全文获取类型
收费全文 | 1717篇 |
免费 | 32篇 |
国内免费 | 4篇 |
专业分类
化学 | 1167篇 |
晶体学 | 26篇 |
力学 | 20篇 |
数学 | 110篇 |
物理学 | 430篇 |
出版年
2023年 | 10篇 |
2021年 | 17篇 |
2020年 | 12篇 |
2019年 | 26篇 |
2018年 | 15篇 |
2017年 | 13篇 |
2016年 | 20篇 |
2015年 | 27篇 |
2014年 | 31篇 |
2013年 | 68篇 |
2012年 | 83篇 |
2011年 | 94篇 |
2010年 | 48篇 |
2009年 | 59篇 |
2008年 | 91篇 |
2007年 | 109篇 |
2006年 | 106篇 |
2005年 | 100篇 |
2004年 | 124篇 |
2003年 | 83篇 |
2002年 | 72篇 |
2001年 | 44篇 |
2000年 | 43篇 |
1999年 | 26篇 |
1998年 | 19篇 |
1997年 | 17篇 |
1996年 | 26篇 |
1995年 | 21篇 |
1994年 | 21篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 14篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 17篇 |
1987年 | 14篇 |
1986年 | 16篇 |
1985年 | 31篇 |
1984年 | 27篇 |
1983年 | 6篇 |
1982年 | 18篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 19篇 |
1978年 | 7篇 |
1977年 | 17篇 |
1976年 | 6篇 |
1975年 | 12篇 |
1974年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有1753条查询结果,搜索用时 15 毫秒
231.
Oxidation of several linear conjugated dienes with tellurium (IV) oxide and lithium bromide in acetic acid affords an isomeric mixture of the corresponding diacetoxyalkenes (1,2- and 1,4-addition products), 1,4-isomer being highly selectively produced when the ratio of LiBr/TeO2, is 5–10. 相似文献
232.
233.
Masao Shiozaki Yukiko Watanabe Toshio Kaneko Daisuke Tanaka Shin-ichi Kurakata 《Tetrahedron》2005,61(21):5101-5122
Lipid A analogues containing glucose in substitution for glucosamine on the reducing end were synthesized, and the inhibitory activities on LPS-induced TNFα production (LPS-antagonistic activity) in vitro using human whole blood cells were measured. The IC50 values (nM) of these ten compounds, 8, 14, 21, 31, 40, 51, 57, 62, 67 and 72, were 11.2, 15.4, 2.7, 0.1, 0.4, 1.3, 3.2, 3.2, 1.4 and 14.4, respectively. And also inhibitory activities (ID50) on TNFα production toward galactosamine loaded C3H/HeN mice in vivo of compounds 21, 31, 57, 62 and 67 were measured. The values of these compounds were 0.29, 0.50, 0.61, not dose-dependent and 0.33 mg/kg, respectively. 相似文献
234.
We report the synthesis of optically active 2-aryl-2-fluoropropionic acids 2 as non-epimerizable mimics of 2-arylpropionic acids 1, a class of compounds which have been widely used as non-steroidal anti-inflammatory drugs (NSAIDs). This is a continuation of our research involving the design, synthesis, and evaluation of chiral fluorine-containing organic molecules as effective analogues of pharmacologically important compounds. 相似文献
235.
Fujiwara Y Yoshizaki M Matsushita S Yahara S Yae E Ikeda T Ono M Nohara T 《Chemical & pharmaceutical bulletin》2005,53(5):584-585
A new pregnane glycoside has been isolated from the overripe fruits of Cherry tomato (Mini tomato), Lycopersicon esculentum var. cerasiforme (DUNAL) ALEF. The structure was determined to be 3-O-beta-lycotetraosyl 3beta-hydroxy-5alpha-pregn-16-en-20-one on the basis of spectroscopic analysis. The seasonal variation of the tomato saponin is discussed. 相似文献
236.
Zgierski MZ Patchkovskii S Fujiwara T Lim EC 《The journal of physical chemistry. A》2005,109(42):9384-9387
The ultrafast radiationless decay of photoexcited uracil and cytosine has been investigated by ab initio quantum chemical methods based on CIS and CR-EOM-CCSD(T) electronic energy calculations at optimized CIS geometries. The calculated potential energy profiles indicate that the S(1) --> S(0) internal conversion of the pyrimidine bases occurs through a barrierless state switch from the initially excited (1)pipi state to the out-of-plane deformed excited state of biradical character, which intersects the ground state at a lower energy. This three-state nonradiative decay mechanism predicts that replacement of the C5 hydrogen by fluorine introduces an energy barrier for the initial state switch, whereas replacement of the C6 hydrogen by fluorine does not. These predictions are borne out by the very different fluorescence yields of 5-fluorinated bases relative to the corresponding 6-fluorinated bases. It is concluded from these results that the origin of the ultrafast radiationless decay is the same for the two pyrimidine bases. 相似文献
237.
Umebayashi Y Mroz B Asada M Fujii K Matsumoto K Mune Y Probst M Ishiguro S 《The journal of physical chemistry. A》2005,109(21):4862-4868
Solvation structure of the zinc(II) ion in N,N-dimethylpropionamide (DMPA) was studied by Raman spectroscopy at varying temperature and by quantum mechanical calculations. No significant ion-pair formation was found for the Zn(ClO4)2 solution in the molality range m(Zn) < 1.5 mol kg(-1), and the solvation number of the zinc(II) ion was determined to be 4, indicating that 6-coordination of DMPA is sterically hindered. Interestingly, DMPA molecules are under equilibrium between planar cis and nonplanar staggered conformers, and the latter is more preferred in the coordination sphere, while the reverse is the case in the bulk. The DeltaG degrees , DeltaH degrees , and TDeltaS degrees values of conformational change from planar cis to nonplanar staggered in the coordination sphere were obtained to be -0.9, -8.5, and -7.5 kJ mol(-1), respectively. Density functional theory (DFT) calculations show that the planar cis conformer is more favorable than the nonplanar staggered one in the 1:2 cluster, as is the case for a single DMPA molecule and H(DMPA)+, indicating that there hardly occurs solvent-solvent interaction through the metal ion in the Zn2+-DMPA 1:2 cluster. On the other hand, the SCF energy of [Zn(planar cis-DMPA)4-n(nonplanar staggered DMPA)n]2+ (n = 0-4) decreases with increasing n, implying that the nonplanar staggered conformer is preferred in the solvate ion. It is thus concluded that solvent-solvent interaction through space, or solvation steric effect, plays a crucial role in the conformational equilibrium in the coordination sphere of the four-solvate metal ion. 相似文献
238.
Treatment of several cyclic olefins, β-pinene, allylbenzene, and estragole with palladium(II) salt in acetic acid in the presence of t-butyl hydroperoxide and tellurium(IV) oxide afforded mainly the corresponding allylic acetates. The reaction proceeded catalytically with palladium(II) salt, t-BuOOH working as a reoxidizing agent. 相似文献
239.
pTa interactions have been studied. The total inelastic cross section, multiplicities, net charge distributions and inclusive momentum and rapidity spectra for produced particles are reported. The correlations between fast and slow particles are presented and the reaction mechanisms for multiparticle production in pTa interactions are discussed. 相似文献
240.
Asymmetric synthesis of β-lactams by means of the reaction of dimethylketene silyl acetal with (S)-alkylidene(1-arylethyl)amines in the presence of titanium tetrachloride was studied. The extent of the asymmetric induction was in the range of 44–78% (diastereomeric purity 72–89%) and the (S)-configuration was turned to be preferentially induced at the 4C position of the resulting β-lactams. 相似文献