首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   24413篇
  免费   651篇
  国内免费   138篇
化学   16714篇
晶体学   123篇
力学   575篇
数学   4334篇
物理学   3456篇
  2022年   174篇
  2021年   258篇
  2020年   301篇
  2019年   317篇
  2018年   234篇
  2017年   193篇
  2016年   481篇
  2015年   452篇
  2014年   490篇
  2013年   1300篇
  2012年   1139篇
  2011年   1429篇
  2010年   727篇
  2009年   709篇
  2008年   1256篇
  2007年   1280篇
  2006年   1255篇
  2005年   1212篇
  2004年   1052篇
  2003年   912篇
  2002年   818篇
  2001年   352篇
  2000年   279篇
  1999年   250篇
  1998年   280篇
  1997年   307篇
  1996年   362篇
  1995年   243篇
  1994年   280篇
  1993年   264篇
  1992年   237篇
  1991年   240篇
  1990年   191篇
  1989年   231篇
  1988年   237篇
  1987年   202篇
  1986年   195篇
  1985年   291篇
  1984年   337篇
  1983年   218篇
  1982年   366篇
  1981年   336篇
  1980年   316篇
  1979年   318篇
  1978年   323篇
  1977年   294篇
  1976年   280篇
  1975年   250篇
  1974年   248篇
  1973年   235篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
The present study uses an unique capillary electrophoresis (CE) approach, that we have termed ion-interaction capillary zone electrophoresis (II-CZE), for the separation of diastereomeric peptide pairs where a single site in the centre of the non-polar face of an 18-residue amphipathic alpha-helical peptide is substituted by the 19 L- or D-amino acids. Through the addition of perfluorinated acids at very high concentrations (up to 400 mM), such concentration levels not having been used previously in chromatography or CE, to the background electrolyte (pH 2.0), we have been able to achieve baseline resolution of all 19 diastereomeric peptide pairs with an uncoated capillary. Since each diastereomeric peptide pair has the same sequence, identical mass-to-charge ratio and identical intrinsic hydrophobicity, such a separation by CZE has previously been considered theoretically impossible. Excellent resolution was achieved due to maximum advantage being taken of even subtle disruption of peptide structure/conformation (due to the presence of D-amino acids) of the non-polar face of the amphipathic alpha-helix and its interaction with the hydrophobic anionic ion-pairing reagents. In addition, due to the excellent resolution of diastereomeric peptide pairs by this novel CZE approach, we have also been able to separate a mixture of these closely-related alpha-helical peptides.  相似文献   
912.
Reaction of [(Tp(Me)2)(2)UI] with KNR(2) (R = C(6)H(5), SiMe(3)) in tetrahydrofuran (THF) afforded the monomeric trivalent actinide amide complexes [(Tp(Me)2)(2)U[N(C(6)H(5))(2)]], 1, and [(Tp(Me)2)(2)U[N(SiMe(3))(2)]], 2. The complexes have been fully characterized by spectroscopic methods and their structures were confirmed by X-ray crystallographic studies. In the solid state 1 and 2 exhibit distorted pentagonal bipyramidal geometries. The U-NR(2) bond lengths in both complexes are the same but in complex 2 the greater steric demands of the N(SiMe(3))(2) ligand led to elongated U-N(pz) bonds, especially those opposite the amido ligand.  相似文献   
913.
A conceptually new, simple and practical method for the syn-nucleophilic displacement of aryl and vinyl epoxides and aryl aziridines with (substituted) phenols, using aryl borates as activating nucleophiles under neutral conditions, is reported.  相似文献   
914.
Conformationally restrained substituted pregnane-20-one derivatives were obtained by an intramolecular nitrene addition onto a C-5/C-6 double bond involving a tethered C-19 sulfamoyl moiety. The resulting aziridine underwent regioselective nucleophilic ring opening at C-5 at room temperature with cyanide, fluoride, and acetate. In the isolated case of acetate, a reversal of regioselectivity was observed at higher temperatures, a result attributed to a rearrangement process involving aziridine ring opening at the C-5 position and subsequent migration of the acetyl moiety to C-6.  相似文献   
915.
A novel [3+2]‐cyclo­addition reaction of alkynyl­boronates and nitrile oxides gave the title compound, C22H32BNO3, as a single regioisomer. The X‐ray crystal structure analysis of this compound shows two independent mol­ecules in the asymmetric unit, each with approximately coplanar isoxazole and boronate rings.  相似文献   
916.
Radio frequency plasma polymerization of vinylidene fluoride was used to modify the surface properties of nitrile rubber. The chemistry and frictional properties of the plasma films were characterized. FTIR transmission spectra and EDX analysis of plasma polymer films deposited on NaCl windows showed that the degree of fluorination of the plasma polymers increased as plasma power was increased from 25 to 50 W, and then decreased monotonically at higher powers. An estimation of the actual F/C ratio from EDX data indicated that the plasma polymer films contained approximately one fluorine atom for every 2–5 carbon atoms. Sliding friction tests on a Delrin countersurface showed that the coefficient of friction of the plasma treated rubbers was lower than untreated rubber, but slighly higher than rubber coated with silicone oil. Repetitive sliding friction testing showed that silicone oil treated samples had a longer lubricating lifetime than plasma treated samples. However, cyclic friction tests conducted with nitrile rubber o-rings yielded similar frictional behavior and lubricating lifetimes for silicone oil and plasma treatments. There was no correlation between chemical composition and the frictional and wear properties of the plasma films. Environmental scanning electron micrographs showed that the plasma films were brittle and tended to crack and flake off during wear testing.  相似文献   
917.
The microwave spectrum of ethyl fluoroformate displays strong a-type R branch transitions from two rotameric forms. One species (extended form) has rotational constants A0 = 9191.3(9) MHz, B0 = 2112.61(1) MHz, C0 = 1756.73(1) MHz which are consistent with a syn-anti (τ1(OCOC) = 0°, r2(cocc) = 180°) planar heavy atom structure. The second species (compact form) has rotational constants A0 = 7760(3) MHz, B0 = 2388.38(4) MHz, C0 = 2102.47(3) MHz which are consistent with a syn-gauche1(ococ) = 0°, τ2(cocc) ˜ 90°) structure. The two conformational forms have approximately equal energy (0 ± 40 cm−1). Four vibrational satellites of the extended species have been analyzed yielding a torsional frequency around the O-ethyl bond of 70(10) cm−1. Three vibrational satellites attributed to the O-ethyl torsion of the compact species have been analyzed yielding a vibrational frequency of 90(10) cm−1. Approximate Fourier coefficients of a three term potential function for internal rotation about the O-ethyl bond have been determined. Vibrational satellites attributed to the first excited states of the O-ester torsion have been analyzed for both conformers. The torsional vibrational frequency around the O-ester bond is 110(15) cm−1 for the extended conformers and 120(20) cm−1 for the compact.  相似文献   
918.
Popa TV  Mant CT  Hodges RS 《Electrophoresis》2004,25(9):1219-1229
The present study compares a charge/hydrophobicity capillary electrophoresis (CE) approach to reversed-phase high-performance liquid chromatography (RP-HPLC) for the separation of three series of four synthetic, random coil peptide standards. Each series has peptides of the same positive charge (+1, +2 and +3 series) and length but differing in hydrophobicity. Complete resolution of the 12 peptides was achieved via a novel CE approach: a capillary zone electrophoresis (CZE) mode effected a separation of identically charged peptides; within each charged group of peptides, the addition of perfluorinated acid anionic ion-pairing reagents allowed resolution of the peptides through a mechanism based on peptide hydrophobicity which we have termed ioninteraction (II)-CZE. The peak capacity and peptide resolution of this CE approach was superior to that of RP-HPLC and stresses an important role for CE for peptide/proteomic applications.  相似文献   
919.
(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   
920.
The intramolecular keto-ene/cyclization tandem reaction of γ-N-allylamino ketones is an effective means of producing 1,3-oxazines. The reaction usually requires high temperatures and/or pressures. We discovered that N,N-diallyl amines undergo the reaction at lower temperatures than their monoallyl analogs. An extreme example, 1-N,N-diallylamino-9,10-anthraquinone, undergoes the keto-ene reaction near ambient temperature. In the case of 1-N,N′-dialkylaminoanthraquinones, electron deficient ene components can even be used, allowing the preparation of a broad spectrum of oxazines. Furthermore, the N-allyl-1,3-oxazine can be easily deallylated to produce a 1,3-oxazine that contains a secondary amine.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号