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   27184篇
  免费   862篇
  国内免费   157篇
化学   18213篇
晶体学   255篇
力学   926篇
综合类   1篇
数学   2347篇
物理学   6461篇
  2023年   154篇
  2022年   410篇
  2021年   493篇
  2020年   432篇
  2019年   448篇
  2018年   333篇
  2017年   317篇
  2016年   727篇
  2015年   631篇
  2014年   793篇
  2013年   1489篇
  2012年   1947篇
  2011年   2135篇
  2010年   1295篇
  2009年   1143篇
  2008年   1741篇
  2007年   1641篇
  2006年   1532篇
  2005年   1428篇
  2004年   1262篇
  2003年   982篇
  2002年   967篇
  2001年   672篇
  2000年   575篇
  1999年   338篇
  1998年   253篇
  1997年   293篇
  1996年   334篇
  1995年   257篇
  1994年   274篇
  1993年   276篇
  1992年   264篇
  1991年   205篇
  1990年   154篇
  1989年   139篇
  1988年   140篇
  1987年   118篇
  1986年   95篇
  1985年   168篇
  1984年   112篇
  1983年   97篇
  1982年   122篇
  1981年   88篇
  1980年   79篇
  1978年   80篇
  1977年   85篇
  1976年   94篇
  1975年   101篇
  1974年   79篇
  1973年   102篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
91.
Asymmetric induction was achieved in the photoaddition of 3-methyl-2-cyclohexenone to optically active enoates 1a–c and the absolute configuration of the major adduct was determined.  相似文献   
92.
Orbital correlation diagrams for the black phosphorus to A7 (arsenic) phase transformation for various points in the Brillouin zone are presented and analyzed in symmetry terms. The level doublings observed at the black phosphorus geometry for several points are directly attributable to the nonsymmorphic nature of the space group (Herring's theorem). Doublings observed at the A7 structure derive from the choice of a four-atom, rather than two-atom, unit cell to view the geometrical transformation. The space group requirements at the beginning and end points of the transformation process simply control the form of orbital correlation diagrams. In general, HOMOLUMO crossings are found at k = (12,α,β) where bonds are made and broken along the x direction, a result with simple analogies to more easily visualized one-dimensional problems.  相似文献   
93.
The origin of the [M–69]+ and [M–111]+ signals in the mass spectrum of taraxasterol was studied through the use of C(18), (19), (21), (22) and/or (30) deuteriated derivatives. The generality of these signals for ring systems with an exocyclic methylene group and a methyl moiety on an adjacent carbon was verified with 2-methylmethylenecyclohexane, 1-methyl-2-methylene-trans-decalin, 1,10-dimethyl-2-methylene-trans-decalin and some of their deuteriated derivatives. The most plausible mechanism for the formation of the [M–69]+ ion appears to involve cleavage of both bonds allylic to the exocyclic methylene group with a 1,3-hydrogen transfer from the adjacent ring. Genesis of the [M–111]+ ion is more complicated but a five-membered allylic ion generated from ring D is proposed.  相似文献   
94.
The catalyst reactivity and reusability for the lanthanide triflate-catalyzed three component synthesis of alpha-amino phosphonates have been examined in room temperature ionic liquids, [bmim][X], in which the catalytic activities were very dependent on the counter anion, X, as well as on the phosphorus nucleophile, and moreover, the catalyst immobilized in an ionic liquid was reused several times without any loss of activity.  相似文献   
95.
Kendomycin, also known as (-)-TAN 2162, is a novel polyketide-derived ansamycin isolated from Streptomyces sp., which exhibits potent antagonist and agonist activities at the endothelin and calcitonin receptors, respectively. This bacterial metabolite also possesses a strong antibiotic activity against a range of gram-positive and -negative bacteria and cytostatic effects on the growth of human cancer cell lines. When a novel macroglycosidation reaction is employed as the key step, the first enantioselective total synthesis of kendomycin has been accomplished. A Friedel-Crafts-type ring closure of the acyclic precursor containing tetrahydropyran and benzofuran moieties produces the macrocycle as a single stereoisomer in good yield, thus establishing the aryl C-glycosidic linkage of the ansa core. This reaction requires a phenolic glycosyl acceptor and appears to proceed through a rapid O-glycosidation followed by a slow rearrangement to an aryl C-glycoside. The requisite secomacrocycle is prepared by the Pd(0)-catalyzed B-alkyl Suzuki-Miyaura cross-coupling of two subunits, which in turn can be expeditiously assembled from readily available building blocks in a modular fashion.  相似文献   
96.
Summary Clenbuterol has been determined in urine by solidphase extraction on a C18 cartridge, diazotization of the eluate with nitrite, coupling of the diazonium ion with 1-(naphthyl)ethylenediamine, and separation of the azo dye formed by HPLC with a C18 column and a micellar mobile phase containing 0.1 M sodium dodecyl sulphate, 12%n-butanol and 0.05 M citrate buffer, pH 3. Recoveries higher than 90% were obtained by mixing the samples with a 20% 0.2 M NaOH before extraction. Limits of detection of 51 and 6.7 ng L−1 were obtained with spectrophotometric and thermal lens spectrometric detection, respectively; respective repeatabilities were 3.1% (5 μg mL−1) and 5.6% (0.16 μg mL−1).  相似文献   
97.
Acyl vinyl phosphonium salts react with amidines to form imidazolyl phosphonium salts. These imidazolyl salts can be readily converted to multifunctional imidazoles with quantitative recovery of triphenyl phosphine.  相似文献   
98.
Chlorpromazine hydrochloride is proposed as an indicator for the microtitration of iron(II), arsenic(III), ascorbic acid and hydroquinone with 0.0005–0.001 N ceric sulphate; the indicator blanks are small. The colorimetric déterminations of μg quantities of cerium(lV) and arsenic(III) using the same reagent are also described.  相似文献   
99.
Sang-Hyuep Lee  Kim D Janda 《Tetrahedron》2004,60(15):3439-3443
The conversion of carbamates into ureas using aluminum amide complexes is reported. This reaction is a convenient method to prepare bi-, tri- and tetra-substituted ureas from carbamate-protected primary or secondary amines by reaction with primary or secondary amines in the presence of stoichometric quantities of trimethylaluminum. A reactivity trend of the various carbamates was observed and methyl and benzyl carbamates were reacted selectively in the presence of t-butyl carbamates.  相似文献   
100.
Lee C  Kim NB  Lee IC  Chung KS 《Talanta》1977,24(4):241-245
A Chelex-100 resin column has been employed for the preliminary concentration of trace elements in water samples before their determination by neutron-activation analysis. The column, filled with a 1:1 mixture of the resin (50-100 mesh) and Pyrex glass powder of the same mesh-size, is shown to maintain a constant flow-rate and give reproducible results. By a combination of preconcentration and neutron-activation analysis it is possible to determine Ba, Ca, Cd, Ce, Co, Cr, Cu, Fe, La, Mg, Mn, Sc, U, V and Zn in sea-water and/or fresh water simultaneously at the parts per milliard level.  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