全文获取类型
收费全文 | 87853篇 |
免费 | 16635篇 |
国内免费 | 19137篇 |
专业分类
化学 | 33945篇 |
晶体学 | 2276篇 |
力学 | 3902篇 |
综合类 | 2052篇 |
数学 | 7863篇 |
物理学 | 25103篇 |
无线电 | 48484篇 |
出版年
2024年 | 497篇 |
2023年 | 1407篇 |
2022年 | 3426篇 |
2021年 | 3220篇 |
2020年 | 2859篇 |
2019年 | 2545篇 |
2018年 | 2511篇 |
2017年 | 3782篇 |
2016年 | 2772篇 |
2015年 | 4396篇 |
2014年 | 5329篇 |
2013年 | 6603篇 |
2012年 | 7165篇 |
2011年 | 7269篇 |
2010年 | 7475篇 |
2009年 | 7701篇 |
2008年 | 8139篇 |
2007年 | 7805篇 |
2006年 | 7446篇 |
2005年 | 6228篇 |
2004年 | 4864篇 |
2003年 | 3403篇 |
2002年 | 3105篇 |
2001年 | 3099篇 |
2000年 | 3046篇 |
1999年 | 1502篇 |
1998年 | 698篇 |
1997年 | 550篇 |
1996年 | 513篇 |
1995年 | 449篇 |
1994年 | 517篇 |
1993年 | 525篇 |
1992年 | 407篇 |
1991年 | 301篇 |
1990年 | 284篇 |
1989年 | 319篇 |
1988年 | 257篇 |
1987年 | 206篇 |
1986年 | 159篇 |
1985年 | 124篇 |
1984年 | 120篇 |
1983年 | 134篇 |
1982年 | 113篇 |
1981年 | 97篇 |
1980年 | 70篇 |
1979年 | 63篇 |
1978年 | 24篇 |
1977年 | 16篇 |
1971年 | 16篇 |
1959年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
DNA works as hereditary material and plays vital role in life science. The study of the binding of small molecules complex to DNA has been the focus of intense investigation, owning to their stereo and sequence specific interaction with the double helix1-… 相似文献
973.
LIU Peng-jun ZHANG Lian-hua SUN Hao CHANG Ying-fei WANG Rong-shun 《高等学校化学研究》2006,22(5):635-638
The reaction mechanisms of HNCS with NH(X^3∑ ) were theoretically investigated. The minimum energy paths (MEP) of the reaction were calculated by using the density functional theory(DFT) at the B3LYP/6-311 + + G^** level. The equilibrium structural parameters, the harmonic vibrational frequencies, the total energies, and the zeropoint energies(ZPE) of all the species were calculated. The single-point energies along the MEP were further refined at the QCISD(T)/6-311 + + G^* * level. It was found that the mechanisms of the HNCS + NH(X^3∑) reaction involve two channels producing the HNC + HNS and the N2H2 + CS products. Channel 1 plays a dominant role and the HNC + HNS are the main preduets. The reaction is exothermie. 相似文献
974.
Addition of one equivalent of LiN(i-Pr)2 or LiN(CH2)5 to carbodiimides, RN=C=NR [R=cyclohexyl (Cy), isopropyl (i-Pr)], generated the corresponding lithium of tetrasubstituted guanidinates {Li[RNC(N R^′2)NR](THF)}2 [R=i-Pr, N R^′2=N(i-Pr)2 (1), N(CH2)5 (2); R=Cy, N R^′2=N(i-Pr)2 (3), N(CH2)5 (4)]. Treatment of ZrCl4 with freshly prepared solutions of their lithium guanidinates provided a series of bis(guanidinate) complexes of Zr with the general formula Zr[RNC(N R^′2)NR]2Cl2 [R=i-Pr, N R^′2=N(i-Pr)2 (5), N(CH2)5 (6); R=Cy, N R^′2=N(i-Pr)2 (7), N(CH2)5 (8)]. Complexes 1, 2, 5-8 were characterized by elemental analysis, IR and ^1H NMR spectra. The molecular structures of complexes 1, 7 and 8 were further determined by X-ray diffraction studies. 相似文献
975.
The cyclodextrins(CDs) are a class of cyclic oligosaccharides made up of six(a), seven(a)or more [a-(1,4)-linked] D-glucopyranose units, and shaped like truncated cones with primary and secondary hydroxyl groups crowning the narrower rim and wider rim respectively. As they have a hydrophobic cavity of appropriate dimension, they can bind with various guest moleculars, such as hydrocarbon, cyclohexane, aromatic compounds, to form inclusion complexes. The cyclodextins inclusion complexation has been considered an ideal model mimicking the enzyme-substrate interaction and a lot of effect has been devoted to it. In this paper, we report our investigation on the inclusion complexation behavior of a-cyclodextrin(a-CD) with diphenyl compounds in order to further explore the molecular recongnition mechanism of 2:1 inclusion complexation of a-CD with aromatic compounds.Figure 1: Possible structures of the inclusion compounds.The inclusion complexation behavior of a-CD with sym-diphenyl-urea, sym-diphenyl-thiourea and diphenyl kotone as respective guest moleclars was studied by ultraviolet spectrometric titrations.The absorption spectral changes observed for the compounds in the absence and presence of a-CD are used to draw the corresponding Benesi-Hildebrand plots and caculate the complex stability constant value (Ks) for the inclusion compounds.The 2:1 inclusion complexations show higher binding constants by cooperative binding of one guest molecular in the closely two hydrophobic cyclodextrin cavites as compared with 1:1 inclusion complexations.The highest value observed for sym-diphenyl-urea could be due to the formation of a hydrogen bond between the carbonyl group and the hydrogen in the hydroxyl group of CD and this is not possible with sym-tiphenyl-thiourea. The lowest value observed for diphenyl kotone indicate the hydrophobic interaction is one of the binding force of cyclodextrin inclusion complex. 相似文献
976.
ZHANG Peng-Fei CHEN Zhen-Chu 《有机化学》2003,23(Z1):87-89
During the last few years alkenyl(phenyl)iodonium salts have risen from mere chemical curiosities to valuable synthetic intermediates. [1] Because of an excellent leaving group ability of a phenyliodonyl moiety, alkenyl(phenyl) iodonium salts undergo nucleophilic vinyl substitutions under mild conditions, thus providing a useful route for the synthesis of various kinds of olefins. Recently, some of functionalized alkenyl iodonium salts have been prepared. [2] 相似文献
977.
The stereoselective synthesis of (2R,4R)-2-N-tert-butyloxycarbonyl amino-4,5-epoxido-valeric acid methyl ester 8,which is the key intermediate for the synthesis of (2′S,2R)-3-trans-nitrocyclopropyl-alanine,was first accomplished. 相似文献
978.
979.
CHEN Kai ZHU Zhong-liang ZHANG De-jun 《高等学校化学研究》2006,22(4):435-438
IntroductionStudies on reversible kinetic systems are consi-dered as a hotspot of chemical and biochemical kineticresearches[1,2]. Of late, some researches have been fo-cused on the simultaneous optimization of the obverseand reverse rate constants[3,4].H… 相似文献
980.
利用酯基锡与缺位Keggin结构杂多阴离子PW9O9-34反应,合成了6种新的杂多阴离子--有机金属配合物M9[(R'OOCCHR"CH2SnOH2)3(PW9O34)2]·xH2O(M=(CH3)4N+,K+;R'=CH3-,CH3CH2-;R"=H,CH3-),通过元素分析、IR光谱、紫外电子光谱、1H
NMR、31P NMP、183w NMR和TGA-DSC热分析等测试手段对标题配合物进行了表征和性质研究,确定该系列配合物为A-β-PW9
型夹心配合物结构. 相似文献