全文获取类型
收费全文 | 75292篇 |
免费 | 15828篇 |
国内免费 | 3252篇 |
专业分类
化学 | 63748篇 |
晶体学 | 975篇 |
力学 | 1613篇 |
综合类 | 41篇 |
数学 | 5372篇 |
物理学 | 9169篇 |
无线电 | 13454篇 |
出版年
2024年 | 47篇 |
2023年 | 169篇 |
2022年 | 296篇 |
2021年 | 672篇 |
2020年 | 1922篇 |
2019年 | 3595篇 |
2018年 | 3229篇 |
2017年 | 3797篇 |
2016年 | 4253篇 |
2015年 | 6527篇 |
2014年 | 6438篇 |
2013年 | 8076篇 |
2012年 | 6624篇 |
2011年 | 6160篇 |
2010年 | 5048篇 |
2009年 | 4746篇 |
2008年 | 5198篇 |
2007年 | 4534篇 |
2006年 | 4122篇 |
2005年 | 3880篇 |
2004年 | 3240篇 |
2003年 | 2875篇 |
2002年 | 3423篇 |
2001年 | 1882篇 |
2000年 | 1622篇 |
1999年 | 794篇 |
1998年 | 194篇 |
1997年 | 171篇 |
1996年 | 154篇 |
1995年 | 144篇 |
1994年 | 132篇 |
1993年 | 82篇 |
1992年 | 70篇 |
1991年 | 42篇 |
1990年 | 58篇 |
1989年 | 38篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1973年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Pb2(OH)2[p‐O2C‐C6H4‐CO2]: Synthesis and Crystal Structure Single crystals of Pb2(OH)2[p‐O2C‐C6H4‐CO2] ( 1 ) were obtained by hydrothermal reaction of terephthalic acid and PbCO3 at 180 °C (10 days). 1 crystallizes in the monoclinic space group P21/c with Z = 2 (a = 1115.6(2) pm, b = 380.10(4) pm, c = 1141.3(2) pm, β = 93.39(1)°, V = 0.4831(1) nm3). The crystal structure is characterized by ladder‐type Pb(OH)3/3 double chains, which are connected to a three‐dimensional framework by terephthalate dianions. 相似文献
992.
Competition STD NMR for the detection of high-affinity ligands and NMR-based screening 总被引:2,自引:0,他引:2
The reported competition STD NMR method combines saturation transfer difference (STD) NMR with competition binding experiments to allow the detection of high-affinity ligands that undergo slow chemical exchange on the NMR time-scale. With this technique, the presence of a competing high-affinity ligand in the compound mixture can be detected by the disappearance or reduction of the STD signals of a low-affinity indicator ligand. This is demonstrated on a BACE1 (beta-site amyloid precursor protein cleaving enzyme 1) protein-inhibitor system. This method can also be used to derive an approximate value, or a lower limit, for the dissociation constant of the potential ligand based on the reduction of the signal intensity of the STD indicator, which is illustrated on an HSA (human serum albumin) model system. This leads to important applications of the competition STD NMR method for lead discovery: it can be used (i) for compound library screening against a broad range of drug targets to identify both high- and low-affinity ligands and (ii) to rank order analogs rapidly and derive structure-activity relationships, which are used to optimize these NMR hits into viable drug leads. 相似文献
993.
Camera E Picardo M Presutti C Catarcini P Fanali S 《Journal of separation science》2004,27(12):971-976
We developed a simple and reliable analytical method for the quantification and the characterization of ceramides extracted from biological samples by high-performance liquid chromatography (HPLC) coupled to electrospray ionisation tandem mass spectrometry (ESI/MS/MS). The chromatographic separation of analytes was carried out in a RP8 column, eluting with a methanol-water mixture in gradient elution mode. The separated lipids were detected by total ion monitoring and characterised by MS/MS spectra; quantitative analysis was performed by integrating the extracted ion peaks obtained in the negative ion mode. Good repeatability was obtained for retention time (0.3-2%), peak area ratio (A(S)/A(IS), 2-8%), as well as limit of detection (LOD, 5-26 pg) and quantification (LOQ, 13-53 pg). The method was validated for the analysis of N-palmitoyl-D-erythro-sphingosine (Cer16), N-stearoyl-D-erythro-sphingosine (Cer18), N-tetracosanoyl-D-erythro-sphingosine (N24:0, lignoceric ceramide, Cer24:0), and N-tetracos-15'-enoyl-D-erythro-sphingosine (N24:1, nervonic ceramide, Cer24:1), giving good results. Lipid mixtures, extracted from skin and epidermal cells, were analysed for their content of the studied ceramides. 相似文献
994.
Hui‐Bo Zhou Wen Dong Mao Liang Dai‐Zheng Liao Zong‐Hui Jiang Shi‐Ping Yan Peng Cheng 《无机化学与普通化学杂志》2004,630(4):498-500
A three‐dimensional cyano‐bridged copper(II) complex, [Cu(dien)Ag(CN)2]2[Ag2(CN)3][Ag(CN)2] ( 1 ) (dien = diethylenetriamine), has been prepared and characterized by X‐ray crystallography. Complex 1 crystallized in the monoclinic space group P21/n with a = 6.988(2), b = 17.615(6), c = 12.564(4) Å, β = 90.790(5)°. The crystal consists of cis‐[Cu(dien)]2+ units bridged by [Ag(CN)2]— to form a zig‐zag chain. The Ag atoms of the free and bridging [Ag(CN)2]— link together to form additional infinite zig‐zag chains with short Ag···Ag distances. The presence of Ag···Ag interactions effectively increases the dimensionality from a 1‐D chain to a 3‐D coordination polymer. 相似文献
995.
A fluorescent aminoacid was designed for selective and sensitive detection of Cu(II) in aqueous solution. The designing of this Cu(II) fluorescent chemosensing molecule, N ± (1‐naphthyl). aminoacetic acid (NAA), was based on the binding of Cu(II) to aminoacetic acid and the novel charge transfer photophysics of 1‐aminonaphthalenes. The fluorescence of NAA was found quenched by Cu (II) and several other metal ions of similar electronic structure such as Co(II), Ni(II) and Zn(II). The quenching was shown to occur via electron transfer within the metal‐NAA complex, which required an optimal combination of high binding affinity and favorable redox properties of the components in the metal‐NAA complex and hence afforded selective fluorometric detection of Cu(II). The calibration graph obeyed Stern‐Volmer theory and was shown for Cu(II) over the range of 0–2.75 ± 10–4 mol/L. The quenching constant of Cu(II) was measured as 8.0 ± 103 mol/L that was two orders of magnitude higher than those of Co(II), Ni(II) and Zn(II). The 3SD limit of detection for Cu(II) was 8.00 ± 10?6 mol/L with a coefficient of variation of 1.65%. Linear range for quantitative detection of Cu(II) was 2.67 ± 10?5‐2.75 ± 10?4 mol/L. The method was applied to synthetic sample measurements which gave recoveries of 105%‐112%. 相似文献
996.
The γ-substituted β-diketonate 2,4-dioxo-3-pentyl 4-[4-(n-octyloxy)cinnamoyl]oxybenzoate 1 and its pyrazole and isoxazole derivatives (2 and 3 respectively) have been synthesized and characterized by the spectroscopic methods and elemental analysis. The mesogenic properties of these compounds have been studied by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). A monotropic nematic mesophase was observed for the β-diketonate 1, in contrast, the pyrazole 2 displays an enantiotropic smectic A and isoxazole 3 exhibits an enantiotropic nematic mesophase. The relationship between the structure and liquid crystalline properties has also been discussed. 相似文献
997.
Grigorii G. Sivets Elena N. Kalinichenko Igor A. Mikhailopulo 《Helvetica chimica acta》2007,90(9):1818-1836
Convergent syntheses of the 9‐(3‐X‐2,3‐dideoxy‐2‐fluoro‐β‐D ‐ribofuranosyl)adenines 5 (X=N3) and 7 (X=NH2), as well as of their respective α‐anomers 6 and 8 , are described, using methyl 2‐azido‐5‐O‐benzoyl‐2,3‐dideoxy‐2‐fluoro‐β‐D ‐ribofuranoside ( 4 ) as glycosylating agent. Methyl 5‐O‐benzoyl‐2,3‐dideoxy‐2,3‐difluoro‐β‐D ‐ribofuranoside ( 12 ) was prepared starting from two precursors, and coupled with silylated N6‐benzoyladenine to afford, after deprotection, 2′,3′‐dideoxy‐2′,3′‐difluoroadenosine ( 13 ). Condensation of 1‐O‐acetyl‐3,5‐di‐O‐benzoyl‐2‐deoxy‐2‐fluoro‐β‐D ‐ribofuranose ( 14 ) with silylated N2‐palmitoylguanine gave, after chromatographic separation and deacylation, the N7‐β‐anomer 17 as the main product, along with 2′‐deoxy‐2′‐fluoroguanosine ( 15 ) and its N9‐α‐anomer 16 in a ratio of ca. 42 : 24 : 10. An in‐depth conformational analysis of a number of 2,3‐dideoxy‐2‐fluoro‐3‐X‐D ‐ribofuranosides (X=F, N3, NH2, H) as well as of purine and pyrimidine 2‐deoxy‐2‐fluoro‐D ‐ribofuranosyl nucleosides was performed using the PSEUROT (version 6.3) software in combination with NMR studies. 相似文献
998.
Vibrational properties (band position, infrared [IR], and Raman intensities) of C?N stretching mode were studied in 65 gas phase hydrogen‐bonded 1:1 complexes of HCN with OH acids and NH acids using density functional theory (DFT) calculations at the B3LYP‐6‐311++G(d,p) level. Furthermore, general characteristics of the hydrogen bonds and vibrational changes in acids OH/NH stretching bands were also considered. Experimentally observed blue shift of the C?N stretching band promoted by hydrogen bonding, which shortens the triple bond length, is very well reproduced and quantitatively depends on the hydrogen bond length. Both IR and Raman ν(C?N) band intensities are enhanced, also in good agreement with the experimental results. IR intensity increase is a direct function of the hydrogen bond energy. However, the predicted Raman intensity raise is a more complex function, depending simultaneously on characteristics of both the hydrogen bond (C?N bond length) and the H‐donating acid (polarizability). With these two parameters, ν (C?N) Raman intensities of the complexes are explained with a mean error of ±2.4%. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 相似文献
999.
Analysis of cyanobacterial toxins (anatoxin-a, cylindrospermopsin, microcystin-LR) by capillary electrophoresis 总被引:4,自引:0,他引:4
Capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) were applied to the simultaneous separation of cyanobacterial toxins (anatoxin-a, microcystin-LR, cylindrospermopsin). The analytical performance data of both methods, optimized for the three toxins, were similar with a precision of migration times smaller than 0.8 RSD% and a detection limit in the range of 1-4 microg/mL, using spectrophotometric detection at 230 nm. Both methods were applied to an analysis of cyanotoxins in water bloom samples and crude cyanobacterial extracts. The results obtained indicate that, for complex matrices, the sequential application of CZE and MEKC is necessary. It is recommended to use both CE techniques for the analysis of the same sample in order to confirm the results by an orthogonal approach. 相似文献
1000.
Hibbs DE Austin-Woods CJ Platts JA Overgaard J Turner P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(5):1075-1084
The experimental electron density distribution in taurine, 2-aminoethane sulfonic acid, 1, has been determined from high-resolution X-ray diffraction data collected at a temperature of 100 K. Taurine crystallizes as a zwitterion in the monoclinic space group P2(1)/c. Topological analysis of the experimental electron density and a comparison with high-level theoretical gas-phase calculations show that the crystal environment has a significant influence on the electronic configuration of the sulfonate moiety in 1, which in the crystal is more delocalized than in the gas phase. This crystal effect is mainly due to hydrogen bonding. 相似文献