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   6890篇
  免费   142篇
  国内免费   13篇
化学   4643篇
晶体学   70篇
力学   135篇
数学   666篇
物理学   1422篇
综合类   109篇
  2021年   45篇
  2020年   50篇
  2019年   74篇
  2016年   108篇
  2015年   104篇
  2014年   126篇
  2013年   268篇
  2012年   202篇
  2011年   266篇
  2010年   181篇
  2009年   175篇
  2008年   254篇
  2007年   245篇
  2006年   223篇
  2005年   237篇
  2004年   214篇
  2003年   165篇
  2002年   163篇
  2001年   114篇
  2000年   134篇
  1999年   134篇
  1998年   109篇
  1997年   121篇
  1996年   116篇
  1995年   109篇
  1994年   121篇
  1993年   152篇
  1992年   112篇
  1991年   94篇
  1990年   80篇
  1989年   89篇
  1988年   69篇
  1987年   103篇
  1986年   100篇
  1985年   102篇
  1984年   102篇
  1983年   82篇
  1982年   100篇
  1981年   92篇
  1980年   118篇
  1979年   79篇
  1978年   97篇
  1977年   63篇
  1976年   103篇
  1975年   92篇
  1974年   72篇
  1973年   77篇
  1972年   50篇
  1971年   41篇
  1967年   53篇
排序方式: 共有7045条查询结果,搜索用时 381 毫秒
101.
Rates of cleavage of some picoyl- and (quinolylmethyl)-trimethylsilanes (RSiMe3, where R = PyCH2 or QnCH2SiMe3) have been measured in “90%” aqueous methanolic sodium methoxide at 50°C. Relative reactivities are: 2-PyCH2, 1.0; 3-PyCH2, 0.030; 4-PyCH2, 8.9; 2-QnCH2, 41; 3-QnCH2, 0.161; 4-QnCH2, 37. The rates correlate well with those for base-catalysed hydrogen-exchange in the parent carbon acids RH. Approximate pKa's (based on the scale of ion-pair acidities in CsNHC6H11H2NC6H11, with pKa of 9-phenylfluorene = 18.6) for the carbon acids, RH, can be derived as follows: 2-PyCH3, 29.5; 3-PyCH3, 34; 4-PyCH3, 27; 2-QnCH3, 25; 3-QnCH3, 32; 4-QnCH3, 25.Rates of cleavage of pyridyl- and quinolyl-trimethylsilanes (PySiMe3 and QnSiMe3) by sodium hydroxide in 4 : 1 v/v Me2SO/H2O at 50°C have also been measured; and the relative reactivities are: 2-Py, 1.0; 3-Py, 2.9; 4-Py, 8.4; 2-Qn, 15.9; 3-Qn, 12.7; 4-Qn, 184. The sequence of reactivity differes from that for base-catalysed hydrogen-exchange at the relevant positions of pyridine and quinoline, indicating that the reactivities are not determined in both cases (if in either) solely by the stabilities of the corresponding carbanions.  相似文献   
102.
In many technical processes, complex multicomponent mixtures have to be handled, for example, in reaction or separation equipment. High-resolution NMR spectroscopy is an excellent tool to study these mixtures and gain insight in their behavior in the processes. For on-line studies under process conditions, flow NMR probes can be used in a wide range of temperature and pressure. A major challenge in engineering applications of NMR spectroscopy is the need for quantitative evaluation. Flow rates, recovery times, and other parameters of the on-line NMR experiments have to be optimized for this purpose. Since it is generally prohibitive to use deuterated solvents in engineering applications, suitable techniques for field homogenization and solvent signal suppression are needed. Two examples for the application of on-line NMR spectroscopic experiments in process engineering are presented, studies on chemical equilibria and reaction kinetics of the technically important system formaldehyde-water-methanol and investigations on reactive gas absorption of CO(2) in aqueous solutions of monoethanolamine.  相似文献   
103.
Enol-esters 1a-1e undergo clean Photo-Fries-rearrangements without side reactions. With anthroyl derivatives the reaction is observed only at 254 nm, not at 366 nm.  相似文献   
104.
The title compound, bis(dimethyl sulfoxide)triiodo­thallium(III), [TlI3(C2H6OS)2], was crystallized from equimolar amounts of TlII and I2 in a dimethyl sulfoxide (DMSO) solution. After the initial redox reaction, the thallium(III)–iodo complex forms and precipitates as a DMSO solvate. In the crystal structure, Tl is surrounded by three iodide ligands in the equatorial plane and two O‐coordinated DMSO mol­ecules in the axial positions, forming a slightly distorted trigonal bipyramid. The complex lies on a twofold rotation axis, making the DMSO mol­ecules and two of the I atoms crystallographically equivalent.  相似文献   
105.
The potential of the earlier reported sandwich-immunoassay principle for the detection of nitroaromatic residues bound to humic acids was examined. A synthetic conjugate derived by coupling a 2,4,6-trinitrotoluene derivative to humic acid was used as a model substance for bound nitroaromatic residues. Spectrophotometric determination gave a content of 14 ± 1.6 μmol/g TNT-derivative in the synthetic conjugate, which was used as a calibration standard. Extensive blocking optimization was necessary to establish the sandwich-immunoassay. Experiments were carried out to check the selectivity of the developed test system. Trace amounts of 2,4,6-trinitrotoluene (25 μg/L) suppressed the assay signal completely by blocking the antibody binding sites. This was a proof for the excellent selectivity of the assay. The potential of quantitative determinations was examined with dilution experiments. During assay optimization strong non-immunological interactions between various proteins and humic acids were observed. This led to a significant improvement of the original sandwich-immunoassay, where the humic acid antibody was substituted by basic proteins. More sensitive calibration curves with higher signal intensities were achieved. This new immunoassay seems to be the method of choice for further immunological investigations of bound residues.  相似文献   
106.
1,3-Dipolar Addition of 2-Benzonitrilio-2-propanid to 7-Methylthieno[2,3-c]pyridine 1,1-Dioxide and Subsequent Reactions The addition of dipole 2 , generated photochemically from 2,2-dimethyl-3-phenyl-2H-azirine ( 1 ), to 7-methylthieno[2,3-c]pyridine 1,1-dioxide yields the pyrroline derivative 4 as a major product and regioisomer 5 in low yield. Compound 4 can be transformed into the pyrrolidine derivative 11 by ring opening, loss of SO2 and hydrogenation. Bromopyrroline derivative 14 gives either by dehydrohalogenation compound 18 or, by substitution, nitrile 17 or ethoxy derivative 19 . Substitution of 14 and ring opening yields methoxypyrrole derivative 20 , which gives access to the unstable hydroxypyrrole and hydroxypyrrolidine derivative 28 resp. 30 . The vinylsulfone 18 is the starting material for addition-ring-cleavage reactions. Oxidation of pyrroline derivative 4 gives epoxy-substituted N-oxide 39 and di-N-oxide 40 ; and oxidative transformation of pyrrolidine derivative 11 yields the (hydroxymethyl)pyridylpyrrolidine derivative 45 .  相似文献   
107.
The energy transfer between argon atoms and ozone complexes O3*, excited in the region of the dissociation threshold, is calculated for fixed temperatures (100 K< or =T < or =2500 K) using classical trajectories. The internal energy of ozone is resolved in terms of vibrational and rotational energies. For all temperatures, energy flows from O3* to Ar. The vibrational energy transfer, relative to k(B)T, is very small below 500 K, but gradually increases towards high temperatures. The relative rotational energy transfer, on the other hand, monotonously decreases with T; around 1100 K it falls below the relative vibrational energy transfer. Thermally averaged cross sections for vibrational and rotational energy transfers are also calculated. The implications for the stabilization of ozone complexes in the energy transfer model are discussed.  相似文献   
108.
Zwitterionic titanoxanes {Cp[η5-C5H4B(C6F5)3]Ti}2O (I) and {(η5-iPrC5H4)[η5-1,3-iPrC5H3B(C6F5)3]Ti}2O (II), which contain two positively charged Ti(IV) centres in the molecule, are able to catalyse the ring-opening polymerization of -caprolactone (-CL) in toluene solution and in bulk. The process proceeds with a noticeable rate even at room temperature and accelerates strongly on raising the temperature to 60 °C. The best results have been obtained on carrying out the reaction in bulk. Under these conditions, the use of I as a catalyst (-CL:I = 1000:1) gives at 60 °C close to quantitative yield of poly--CL with the molecular mass of 197 000. An increase in the -CL:I ratio to 6000:1 increases the molecular mass of poly--CL to 530 000. Tetrahydrofuran (THF) is also polymerized under the action of I albeit with a lesser rate. However, the molecular mass of the resulting poly-THF can reach rather big values under optimal conditions (up to 217 000 at 20 °C and the THF:I ratio of 770:1). A rise in the reaction temperature from 20 to 60 °C results here to a decrease in the efficiency of the process. Titanoxane II is close to I in its catalytic activity in the -CL polymerization but it is much less active in the polymerization of THF. Propylene oxide (PO), in contrast to -CL and THF, gives with I only liquid oligomers in wide temperature and PO:I molar ratio ranges (−30 to +20 °C, PO:I = 500–2000:1). γ-Butyrolactone and 1-methyl-2-pyrrolidone are not polymerized under the action of I at room temperature. The reactions found are the first examples of catalysis of the cationic ring-opening polymerization by zwitterionic metallocenes of the group IVB metals.  相似文献   
109.
4-tert-Butyl-2-cyclohexen-1-one dimerizes in THF solution via its kinetic enolate, leading to di-tert-butylditwistane 8 in up to 36% yield (−78 °C → room temp., protonolysis, flash chromatography). X-ray crystallography shows that 8 incorporates one R and one S enantiomer of the starting ketone; none of the diastereomeric ditwistanes epi-8, epi’-8 or iso-8 was isolated. This means that the formation of 8 proceeds with mutual kinetic resolution and 100% induced diastereoselectivity.  相似文献   
110.
The photochemical reactions of different allyl aryl ethers (Scheme 3) were investigated in hydrocarbons (Chap. 3.1) and in alcoholic solvents (Chap. 3.2). The composition of the photoproducts depended very much on the nature of the solvent. Irradiation (3–95 h) of different methyl substituted allyl aryl ethers ( 1, 3, 5, 7 and 11 ) with a low pressure mercury lamp (λEmiss. = 254 nm; 6 or 15 Watt) under argon (quartz vessel) resulted in the formation of 2-, 3– and 4-substituted phenols, dienones and products of consecutive reactions (Tables 1–4 and 6). The results suggested that all products were formed by homolytic cleavage of the C? O bond in the singlet state of the ethers to intermediate radical-geminates (Scheme 5) followed by radical recombination of the two fragments. No products were formed by concerted processes (Table 5, Schemes 5 and 6). Upon irradiation of allyl aryl ethers lacking alkyl substituents at position 4 ( 1 and 5 ) in protic solvents, mainly 2- and 4-allylated phenols were obtained (Tables 1 and 4); 3-allylated phenols were formed only in small amounts (0.02%). However, in aromatic hydrocarbons or cyclohexane 3-allylated phenols were obtained from 1 , 5 and 11 in significant amounts (3–11%; Tables 1, 4 and 6). E.g., upon irradiation of allyl-2,6-dimethyl-2,4-cyclohexadien-1-one ( 6 ) besides 3- and 4-allyl-2, 6-dimethyl-phenol ( 23 and 24 ). Irradiation of 5 in methanol afforded 23 and 6 only in traces, whereas 24 was the main product.  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