全文获取类型
收费全文 | 4139篇 |
免费 | 86篇 |
国内免费 | 11篇 |
专业分类
化学 | 3011篇 |
晶体学 | 12篇 |
力学 | 64篇 |
数学 | 719篇 |
物理学 | 430篇 |
出版年
2020年 | 26篇 |
2019年 | 27篇 |
2016年 | 55篇 |
2015年 | 65篇 |
2014年 | 58篇 |
2013年 | 153篇 |
2012年 | 120篇 |
2011年 | 156篇 |
2010年 | 98篇 |
2009年 | 101篇 |
2008年 | 124篇 |
2007年 | 128篇 |
2006年 | 148篇 |
2005年 | 137篇 |
2004年 | 166篇 |
2003年 | 117篇 |
2002年 | 126篇 |
2001年 | 69篇 |
2000年 | 69篇 |
1999年 | 69篇 |
1998年 | 53篇 |
1997年 | 63篇 |
1996年 | 68篇 |
1995年 | 66篇 |
1994年 | 66篇 |
1993年 | 59篇 |
1992年 | 75篇 |
1991年 | 52篇 |
1990年 | 47篇 |
1989年 | 50篇 |
1988年 | 57篇 |
1987年 | 42篇 |
1986年 | 63篇 |
1985年 | 66篇 |
1984年 | 84篇 |
1983年 | 63篇 |
1982年 | 64篇 |
1981年 | 82篇 |
1980年 | 84篇 |
1979年 | 69篇 |
1978年 | 88篇 |
1977年 | 60篇 |
1976年 | 71篇 |
1975年 | 55篇 |
1974年 | 53篇 |
1973年 | 61篇 |
1972年 | 42篇 |
1971年 | 40篇 |
1970年 | 38篇 |
1968年 | 24篇 |
排序方式: 共有4236条查询结果,搜索用时 31 毫秒
91.
Investigation of drift gas selectivity in high resolution ion mobility spectrometry with mass spectrometry detection 总被引:3,自引:0,他引:3
Matz LM Hill HH Beegle LW Kanik I 《Journal of the American Society for Mass Spectrometry》2002,13(4):300-307
Recent studies in electrospray ionization (ESI)/ion mobility spectrometry (IMS) have focussed on employing different drift gases to alter separation efficiency for some molecules. This study investigates four structurally similar classes of molecules (cocaine and metabolites, amphetamines, benzodiazepines, and small peptides) to determine the effect of structure on relative mobility changes in four drift gases (helium, nitrogen, argon, carbon dioxide). Collision cross sections were plotted against drift gas polarizability and a linear relationship was found for the nineteen compounds evaluated in the study. Based on the reduced mobility database, all nineteen compounds could be separated in one of the four drift gases, however, the drift gas that provided optimal separation was specific for the two compounds. 相似文献
92.
93.
Stasch A Ferbinteanu M Prust J Zheng W Cimpoesu F Roesky HW Magull J Schmidt HG Noltemeyer M 《Journal of the American Chemical Society》2002,124(19):5441-5448
The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods. 相似文献
94.
95.
The ZnCl2 catalyzed reaction of p-methoxybenzyl chloride with alkenes yields the 1:1 addition products 3, which are converted into the γ-lactones 4 via Ru(VIII) catalyzed oxidative degradation of the aromatic ring. 相似文献
96.
97.
Proportionally spaced cubicB-splines are an appropriate function system for the approximation of titration curves and other sigmoidal functions. They give a smooth weighted leastsquares fit without instability problems. The degree of smoothing depends only on the number of spline function which can be chosen by the user. The equivalence points of titration curves can be estimated with high accuracy from the zeros of the second derivative.The method gives good approximation curves even in the case of empty data regions, i. e. there are no artefacts in subranges where no data points exist. The routine has been tested successfully with large series of simulated and experimental data. 相似文献
98.
The triphenylallenyl cation (8), generated from triphenylpropynol (7) and FSO3H, reacts with alkenes to give the allyl cations 12, which may be deprotonated to yield the methyleneccylobutenes 14. Alternatively, 12 can be converted into the 2-vinyl-indenes 13 two subsequent electrocyclic reactions. 相似文献
99.
Marx T Mosel B Pantenburg I Hagen S Schulze H Wesemann L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4472-4478
The tetrasubstituted polyanions of platinum, palladium, and gold [M(SnB(11)H(11))(4)](x-) (x=6, M=Pd, Pt; x=5, M=Au) have been prepared and characterized by single-crystal X-ray diffraction, elemental analysis, IR, Raman, (11)B, and (119)Sn heteronuclear NMR spectroscopy. In the case of the platinum derivative [Bu(3)MeN](6)[Pt(SnB(11)H(11))(4)] (2) (119)Sn M?ssbauer spectroscopy has been carried out. The isolated salts are stable towards moisture and air and the complexes 2 and 3 were treated with 1,3-bis(diphenylphosphino)propane (dppp) to give the respective substitution products [Bu(3)MeN](2)[(dppp)M(SnB(11)H(11))(2)] (M=Pd, Pt). 相似文献
100.
Gerhard Cordier Christoph Schwidetzky Herbert Schäfer 《Journal of solid state chemistry》1984,54(1):84-88
The new compound BaSb2S4 crystallizes in the monoclinic system (space group: , No. 14) with a = 8.985(2) Å, b = 8.203(3) Å, c = 20.602(5) Å, β = 101.36(3)°. SbS3 ψ tetrahedra and ψ-trigonal SbS4 bipyramids are connected by common corners and edgers to infinite strings. These are arraged cross-wise in sheets perpendicular to the c axis. 相似文献