全文获取类型
收费全文 | 54774篇 |
免费 | 2975篇 |
国内免费 | 3974篇 |
专业分类
化学 | 41754篇 |
晶体学 | 390篇 |
力学 | 557篇 |
综合类 | 676篇 |
数学 | 7690篇 |
物理学 | 10656篇 |
出版年
2023年 | 458篇 |
2022年 | 1943篇 |
2021年 | 1716篇 |
2020年 | 1150篇 |
2019年 | 1548篇 |
2018年 | 1023篇 |
2017年 | 1334篇 |
2016年 | 1268篇 |
2015年 | 1322篇 |
2014年 | 1835篇 |
2013年 | 3680篇 |
2012年 | 2255篇 |
2011年 | 2473篇 |
2010年 | 2165篇 |
2009年 | 2993篇 |
2008年 | 3301篇 |
2007年 | 3352篇 |
2006年 | 2751篇 |
2005年 | 2045篇 |
2004年 | 1993篇 |
2003年 | 1928篇 |
2002年 | 4155篇 |
2001年 | 1719篇 |
2000年 | 1234篇 |
1999年 | 1011篇 |
1998年 | 934篇 |
1997年 | 689篇 |
1996年 | 765篇 |
1995年 | 695篇 |
1994年 | 681篇 |
1993年 | 670篇 |
1992年 | 688篇 |
1991年 | 423篇 |
1990年 | 368篇 |
1989年 | 309篇 |
1988年 | 335篇 |
1987年 | 242篇 |
1986年 | 238篇 |
1985年 | 343篇 |
1984年 | 270篇 |
1983年 | 164篇 |
1982年 | 312篇 |
1981年 | 495篇 |
1980年 | 445篇 |
1979年 | 485篇 |
1978年 | 384篇 |
1977年 | 285篇 |
1976年 | 247篇 |
1974年 | 109篇 |
1973年 | 196篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
861.
Sucrose ethyleneoxide adducts have been prepared by reaction of sucrose with various amounts of ethyleneoxide inDMSO. The resulting polypode molecules were found to be efficient phase transfer catalysts in nucleophilic substitutions, oxidation and dichlorcarbene generation. Polymerisable polypodes have been obtained by reaction of sucrose ethyleneoxide adducts with methacrylic anhydride or methacryloylchloride in pyridine. Free radical polymerisation of the resulting mixtures of mono- and polyfunctional methacrylic esters of sucrose ethyleneoxide adducts yielded crosslinked gels. These polymer-supported octopus-molecules were found to be efficient triphase catalysts. 相似文献
862.
The metalation of allyl benzyl thioether involves the benzylic or the allylic hydrogens. The benzylic carbanion undergoes a rapid[2,3] sigmatropic shift whereas the allylic carbanion gives rise to various rearrangements, among them migration of the allylic unit to the para position with allylic inversion. The temperature dependence of the ratio of products arising from the benzylic carbanion vs those from the allylic carbanion shows that the allylic-to-benzylic carbanion transformation occurs only under special conditions: (a) with slow addition of the base; (b) with thioether in excess relative to the base, and (c) on raising the temperature of the reaction medium from ?78° to ?15°. In the last instance, the proton transfer is intramolecular as shown with labeled thioethers. The extent of the different rearrangements depends on the temperature and solvent. A choice of mechanism cannot be made at this time for the para migration 5→9a. A leaving group effect on the reaction regioselectivity of the carbanion from allyl methyl thioether with benzyl halides has been noticed. The presence of dibenzyl indicates that, in addition to SN2 reactions, some electron transfer process is occurring. 相似文献
863.
Functionalization of polymers by grafting monomeric species on to the backbone of molecular chains with the use of γ-radiation
has been used extensively. In this work methacrylic acid was grafted onto a commercial grade of polycaproamide (Nylon 6) by
preirradiating the polymer granules to 15 kGy at a rate of 1.0 kGy per hour and subsequently immersing these in a 10% aqueous
solution of methacrylic acid in the presence of small quantities of FeSO4 as homopolymerization inhibitor. The polymer was subsequently neutralized by mixing it with zinc acetylacetonate in a laboratory
scale melt mixing device. The acid-grafting polymer modification resulted in an increase in glass transition temperature,
while the addition of zinc acetylacetonate gave rise to two transitions: The lower transition corresponds to a miscible mixture
of free polyamide and acid-grafted polymer, both plasticized with undecomposed zinc compound, while the upper transition corresponds
to the zinc salt of the acid grafted polyamide. Through rheological measurements it was shown that both the acid-grafted polymer
and the derived zinc salt have a branched structure, possibly containing also some crosslinked domains. Large improvements
in solvent resistance were observed for both type of polymer modifications.
Received: 13 December 1996 Accepted: 10 February 1997 相似文献
864.
Under various basic solutions and reaction durations, 3-substituted 4-formyIsydnones and nitroalkanes containing α-active hydrogen (e.g. nitromethane and nitroethane) are converted to β-nitroalcohols, nitroalkenes and dinitroalkanes. β-Nitroalcohols are obtained only by electrochemical reaction on an electrogenerated base (EGB). The mechanism of overall reaction steps in particular is discussed. 相似文献
865.
新显色剂二溴硝基苯基重氮氨基偶氮苯的合成及与镉的显色反应的研究 总被引:10,自引:2,他引:10
作者合成了新显色剂2,6-二溴-4-硝基苯基重氮氨基偶氮苯(DBNDAA)。在pH8.5的硼砂-盐酸缓冲介质中,在Tween 80-SDBS存在下,该试剂与镉(Ⅱ)生成络合比为3:1的红色络合物,λ_(max)=500nm,ε_(500)=1.52×10~5L·mol~(-1)·cm~(-1)。镉量在0~28μg/25ml范围内遵守比耳定律。并测定了试剂的表观离解常数和络合物的表观不稳定常数。方法有良好的选择性。用该试剂测定了铝合金标样、环境水样及废水中的痕量镉,结果满意。 相似文献
866.
The reaction of trimethylsilyl- and pentafluorophenyl-N-sulfinylamine respectively with 3,5-dihalogeno-1,2,4-trithia-3,5-diborolanes yields the 1,2-dithia-4-aza-3,5-diborolidines1-3.Tert-butyl-N-sulfinylamine and 3,5-dimethyl-1,2,4-trithia-3,5-diborolane react analogous. OtherN-sulfinylamines however split the disulfane bridge in 3,5-dimethyl-1,2,4-trithia-3,5-diborolane and the 1,4-dithia-2-aza-3,5-diborolidines5A-7(A) are formed. Besides of boroxines, cyclo-2,4,6-trimethyl-1,3-dioxa-5-aza-2,4,6-triboranes and cyclo-2,4,6-trimethyl-1-oxa-3,5-diaza-2,4,6-triboranes are formed as byproducts,8–10 have been isolated. In 1,2,4-trithia-3,5-diborolanes and 1,2-dithia-4-aza-3,5-diborolidines the bromo-atoms can be substituted by alkyl (13, 14), by amino (15–20) and by isothiocyanato groups. The compounds were characterised analytically and spectroscopically (MS; NMR:1H,11B,19F,29Si; IR). 相似文献
867.
David Ajó Maurizio Casarin Gaetano Granozzi Ignazio Fragalá 《Chemical physics letters》1981,80(1):188-191
The He(I) photoelectron spectra of 5-azauracil (1,3,5-triazine-2,4(1H,3H)-dione) and 6-azauracil (1,2,4-triazine-3, 5(2H,4H)-dione) are presented. The spectra are assigned by using INDO/S calculations and comparison arguments with uracil. The first ionization energy in the title compounds is higher than in the usual nucleic acid bases. 相似文献
868.
A high-performance thin-layer chromatographic (HPTLC) method is described for the determination of tributyltin compounds (bis(tri-n-butyltin) oxide, TBTO, and tri-n-butyltin naphthenate, TBTN) and their degradation products (dibutyltin and monobutyltin compounds). The organotin compounds are extracted from wood with ethanol containing 0.5% (v/v) of hydrochloric acid and the separation of the defferent kinds of organotin compounds is achieved by thin-layer chromatography. The sample spots are measured using a scanning densitometer after decomposing the organotin compounds to inorganic tin by ultraviolet irradiation and visualization of the spots with pyrocatechol violet. Applications of the method to detection and quantification of organotin compounds in preservative solutions, in recently impregnated wood, and in wood samples from five-year-old window frames are described. 相似文献
869.
Trigonal Planar CuX3-Groups in Cu2Mo6X14, X = Cl, Br, I Cu2Mo6Cl14 (I), Cu2Mo6Br14 (II) and Cu2Mo6I14 (III) were synthesized by thermal treatment of corresponding mixtures of copper(I) and molybdenum(II) halides. The crystal structures were determined by single crystal X-ray analyses. I and II show isotypism, cubic, Pn3 (no. 201, sec. setting), Z = 4, I: a = 12.772(3) Å, II: a = 13.350(2) Å. III shows a new structural type, orthorhombic, Pbca (No. 61), Z = 4, a = 16.058(3) Å, b = 10.643(2) Å, c = 16.963(3) Å. Trigonal planar CuX3 units were found in I? III. Structural behaviour relations are discussed, especially with regard to ionic conductivity. 相似文献
870.
含氮给体结构对丙烯腈电荷转移光聚合的影响 总被引:1,自引:0,他引:1
<正> 对于正性烯类单体,主要是乙烯基咔唑的电荷转移聚合,诚田已作过详细综述。负性烯类单体的电荷转移聚合主要研究的单体是丙烯腈(AN)及甲基丙烯酸甲酯(MMA)。Barton等研究了芳烃为引发剂的MMA光聚合。我们研究了芳胺为引发剂的光聚合。 芳胺,尤其是芳叔胺,是较强的电子给体,与负性单体在光照下经激基复合物而引发聚合,我们的实验表明,吡啶及其同系物(喹啉与吖啶)并不象吡咯及其同系物(吲哚与咔唑)那样有效地引发AN等负性单体的光聚合(见表1)。从图1所示的结构看,它们有 相似文献