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   9244篇
  免费   369篇
  国内免费   156篇
化学   4554篇
晶体学   153篇
力学   261篇
综合类   24篇
数学   1854篇
物理学   2923篇
  2023年   183篇
  2022年   230篇
  2021年   333篇
  2020年   371篇
  2019年   223篇
  2018年   304篇
  2017年   304篇
  2016年   322篇
  2015年   288篇
  2014年   498篇
  2013年   790篇
  2012年   531篇
  2011年   590篇
  2010年   544篇
  2009年   538篇
  2008年   406篇
  2007年   430篇
  2006年   445篇
  2005年   303篇
  2004年   245篇
  2003年   202篇
  2002年   146篇
  2001年   214篇
  2000年   103篇
  1999年   147篇
  1998年   91篇
  1997年   85篇
  1996年   49篇
  1995年   53篇
  1994年   50篇
  1993年   39篇
  1992年   48篇
  1991年   37篇
  1990年   49篇
  1989年   41篇
  1988年   180篇
  1987年   96篇
  1986年   28篇
  1985年   28篇
  1984年   40篇
  1983年   12篇
  1982年   12篇
  1981年   17篇
  1980年   13篇
  1979年   12篇
  1978年   9篇
  1977年   15篇
  1976年   9篇
  1973年   15篇
  1971年   8篇
排序方式: 共有9769条查询结果,搜索用时 31 毫秒
151.
The crystal structures of the four title clathrate compounds Cd(NH3)2Cd(CN)4 · 2C6H6,I, Cd(NH3)2Cd(CN)4 · 2C6H5NH2,II, Cd(NH2CH2CH2NH2)Cd(CN)4 · 2 C6H5NH2,III, and Cd(C6H5NH2)2Cd(CN)4 · 0.5C6H5NH2,IV, have been analyzed by single crystal X-ray diffraction methods. CompoundI crystallizes in the monoclinic space groupC2/c,a = 12.063(2),b = 12.174(2),c = 14.621(1) Å,β = 90.976(9)°,Z = 4,R = 0.042 for 2388 reflections;II: monoclinic C2/c,a = 12.1951(9),b = 12.078(1),c = 14.6921(7) Å,β = 93.436(5)°,Z = 4,R = 0.039 for 2374 reflections;III: monoclinicCc,a = 11.027(1),b = 12.0767(9),c = 15.837(1) Å,β = 92.059(9)°,Z = 4,R = 0.041 for 2883 reflections; andIV: monoclinicP21/n,a = 15.169(2),b = 16.019(2),c = 8.866(1) Å,β = 95.73(1)°,Z = 4,R = 0.052 for 3612 reflections. The three-dimensionalcatena-[diamminecadmium(II) tetra-μ-cyanocadmate(II)] hosts ofI andII are substantially isostructural to that of the already known Hofmann-Td-type Cd(NH3)2Hg(CN)4 · 2C6H6. The three-dimensional en-Td-typecatena-[catena-μ-ethylenediaminecadmium(II) tetra-μ-cyanocadmate(II)] host ofIII, reinforced by the catena-μ-en linking between the octahedral Cd atoms, accommodates the aniline as the guest with a monoclinic distortion from the tetragonal symmetry of the previously reported en-Td-type benzene clathrate. InIV dual behavior of aniline, one as the unidentate ligand in the three-dimensional host and the other as the guest in the cage-like cavity, has been demonstrated.  相似文献   
152.
The synthesis, spectroscopy, and redox-reactivity towards PPh(3) of bis(3,5-di-t-butyl-1,2-benzoquinone 1-oximato)manganese(II), Mn(ox)(2), and its mixed-ligand complexes, Mn(ox)L(x) [L(x)=8-hydroxyquinolinato (L(1)), 2,2'-bipyridine (bpy) (L(2)), 1,10-Phenanthroline (phen) (L(3)) and N-(2-hydroxyphenyl)-3,5-di-t-butylsalicylaldiminato (L(4))] complexes, are presented. While some complexes exhibit solid-state ESR spectra they were ESR-silent in toluene or CHCl(3) at 300 K. The reduction of all ESR-silent complexes with an excess of PPh(3), independently from the nature of secondary L(x) ligands, results in the appearance of a six-line hyperfine coupling of 55Mn (I=5/2) (A(Mn)=95-100 G) and a seven-line radical signal, which was assigned to phosphiniminophenoxyl type radicals (g=2.0056-2.0075, A(P)=8. 5 G and A(N)=1.875 G).  相似文献   
153.
采用密度泛函理论(DFT), 在B3LYP/6-311++G(d,p)基组上计算得到了21种N8H8链状异构体, 并研究了这些异构体间可能的互变异构情况. 为了得到更为精确的能量信息, 计算了QCISD(T)/6-311G(d,p)基组水平上各物质的能量. 所得的21种异构体分为4类(4种类型链状化合物): A为直链, B有一个支链, C有2个支链, D有3个支链; D类只有一种, A类稳定构型2种, B类稳定构型12种, C类稳定构型6种; 相对稳定的分别为: B2-1构型, B2-3构型和C23-2构型. 我们研究发现N8H8链状异构体中含有明显N=N双键特征有利于化合物稳定性的提高.  相似文献   
154.
H. Cesur 《Chemical Papers》2007,61(5):342-347
A new solid-phase extraction method has been developed for the selective extraction of Cu(II) in an aqueous system using freshly precipitated lead diethyldithiocarbamate (Pb(DDTC)2) as a reagent. The method is based on the quantitative replacement of Pb(II) ions in the solid Pb(DDTC)2 phase by Cu(II) ions present in aqueous phase. The obtained solid Cu(DDTC)2 phase was dissolved in chloroform and determined spectrophotometrically at 435 nm. Beer’s law was obeyed over the concentration range of 0.2–5 mg dm−3. The molar absorptivity and the Sandell’s sensitivity coefficients of the solutions were 1.0689 × 104 dm3 mol−1 cm−1 and 0.0060 μg cm−2, respectively. The optimum conditions for each parameter were experimentally determined and possible interferences of various salts were also studied. The method has been validated and applied to determine Cu(II) in various alloys and water samples.  相似文献   
155.
The kinetics of the oxidation of benzyl alcohol by potassium chlorochromate, KCrClO3, has been studied in dimethyl sulfoxide-dichloromethane medium. The reaction is catalyzed by acid. The effects of temperature and solvent composition were studied and activation parameters evaluated. Probable mechanisms are discussed.
, KCrClO3, . . . .
  相似文献   
156.
Sahin F  Volkan M  Howard AG  Ataman OY 《Talanta》2003,60(5):1003-1009
Silica gel modified with 3-mercaptopropyl-trimethoxysilane was used for the selective separation and pre-concentration of selenite (Se(IV)) from aqueous solutions containing Se(IV) and selenate (Se(VI)). Over a wide range of acidity, from 2 mol l−1 HCl to pH 9.00, Se(IV) was taken up by the mercaptopropyl-silica with nearly 100% efficiency; Se(VI) however was unretained. Se(IV) content was determined by hydride generation atomic absorption spectrometry (HGAAS), following batch release of the selenium from the pre-concentration medium by acidic periodate. The overall pre-concentration efficiency, including both take-up and elution, in the range of 89-106%. The method was applied to spiked seawater samples containing as low as 800 ng l−1 Se in selenite form. This solid-phase extraction system offers several major advantages over conventional solvent extraction procedures. It firstly exhibits high selectivity for Se(IV) over Se(VI). Using the solid-phase media, pre-concentration of Se(IV) in dilute water samples can be carried out in the field, stabilizing the selenite-selenium in a convenient form for transport and storage. In addition, selenium stored on silica is derived solely from Se(IV) overcoming problems of selenium redox speciation changes and loss during storage.  相似文献   
157.
The preparation and spectroscopic characterization of a series of new bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylaldiminato]nickel(II) complexes, [Ni(LX)2], bearing one or two OH and MeO substituents on the salicylaldehyde moiety, as well as radical species generated from these compounds by the oxidation with PbO2, are reported. The [Ni(LX)2] chelates, which appear to be tetrahedral in the solid state and in dioxane solution, are converted into a square-planar configuration in non-donor solvents. The OH-substituted complexes, unlike their MeO analogues, form six-coordinate adducts in pyridine, DMF and DMSO. These new compounds, unlike their analogues with electron-withdrawing substituents (Cl, Br, NO2), are easily oxidized by PbO2 to produce NiII-stabilized phenoxy radicals in which the unpaired electrons are delocalized over the ligand and do not couple with the second radical center. No e.s.r. signals were observed that could be attributed to a M = ±2 transition of the triplet state biradicals.  相似文献   
158.
Ab initio calculations on the different associated structures of 2-pyrrolidinone with water and with itself were carried out using 3-21G and 6-31G* basis sets at the Hartree–Fock level, including electron correlation using second-order Møller–Plesset perturbation theory. The calculated free energy changes for the intermolecular hydrogen bonded dimer and hydrated species indicated that the molecular systems with cyclic dimerization and association with two water molecules are dominant. The results are compared to the available experimental data in the literature.  相似文献   
159.
A variety of symmetrically or unsymmetrically 3,4-disubstituttd furoxans such as dicyano, dialkyl, diacyl, bis(phenylsulfonyl), N.N'-dialkyldicarbamoyl, 3(or 4)-methyl-4(or -3)-phenyl(or nitro, ethoxy, phenoxy, phenylthio, pyrrolidinyl, phenylsulfonyl), 3(or 4)-ethyl-4(or -3)phcnyl, and 3(or 4)-ethoxy-4(or -3)-phenylsulfonylruroxan reacted with dipolarophiles in toluene or xylene at the refluxing temperature to give nitrone-type 1,3-dipolar cycloadducts, 5-substituted 1-aza-2,8-dioxabicyclo-[3.3.0]octanes and/or 3-substituted 2-isoxazoline 2-oxides. On the other hand, some of the furoxans gave 2-isoxazolines via nitrile oxide 1,3-dipolar cycloaddition in a toluene (or xylene)-DMF solvent at the refluxing temperature.  相似文献   
160.
A silver(I)-saccharinato (sac) complex with nicotinamide (nia), [Ag(sac)(nia)] n has been synthesized and characterized by elemental analysis, IR spectroscopy, DTA-TG analyses and single crystal X-ray diffractometry. The complex crystallizes in monoclinic space group P21/n with unit cell parameters of a=7.0258(4) Å, b=24.3784(10) Å, c=8.4301(5) Å, β=109.407(5)°, V=1361.85(13) Å3 and Z=4. [Ag(sac)(nia)] n contains [Ag(sac)(nia)] units, which are doubly bridged by both nia and sac ligands, leading to a linear one-dimensional polymeric chains running along the a axis. The silver(I) ion has a highly distorted AgN2O2 tetrahedral geometry and the coordination polymer exhibits relatively short intra-chain ligand supported Ag···Ag separations of 3.1593(4) Å. The one-dimensional chains are crosslinked by N–H···O hydrogen bonds and aromatic π(sac)···π(nia) stacking interactions to generate a two-dimensional layer structure. IR spectra and thermal analysis data are in agreement with the crystal structure.  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