全文获取类型
收费全文 | 8836篇 |
免费 | 385篇 |
国内免费 | 77篇 |
专业分类
化学 | 6247篇 |
晶体学 | 39篇 |
力学 | 208篇 |
数学 | 1520篇 |
物理学 | 1284篇 |
出版年
2023年 | 64篇 |
2022年 | 189篇 |
2021年 | 288篇 |
2020年 | 181篇 |
2019年 | 191篇 |
2018年 | 184篇 |
2017年 | 146篇 |
2016年 | 341篇 |
2015年 | 316篇 |
2014年 | 339篇 |
2013年 | 573篇 |
2012年 | 595篇 |
2011年 | 740篇 |
2010年 | 457篇 |
2009年 | 368篇 |
2008年 | 604篇 |
2007年 | 575篇 |
2006年 | 533篇 |
2005年 | 455篇 |
2004年 | 400篇 |
2003年 | 297篇 |
2002年 | 274篇 |
2001年 | 88篇 |
2000年 | 92篇 |
1999年 | 66篇 |
1998年 | 79篇 |
1997年 | 73篇 |
1996年 | 79篇 |
1995年 | 54篇 |
1994年 | 54篇 |
1993年 | 54篇 |
1992年 | 52篇 |
1991年 | 44篇 |
1990年 | 33篇 |
1989年 | 24篇 |
1988年 | 19篇 |
1987年 | 23篇 |
1986年 | 25篇 |
1985年 | 37篇 |
1984年 | 44篇 |
1983年 | 31篇 |
1982年 | 37篇 |
1981年 | 34篇 |
1980年 | 21篇 |
1979年 | 20篇 |
1978年 | 24篇 |
1977年 | 17篇 |
1976年 | 8篇 |
1974年 | 6篇 |
1973年 | 11篇 |
排序方式: 共有9298条查询结果,搜索用时 31 毫秒
951.
Mendes G Soares AR Sigiliano L Machado F Kaiser C Romeiro N Gestinari L Santos N Romanos MT 《Molecules (Basel, Switzerland)》2011,16(10):8437-8450
In this paper, we evaluated the antiviral activity against HMPV replication of crude extract of the marine algae Stypopodium zonale and of two meroditerpenoids obtained from it, atomaric acid and epitaondiol, and a methyl ester derivative of atomaric acid. Their selectivity indexes were 20.78, >56.81, 49.26 and 12.82, respectively. Compared to ribavirin, the substances showed a relatively low cytotoxicity on LLC-MK2 cells, with a significant antiviral activity, inhibiting at least 90% of viral replication in vitro, which demonstrates the potential of these marine natural products to combat infections caused by HMPV in vitro. 相似文献
952.
The double cyclocondensation of symmetric pyridyl bis(oxoacids) 2b and 3b with (R)-phenylglycinol stereoselectively gave access to bis-phenylglycinol-derived oxazolopyrrolidine 9 and oxazolopiperidone 10, respectively. Application of the stereocontrolled cyclocondensation reaction to phenyl bis-γ-oxoacid 4b provided 11, which was converted to the corresponding enantiopure di(pyrrolidinyl)benzene 22. The absolute configuration of the new stereogenic centers generated in the key cyclocondenstion step was unambiguously established by X-ray crystallographic analysis. 相似文献
953.
Ponsico S Gulyas H Martínez-Belmonte M Escudero-Adán EC Freixa Z van Leeuwen PW 《Dalton transactions (Cambridge, England : 2003)》2011,40(40):10686-10697
Chelating diphosphines were constructed using dinuclear Zn(II) complexes of Robson macrocycles (Zn-RMCs) as templates. The assembly process is driven by the interaction between the metal centers (Lewis acids) with anionic and neutral Lewis base-functionalized monophosphines. The stability of the final structure depends on the geometry and the affinity of the functional groups of the ditopic phosphines and on the structure of the RMC. In the free ligand the ditopic phosphines coordinate at opposite faces of the pseudo-planar macrocycle as is shown in the molecular structure of several of the assemblies, according to X-ray diffraction. Pre-organization of the system by coordinating the phosphorus atoms to a transition metal center enforced coordination of the functional groups at the same face of the RMC. For several templated diphosphines cis-PtCl(2) complexes were identified by NMR. The in situ assembled diphosphines showed a chelating effect in the rhodium catalyzed hydroformylation of 1-octene. Combination of Zn-RMC 3 and phosphine A gave the highest l/b ratio (13) in acetonitrile. 相似文献
954.
Westrup KC Gregório T Stinghen D Reis DM Hitchcock PB Ribeiro RR Barison A Back DF de Sá EL Nunes GG Soares JF 《Dalton transactions (Cambridge, England : 2003)》2011,40(13):3198-3210
The reversible thermochromic behaviour of homoleptic [{V(OR)(4)}(n)] complexes in solution [R = Pr(i) (product I), Bu(s) (B(s)), Nep (N) and Cy (C)] is accounted for the existence of an aggregation equilibrium involving dimeric and monomeric species in which vanadium(iv) is respectively five- and four-coordinate. Bulky R groups such as Bu(t) and Pe(t) (tert-pentoxide) prevent aggregation and therefore give rise to exclusively mononuclear compounds (B(t) and P(t), respectively) that are not thermochromic. The complexes and their temperature-dependent interconversion were characterised by single crystal X-ray diffractometry, magnetic susceptibility measurements and electronic, FTIR and EPR spectroscopies in a wide temperature range. Equilibrium constants and enthalpy and entropy changes for the dimerization reactions have been determined and compared with literature data. 相似文献
955.
Jose DA Mon I Fernández-Pérez H Escudero-Adán EC Benet-Buchholz J Vidal-Ferran A 《Organic letters》2011,13(14):3632-3635
The synthesis of new P═O-disubstituted receptors with appended crown ethers and their properties as receptors for dicarboxylic acids have been studied. High affinities have been observed (oxalic and malonic acids with 4-, 5-, 6-, or 8-crown ethers). Binding of a cationic effector within the crown ether unit resulted in a positive "allosteric" effect, which has been determined to be K(rel) = 7 in the best case (binding of malonic acid with Li(+) @ rac-3b). 相似文献
956.
Pharmacophore search is a key component of many drug discovery efforts. Pharmer is a new computational approach to pharmacophore search that scales with the breadth and complexity of the query, not the size of the compound library being screened. Two novel methods for organizing pharmacophore data, the Pharmer KDB-tree and Bloom fingerprints, enable Pharmer to perform an exact pharmacophore search of almost two million structures in less than a minute. In general, Pharmer is more than an order of magnitude faster than existing technologies. The complete source code is available under an open-source license at http://pharmer.sourceforge.net . 相似文献
957.
958.
Sáiz J Bravo JC Velasco Ávila E Torre M García-Ruiz C 《Journal of separation science》2011,34(23):3353-3358
In this study, a sequential extraction method using water and methanol to recover ethylene glycol dinitrate or nitroglycol (EGDN) contained in Goma-2 ECO dynamite was developed. After, an HPLC method was used for the determination of EGDN in the two extracted phases. The analytical method was validated by evaluating its selectivity, sensitivity, linearity, and linear working concentration range, limit of detection and quantitation, precision (as repeatability and intermediate precision), accuracy, and robustness, providing appropriate values (i.e. RSD values for precision about 6% and accuracy about 100%). Finally, the EGDN content of a sample of the Goma-2-ECO dynamite was determined obtaining a concentration of 30.29%, which is in accordance with the manufacturer's specifications for this dynamite (25.7-31.4%). 相似文献
959.
Garrido Arteaga R Veloso Pita RC López López MA González Labaut JA Rodríguez Montero Mdel C Vélez Castro H Cremata Alvarez JA 《Analytical and bioanalytical chemistry》2011,400(10):3675-3680
Gangliosides are membrane-associated glycosphingolipids. N-Acetyl GM3 and N-glycolyl GM3 are two tumor-associated antigens expressed in cancer tissues such as melanoma and mammalian cancer. In order
to use these antigens in GM3-based vaccines for patients with early stage cancer, the synthetic version is recommended to
avoid the risk of animal virus transmission from the source. However, the isolation of natural gangliosides is of comparative
value for the structural characterization. The structures of N-acetyl and N-glycolyl GM3 extracted from dog and horse erythrocytes were evaluated by matrix-assisted laser desorption/ionization time-of-flight
mass spectrometry and nuclear magnetic resonance techniques; additionally, the natural N-acetyl ganglioside was compared to a synthetic one. In addition to the main compound with C24:0 fatty acid chain, a minor
component with an additional unsaturation in the ceramide chain was detected, in both the dog and the horse gangliosides.
This paper shows spectroscopic evidence of the aforementioned compounds. 相似文献
960.
Ellis G Cano P Jadraque M Martín M López L Núñez T de la Peña E Marco C Garrido L 《Analytical and bioanalytical chemistry》2011,399(7):2379-2388
Flexible and biodegradable film substrates prepared by solvent casting from poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) solutions in chloroform were microperforated by ultraviolet laser ablation and subsequently characterized
using infrared (IR) microspectroscopy and imaging techniques and scanning electron microscopy (SEM). Both transmission synchrotron
IR microspectroscopy and attenuated total reflectance microspectroscopy measurements demonstrate variations in the polymer
at the ablated pore rims, including evidence for changes in chemical structure and crystallinity. SEM results on microperforated
PHBHV substrates after cell culture demonstrated that the physical and chemical changes observed in the biomaterial did not
hinder cell migration through the pores. 相似文献