全文获取类型
收费全文 | 1622篇 |
免费 | 65篇 |
国内免费 | 4篇 |
专业分类
化学 | 1330篇 |
晶体学 | 15篇 |
力学 | 17篇 |
数学 | 61篇 |
物理学 | 268篇 |
出版年
2023年 | 8篇 |
2022年 | 8篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 40篇 |
2018年 | 24篇 |
2017年 | 16篇 |
2016年 | 36篇 |
2015年 | 42篇 |
2014年 | 46篇 |
2013年 | 62篇 |
2012年 | 109篇 |
2011年 | 120篇 |
2010年 | 71篇 |
2009年 | 66篇 |
2008年 | 74篇 |
2007年 | 102篇 |
2006年 | 98篇 |
2005年 | 82篇 |
2004年 | 64篇 |
2003年 | 83篇 |
2002年 | 73篇 |
2001年 | 31篇 |
2000年 | 21篇 |
1999年 | 32篇 |
1998年 | 14篇 |
1997年 | 14篇 |
1996年 | 21篇 |
1995年 | 15篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 6篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 15篇 |
1984年 | 23篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 6篇 |
1974年 | 5篇 |
1930年 | 4篇 |
排序方式: 共有1691条查询结果,搜索用时 328 毫秒
941.
Isamu Shiina Prof. Dr. Kenya Nakata Dr. Keisuke Ono Masuhiro Sugimoto Akihiro Sekiguchi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(1):167-172
A variety of optically active 2‐hydroxyalkanoates and the corresponding 2‐acyloxyalkanoates are produced by the kinetic resolution of racemic 2‐hydroxyalkanoates by using achiral 2,2‐diarylacetic acid with hindered carboxylic anhydrides as the coupling reagents. The combined use of diphenylacetic acid, pivalic anhydride, and (+)‐(R)‐benzotetramisole ((R)‐BTM) effectively produces (S)‐2‐hydroxyalkanoates and (R)‐2‐acyloxyalkanoates from the racemic 2‐hydroxyalkanoates (s‐values=47–202). This protocol directly provides the desired chiral 2‐hydroxyalkanoate derivatives from achiral diarylacetic acid and racemic secondary alcohols that do not include the sec‐phenethyl alcohol moiety by using the transacylation process to generate the mixed anhydrides from the acid components with bulky carboxylic anhydrides under the influence of the chiral acyl‐transfer catalyst. The transition state that provides the desired (R)‐2‐acyloxyalkanoate from (R)‐2‐hydroxyalkanoate included in the racemic mixture is disclosed by DFT calculations, and the structural features of the transition form are also discussed. 相似文献
942.
Hori T Aratani N Takagi A Matsumoto T Kawai T Yoon MC Yoon ZS Cho S Kim D Osuka A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(5):1319-1327
Starting from a 1,3-phenylene-linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag(I)-promoted coupling reactions provided linear oligomers 4ZA, 6ZA, 8ZA, and 12ZA. The intramolecular cyclization reaction of 12ZA under dilute conditions (1x10(-6) M) gave porphyrin ring C12ZA with a diameter of approximately 35 A in 60% yield. This synthetic strategy has been applied to a 1,3-phenylene-linked tetraporphyrin 4ZB to provide 8ZB, 12ZB, 16ZB, 24ZB, and 32ZB. The intramolecular coupling reaction of 24ZB gave a larger 24-mer porphyrin ring C24ZB with a diameter of approximately 70 A in 34% yield. These two large porphyrin rings were characterized by means of 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, UV-visible spectroscopy, gel permeation chromatography (GPC) analysis, and scanning tunneling microscopy (STM) techniques. The STM images of C12ZA reveal largely circular structures, whereas those of C24ZB exhibit mostly ellipsoidal shapes, indicating more conformational flexibility of C24ZB. Similar to the case of C12ZA, the efficient excitation energy transfer along the ring has been confirmed for C24ZB by using the time-correlated single-photon counting (TCSPC) and picosecond transient absorption anisotropy (TAA) measurements, and occurs with a rate of (35 ps)(-1) for energy hops between neighboring tetraporphyrin subunits. Collectively, the present work provides an important step for the construction of large cyclic-arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna. 相似文献
943.
Akashi S Suzuki K Arai A Yamada N Suzuki E Hirayama K Nakamura S Nishimura Y 《Rapid communications in mass spectrometry : RCM》2006,20(12):1932-1938
A system of microchip capillary electrophoresis/electrospray ionization mass spectrometry (microchip-CE/ESI-MS) for rapid characterization of proteins has been developed. Capillary electrophoresis (CE) enables rapid analysis of a sample present in very small quantity, such as at femtomole levels, at high resolution. Faster CE/MS analysis is expected by downsizing the normal capillary to the microchip (microchip) capillary. Although rapidity and high resolution are advantages of CE separation, electroosmotic flow (EOF) instability caused by the interaction between proteins and the microchannel surface results in low reproducibility in the analysis of basic proteins under neutral pH conditions. By coating the microchannel surface with a basic polymer, polyE-323, basic proteins, which have pI values of over 7.5, could be separated and detected by microchip-CE/MS on quadrupole (Q) and time-of-flight (TOF) hybrid instruments. By increasing the cone and collision voltages during the analysis by microchip-CE/ESI-MS of a small protein, some product ions, which contain the sequence information, could also be obtained, i.e., 'top-down' analysis of the protein could be accomplished with this microchip-CE/MS system. To our knowledge, this is the first report of 'top-down' analysis of a protein by microchip-CE/MS. Since it requires a much shorter time and a smaller sample amount for analysis than the conventional liquid chromatography (LC)/ESI-MS method, microchip-CE/MS promises to be suitable for the high-throughput characterization of proteins. 相似文献
944.
The temperature dependence of the Hall-Lorenz number () for the optimally doped La1.855Sr0.145CuO4 (LSCO) has been obtained from the experimentally determined transverse and longitudinal transport coefficients. A comparison between Lxy(T) dependence found for LSCO and Lxy(T) reported previously for copper indicates that the Hall-Lorenz number in LSCO follows standard metallic behavior from room temperature down to . Below this temperature the Lxy coefficient deviates from regular metallic dependence in a way characteristic of an electronic system with lowered density of electronic states at the Fermi level. We present results of calculations provided in terms of the Boltzmann equation for a two-dimensional model of the electronic structure with a d-symmetrical pseudogap. A temperature Tmax, where a maximum in the Lxy(T) dependence appears, turns out to be dependent on the width of the supposed pseudogap . The best agreement between the model and the experimental data was obtained for , which corresponds well with values reported previously by other groups. 相似文献
945.
Takahiro Hamada Akihiro Ito Eiji Fujii Dewei Chu Kazumi Kato Yoshitake Masuda 《Journal of Crystal Growth》2009,311(14):3687-3691
High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al2O3 (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al2O3 (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×1019 cm−3 and high carrier mobility of 22.6 cm2 V−1 s−1. 相似文献
946.
We explicitly construct all stationary, non-static, extremal near horizon geometries in D dimensions that satisfy the vacuum Einstein equations, and that have D−3 commuting rotational symmetries. Our work generalizes [arXiv:0806.2051] by Kunduri and Lucietti, where such a classification
had been given in D = 4,5. But our method is different from theirs and relies on a matrix formulation of the Einstein equations. Unlike their
method, this matrix formulation works for any dimension. The metrics that we find come in three families, with horizon topology
S
2 × T
D-4, or S
3 × T
D-5, or quotients thereof. Our metrics depend on two discrete parameters specifying the topology type, as well as (D − 2)(D − 3)/2 continuous parameters. Not all of our metrics in D ≥ 6 seem to arise as the near-horizon limits of known black hole solutions. 相似文献
947.
Understanding the effect of guest species on the host framework is important for the development of structure-based properties of inclusion compounds. Herein, the crystal structures of the noble gas hydrates encapsulating Xe, Kr, and Ar were studied by powder X-ray diffraction measurements. The crystal structures and hydration numbers of these noble gas hydrates were solved by Rietveld refinements using optimized models with the direct-space technique. It was revealed that host cage size of these hydrates changed depending on the type of guest species even though their unit-cell parameters were the same. Based on the structure models obtained, the densities of Xe, Kr, and Ar gas hydrates were also determined to be 1.837, 1.445 and 1.097 g/cm3 at 93 K, respectively. Our findings, from a crystallographic point of view, may give insight into further understanding the thermodynamic stability and physical properties of gas hydrates encapsulating small guests. 相似文献
948.
Waka Nakanishi Shohei Saito Naoki Sakamoto Akihiro Kashiwagi Shigehiro Yamaguchi Hideki Sakai Katsuhiko Ariga 《化学:亚洲杂志》2019,14(16):2869-2876
The air–water interface, which is the boundary of two phases with a large difference in polarity, gives a distinct environment compared with bulk water or air. Since the interface provides a field for various biomolecules to work, it is important to understand the molecular behaviors at the interface. Here, polarity‐independent flapping viscosity probes (FLAP) equipped with hydrophobic/hydrophilic substituents have been synthesized and studied at the air–water interface. In situ fluorescence (FL), which is related to the internal motion and orientation, of three different FLAPs were investigated at the interface, and the internal motion of the molecule was indicated to be suppressed at the interface. In addition, the molecular response was compared with that of conventional viscosity probes (molecular rotors), which indicates the different behaviors of FLAP probably due to the distinct molecular orientation as well as molecular motion. 相似文献
949.
Sam Benson Antonio Fernandez Nicole D. Barth Fabio deMoliner Mathew H. Horrocks C. Simon Herrington Jose Luis Abad Antonio Delgado Lisa Kelly Ziyuan Chang Yi Feng Miyako Nishiura Yuichiro Hori Kazuya Kikuchi Marc Vendrell 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(21):6985-6989
The transport and trafficking of metabolites are critical for the correct functioning of live cells. However, in situ metabolic imaging studies are hampered by the lack of fluorescent chemical structures that allow direct monitoring of small metabolites under physiological conditions with high spatial and temporal resolution. Herein, we describe SCOTfluors as novel small‐sized multi‐colored fluorophores for real‐time tracking of essential metabolites in live cells and in vivo and for the acquisition of metabolic profiles from human cancer cells of variable origin. 相似文献
950.
Ken‐ichi Takao Hirotaka Kai Ai Yamada Yuuki Fukushima Daisuke Komatsu Akihiro Ogura Keisuke Yoshida 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(29):9956-9960
The short, efficient total synthesis of (+)‐aquatolide was achieved by a biomimetic transannular [2+2] photocycloaddition, and provides the first example of constructing a 5/5/4/8‐ring system from asteriscunolides. Furthermore, the reaction leading to a 5/4/4/7‐ring system, the originally proposed structure of aquatolide, was also developed. This strategy achieved syntheses of five more humulanolides, (?)‐asteriscunolides A, C, D, and I, and (+)‐tetradehydroasteriscanolide. 相似文献