全文获取类型
收费全文 | 1995篇 |
免费 | 43篇 |
国内免费 | 18篇 |
专业分类
化学 | 1426篇 |
晶体学 | 26篇 |
力学 | 31篇 |
数学 | 273篇 |
物理学 | 300篇 |
出版年
2022年 | 8篇 |
2021年 | 23篇 |
2020年 | 22篇 |
2019年 | 11篇 |
2018年 | 12篇 |
2017年 | 9篇 |
2016年 | 30篇 |
2015年 | 31篇 |
2014年 | 26篇 |
2013年 | 85篇 |
2012年 | 85篇 |
2011年 | 122篇 |
2010年 | 66篇 |
2009年 | 74篇 |
2008年 | 130篇 |
2007年 | 141篇 |
2006年 | 127篇 |
2005年 | 111篇 |
2004年 | 101篇 |
2003年 | 102篇 |
2002年 | 103篇 |
2001年 | 41篇 |
2000年 | 47篇 |
1999年 | 28篇 |
1998年 | 22篇 |
1997年 | 24篇 |
1996年 | 43篇 |
1995年 | 19篇 |
1994年 | 19篇 |
1993年 | 18篇 |
1992年 | 27篇 |
1991年 | 15篇 |
1990年 | 11篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1986年 | 14篇 |
1985年 | 19篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 23篇 |
1981年 | 35篇 |
1980年 | 18篇 |
1979年 | 22篇 |
1978年 | 21篇 |
1977年 | 14篇 |
1976年 | 16篇 |
1975年 | 23篇 |
1974年 | 11篇 |
1973年 | 11篇 |
排序方式: 共有2056条查询结果,搜索用时 15 毫秒
81.
van Heijnsbergen D von Helden G Meijer G Maitre P Duncan MA 《Journal of the American Chemical Society》2002,124(8):1562-1563
The organometallic ions V+-(benzene) and V+-(benzene)2 are produced by laser vaporization in a pulsed nozzle source. They are trapped and mass selected in an ion-trap/time-of-flight mass spectrometer, and their infrared spectra are measured with resonance-enhanced multiphoton photodissociation (IR-REMPD) spectroscopy with a tunable free-electron laser. Vibrational bands in the 600-1800 cm-1 region are characteristic of the benzene molecular moiety perturbed by the metal cation bonding. Experimental data are compared to the IR spectra derived from density functional calculations. Vibrational patterns in V+-(C6H6) indicate that the metal is bound in an eta6 pi-bonding configuration, while V+-(C6H6)2 is a sandwich. Trapped-ion IR-REMPD is a general method to access the vibrational spectroscopy of organometallic ions and their clusters. 相似文献
82.
Abdelkrim El‐Ghayoury Albertus P. H. J. Schenning E. W. Meijer 《Journal of polymer science. Part A, Polymer chemistry》2002,40(22):4020-4023
An oligo(p‐phenylene vinylene) that contains terpyridine ligands has been synthesized. Upon addition of metal ions, a π‐conjugated metallo polymer is formed in which the well‐defined character of oligomers and the material properties of polymers are combined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4020–4023, 2002 相似文献
83.
Priscilla E. Greenwood Gerard Hooghiemstra 《Probability Theory and Related Fields》1991,89(2):201-210
Summary Between the operations which produce partial maxima and partial sums of a sequenceY
1,Y
2, ..., lies the inductive operation:X
n
=X
n-1(X
n-1+Y
n
),n1, for 0<<1. If theY
n
are independent random variables with common distributionF, we show that the limiting behavior of normed sequences formed from {X
n
,n1}, is, for 0<<1, parallel to the extreme value case =0. ForFD() we give a full proof of the convergence, whereas forFD()D(), we only succeeded in proving tightness of the involved sequence. The processX
n
is interesting for some applied probability models. 相似文献
84.
85.
86.
87.
Henk de Vries Peter J. Hemelaar Alice C. M. Gevers Gerard M. J. Beyersbergen van Henegouwen 《Photochemistry and photobiology》1994,60(3):249-252
Abstract The negative side effects of chlorarnphenicol (CAP) mostly involve blood dyscrasias (e.g. irreversible nondose-dependent aplastic anemia), allergic skin reactions and eye damage. To learn the cause of these side effects, most research focuses on metabolically formed nitroso- and hydroxylamino derivatives in the predisposed patient. In previous investigations it was demonstrated that photochemical decomposition of CAP in vitro by UV-A leads to formation of p-nitrobenzaldehyde (pNB), p-nitrobenzoic acid (pNBA) and p-nitrosobenzoic acid (pNOBA); the latter comprises up to 45 mol% of the starting amount of CAP. Incubation of these photoproducts in rat blood showed that pNB and pNOBA rapidly react and that PNBA is stable under these conditions. Reaction products from pNB (half-life 1.7 min) proved to be pNBA and p-nitrobenzyl alcohol (pNBOH) while pNOBA (half-life 3.7 min) was converted into p-aminobenzoic acid (pABA). Exposure of CAP in rat blood to UV-A yielded the same end products: pNBA, PABA and pNBOH. To estimate the amount of oxidative stress generated in vivo by these compounds, the ability to form methemoglobin (MetHb) in erythrocytes was tested; only pNOBA and p-hydroxylaminobenzoic acid (pHABA), a possible intermediate in the decomposition of pNOBA, proved to be reactive. Ultraviolet-A exposure of rats, after intraperitoneal injection of CAP, led to 3.6 times the basic level of MetHb. In addition, covalent binding of 3H-labeled CAP photoproducts to the skin of the back and to the ears was found, which was 9.1 and 3.2 times higher, respectively, than the dark values. Toxicity toward bone marrow cells of all photoproducts was established in vitro. p-Nitrobenzaldehyde, pNOBA andpHABA were 20, 6 and 6 times more toxic than CAP, respectively. These results show that photodecomposition of CAP in vivo does occur. Its reactive photoproducts are able to cause damage that may lead to (systemic) side effects. The latter is supported by the fact that the nature of the reactive products, nitroso- and hydroxylamino derivatives, is the same as the expected metabolites. 相似文献
88.
Antonius F.M.J. Van Der Ploeg Cornelus E.M. Van Der Kolk Gerard Van Koten 《Journal of organometallic chemistry》1981,212(3):283-290
Arylmercury compounds of the type Ar2Hg and ArHgX (X = Cl, OAc) have been synthesized and characterized by 1H and 13C NMR spectroscopy; the Ar group was either 2-Me2NCH2C6H4 or (S)-2-Me2NCH(Me)C6H4, both of which contain N-donor ligands. The observation of anisochronous NMe resonances in (S)-2-Me2NCH(Me)C6H4HgX (X = Cl, OAc) at low temperature indicates that in solution the mercury centre is three-coordinate as a result of stable intramolecular HgN coordination 相似文献
89.
Kronenburg CM Rijnberg E Jastrzebski JT Kooijman H Lutz M Spek AL Gossage RA van Koten G 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):253-261
(R)-[1-(Dimethylamino)ethyl]benzene reacts with nBuLi in a 1:1 molar ratio in pentane to quantitatively yield a unique hetero-aggregate (2 a) containing the lithiated arene, unreacted nBuLi, and the complexed parent arene in a 1:1:1 ratio. As a model compound, [Li(4)(C(6)H(4)CH(Me)NMe(2)-2)(2)(nBu)(2)] (2 b) was prepared from the quantitative redistribution reaction of the parent lithiated arene Li(C(6)H(4)CH(Me)NMe(2)-2) with nBuLi in a 1:1 molar ratio. The mono-Et(2)O adduct [Li(4)(C(6)H(4)CH(Me)NMe(2)-2)(2)(nBu)(2)(OEt(2))] (2 c) and the bis-Et(2)O adduct [Li(4)(C(6)H(4)CH(Me)NMe(2)-2)(2)(nBu)(2)(OEt(2))(2)] (2 d) were obtained by re-crystallization of 2 b from pentane/Et(2)O and pure Et(2)O, respectively. The single-crystal X-ray structure determinations of 2 b-d show that the overall structural motifs of all three derivatives are closely related. They are all tetranuclear Li aggregates in which the four Li atoms are arranged in an almost regular tetrahedron. These structures can be described as consisting of two linked dimeric units: one Li(2)Ar(2) dimer and a hypothetical Li(2)nBu(2) dimer. The stereochemical aspects of the chiral Li(2)Ar(2) fragment are discussed. The structures as observed in the solid state are apparently retained in solution as revealed by a combination of cryoscopy and (1)H, (13)C, and (6)Li NMR spectroscopy. 相似文献
90.
Gerard A. van Albada Ilpo Mutikainen Urho Turpeinen Jan Reedijk 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(3):m179-m181
The title mononuclear complex, [Cu(CO3)(C8H7N5)(H2O)]·2H2O, was obtained by fixation of CO2 by a mixture of copper(II) tetrafluoroborate and the ligand bis(pyrimidin‐2‐yl)amine in ethanol/water. The CuII ion of the complex has a distorted square‐pyramidal environment, with a basal plane formed by two N atoms of the ligand and two chelating O atoms of the carbonate group, while the apical position is occupied by the O atom of the coordinating water molecule. In the solid state, hydrogen‐bonding interactions are dominant, the most unusual being the Watson–Crick‐type coplanar ligand pairing through two N—H?N bonds. Lattice water molecules also participate in hydrogen bonding. 相似文献