全文获取类型
收费全文 | 4803篇 |
免费 | 154篇 |
国内免费 | 12篇 |
专业分类
化学 | 2889篇 |
晶体学 | 88篇 |
力学 | 152篇 |
数学 | 362篇 |
物理学 | 1478篇 |
出版年
2023年 | 45篇 |
2022年 | 94篇 |
2021年 | 87篇 |
2020年 | 105篇 |
2019年 | 99篇 |
2018年 | 119篇 |
2017年 | 88篇 |
2016年 | 143篇 |
2015年 | 117篇 |
2014年 | 170篇 |
2013年 | 273篇 |
2012年 | 357篇 |
2011年 | 351篇 |
2010年 | 206篇 |
2009年 | 178篇 |
2008年 | 204篇 |
2007年 | 199篇 |
2006年 | 190篇 |
2005年 | 154篇 |
2004年 | 119篇 |
2003年 | 124篇 |
2002年 | 90篇 |
2001年 | 75篇 |
2000年 | 62篇 |
1999年 | 56篇 |
1998年 | 35篇 |
1997年 | 54篇 |
1996年 | 64篇 |
1995年 | 46篇 |
1994年 | 77篇 |
1993年 | 82篇 |
1992年 | 82篇 |
1991年 | 53篇 |
1990年 | 51篇 |
1989年 | 54篇 |
1988年 | 35篇 |
1987年 | 39篇 |
1986年 | 37篇 |
1985年 | 37篇 |
1984年 | 33篇 |
1983年 | 37篇 |
1982年 | 39篇 |
1981年 | 50篇 |
1980年 | 40篇 |
1979年 | 31篇 |
1978年 | 26篇 |
1977年 | 34篇 |
1975年 | 24篇 |
1974年 | 27篇 |
1973年 | 33篇 |
排序方式: 共有4969条查询结果,搜索用时 0 毫秒
161.
A free acid Mo(C2O4H)(OH)3(H2O) has been prepared. The corresponding K+, NH4+ and Pb2+ salts have been synthesized. Starting from this, some chloro-oxalato derivatives were prepared. All the compounds are characterised by chemical and physico-chemical studies and their structures are proposed. 相似文献
162.
163.
Arylazoimidazoles (2) are N,N-chelating ligands. The polymerization trend of the azolate system is restricted via N(1)-benzylation. The parent molecules (2), N(1)-benzylated products (3) and palladium complexes (4) were made by standard methods. The ligands (3) and complexes (4) are new. They have been characterized by elemental analysis, i.r., u.v.-vis. and high resolution 1H-n.m.r. spectral data. Redox studies were carried out by cyclic voltammetry. On complexation, azo reduction is shifted anodically. 相似文献
164.
Repeated irradiation enhances the precision and sensitivity of INAA based on short-live radionuclides, but entail systematic losses due to dead time. The relative standard deviation in the net peak area decreases with the square root of the number of cycles while the systematic bias increases with it. The limits of decision, detection and determination decrease in a somewhat more complicated way with the number of cycles. The derived formulation is applied to the determination of selenium in hair by the 161.9 keV photopeak of77mSe,T
1/2=17.8 s. 相似文献
165.
Ashwani Kumar Sahanaz Parvin Dr. Raj Kumar Das Prof. Sayan Bhattacharyya 《化学:亚洲杂志》2021,16(21):3444-3452
Metal-organic coordination polymers (CP) have attracted the scientific attention for electrochemical water oxidation as it has the similar coordination structure like natural photosynthetic coordinated complex. However, the harsh synthesis conditions and bulky nature pose a major challenge in the field of catalysis. Herein, 3–5 nm CP particles synthesized at room temperature using aqueous solutions of Ni2+/Cu2+ and 2,5-dihydroxyterepthalic acid as precursor were applied for alkaline water and urea electrolysis. The overpotential required is only 300 mV at 10 mA cm−2 by Nano-Ni CP for water oxidation, with turnover frequency (TOF) of 21.4 s−1 which is around 8 times higher than its bulk-counterpart. Overall water and urea splitting were achieved with Nano-Cu (−) ∥ Nano-Ni (+) couple on Ni foam at 1.69 and 1.52 V to achieve 10 mA cm−2, respectively. High electrochemical surface area (ECSA), high TOF, and enhanced mass diffusion are found to be the key parameters responsible for the state-of-the-art water and urea splitting performances of nano-CPs as compared to their bulk counterparts. 相似文献
166.
Vinod Kumar Vashistha Kumar Anuj Tevatia Prashant Das Deepak Kumar 《Russian Journal of Electrochemistry》2021,57(4):348-356
Russian Journal of Electrochemistry - Herein, we synthesized [12] membered pyridine based transition metal macrocyclic complexes [MIILCl2] (M = Fe(II) and Ni(II), L = 6,12,5,11-tetraphenyl... 相似文献
167.
Peter Bartner Birendra N. Pramanik Anil K. Saksena Yan-Hui Liu Pradip R. Das Olga Sarre Ashit K. Ganguly 《Journal of the American Society for Mass Spectrometry》1997,8(11):1134-1140
The structural characterization of a new oligosaccharide antibiotic, Everninomicin-6 (EV-6), is described. Detailed fast-atom bombardment mass spectrometry (FAB-MS) studies along with NMR and chemical degradation methods were conducted to elucidate the structure of EV-6. The effects of the use of various matrices, including salt addition, on the quality of the FAB-MS were explored. The use of 3-nitro benzyl alcohol, dimethyl sulfoxide (DMSO), and NaCl produced the best results: an intense sodiated molecular ion plus structurely informative fragmentation. FAB-MS yields information providing the complete sugar sequence information for everninomicins, which is quite valuable to the elucidation of the structure of this complex oligosaccharide antibiotic. In addition, the results of accurate mass work with the molecular ion are consistent with the assigned structure. The use of electrospray ionization mass spectrometry (ESI-MS) and ESI-MS/MS for the study of EV-6 was investigated and was found to produce an abundant molecular ion with limited structural information. These results revealed that EV-6 resembled EV-D quite closely except for the absence of the nitrosugar and the replacement on ring g of the -CH2OCH3 group with a -CH2OH group. 相似文献
168.
169.
Seik Weng Ng V. G. Kumar Das Jaroslav Hole
ek Antonin Ly
ka Marcel Gielen Michael G. B. Drew 《应用有机金属化学》1997,11(1):39-45
Bis(dicyclohexylammonium) bis(2,6-pyridinedicarboxylato)dibutylstannate is assigned seven-fold coordination at tin on the basis of its 119Sn CP/MAS NMR chemical shift (δ=−424.9 ppm). The assignment has been corroborated by a crystal structure determination of its monohydrate, whose tin atom has the trans -C2SnNO4 pentagonal bipyramidal [Sn–C=2.040(9), 2.067(8) Å; C–Sn–C =168.9(5)°] geometry. One 2,6-pyridine- dicarboxylato group chelates to the tin atom (Sn–O=2.234(4), 2.260(4); Sn–N =2.279(5) Å) whereas the other binds through only one carboxyl –CO2 end (Sn–O=2.416(5), 2.441(5) Å). Hydrogen bonds link the cation and the stannate into a linear chain parallel to the b -axis. The lattice water molecule is hydrogen-bonded to the free carboxyl end. The anhydrous compound showed higher in vitro antitumor activity than those of carboplatin and cisplatin when screened against breast (MCF-7, EVSAT), colonic (WiDr), ovarian (IGROV) and renal (A498) carcinoma, and melanoma (M19 MEL) cell lines. © 1997 by John Wiley & Sons, Ltd. 相似文献
170.
Das C Berezovska O Diehl TS Genet C Buldyrev I Tsai JY Hyman BT Wolfe MS 《Journal of the American Chemical Society》2003,125(39):11794-11795
gamma-Secretase cleaves the transmembrane domain of the amyloid precursor protein, a process implicated in the pathogenesis of Alzheimer's disease, and this enzyme is a founding member of an emerging class of intramembrane proteases. Modeling and mutagenesis suggest a helical conformation for the substrate transmembrane domain upon initial interaction with the protease. Moreover, biochemical evidence supports the presence of an initial docking site for substrate on gamma-secretase that is distinct from the active site, a property predicted to be generally true of intramembrane proteases. Here we show that short peptides designed to adopt a helical conformation in solution are inhibitors of gamma-secretase in both cells and enzyme preparations. Helical peptides with all d-amino acids are the most potent inhibitors and represent potential therapeutic leads. Subtle modifications that disrupt helicity also substantially reduce potency, suggesting that this conformation is critical for effective inhibition. Fluorescence lifetime imaging in intact cells demonstrates that helical peptides disrupt binding between substrate and protease, whereas an active site-directed inhibitor does not. These findings are consistent with helical peptides interacting with the initial substrate docking site of gamma-secretase, suggesting a general strategy for the development of potent and specific inhibitors of intramembrane proteases. 相似文献