV) with TBP/n-dodecane system have also been compared under similar experimental conditions. 相似文献
77.
Hamada Fumio Kondo Yoshihiko Ito Riko Suzuki Iwao Osa Tetsuo Ueno Akihiko 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(3):273-281
The crystal structure of thiamine iodide sesquihydrate has been determined by X-ray diffraction methods as a host-guest model for coenzyme-substrate interactions. The asymmetric unit contains two chemical units. Both the thiamine molecules A and B, which are crystallographically independent, assume the usualF conformation and have a disordered hydroxyethyl side chain. An iodide anion (or a water molecule) bridges the pyrimidine and thiazolium rings of molecule A (or B) by forming a hydrogen bond with the amino group and an electrostatic contact with the thiazolium ring to stabilize the molecular conformation. In the crystal the thiamine molecules self-associate to form a pipe-like polymeric structure, in which four thiamine hosts surround an iodide guest and hold it through C(2)-H...I hydrogen bonds and thiazolium...I electrostatic interactions. Crystal data: C12H17N4OS+·I– · 1.5 H2O, monoclinic,P21/c, a=12.585(2), b=25.303(5), c=12.030(2) Å, =115.15(1)°,V=3468(1) Å3,Z=8,D
c=1.606 g cm–3,R=0.045 for 3328 observed reflections.
Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP. 82156 (13 pages). 相似文献
78.
Three novel diepoxy tetrahydrochromones, oxidoagarochromones A (1), B (2), and C (3), were isolated from agarwood artificially produced by intentional wounding of Aquilaria crassna. Inductive production of these compounds was also confirmed at the early stage of wounding in A. sinensis and A. crassna. These diepoxy tetrahydrochromones would play an important role in understanding the biosynthesis of chromone derivatives in agarwood. 相似文献
79.
An industrial and economic carbonylation of amines with carbon monoxide and sulfur has been developed for the synthesis of S-alkyl thiocarbamate herbicides. In the presence of potassium carbonate and solvent DMSO, S-alkyl thiocarbamates, such as thiobencarb and orbencarb (herbicides) are synthesized in excellent yields from amines, carbon monoxide, sulfur, and alkyl halides under mild conditions (1 atm, 20 °C). 相似文献
80.
The structures of two new dimeric acridone alkaloids, glycobismine-D (1) and -E (2), having a novel linkage as binary acridones, three monomeric acridones, glycocitrine-IV (3), -V (4), and -VI (5), and three quinolone alkaloids, glycocitlone-A (6), -B (7), and -C (8) from Glycosmis citrifolia (Willd.) Lindl. (Rutaceae) have been elucidated by spectrometric studies. 相似文献