全文获取类型
收费全文 | 6306篇 |
免费 | 239篇 |
国内免费 | 26篇 |
专业分类
化学 | 4457篇 |
晶体学 | 47篇 |
力学 | 177篇 |
数学 | 964篇 |
物理学 | 926篇 |
出版年
2024年 | 6篇 |
2023年 | 57篇 |
2022年 | 187篇 |
2021年 | 201篇 |
2020年 | 161篇 |
2019年 | 157篇 |
2018年 | 115篇 |
2017年 | 116篇 |
2016年 | 253篇 |
2015年 | 213篇 |
2014年 | 226篇 |
2013年 | 334篇 |
2012年 | 398篇 |
2011年 | 492篇 |
2010年 | 281篇 |
2009年 | 259篇 |
2008年 | 388篇 |
2007年 | 364篇 |
2006年 | 366篇 |
2005年 | 338篇 |
2004年 | 307篇 |
2003年 | 245篇 |
2002年 | 209篇 |
2001年 | 80篇 |
2000年 | 70篇 |
1999年 | 76篇 |
1998年 | 49篇 |
1997年 | 52篇 |
1996年 | 79篇 |
1995年 | 48篇 |
1994年 | 46篇 |
1993年 | 39篇 |
1992年 | 49篇 |
1991年 | 39篇 |
1990年 | 23篇 |
1989年 | 22篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 21篇 |
1983年 | 22篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1977年 | 9篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有6571条查询结果,搜索用时 15 毫秒
161.
Andrea Schfer Manfred Hesse Max Viscontini Henning Paul Berthold Fischer 《Helvetica chimica acta》1995,78(7):1763-1776
The experimental conditions developed for the detection of rather stable radical cations in solution by electrospray-ionization mass spectrometry (ESI-MS) of a FeII complex of 2-amino-5,6,7,8-tetrahydro-5-methylpteridin-4 (3H)-one ( 1c ) are used to observe the formation of the more unstable radical cations formed from 2-amino-5,6,7,8-tetrahydropteridin-4(3H)-one ( 1a ) and tris(pentane-2,4-dionato)iron(III) ([FeIII(acac)3]; 4 ) and to monitor their oxidation to the corresponding p-quinonoid dihydropterin complexes. These results contribute to the understanding of the important role played by 6β-5,6,7,8-tetrahydro-L -biopterin ( 1b ; a homologue of 1a ) together with iron as constituent of some cofactors. The complexes obtained from 1a and iron may be considered, e.g. as a model of the cofactor of the phenylalanine hydroxylase. Moreover, we describe an improved synthesis of 1c . 相似文献
162.
163.
Baldelli E Battaglia A Bombardelli E Carenzi G Fontana G Gelmi ML Guerrini A Pocar D 《The Journal of organic chemistry》2004,69(20):6610-6616
New taxanes 15 and 18, containing the unsaturated and saturated baccatin[14,1-d]furan-2-one nucleus, respectively, were prepared starting from the readily available 13-oxo-7-Tes-baccatin III (3). Sequential formation of the enolate of 3 and reaction with ethyl glyoxylate gave the 13-oxo-7-Tes-baccatin[14,1-d]-3,4-dehydrofuran-2-one 4. The reduction of 4 can result in the formation of a mixture of compounds corresponding to 13-hydroxy alcohol 5 and 13-enol derivative 6. Both 5 and 6 were transformed into 13-oxo-7-Tes-baccatin[14,1-d]furan-2-one 8 by treatment with a base. Further reduction of 8 gave 13-hydroxy compound 9. Esterification of 6 and 9 with N,O-protected norstatine 12, followed by deprotection, gave the new promising anticancer taxanes 15 and 18, respectively. 相似文献
164.
At pH 4.5 (citrate buffer), D -gluconhydroximo-lactone ( 2 ), the N-methylurethane 3 and the N-phenylurethane 4 inhibit competitively the hydrolysis of p-nitrophenyl β-D -glucopyranoside by emulsin. The IC50 values of 2, 3 , and 4 were 1.6 × 10?4, 1.0 × 10?4, and 5.8 × 10?6 M , respectively. The Ki values of 2 and 4 were 9.8 × 10?5 and 2.3 × 10?6 M , respectively, while D-glucono-1,5-lactone ( 1 ) showed IC50 = 1.1 × 10?4 M and Ki = 3.7 × 10?5 M . 相似文献
165.
166.
Amidoalkyl radicals, generated from amides by Mn(OAc)3 in acetic acid react with phenyl substituted alkenes to generate five-membered lactones or lactams. Ultrasound at ambient temperature significantly accelerates these single electron transfer reactions even when compared with conventional conditions under reflux or simple mechanical stirring. In some cases sonication leads to unusual products. 相似文献
167.
α-acyloxyaminoamides can be prepared, with yields ranging from fair to very good (up to 89%), through an Ugi four component reaction by mixing a carbonyl component, a carboxylic acid, an isocyanide and an N-alkylated hydroxylamine in methanol. Preformed nitrones furnish the same final compounds with comparable yields. 相似文献
168.
A numerical model for the adsorption kinetics of proteins on the walls of a microchannel has been developed using the finite element method (FEM) to address the coupling with diffusion phenomena in the restricted microchannel volume. Time evolutions of the concentration of one species are given, both in solution and on the microchannel walls. The model illustrates the adsorption limitation sometimes observed when the microdimensions of these systems induce a global depletion of the bulk solution. A new non-dimensional parameter is introduced to predict the final value of the coverage of any microsystem under static adsorption. A working curve and a criteria (h/K[Gamma](max) > 10) are provided in order to choose, for given adsorption characteristics, the value of the volume-to-surface ratio (i.e. the channel height h) avoiding depletion effects on the coverage (relative coverage greater than 90% of the theoretical one). Simulations were compared with confocal microscopy measurements of IgG antibody adsorption on the walls of a PET microchannel. The fit of the model to the experimental data show that the adsorption is under apparent kinetic control. 相似文献
169.
Chun-Long?Chen Andrea?M.?Goforth Mark?D.?Smith William?R.?Gemmill Cheng-Yong?Su Hans-Conrad?zur?LoyeEmail author 《Journal of Cluster Science》2005,16(4):477-487
A new flexible ligand, 1,5-bis(8-oxaquinoline)-3-methylpentane (C5MeOQ), was synthesized and used in the preparation of the
new dinuclear copper(II) complex 1: [Cu2(μ-OMe)2(μ-C5MeOQ)(NO3)2]. Single crystal X-ray analysis revealed that complex 1 contains dinuclear Cu(II) units with bridging methoxo groups. The ligand, C5MeOQ, takes on a bent conformation in order to
connect to and complete the square planar coordination environment of both copper centers; and one of the two Cu(II) centers
adopts a distorted square planar configuration as a result of the ligand conformation. The magnetic properties of complex
1 were investigated by variable-temperature magnetic susceptibility measurements in the 100–300 K range. Interestingly, the
susceptibility data for complex 1 reveals that the compound is diamagnetic, suggesting the presence of very strong antiferromagnetic coupling interactions
between the adjacent Cu(II) centers. 相似文献
170.
Dalvit C Hadden DT Sarver RW Ho AM Stockman BJ 《Combinatorial chemistry & high throughput screening》2003,6(5):445-453
High-throughput ligand-based proton NMR screening performed in the presence of a spy molecule and a control molecule is a valuable tool for identifying drug leads. A limitation of the technique is represented by the severe overlap encountered in the screening of large chemical mixtures. An approach for overcoming this overlap problem is the use of multi-selective R(1) filtered and COSY or TOCSY experiments. Application of this methodology to compounds binding to the Sudlow site I of human serum albumin is presented. The screening is performed by simply monitoring the intensity of two signals. The precise measurement of the relative intensity of the two resonances permits determination of the binding constant of the NMR-hit. For a simple competition binding mechanism, the rapidly-derived NMR binding constants are in good agreement with the values derived from full-titration ITC and fluorescence spectroscopy measurements. 相似文献