全文获取类型
收费全文 | 4337篇 |
免费 | 99篇 |
国内免费 | 39篇 |
专业分类
化学 | 2972篇 |
晶体学 | 67篇 |
力学 | 81篇 |
数学 | 731篇 |
物理学 | 624篇 |
出版年
2021年 | 42篇 |
2020年 | 39篇 |
2019年 | 49篇 |
2018年 | 46篇 |
2017年 | 36篇 |
2016年 | 81篇 |
2015年 | 102篇 |
2014年 | 102篇 |
2013年 | 233篇 |
2012年 | 193篇 |
2011年 | 249篇 |
2010年 | 118篇 |
2009年 | 103篇 |
2008年 | 205篇 |
2007年 | 221篇 |
2006年 | 198篇 |
2005年 | 214篇 |
2004年 | 202篇 |
2003年 | 185篇 |
2002年 | 122篇 |
2001年 | 41篇 |
2000年 | 52篇 |
1999年 | 55篇 |
1998年 | 47篇 |
1997年 | 42篇 |
1996年 | 66篇 |
1995年 | 34篇 |
1994年 | 58篇 |
1993年 | 51篇 |
1992年 | 42篇 |
1991年 | 53篇 |
1990年 | 41篇 |
1989年 | 34篇 |
1988年 | 44篇 |
1987年 | 44篇 |
1986年 | 44篇 |
1985年 | 64篇 |
1984年 | 67篇 |
1983年 | 46篇 |
1982年 | 55篇 |
1981年 | 54篇 |
1980年 | 38篇 |
1979年 | 67篇 |
1978年 | 60篇 |
1977年 | 67篇 |
1976年 | 44篇 |
1975年 | 50篇 |
1974年 | 46篇 |
1973年 | 37篇 |
1972年 | 36篇 |
排序方式: 共有4475条查询结果,搜索用时 15 毫秒
81.
Zahid S Udenigwe CC Ata A Eze MO Segstro EP Holloway P 《Natural product research》2006,20(14):1283-1289
Chemical studies on the crude methanolic extract of Coprinus micaceus, collected from the Canadian Prairie region, have resulted in the isolation of two new natural products, micaceol (1), a sterol and (Z,Z)-4-oxo-2,5-hetpadienedioic acid (2). Compound 2 was isolated for the first time as a natural product. Structures of these new compounds were established with the aid of extensive NMR spectroscopic studies. Compound 1 has shown anti-bacterial activity against Corynebacterium xerosis and Staphylococcus aureus, whereas compound 2 exhibited glutathione S-transferase inhibition. 相似文献
82.
Dey A Hocking RK Larsen P Borovik AS Hodgson KO Hedman B Solomon EI 《Journal of the American Chemical Society》2006,128(30):9825-9833
Iron L-edge, iron K-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [Fe(III)H(3)buea(X)](n-) (X = S(2-), O(2-), OH(-)). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metal-ligand bonding and to evaluate the effects of H-bonding in Fe(III)(-)O vs Fe(III)(-)S complexes. It was found that the Fe(III)(-)O bond, while less covalent, is stronger than the Fe(III)(-)S bond. This dominantly reflects the larger ionic contribution to the Fe(III)(-)O bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the Fe-O bond. These results were extended to consider an Fe(IV)(-)O complex with the same ligand environment. It was found that hydrogen bonding to Fe(IV)(-)O is less energetically favorable than that to Fe(III)(-)O, which reflects the highly covalent nature of the Fe(IV)(-)O bond. 相似文献
83.
84.
Taek-Hyoun Chang Edward B. Walker Pill-Soon Song 《Photochemistry and photobiology》1981,33(6):933-936
Abstract— Two simple methods of phototaxis measurements have been applied to study the effects of ionophores on the negative phototactic response in Stentor coeruleus. The inhibitory effects of Ca2+ -ionophore (A23187), Ca2+ -blocking agent (Ruthenium Red), and K+ -ionophore (valinomycin) on photo-taxis have been determined. Results suggest that the influx of Ca2+ plays a transducing role in the phototaxis of Stentor. 相似文献
85.
An extended system molecular dynamics method for the isomolar semigrand ensemble (fixed number of particles, pressure, temperature, and fugacity fraction) is developed and applied to the calculation of liquid-liquid equilibria (LLE) for two Lennard-Jones mixtures. The method utilizes an extended system variable to dynamically control the fugacity fraction xi of the mixture by gradually transforming the identity of particles in the system. Two approaches are used to compute coexistence points. The first approach uses multiple-histogram reweighting techniques to determine the coexistence xi and compositions of each phase at temperatures near the upper critical solution temperature. The second approach, useful for cases in which there is no critical solution temperature, is based on principles of small system thermodynamics. In this case a coexistence point is found by running N-P-T-xi simulations at a common temperature and pressure and varying the fugacity fraction to map out the difference in chemical potential between the two species A and B (mu(A)-mu(B)) as a function of composition. Once this curve is known the equal-distance/equal-area criterion is used to determine the coexistence point. Both approaches give results that are comparable to those of previous Monte Carlo (MC) simulations. By formulating this approach in a molecular dynamics framework, it should be easier to compute the LLE of complex molecules whose intramolecular degrees of freedom are often difficult to properly sample with MC techniques. 相似文献
86.
87.
The asymmetric total synthesis of the α-glucosidase inhibitor (+)-castanospermine is reported. The central theme in our approach to this polyhydroxylated alkaloid is the simultaneous generation of the piperidine ring and the C-1/8a erythro stereodiad through the diastereoselective, oxamidation of an unsaturated O-alkyl hydroxamate. This process is believed to proceed sequentially via singlet acylnitrenium and aziridinium ion intermediates. 相似文献
88.
Zech SG Olejniczak E Hajduk P Mack J McDermott AE 《Journal of the American Chemical Society》2004,126(43):13948-13953
A novel approach for detection of ligand binding to a protein in solid samples is described. Hydrated precipitates of the anti-apoptotic protein Bcl-xL show well-resolved (13)C-(13)C 2D solid-state NMR spectra that allow site-specific assignment of resonances for many residues in uniformly (13)C-enriched samples. Binding of a small peptide or drug-like organic molecule leads to changes in the chemical shift of resonances from multiple residues in the protein that can be monitored to characterize binding. Differential chemical shifts can be used to distinguish between direct protein-ligand contacts and small conformational changes of the protein induced by ligand binding. The agreement with prior solution-state NMR results indicates that the binding pocket in solid and liquid samples is similar for this protein. Advantages of different labeling schemes involving selective (13)C enrichment of methyl groups of Ala, Val, Leu, and Ile (Cdelta1) for characterizing protein-ligand interactions are also discussed. It is demonstrated that high-resolution solid-state NMR spectroscopy on uniformly or extensively (13)C-enriched samples has the potential to screen proteins of moderate size ( approximately 20 kDa) for ligand binding as hydrated solids. The results presented here suggest the possibility of using solid-state NMR to study ligand binding in proteins not amenable to solution NMR. 相似文献
89.
Leach ES Hopkinson A Franklin K van Duijneveldt JS 《Langmuir : the ACS journal of surfaces and colloids》2005,21(9):3821-3830
It is shown how stable, nonaqueous suspensions of fully exfoliated smectite clays such as Laponite and montmorillonite can be obtained. Suspensions in toluene and in a branched aliphatic solvent (polydecene) were characterized using elemental analysis, rheometry, and small-angle X-ray scattering. For Laponite, stable suspensions were obtained by adsorbing a dichain poly(isobutylene) based stabilizer to the particles. This approach did not result in full exfoliation for montmorillonite particles, possibly because the stabilizer was able to connect individual clay sheets at the edges during the treatment process. Instead, a quaternary ammonium surfactant, dimethyl dioctadecylammonium bromide (DODAB), was first adsorbed to the clay. Subsequently, adsorption of the poly(isobutylene) based stabilizer to this pretreated clay resulted in fully exfoliated suspensions of montmorillonite. Suspensions of clay particles treated only with DODAB have been studied by several authors before and are included in this study for comparison. A detailed discussion of the suspension behavior of the different clay samples is given. The suspension routes presented here may enable further studies of the structure and flow behavior of suspensions of thin, flexible clay platelets as a function of aspect ratio. 相似文献
90.
Baki B. Sadi Chunsheng Li Gary H. Kramer Cassandra L. Johnson Queenie Ko Edward P. C. Lai 《Journal of Radioanalytical and Nuclear Chemistry》2011,290(2):415-425
A new radioanalytical method was developed for rapid determination of 226Ra in drinking water samples. The method is based on extraction and preconcentration of 226Ra from a water sample to an organic solvent using a dispersive liquid-liquid microextraction (DLLME) technique followed by
radiometric measurement using liquid scintillation counting. In DLLME for 226Ra, a mixture of an organic extractant (toluene doped with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone) and a disperser
solvent (acetonitrile) is rapidly injected into the water sample resulting in the formation of an emulsion. Within the emulsion,
226Ra reacts with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone and partitions into the fine droplets of toluene. The water/toluene
phases were separated by addition of acetonitrile as a de-emulsifier solvent. The toluene phase containing 226Ra was then measured by liquid scintillation counting. Several parameters were studied to optimize the extraction efficiency
of 226Ra, including water immiscible organic solvent, disperser and de-emulsifier solvent type and their volume, chelating ligands
for 226Ra and their concentrations, inorganic salt additive and its concentration, and equilibrium pH. With the optimized DLLME conditions,
the accuracy (expressed as relative bias, B
r
) and method repeatability (expressed as relative precision, S
B
) were determined by spiking 226Ra at the maximum acceptable concentration level (0.5 Bq L−1) according to the Guidelines for Canadian Drinking Water Quality. Accuracy and repeatability were found to be less than −5%
(B
r
) and less than 6% (S
B
), respectively, for both tap water and bottled natural spring water samples. The minimum detectable activity and sample turnaround
time for determination of 226Ra was 33 mBq L−1 and less than 3 h, respectively. The DLLME technique is selective for extraction of 226Ra from its decay progenies. 相似文献