全文获取类型
收费全文 | 2170篇 |
免费 | 59篇 |
国内免费 | 22篇 |
专业分类
化学 | 1139篇 |
晶体学 | 9篇 |
力学 | 31篇 |
数学 | 272篇 |
物理学 | 485篇 |
无线电 | 315篇 |
出版年
2020年 | 18篇 |
2019年 | 25篇 |
2018年 | 22篇 |
2017年 | 14篇 |
2016年 | 42篇 |
2015年 | 36篇 |
2014年 | 45篇 |
2013年 | 91篇 |
2012年 | 90篇 |
2011年 | 112篇 |
2010年 | 70篇 |
2009年 | 66篇 |
2008年 | 122篇 |
2007年 | 109篇 |
2006年 | 124篇 |
2005年 | 102篇 |
2004年 | 117篇 |
2003年 | 82篇 |
2002年 | 85篇 |
2001年 | 65篇 |
2000年 | 52篇 |
1999年 | 41篇 |
1998年 | 33篇 |
1997年 | 33篇 |
1996年 | 46篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 54篇 |
1992年 | 34篇 |
1991年 | 31篇 |
1990年 | 23篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 10篇 |
1986年 | 21篇 |
1985年 | 20篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 22篇 |
1978年 | 22篇 |
1977年 | 14篇 |
1976年 | 22篇 |
1975年 | 16篇 |
1974年 | 12篇 |
1973年 | 12篇 |
1972年 | 10篇 |
1971年 | 11篇 |
排序方式: 共有2251条查询结果,搜索用时 31 毫秒
991.
Lipton AS Heck RW Staeheli GR Valiev M De Jong WA Ellis PD 《Journal of the American Chemical Society》2008,130(19):6224-6230
We present here a (67)Zn solid-state NMR investigation of Zn(2+) substituted rubredoxin. The sample has been prepared as both a dry powder and a frozen solution to determine the effects of static disorder on the NMR line shape. Low-temperature experiments have been performed at multiple fields to determine the relative contributions to the NMR line shape from the electric field gradient and the anisotropic shielding tensors. Finally we present the theoretical interpretation of the experimental results utilizing a combined quantum mechanical molecular mechanics (QM/MM) approach. Theory predicts a sizable contribution from anisotropic shielding as compared with previously examined model systems. This is in good agreement with the experimental data. 相似文献
992.
993.
994.
Liquid azide salts 总被引:1,自引:0,他引:1
Schneider S Hawkins T Rosander M Mills J Brand A Hudgens L Warmoth G Vij A 《Inorganic chemistry》2008,47(9):3617-3624
Ionic liquid azides from azidoethyl, alkyl, and alkenyl substituted derivatives of 1,2,4- and 1,2,3-amino-triazoles were prepared and examined for the first time to investigate their structural and physical properties. All reported salts possess melting points below 100 degrees C. The unique character of these newly discovered ionic liquid azides is based upon the fact that these molecules are not simple protonated salts like previously reported substituted hydrazinium azides. The presence of quaternary nitrogen confers both thermal stability and negligible volatility. 相似文献
995.
We report a technique for carrying out in situ solid-state NMR studies of crystallization from solution, allowing the evolution of different solid state structures (polymorphs) produced during the crystallization process to be identified. The technique exploits selectivity in NMR properties (specifically, the efficiency of cross-polarization from (1)H to (13)C) between molecules in the solid and solution states, such that the first solid particles produced during the crystallization process are observed selectively, without detecting any signal from dissolved solute (or solvent) molecules. The application of the technique is demonstrated to reveal new insights concerning an isotope effect on the polymorphic outcome of crystallization of glycine from water. As revealed by this example, the in situ solid-state NMR approach reported here creates significant new opportunities for probing and understanding details of the evolution of solid state structures produced during crystallization from solution. 相似文献
996.
Thompson AE Hughes G Batsanov AS Bryce MR Parry PR Tarbit B 《The Journal of organic chemistry》2005,70(1):388-390
A range of halogenated aromatics and heteroaromatics bearing a primary amine group are shown to be suitable substrates for Suzuki cross-coupling reactions with arylboronic acids and pyridylboronic acids under standard conditions, without the need for protection/deprotection steps. New amino-substituted arylpyridines, bipyridines, and pyrazinopyridines have thereby been obtained. Conditions for the efficient syntheses of 2-methoxy-5-pyridylboronic acid 1 and 2-methoxy-3-pyridylboronic acid 2 in ca. 75 g batches have been defined. A 2-fold reaction of 2-amino-5-bromopyrazine with 2,5-dimethoxy-1,4-benzenediboronic acid affords 1,4-dimethoxy-2,5-bis[2-(5-aminopyrazyl)]benzene 31. The X-ray crystal structures of 1 and 31.DMF are reported. 相似文献
997.
[reaction: see text] A concise synthesis of rhazinilam through direct, palladium-catalyzed, intramolecular coupling is described. 相似文献
998.
A porphyrin-ferrocene dyad has been synthesized in which there is close face-to-face contact between the two aromatic systems, providing a model for heterobimetallic polymers based on the same repeating unit. Attempts to synthesize the 2:1 adduct instead led to a remarkable intramolecular Heck-type cyclization which planarizes the system and extends the conjugation. [structure: see text] 相似文献
999.
Hughes S 《Optics letters》2005,30(11):1393-1395
A theoretical quantum-optical study of the modified spontaneous emission dynamics from a single quantum dot in a photonic crystal nanocavity is presented. By use of a photon Green function technique, enhanced single-photon emission and pronounced vacuum Rabi flops are demonstrated, in qualitative agreement with recent experiments. 相似文献
1000.
Substituted biphenyl I (BMS-207940), a selective antagonist of the endothelin A (ETA) receptor, has been proposed for the treatment of congestive heart failure. The structure of I possesses a stereogenic axis due to the hindered rotation about the biphenyl bond in the presence of its large ortho-substituents. As a result, I exhibits atropisomerism in which two nonplanar, axially enantiomers exist, which will be generically referred to as isomers A and B. Within the pharmaceutical industry, both from a scientific and regulatory point of view, characterization of enantiomeric drugs has become an important step in the development process. To investigate the configurational stability of I atropisomers, normal phase enantiomeric LC with tandem UV and laser polarimetric detection was used under pseudo-physiological conditions: first in a simple aqueous medium at 37 degrees C, and then in human serum at 37 degrees C. Kinetic studies indicated that the half-life of I enantiomerization in an aqueous medium at 37 degrees C was ca. 15 h. Enantiomerization of I atropisomers was greatly accelerated in the presence of human serum and human serum albumin, and the rate of enantiomerization depended on the concentration of I. The sera-concentration-dependent enantiomerization behavior of I strongly suggests a restricted site-specific substrate/I interaction mechanism. It was therefore demonstrated that atropisomeric interconversion studies for the compound studied required consideration of temperature, presence of plasma proteins, and drug concentration to account for the kinetic data. 相似文献