全文获取类型
收费全文 | 60813篇 |
免费 | 3567篇 |
国内免费 | 3766篇 |
专业分类
化学 | 25348篇 |
晶体学 | 507篇 |
力学 | 4072篇 |
综合类 | 574篇 |
数学 | 15506篇 |
物理学 | 22139篇 |
出版年
2023年 | 484篇 |
2022年 | 1173篇 |
2021年 | 1204篇 |
2020年 | 1198篇 |
2019年 | 1304篇 |
2018年 | 1119篇 |
2017年 | 1262篇 |
2016年 | 1369篇 |
2015年 | 1100篇 |
2014年 | 1799篇 |
2013年 | 3395篇 |
2012年 | 2195篇 |
2011年 | 2619篇 |
2010年 | 2187篇 |
2009年 | 3484篇 |
2008年 | 3649篇 |
2007年 | 3981篇 |
2006年 | 3432篇 |
2005年 | 2653篇 |
2004年 | 2174篇 |
2003年 | 2407篇 |
2002年 | 2827篇 |
2001年 | 2138篇 |
2000年 | 2039篇 |
1999年 | 1748篇 |
1998年 | 1665篇 |
1997年 | 1005篇 |
1996年 | 977篇 |
1995年 | 818篇 |
1994年 | 872篇 |
1993年 | 646篇 |
1992年 | 706篇 |
1991年 | 493篇 |
1990年 | 443篇 |
1989年 | 368篇 |
1988年 | 311篇 |
1987年 | 312篇 |
1986年 | 275篇 |
1985年 | 953篇 |
1984年 | 733篇 |
1983年 | 575篇 |
1982年 | 668篇 |
1981年 | 545篇 |
1980年 | 417篇 |
1979年 | 431篇 |
1978年 | 354篇 |
1977年 | 343篇 |
1976年 | 350篇 |
1975年 | 272篇 |
1973年 | 285篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The intramolecular cyclization reactions of tri-1-hex-5-enyl Group III derivatives and of 1-hex-5-enyllithium have been investigated and it is proposed that the reactions may be depicted as internal addition across the terminal double bonds of the alkenyl species. The cyclization reaction proceeds more readily for the 1-hex-5-enyl derivatives than for any other chain length and is dependent on temperature, concentration and solvent. 1H NMR data and cryoscopic molecular weight data are presented for several diisobutyialkenyl- and diisobutylalkyl-aluminum derivatives. These data have been interpreted in terms of a direct intramolecular interaction between the metal center and the π-electron system of the olefinic site. The intramolecular cyclization reactions for the 1-hept-6-enyl, 1-oct-7-enyl- and 1-undec-10-enylaluminum derivatives also were investigated and it was found that cyclization occurred only for the 1-hept-6-enyl derivative. 相似文献
992.
The crystal structure of a second high-pressure copper vanadate phase, CuVO3(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (Rw = 0.051). The space group is rhombohedral, , with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [aR = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu+V5+O3 is proposed. This represents a unique example of Cu+ in an octahedral environment. 相似文献
993.
994.
The treatment of the non-linear isotherm in chromatography by one of the authors [1] is extended to other cases of practical interest. The original paper dealt with the case of an initially thin zone resulting in a Poisson distribution the asymmetry of which is a function of the plate number and the non-linearity constant. The present work shows that the same relations apply to initially thick zones and to the cases of washing and deposition leading to simple relations for breakthrough curves. For significantly non-linear isotherms other quasi-Gaussian distributions can give a better fit. 相似文献
995.
簇合物[Fe(DMF)6][W2S6]的合成,结构和性能研究 总被引:4,自引:0,他引:4
以(NH_4)_2WS_4、FeCl_3和NaS_2CNC_5H_(10)为原料,合成出[Fe(DMF)_6][W_2S_6]簇合物,用X射线衍射法测定了晶胞结构.其晶胞参数为a=0.8778(1)nm,b=0.9467(3)nm,c=1.1362(6)nm,α=71.506(1)°,β=74.775(2)°,γ=74.775(2)°,ν=0.84806nm~3,Z=1.晶体结构经块矩阵最小二乘法精修后,最终偏离因子R=0.0811.此外还进行了红外光谱、紫外可见光谱的测定. 相似文献
996.
W. Gołkiewicz 《Chromatographia》1986,21(5):259-264
Summary Equations describing multi-step gradient elution with a mobile phase of constant composition in each step were derived. These
equations useful for calculating the retention volumes in both gradient HPLC and TLC were derived on the basis of the relationship
between the isocratic capacity factor and the volume fraction of the organic modifier. The validity of the equations was experimentally
verified in a LiChrosorbRP-18-water/methanol system for 11 methyl- and chlorobenzenes and phenols. A satisfactory agreement
between the theoretical and experimental k′ values was found. 相似文献
997.
NH3(MoO3)3 crystallizes with hexagonal symmetry, space group , lattice constants a = 10.568 Å, c = 3.726 Å, and Z = 2. The crystal structure has been determined by Patterson synthesis and refined assuming isotropic temperature factors to a final conventional R value of 0.085. The structure shows a three-dimensional arrangement built up of double chains of distorted MoO6 octahedra, parallel to the [001] direction. The octahedral double chains are linked among each other through common oxygen atoms. In addition to the shared oxygen atoms, each molybdenum is coordinated to one terminal oxygen. MoO distances range from 1.645 to 2.378 Å and OMoO angles from 74.3 to 114.3°. These results are consistent with the fact that molybdenum in high-valence states shows octahedral coordination with terminal oxygens. 相似文献
998.
It is well known that porphyrin derivatives play a key role in the primary process of photo-synthesis[1], in which porphyrins directly absorb the sunlight or indirectly acquire excitation en-ergy from light-harvesting antenna system to reach their excited state, and then donate electrons to quinone acceptors to yield a series of charge-separated species. In general, only first singlet ex-cited state of porphyrins is involved in energy transfer process[2]. However, highly excited state (S2 stat… 相似文献
999.
1000.