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   10267篇
  免费   329篇
  国内免费   82篇
化学   7329篇
晶体学   45篇
力学   235篇
数学   1570篇
物理学   1499篇
  2022年   69篇
  2021年   120篇
  2020年   154篇
  2019年   153篇
  2018年   106篇
  2017年   79篇
  2016年   236篇
  2015年   201篇
  2014年   237篇
  2013年   572篇
  2012年   576篇
  2011年   741篇
  2010年   327篇
  2009年   259篇
  2008年   563篇
  2007年   643篇
  2006年   688篇
  2005年   556篇
  2004年   496篇
  2003年   451篇
  2002年   394篇
  2001年   163篇
  2000年   105篇
  1999年   97篇
  1998年   91篇
  1997年   116篇
  1996年   134篇
  1995年   105篇
  1994年   94篇
  1993年   108篇
  1992年   94篇
  1991年   70篇
  1990年   99篇
  1989年   61篇
  1988年   82篇
  1987年   74篇
  1986年   93篇
  1985年   133篇
  1984年   128篇
  1983年   80篇
  1982年   108篇
  1981年   109篇
  1980年   107篇
  1979年   88篇
  1978年   103篇
  1977年   81篇
  1976年   85篇
  1975年   77篇
  1974年   53篇
  1973年   76篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
A concise enantioselective synthesis of the cyclic ether core of the marine natural product (+)-laurenyne has been accomplished using ring-closing metathesis for medium-ring construction.  相似文献   
152.
The directed three-dimensional self-assembly of microstructures and nanostructures through the selective hybridization of DNA is the focus of great interest toward the fabrication of new materials. Single-stranded DNA is covalently attached to polystyrene latex microspheres. Single-stranded DNA can function as a sequence-selective Velcro by only bonding to another strand of DNA that has a complementary sequence. The attachment of the DNA increases the charge stabilization of the microspheres and allows controllable aggregation of microspheres by hybridization of complementary DNA sequences. In a mixture of microspheres derivatized with different sequences of DNA, microspheres with complementary DNA form aggregates, while microspheres with noncomplementary sequences remain suspended. The process is reversible by heating, with a characteristic "aggregate dissociation temperature" that is predictably dependent on salt concentration, and the evolution of aggregate dissociation with temperature is observed with optical microscopy.  相似文献   
153.
Bell KE  Kelly HC 《Inorganic chemistry》1996,35(25):7225-7228
The rate of reduction of nitrite by trimethylamine-borane was followed by observing the decrease in nitrite absorbance under pseudo-first-order conditions. The reaction is acid-catalyzed and exhibits a first-order dependence on both amine-borane and total nitrite concentration. The molar equivalence of NaNO(2) to (CH(3))(3)NBH(3) = 2:1. Equimolar amounts of hydrogen and nitrous oxide are formed, and the molar ratio of nitrite reacted to N(2)O produced is 2:1. In concentrated HCl or H(2)SO(4), a correlation of rate with the Hammett acidity function, h(o), is observed. The reaction is subject to a pronounced inversesolvent isotope effect (k(D)()2(O)/k(H)()2(O) approximately 2.7) and a modest normal substrate effect (k((CH)()3())()3(N.BH)()3/k((CH)()3())()3(N.BD)()3 approximately 1.4). The reaction is first-order in H(3)O(+) in the region pH 0.7-2.7, but a second-order dependence is observed above pH 4 with the transition occurring at pH approximately pK(a) for HNO(2). Results are consistent with a mechanistic model involving preequilibration protonation of molecular nitrous acid followed by rate-limiting hydride attack on H(2)ONO(+) or free NO(+) to produce nitrosyl hydride as a reactive intermediate.  相似文献   
154.
    
The synthesis and characterisation of five newβ-aminodiketones is reported: (a) 3,3,5,8,10,10-hexamethyl-5,8-diazadodecane-2,11-dione; (b) 3,3,11,11-tetramethyl-5,8-diazatridecane-2,12-dione; (c) 3,3,12,12-tetramethyl-5,10-diazatetradecane-2,13-dione; (d) 3,3,10,10-tetramethyl-5,8-diphenyl-5,8-diazadodecane-2,11,dione and (e) 6,7-benzo-3,3,10,10-tetramethyl-5,8-diazadodecane-2,11-dione. These compounds are discussed as potential precursors in the synthesis of tetra-aza macrocyclic complexes.  相似文献   
155.
The development of an effective chiral auxiliary for hydroxyalkyl radicals is delineated. Both the 2-tetrahydropyranyl (THP) and tri-O-benzyl-2-deoxy-alpha-D-glucopyranosyl (GLU) auxiliaries resulted in diastereoselective radical additions to methyl acrylate at -78 degrees C (ds = 6/1 and 11/1, respectively). The developing stereochemistry at the radical center was completely under auxiliary control. Correlation experiments showed that the D-GLU auxiliary led to attack on the radical Si-face. The selectivity of these radical additions dropped-off considerably when the more reactive 2-nitropropene trap was employed. Computational studies suggested that the observed facial selectivity was due primarily to entropic factors in the transition state but that a smaller temperature-dependent enthalpic contribution was also involved. It was hypothesized that incorporation of a quaternary center at C-6 (THP numbering) would restore the facial selectivity with more reactive radical traps by restricting the orientations available to the incoming alkene. In the event, the trans-6-tert-butyltetrahydropyranyl (tBu-THP) auxiliary resulted in very good diastereoselection with 2-nitropropene (ds = 35/1 at -78 degrees C, 15/1 at 0 degrees C, and 8/1 at RT) as did the tri-O-benzyl-6,6-dimethyl-2-alpha-D-deoxyglucopyranosyl (diMe-GLU) auxiliary during additions to ethyl alpha-trifluoroacetoxyacrylate (ds = 10/1 at 0 degrees C). A protocol for recovery of the sugar-derived chiral auxiliaries was also established. This work sets the stage for the development of a novel approach to 1, 3, 5.(2n + 1) polyols based on iterative radical homologation as well as the application of these pyranosidic auxiliaries to other synthetically important reactions.  相似文献   
156.
The flavanone glycoside naringin hydrate is widely abundant in various citrus plants. As an ongoing effort toward the exploitation of natural products as scaffolds for chemical diversification at readily accessible positions, we have prepared a series of analogues of naringin in which the 6-hydroxyl group of the beta-d-glucopyranosyl subunit was converted to sulfonamides, amides, urethanes, and secondary and tertiary amines via the corresponding 6-amino derivative using a solution-phase parallel array protocol.  相似文献   
157.
A quantitative study has been made of the solvent effects on the fluorescence properties of 1- and 3-methyl indole, with the aim of further understanding the origin of the unusually large Stokes shift in polar solvents. For the derivatives considered here the fluorescence transition probability is decreased in solvents of moderate and high polarities, and the spectrum shifts to the red. The data (in two-component, solute and solvent, systems) can be interpreted on the basis of the stabilisation, by solvent-solute relaxation, of a state with an increased charge-transfer character, relative to the initially excited state. Å consideration of the decay data for other indole derivatives suggests that this state has its origin in the 1L4 state (S2 in non-polar media). Thus we conclude that the appropriate label of the fluorescent state of many substituted indoles in polar solvents is 1La/CT. This is consistent with the observed solvent, temperature, time and substituent dependence of the decay kinetics of these derivatives.  相似文献   
158.
The synthesis and characterization of 2-{1-{3,5-bis(1,1-dimethylethyl)-2-{[2,4,8,10-tetrakis(1,1-dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin-6-yl]oxy}phenyl}ethyl}-4,6-bis(1,1-dimethylethyl)phenyl diphenyl phosphite ( 6 ) is described. In the 31P-NMR spectrum (1H-decoupled) of 6 , an unprecedented eight-bond P,P coupling of J = 72.8 Hz is observed. In the X-ray crystal structure of 6 , an intramolecular P–P distance of 3.67 Å is found, which is within the sum of the van-der-Waals radii of the P-atoms. The observed intramolecular P–P distance suggests that a through-space coupling mechanism is operative. The solid-state conformation of 6 is compared to the conformation obtained by semi-empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid-state structure is near a true energy minimum, but that crystal-packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal-packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6 . The proximity of the P-atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable-temperature 31P-NMR spectroscopy. Semi-empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O-atoms in a common plane, with the P-atom lying above this plane.  相似文献   
159.
Gaseous di-t-butylamine, NHBut2, has been studied by electron diffraction at a nozzle temperature of ca. 293 K. The diffraction data reveal that this amine is exceedingly hindered. Repulsive steric interactions between the But groups are mainly relieved by an opening of the CNC valence angle to 135(3)°. Evidence of steric strain may also be found in the non-zero tilts (2–3°) of the But groups, defining the angle of rotation of these groups about axes through the N-bonded carbon atoms orthogonal to the NC2 plane. The N---C and C---C bonds, 1.467(13) and 1.561(6) Å, respectively, are on the other hand relatively unstrained. Other principal geometrical parameters are unexceptional: C---H = 1.103(10) Å, NCC = 111.3(1.5)° (mean value) and CCH = 106(2)°. The position of the N-bonded hydrogen atom relative to the NC2 plane could not be determined. The torsional positions of the But groups, with respect to the N---C bonds, could not be derived from least-squares refinements. A large number of models with But groups in various fixed positions were considered; the best fit between the observed and theoretical intensity data was obtained with one But group being twisted 19° away from the position typified by one C---C bond of this group being anti to the remote N---C bond, and the other But group similarly twisted by 30°. When viewed along the N---C bonds, the But groups are twisted in the same direction. Viewed along the C(N)C axis, these groups come close to being staggered with respect to each other. The values of the above bond distances are those of ra parameters. Parenthesized values denote error limits, being least-squares standard deviations multiplied by a factor of three.  相似文献   
160.
The principle of an assay of the major thyroid hormones by an electrochemical technique is demonstrated. The separation of 3,3',5-triiodothyronine, 3,3',5'-triiodothyronine, and thyroxine, by reverse-phase high-performance liquid chromatography is followed by their electrochemical oxidation in a thin-layer electrochemical detection cell with a low-temperature isotropic carbon working electrode. The limits of detection found were in the subnanogram range with linear response in the ranges 0–125 ng for T3 and 0–500 ng for T4. The approach makes the simultaneous assay of total serum thyroid hormones feasible.  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