全文获取类型
收费全文 | 2539篇 |
免费 | 67篇 |
国内免费 | 31篇 |
专业分类
化学 | 1620篇 |
晶体学 | 14篇 |
力学 | 77篇 |
数学 | 468篇 |
物理学 | 458篇 |
出版年
2021年 | 28篇 |
2020年 | 27篇 |
2019年 | 16篇 |
2018年 | 27篇 |
2017年 | 17篇 |
2016年 | 53篇 |
2015年 | 30篇 |
2014年 | 45篇 |
2013年 | 111篇 |
2012年 | 99篇 |
2011年 | 139篇 |
2010年 | 73篇 |
2009年 | 72篇 |
2008年 | 110篇 |
2007年 | 126篇 |
2006年 | 142篇 |
2005年 | 147篇 |
2004年 | 129篇 |
2003年 | 115篇 |
2002年 | 108篇 |
2001年 | 68篇 |
2000年 | 43篇 |
1999年 | 21篇 |
1998年 | 24篇 |
1997年 | 29篇 |
1996年 | 47篇 |
1995年 | 33篇 |
1994年 | 23篇 |
1993年 | 39篇 |
1992年 | 30篇 |
1991年 | 27篇 |
1990年 | 16篇 |
1989年 | 24篇 |
1988年 | 27篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 38篇 |
1984年 | 28篇 |
1983年 | 23篇 |
1982年 | 39篇 |
1981年 | 47篇 |
1980年 | 39篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 30篇 |
1975年 | 24篇 |
1974年 | 31篇 |
1973年 | 30篇 |
1972年 | 20篇 |
排序方式: 共有2637条查询结果,搜索用时 15 毫秒
101.
Gan Wang Clint A. JamesNicholas A. Meanwell Lawrence G. HamannMakonen Belema 《Tetrahedron letters》2013
A stereoselective and scalable synthesis of (1R,3S,5R)-2-(tert-butoxycarbonyl)-2-azabicyclo[3.1.0]hexane-3-carboxylic acid (3a) is described. Key to the success of the devised route was the realization that the stereoselectivity of a cyclopropanation step could be controlled by the composition of the functional group at C-α. 相似文献
102.
Benjamin L. Moore Lawrence A. Kelley James Barber James W. Murray James T. MacDonald 《Journal of computational chemistry》2013,34(22):1881-1889
Coarse‐grained protein structure models offer increased efficiency in structural modeling, but these must be coupled with fast and accurate methods to revert to a full‐atom structure. Here, we present a novel algorithm to reconstruct mainchain models from C traces. This has been parameterized by fitting Gaussian mixture models (GMMs) to short backbone fragments centered on idealized peptide bonds. The method we have developed is statistically significantly more accurate than several competing methods, both in terms of RMSD values and dihedral angle differences. The method produced Ramachandran dihedral angle distributions that are closer to that observed in real proteins and better Phaser molecular replacement log‐likelihood gains. Amino acid residue sidechain reconstruction accuracy using SCWRL4 was found to be statistically significantly correlated to backbone reconstruction accuracy. Finally, the PD2 method was found to produce significantly lower energy full‐atom models using Rosetta which has implications for multiscale protein modeling using coarse‐grained models. A webserver and C++ source code is freely available for noncommercial use from: http://www.sbg.bio.ic.ac.uk/phyre2/PD2_ca2main/ . © 2013 Wiley Periodicals, Inc. 相似文献
103.
104.
A combined computational and 13C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method. 相似文献
105.
Thomas H. Walter Bonnie A. Alden Kenneth Berthelette Jessica A. Field Nicole L. Lawrence Justin McLaughlin Amit V. Patel 《Journal of separation science》2022,45(8):1389-1399
We have characterized a sulfobetaine stationary phase based on 1.7 μm ethylene-bridged hybrid organic–inorganic particles, which is intended for use in hydrophilic interaction chromatography. The efficiency of a column packed with this material was determined as a function of flow rate, demonstrating a minimum reduced plate height of 2.4. The batch-to-batch reproducibility was assessed using the separation of a mixture of acids, bases, and neutrals. We compared the retention and selectivity of the hybrid sulfobetaine stationary phase to that of several benchmark materials. The hybrid sulfobetaine material gave strong retention for polar neutrals and high selectivity for methyl groups, hydroxy groups, and configurational isomers. Large differences in cation and anion retention were observed among the columns. We characterized the acid and base stability of the hybrid sulfobetaine stationary phase, using accelerated tests at pH 1.3 and 11.0, both at 70°C. The results support a recommended pH range of 2–10. We also investigated the performance of columns packed with this material for metal-sensitive analytes, comparing conventional stainless steel column hardware to hardware that incorporates hybrid surface technology to mitigate interactions with metal surfaces. Compared to the conventional columns, the hybrid surface technology columns showed a greatly improved peak shape. 相似文献
106.
Neel Sisodia Monica Miranda Kay L. McGuinness Jay D. Wadhawan Nathan S. Lawrence 《Electroanalysis》2021,33(3):559-562
This work presents the electrochemical response of a 2-(methylthio)phenol glassy carbon based electrode for a promising voltammetric pH sensor in both buffered and low-buffered solutions. Electropolymerization of the redox species was performed with the resulting polymer presenting a Nernstian response in buffered media, with a sensitivity of 51 mV/pH unit. The effectiveness of the sulfhydryl bond to facilitate proton transfer from the bulk solution to the phenol molecules has been confirmed, providing an accurate pH measurement of 8.28 in sea water media, compared to that measured with a calibrated glass pH probe of 8.30. 相似文献
107.
Oleg B. Shchekin Peter J. Schmidt Fahong Jin Nate Lawrence Kenneth J. Vampola Helmut Bechtel Danielle R. Chamberlin Gerd O. Mueller 《固体物理学:研究快报》2016,10(4):310-314
Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu2+ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm2. The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu2+ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
108.
Queiroz Ana Luiza P. Kerins Brian M. Yadav Jayprakash Farag Fatma Faisal Waleed Crowley Mary Ellen Lawrence Simon E. Moynihan Humphrey A. Healy Anne-Marie Vucen Sonja Crean Abina M. 《Cellulose (London, England)》2021,28(14):8971-8985
Cellulose - Microcrystalline cellulose (MCC) is a semi-crystalline material with inherent variable crystallinity due to raw material source and variable manufacturing conditions. MCC crystallinity... 相似文献
109.
Three-dimensional fluorescence spectroscopy coupled with parallel factor analysis and neural network was applied to the year discrimination of mild aroma Chinese liquors. The excitation-emission fluorescence matrices (EEMs) of 120 samples with various years were measured by FLS920 fluorescence spectrometer. The trilinear decomposition of the data array was performed and the loading scores of and the excitation-emission profiles of four components were also obtained. The scores were employed as the inputs of the BP neural networks and the PARAFAC-BP identification model was constructed. 10 samples were collected from 10, 20 and 30 years of liquors respectively, and 30 samples were selected as the test sets. The remaining 90 samples were used as the training sets to build the training model. The year prediction of unknown samples was also carried out, and the prediction accuracy was 90%, 100% and 100%, respectively. Meanwhile, the discrimination analysis method and the multi way partial least squares discriminant analysis were compared, namely PARAFAC-BP and NPLS-DA. The results indicated that parallel factor combined with the neural network (PARAFAC-BP) has higher prediction accuracy. The proposed method can effectively extract the spectral characteristics, and also reduce the dimension of the input variables of neural network. A good year discrimination result was finally achieved. 相似文献
110.
Qian Li Christopher Batchelor-McAuley Nathan S. Lawrence Robert S. Hartshorne Charles J. V. Jones Richard G. Compton 《Journal of Solid State Electrochemistry》2014,18(5):1215-1221
In this work, a reticulated vitreous carbon electrode (RVCE, 96.5 % porosity, 24 cm?1) was modified with 2-anthraquinonyl groups to electrocatalytically reduce dissolved oxygen in neutral aqueous solution (0.1 M phosphate buffer solution supported with 3 M potassium chloride, pH of 6.7) to hydrogen peroxide (H2O2) at 25 °C under atmospheric pressure. The obtained current density was ca. 3 mA cm?2. For the first time, the oxygen reduction was investigated on a novelly designed RVCE housed in a gravity-feed flow system. Fractional current conversions obtained on the RVC flow cell were compared and contrasted with those on a two-dimensional electrode, viz. a tubular flow electrode. The modified-on catalyst has the benefit in terms of easy separation of the product from the catalyst. The in situ generated low concentration of H2O2 provides potential applications to water purification processes and disinfection for water and food. 相似文献