全文获取类型
收费全文 | 13426篇 |
免费 | 859篇 |
国内免费 | 1997篇 |
专业分类
化学 | 10204篇 |
晶体学 | 161篇 |
力学 | 436篇 |
综合类 | 12篇 |
数学 | 547篇 |
物理学 | 4922篇 |
出版年
2024年 | 43篇 |
2023年 | 426篇 |
2022年 | 361篇 |
2021年 | 311篇 |
2020年 | 347篇 |
2019年 | 405篇 |
2018年 | 270篇 |
2017年 | 419篇 |
2016年 | 547篇 |
2015年 | 480篇 |
2014年 | 561篇 |
2013年 | 1073篇 |
2012年 | 725篇 |
2011年 | 931篇 |
2010年 | 749篇 |
2009年 | 969篇 |
2008年 | 871篇 |
2007年 | 957篇 |
2006年 | 885篇 |
2005年 | 677篇 |
2004年 | 597篇 |
2003年 | 462篇 |
2002年 | 354篇 |
2001年 | 302篇 |
2000年 | 315篇 |
1999年 | 257篇 |
1998年 | 279篇 |
1997年 | 219篇 |
1996年 | 210篇 |
1995年 | 172篇 |
1994年 | 160篇 |
1993年 | 116篇 |
1992年 | 111篇 |
1991年 | 104篇 |
1990年 | 90篇 |
1989年 | 92篇 |
1988年 | 103篇 |
1987年 | 71篇 |
1986年 | 31篇 |
1985年 | 32篇 |
1984年 | 33篇 |
1983年 | 16篇 |
1982年 | 30篇 |
1981年 | 27篇 |
1980年 | 27篇 |
1979年 | 23篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
82.
83.
高效液相色谱法测定细胞分裂素的方法研究 总被引:4,自引:0,他引:4
利用高效液相色谱法测定水稻愈伤组织和微生物发酵产品中的玉米素,二氢玉米素,核糖基玉米素和核糖基二氢玉米素。样品用抽提液抽提,经Bio-Rex70弱阳离子交换柱和XAD-7/Sep-pakC18复合柱净化和富集后,直接用HPLC分析,方法回收率达94%~84%,相对标准偏差小于3.3%。最低检测限达2.0ng。 相似文献
84.
The structures of two derivatives of the title compound [C24H22N2OS, Mr = 386. 5 for (1); C25H22N2O3S, Mr = 430. 5 for (2)] were determined by using X-ray single-crystal strcuture analysis method. The final discrepancies are R = 0. 071 and 0. 077, respectively, for the reflections measured on a four-circle diffractometer. The space group for compound (1) is P bca with a=1. 6639(4), b=2.0286(3), c= 1.1742(1) nm, V = 3. 964(1) nm3, F(000) = 1632 e, Z=8; and (2) belongs to P21/n space group, and the cell dimensions are a=1. 1115(4), b=0. 8932(7), c=2. 186(3) nm, β=97. 52 (1)°? V=2.151(4) nm3, F(000) = 904 e, Z=4.The molecular backbones are very similar, each is a tricyclic system. The central seven-membered ring is in a twisted-boat conformation, and is cis-fused to 1,2,4-oxa-diazolino ring, while the latter moiety is in an envelope form. There are conjugated but non-coplanar effects in each structure. All bond lengths and angles in the molecules are normally acceptable. The crystal structure on the whol 相似文献
85.
86.
Brian D. Hudson Ashley R. George Martyn G. Ford David J. Livingstone 《Journal of computer-aided molecular design》1992,6(2):191-201
Summary Molecular dynamics simulations have been performed on a number of conformationally flexible pyrethroid insecticides. The results indicate that molecular dynamics is a suitable tool for conformational searching of small molecules given suitable simulation parameters. The structures derived from the simulations are compared with the static conformation used in a previous study. Various physicochemical parameters have been calculated for a set of conformations selected from the simulations using multivariate analysis. The averaged values of the parameters over the selected set (and the factors derived from them) are compared with the single conformation values used in the previous study. 相似文献
87.
脑神经退行性疾病中的有机化学-朊病毒(疯牛病)中的蛋白物理有机化学和自由基化学 总被引:5,自引:0,他引:5
杨池明 《高等学校化学学报》2002,23(2):243-250
通过关于“普里昂”蛋白病毒疾病的已有临床、医学生理、免疫和化学等方面的现象,讨论了朊病毒当中的部分蛋白氧化损伤和蛋白自由基化学本质。 相似文献
88.
Svein G. Dahl Peter A. Kollman Shashidhar N. Rao U. Chandra Singh 《Journal of computer-aided molecular design》1992,6(3):207-222
Summary The side-chain conformations of psychoactive phenothiazine drugs in crystals are different from those of biologically inactive ring sulfoxide metabolites. This study examines the potential energies, molecular conformations and electrostatic potentials in chlorpromazine, levomepromazine (methotrimeprazine), their sulfoxide metabolites and methoxypromazine. The purpose of the study was to examine the significance of the different crystal conformations of active and inactive phenothiazine derivatives, and to determine why phenothiazine drugs lose most of their biological activity by sulfoxidation. Quantum mechanics and molecular mechanics calculations demonstrated that conformations with the side chain folded over the ring structure had lowest potential energy in vacuo, both in the drugs and in the sulfoxide metabolites. In the sulfoxides, side chain conformations corresponding to the crystal structure of chlorpromazine sulfoxide were characterized by stronger negative electrostatic potentials around the ring system than in the parent drugs. This may weaken the electrostatic interaction of sulfoxide metabolites with negatively charged domains in dopamine receptors, and cause the sulfoxides to be virtually inactive in dopamine receptor binding and related pharmacological tests. 相似文献
89.
Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances. 相似文献
90.
Rivard C. J. Himmel M. E. Vinzant T. B. Adney W. S. Wyman C. E. Grohmann K. 《Applied biochemistry and biotechnology》1989,(1):461-478
Economic evaluations of the capital costs for anaerobic digestion systems for gas production show that the reactor is a significant
cost component. The successful application of high solids digestion of processed MSW (e.g., greater than 10% solids within
the digester) would allow a decrease in reactor volume with maintenance of relatively high gas production rates. However,
high solids slurries do not mix well in conventional stirred tank reactors. A horizontal shaft, hydraulically driven reactor
was designed and fabricated to test the anaerobic digestion of high solids concentrations. Digester performance was evaluated
as a function of experimental parameters such as nutrient requirements, feeding rates, pH control, and agitator design/ rotation
speed; horsepower of mixing was also evaluated for the reactor. Several startup protocols were examined to obtain a biologically
stable anaerobic fermentation at high solids levels. 相似文献