全文获取类型
收费全文 | 4637篇 |
免费 | 240篇 |
国内免费 | 40篇 |
专业分类
化学 | 3271篇 |
晶体学 | 56篇 |
力学 | 105篇 |
数学 | 380篇 |
物理学 | 1105篇 |
出版年
2024年 | 4篇 |
2023年 | 34篇 |
2022年 | 53篇 |
2021年 | 129篇 |
2020年 | 99篇 |
2019年 | 116篇 |
2018年 | 88篇 |
2017年 | 79篇 |
2016年 | 184篇 |
2015年 | 153篇 |
2014年 | 188篇 |
2013年 | 343篇 |
2012年 | 356篇 |
2011年 | 422篇 |
2010年 | 232篇 |
2009年 | 220篇 |
2008年 | 311篇 |
2007年 | 280篇 |
2006年 | 266篇 |
2005年 | 255篇 |
2004年 | 201篇 |
2003年 | 167篇 |
2002年 | 159篇 |
2001年 | 79篇 |
2000年 | 87篇 |
1999年 | 51篇 |
1998年 | 45篇 |
1997年 | 37篇 |
1996年 | 35篇 |
1995年 | 28篇 |
1994年 | 30篇 |
1993年 | 41篇 |
1992年 | 25篇 |
1991年 | 17篇 |
1990年 | 24篇 |
1989年 | 14篇 |
1988年 | 8篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1975年 | 4篇 |
1968年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有4917条查询结果,搜索用时 46 毫秒
121.
Choi YA Lee DJ Lim HK Jeong JH Sonn JK Kang SS Baek SH 《Experimental & molecular medicine》2004,36(3):226-232
IL-1beta is known promote cyclooxygenase-2 (COX- 2) and matrix metalloproteinase-2 (MMP-2) expression. This study focuses on the characterization of the signaling cascade associated with IL-1beta-induced matrix metalloproteinase-2 (MMP-2) regulation in human chondrocytes. The decrease in collagen levels in the conditioned media was prevented by a broad spectrum MMP inhibitor, suggesting that IL-1beta promotes the proteolytic process leading to MMP-2 activation. IL-1beta-related MMP-2 expression was found to be dependent on prostaglandin E2 (PGE2) production. In addition, the induction of COX-2 and MMP-2 was inhibited by the pretreatment of chondrocytes with a SB203580 or Ro 31-8220, indicating the involvement of protein kinase C (PKC) or p38 mitogen-activated protein kinase (MAPK). However, there is no cross-talk between PKC and p38 MAPK in the IL-1beta-induced MMP-2 activation. Taken together, these results demonstrated that IL-1beta induces MMP-2 expression through the PGE2-dependent mechanism in human chondrocytes. 相似文献
122.
Jew SS Jeong BS Lee JH Yoo MS Lee YJ Park BS Kim MG Park HG 《The Journal of organic chemistry》2003,68(11):4514-4516
Systematic investigations to develop an efficient enantioselective synthetic method for alpha-alkyl-alanine by catalytic phase-transfer alkylation were performed. The alkylation of 2-naphthyl aldimine tert-butyl ester, 1E, with RbOH and O(9)-allyl-N-2',3',4'-trifluorobenzylhydrocinchonidinium bromide, 6, at -35 degrees C showed the highest enantioselectivities, up to 96% ee. 相似文献
123.
Chan Sik Cho Na Young Lee Tae‐Jeong Kim Sang Chul Shim 《Journal of heterocyclic chemistry》2004,41(3):423-429
Nitroarenes are reductively cyclized with an array of tetraalkylammonium halides and trialkylarnmonium chlorides in the presence of a catalytic amount of a ruthenium catalyst along with tin(II) chloride dihydrate at 180° to afford the corresponding quinolines in moderate to good yields. The addition of tin(II) chloride dihydrate is necessary for the effective formation of quinolines and toluene is the solvent of choice. A reaction pathway involving initial reduction of nitroarenes to anilines and conversion of alkylammonium halides to alkylamines, alkyl group transfer from alkylamines to anilines to form an imine, dimerization of imine, and heteroannulation is proposed for this catalytic process. 相似文献
124.
Kinetic studies of the addition of benzylamines to a noncyclic dicarbonyl group activated olefin, ethyl alpha-acetyl-beta-phenylacrylate (EAP), in acetonitrile at 25.0 degrees C are reported. The rates are lower than those for the cyclic dicarbonyl group activated olefins. The addition occurs in a single step with concurrent formation of the Calpha-N and Cbeta-H bonds through a four-center hydrogen bonded transition state.The kinetic isotope effects (kH/kD > 1.0) measured with deuterated benzylamines (XC6H4CH2ND2) increase with a stronger electron acceptor substituent (deltasigmaX > 0) which is the same trend as those found for other dicarbonyl group activated series (1-4), but is in contrast to those for other (noncarbonyl) group activated series (5-9). For the dicarbonyl series, the reactivity-selectivity principle (RSP) holds, but for others the anti-RSP applies. These are interpreted to indicate an insignificant imbalance for the former, but substantial lag in the resonance delocalization in the transition state for the latter series. 相似文献
125.
Five metallocycles 1 a-e have been self-assembled from S-shaped bispyridyl ligands 2 a-e and a palladium complex, [Pd(dppp)(OTf)(2)] (dppp=1,3-bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including (1)H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen-bonding sites based on pyridine-2,6-dicarboxamide units. The binding properties of the metallocycles with N,N,N',N'-tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and (1)H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. (1)H NMR studies on 1 a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1 a and G, showing maximum complexation at approximately 0.33 mol fraction of the metallocycle 1 a in CDCl(3). The binding constants K(1) and K(2) are calculated to be 1600 and 1400 M(-1) (+/-10 %), respectively, at 25 degrees C in CDCl(3), indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl(3), for comparison purposes the binding properties of the metallocycles 1 b-e were investigated in a more polar medium, 3 % CD(3)CN/CDCl(3). (1)H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles. 相似文献
126.
127.
Eun Jeong ChoFrank V Bright 《Analytica chimica acta》2002,470(1):101-110
We report a new, solid-state, integrated optical array sensor platform. By using pin printing technology in concert with sol-gel-processing methods, we form discrete xerogel-based microsensor elements that are on the order of 100 μm in diameter and 1 μm thick directly on the face of a light emitting diode (LED). The LED serves as the light source to excite chemically responsive luminophores sequestered within the doped xerogel microsensors and the analyte-dependent emission from within the doped xerogel is detected with a charge coupled device (CCD). We overcome the problem of background illumination from the LED reaching the CCD and the associated biasing that results by coating the LED first with a thin layer of blue paint. The thin paint layer serves as an optical filter, knocking out the LEDs red-edge spectral tail. The problem of the spatially-dependent fluence across the LED face is solved entirely by performing ratiometric measurements. We illustrate the performance of the new sensor scheme by forming an array of 100 discrete O2-responsive sensing elements on the face of a single LED. The combination of pin printing with an integrated sensor and light source platform results in a rapid method of forming (∼1 s per sensor element) reusable sensor arrays. The entire sensor array can be calibrated using just one sensor element. Array-to-array reproducibly is <8%. Arrays can be formed using single or multiple pins with indistinguishable analytical performance. 相似文献
128.
Kim SW Oh HM Kim BS Chung HT Han WC Kim EC Kim TH Seo GS Lyou JH Nah YH Jung JC Choi SC Jun CD 《Experimental & molecular medicine》2003,35(1):53-59
Tumor target-derived soluble secretary factor has been known to influence macrophage activation to induce nitric oxide (NO) production. Since heme oxygenase-1 (HO-1) is induced by a variety of conditions associated with oxidative stress, we questioned whether soluble factor from tumor cells induces HO-1 through NO-dependent mechanism in macrophages. We designated this factor as a tumor-derived macrophage-activating factor (TMAF), because of its ability to activate macrophages to induce iNOS. Although TMAF alone showed modest activity, TMAF in combination with IFN-gamma significantly induced iNOS expression and NO synthesis. Simultaneously, TMAF induced HO-1 and this induction was slightly augmented by IFN-gamma. Surprisingly, however, induction of HO-1 by TMAF was not inhibited by the treatment with the highly selective iNOS inhibitor, 1400 W, indicating that TMAF induces the HO-1 enzyme by a NO-independent mechanism. While rIFN-gamma alone induced iNOS, it had no effect on HO-1 induction by itself. Collectively, the current study reveals that soluble factor from tumor target cells induces HO-1 enzyme in macrophages. However, overall biological significance of this phenomenon remains to be determined. 相似文献
129.
[reaction: see text] The stereoselective aldol reaction of 3-silyloxyfurans with aldehydes in the presence of a Lewis acid is described. N-Bromosuccinimide (NBS)-mediated cyclization of the aldol product leads to the formation of the 2,7-dioxa-bicyclo[2.2.1]heptan-3-one ring system, which represents the formal product of hetero Diels-Alder reaction of the furan with the aldehyde. 相似文献
130.