全文获取类型
收费全文 | 1879篇 |
免费 | 53篇 |
国内免费 | 11篇 |
专业分类
化学 | 1171篇 |
晶体学 | 10篇 |
力学 | 62篇 |
综合类 | 1篇 |
数学 | 193篇 |
物理学 | 506篇 |
出版年
2020年 | 13篇 |
2019年 | 16篇 |
2017年 | 22篇 |
2016年 | 34篇 |
2015年 | 22篇 |
2014年 | 27篇 |
2013年 | 83篇 |
2012年 | 82篇 |
2011年 | 103篇 |
2010年 | 44篇 |
2009年 | 53篇 |
2008年 | 87篇 |
2007年 | 94篇 |
2006年 | 89篇 |
2005年 | 79篇 |
2004年 | 80篇 |
2003年 | 60篇 |
2002年 | 49篇 |
2001年 | 38篇 |
2000年 | 40篇 |
1999年 | 42篇 |
1998年 | 21篇 |
1997年 | 20篇 |
1996年 | 24篇 |
1995年 | 40篇 |
1994年 | 35篇 |
1993年 | 35篇 |
1992年 | 45篇 |
1991年 | 21篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 15篇 |
1985年 | 33篇 |
1984年 | 14篇 |
1983年 | 19篇 |
1982年 | 26篇 |
1981年 | 23篇 |
1980年 | 16篇 |
1979年 | 26篇 |
1978年 | 25篇 |
1977年 | 25篇 |
1976年 | 20篇 |
1975年 | 21篇 |
1974年 | 29篇 |
1973年 | 23篇 |
1972年 | 12篇 |
1971年 | 13篇 |
1934年 | 16篇 |
排序方式: 共有1943条查询结果,搜索用时 15 毫秒
61.
A method based on Ge(Li) γ-ray spectrométry is applied to the determination of 224Ra (t= 3.64 days) in natural waters. The 224Ra is first removed from several hundred liters of water by preconcentration onto manganese dioxide-impregnated acrylic fibers. The fibers are leached, radium is coprecipitated with barium sulfate, and the γ-ray activity is counted so that activity ratios among 224Ra, 225Ra and 226Ra can be calculated. Concentrations are determined by using the 226Ra concentration determined on a small separate sample. Results from samples collected from ground water, estuarine, and continental shelf environments are presented. 相似文献
62.
R. B. King 《Russian Chemical Bulletin》1993,42(8):1283-1291
Methods derived from topology and graph theory indicate that the deltahedral boranes B
n
H
n
2– and the corresponding carboranes C2Bn–2H
n
(6 n 12) may be regarded as three-dimensional delocalized aromatic systems in which surface bonding and core bonding correspond to -bonding and -bonding, respectively, in planar polygonal two-dimensional hydrocarbons CinnH
n
(n–6)+ (n=5/7). The two extreme types of topologies which may be used to model core bonding in deltahedral boranes and carboranes are the deltahedral (D
n
) topology based on the skeleton of the underlying deltahedron and the complete (K
n
) topology based on the corresponding complete graph. Analyses of the Hoffmann-Lipscomb LCAO extended Hückel computations, the Armstrong—Perkins—Stewart self-consistent molecular orbital computations, and SCF MOab initio GAUSSIAN-82 computations on B6H6
2– indicate that the approximation of the atomic orbitals by the sum of the molecular orbitals, as is typical in modernab initio computations, leads to significantly weaker apparent core bonding approximated more closely by deltahedral (D
n
) topology than by complete (K
n
) topology.This work was presented at the Workshop The Modern Problems of Heteroorganic Chemistry sponsored by the A. N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (May 8–13, 1993).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1353–1360, August, 1993. 相似文献
63.
R. B. King 《Journal of mathematical chemistry》1991,7(1):69-84
Most chiral molecules can be dissected into a collection of ligands attached to an underlying skeleton. Application of permutation group theory and group representation theory to such a model can lead to chirality functions which can be used to approximate pseudoscalar measurements such as optical rotation or circular dichroism. Such chirality functions have been tested experimentally for the following skeletons: (1) The polarized triangle of phosphines and phosphine oxides; (2) the tetrahedron of methane derivatives; (3) the disphenoid of allene and 2, 2-spirobiindane derivatives; (4) the polarized rectangle of [2, 2]-metacyclophanes; (5) the polarized pentagon of heterodisubstituted ferrocenes. The success of this method is fair to good for the polarized triangle, tetrahedron, and disphenoid skeletons but deteriorates rapidly for the polarized rectangle and polarized pentagon skeletons, in accord with the greater group-theoretical complexity of the latter skeletons. 相似文献
64.
65.
P-Nitrosophosphates, such as 9, react as N-O heterodienophiles with 1,3-dienes to form highly functionalized cycloadducts that can be directly transformed into allylic phosphoramidates. The in situ periodate oxidation of the unstable N-hydroxyphosphoramidate precursors provides an efficient preparation of these new reactive intermediates. P-Nitrosophosphate (9) regioselectively reacts with 1-methoxy-1,3-butadiene to provide cycloadduct 16. P-Nitrosophosphate (9) also reacts with 9,10-dimethylanthracene to give cycloadduct 17, which undergoes retro Diels-Alder dissociation to re-form 9. In the absence of a 1,3-diene, the decomposition of 17 produces nitrous oxide, evidence for nitroxyl, the one-electron-reduced form of nitric oxide. An asymmetric P-nitrosophosphate reacted with 1,3-cyclohexadiene to form a mixture of diastereomeric cycloadducts (19 and 20) in a 1.6:1 ratio. These results identify P-nitrosophosphates as new species that react similarly to acyl nitroso compounds, making them useful synthetic intermediates and potential nitroxyl delivery agents. 相似文献
66.
67.
68.
69.
70.
Schumm BA Koetke DS Adolphsen CE Alexander JP Averill D Barish BC Barklow T Barnett BA Blockus D Boyarski A Brabson B Breakstone A Bulos F Burchat PR Burke DL Cence RJ Chapman J Chmeissani M Cords D Coupal DP Dauncey P DeStaebler HC Dorfan JM Drell PS Drewer DC Durrett D Elia R Feldman GJ Field RC Ford WT Fordham C Frey R Fujino D Gan KK Gero E Gidal G Glanzman T Goldhaber G Gomez Cadenas JJ Gratta G Hanson G Harr R Harral B Harris FA Hayes K Hearty C Heusch CA Hildreth MD Himel T Hinshaw DA 《Physical review D: Particles and fields》1992,46(1):453-456