全文获取类型
收费全文 | 4058篇 |
免费 | 120篇 |
国内免费 | 35篇 |
专业分类
化学 | 3000篇 |
晶体学 | 33篇 |
力学 | 88篇 |
综合类 | 1篇 |
数学 | 571篇 |
物理学 | 520篇 |
出版年
2023年 | 20篇 |
2022年 | 27篇 |
2021年 | 31篇 |
2020年 | 59篇 |
2019年 | 57篇 |
2018年 | 33篇 |
2017年 | 39篇 |
2016年 | 93篇 |
2015年 | 90篇 |
2014年 | 101篇 |
2013年 | 218篇 |
2012年 | 209篇 |
2011年 | 322篇 |
2010年 | 121篇 |
2009年 | 127篇 |
2008年 | 258篇 |
2007年 | 261篇 |
2006年 | 257篇 |
2005年 | 254篇 |
2004年 | 224篇 |
2003年 | 165篇 |
2002年 | 159篇 |
2001年 | 42篇 |
2000年 | 49篇 |
1999年 | 40篇 |
1998年 | 39篇 |
1997年 | 45篇 |
1996年 | 60篇 |
1995年 | 36篇 |
1994年 | 47篇 |
1993年 | 46篇 |
1992年 | 35篇 |
1991年 | 37篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 26篇 |
1987年 | 17篇 |
1986年 | 19篇 |
1985年 | 47篇 |
1984年 | 44篇 |
1983年 | 33篇 |
1982年 | 57篇 |
1981年 | 44篇 |
1980年 | 41篇 |
1979年 | 33篇 |
1978年 | 41篇 |
1977年 | 33篇 |
1976年 | 27篇 |
1975年 | 33篇 |
1974年 | 20篇 |
排序方式: 共有4213条查询结果,搜索用时 31 毫秒
221.
222.
R-5-Methylcyclohex-2-enone 1 reacts successively with the phenyldimethylsilylzincate reagent and acetaldehyde to give with regiocontrol the aldols 7, dehydration of which creates the E-exocyclic double bond of the alpha,beta-unsaturated ketone 2. Conjugate addition of the ethylcuprate reagent to this compound takes place with high (96:4) selectivity in favour of the R stereoisomer 12, hydrolysis of which gives (2R,3R,5S,2'R)-2-(but-2'-yl)-3-dimethyl(phenyl)silyl-5-methylcyclohexanone 3. The oxime acetate of this ketone undergoes fragmentation in the presence of trimethylsilyl trifluoromethanesulfonate to give 3R,7R,5E-3,7-dimethylnon-5-enonitrile 4, in which an open-chain 1,5-stereochemical relationship is set up with a high level of stereocontrol. A similar sequence adding 4-methylpentylcuprate to the enone 2, and fragmentation gives 3R,7R,5E-3,7,11-trimethyldodec-5-enonitrile 20. Reduction and hydrogenation of this nitrile gives 3R,7R-3,7,11-trimethyldodecanal 22, which can be converted into phytol 25. The ketoaldehyde 29 reacts with samarium iodide to give only the alcohol 30, in which the radical anion has attacked from the top surface just like the cuprate reagents in their reactions with the ketone 2. 相似文献
223.
van Sprang EN Ramaker HJ Boelens HF Westerhuis JA Whiteman D Baines D Weaver I 《The Analyst》2003,128(1):98-102
Many high quality products are produced in a batch wise manner. One of the characteristics of a batch process is the recipe driven nature. By repeating the recipe in an identical manner a desired end-product is obtained. However, in spite of repeating the recipe in an identical manner, process differences occur. These differences can be caused by a change of feed stock supplier or impurities in the process. Because of this, differences might occur in the end-product quality or unsafe process situations arise. Therefore, the need to monitor an industrial batch process exists. An industrial process is usually monitored by process measurements such as pressures and temperatures. Nowadays, due to technical developments, spectroscopy is more and more used for process monitoring. Spectroscopic measurements have the advantage of giving a direct chemical insight in the process. Multivariate statistical process control (MSPC) is a statistical way of monitoring the behaviour of a process. Combining spectroscopic measurements with MSPC will notice process perturbations or process deviations from normal operating conditions in a very simple manner. In the following an application is given of batch process monitoring. It is shown how a calibration model is developed and used with the principles of MSPC. Statistical control charts are developed and used to detect batches with a process upset. 相似文献
224.
The asymmetric hydrogenation of a selectively crystallised (E)-4,4-diaryl-3-butenoate with a rhodium-PhanePhos catalyst is described, providing an intermediate to the antidepressant sertraline. 相似文献
225.
We treat the time discretization of an initial-value problem for a homogeneous abstract parabolic equation by first using a representation of the solution as an integral along the boundary of a sector in the right half of the complex plane, then transforming this into a real integral on the finite interval , and finally applying a standard quadrature formula to this integral. The method requires the solution of a finite set of elliptic problems with complex coefficients, which are independent and may therefore be done in parallel. The method is combined with spatial discretization by finite elements.
226.
Terry A. Egerton Lorna M. Kessell Ian R. Tooley Liwei Wang 《Journal of nanoparticle research》2007,9(2):251-260
Photogreying, the change in brightness on UV irradiation in the absence of oxygen, of TiO2 nanoparticulate dispersions is shown to depend on the nature of the liquid, consistent with a surface reaction. Measurements
on a series of TiO2 particles (mainly 75×10 nm) dispersed in, e.g., alkyl benzoate correlate well with those on the same TiO2’s dispersed in a second liquid (e.g. propan-2-ol). Photogreying in propan-2-ol is paralleled by photocatalytic-oxidation
activity, indicating a common origin – UV-generation of charge carriers. Further, photogreying parallels Ti3+ formation. Hence, although appearance and the visible spectra of photogreyed particles both differ from those of Ti3+ in ≤10 nm colloidal TiO2, we suggest that photogreying is caused by capture of UV excited electrons to form Ti3+. Surface treatment reduces photogreying, and we speculate that differences between uncoated samples reflect differences in
the number of potentially reducible Ti’s. 相似文献
227.
Pyridine N-oxides were converted to 2-aminopyridines in a one-pot fashion using Ts2O-t-BuNH2 followed by in situ deprotection with TFA. The amination proceeded in high yields, excellent 2-/4-selectivity, and with good functional group compatibility. 2-Amino (iso)quinolines were also obtained in the same manner. Combined with the simple oxidation of pyridines to pyridine N-oxides, this method provides a general and efficient way for amination of 2-unsubstituted pyridines. 相似文献
228.
Loftus N Miseki K Iida J Gika HG Theodoridis G Wilson ID 《Rapid communications in mass spectrometry : RCM》2008,22(16):2547-2554
High mass accuracy electrospray ionisation multistage tandem mass spectrometry (MS(n)) was applied to metabolite profiling studies on plasma samples derived from two strains of rat (the Zucker (fa/fa) obese strain and the normal wild type). Using a quadrupole ion trap time-of-flight (QIT-TOF) mass spectrometer, metabolite profiling software was applied to locate components of biological significance that could account for the differences between the two strains of rat and a formula prediction software tool was used to help identify individual components. The primary factor discriminating between the two populations was the concentration of endogenous lipids. In the Zucker (fa/fa) obese strain, the dominant ion signals and MS(n) spectra were in agreement with lysoglycerophosphocholine components such as palmitoyllysophosphatidylcholine, 1-oleoylglycerophosphocholine, 1-octadecyl-sn-glycero-3-phosphocholine and 1-stearoylglycerophosphocholine and these were found in relatively higher concentrations compared to the normal wild type. Components were identified using high mass accuracy MS(n) data, formula prediction software and by agreement with published mass spectra through internet databases, rather than using a conventional approach with authentic standards. This application shows that the use of high mass accuracy electrospray ionisation MS(n) together with a software tool can be used effectively to detect and characterise unknown analytes in complex matrices, and represents a promising approach for future profiling studies. 相似文献
229.
Wiklander J Karlsson BC Aastrup T Nicholls IA 《Analytical and bioanalytical chemistry》2011,400(5):1397-1404
A series of streptavidin-mimicking molecularly imprinted polymers has been developed and evaluated for their biotin binding characteristics. A combination of molecular dynamics and NMR spectroscopy was used to examine potential polymer systems, in particular with the functional monomers methacrylic acid and 2-acrylamidopyridine. The synthesis of copolymers of ethylene dimethacrylate and one or both of these functional monomers was performed. A combination of radioligand binding studies and surface area analyses demonstrated the presence of selectivity in polymers prepared using methacrylic acid as the functional monomer. This was predicted by the molecular dynamics studies showing the power of this methodology as a prognostic tool for predicting the behavior of molecularly imprinted polymers.
The biotin binding characteristics of a series of molecularly imprinted polymers have been evaluated and correlated to predictions made by molecular dynamics simulations and 1H-NMR titrations 相似文献
230.
Recently a method termed constrained fluid lambda-integration was proposed for calculating the free energy difference between bulk solid and liquid reference states via the construction of a reversible thermodynamic integration path; coupling the two states in question. The present work shows how the application of the constrained fluid lambda-integration concept to solid/liquid slab simulation cells makes possible a generally applicable computer simulation methodology for calculating the free energy of any surface and/or surface defect structure, including surfaces requiring variations in surface atom or density number, such as the (1 x 5) Au(100) or (1 x 2) missing row Au(110) reconstructed surfaces or excess adatom/vacancy/step populated surfaces. We evaluate the methodology by calculating the free energy of various disordered high temperature Au(110) embedded atom method surfaces constrained to differing excess surface atom numbers [including those corresponding to the (1 x 2) missing row reconstructed surface] and obtained the interesting result that at 1000 K (as distinct from lower temperatures) the free energy difference between these surfaces is reduced to zero; a result which is consistent with an expected order-disorder phase transition for the Au(110) surface at such high temperatures. 相似文献