全文获取类型
收费全文 | 7781篇 |
免费 | 196篇 |
国内免费 | 34篇 |
专业分类
化学 | 4412篇 |
晶体学 | 67篇 |
力学 | 248篇 |
数学 | 936篇 |
物理学 | 2348篇 |
出版年
2022年 | 81篇 |
2021年 | 93篇 |
2020年 | 114篇 |
2019年 | 106篇 |
2018年 | 106篇 |
2017年 | 91篇 |
2016年 | 155篇 |
2015年 | 126篇 |
2014年 | 149篇 |
2013年 | 400篇 |
2012年 | 348篇 |
2011年 | 409篇 |
2010年 | 234篇 |
2009年 | 231篇 |
2008年 | 349篇 |
2007年 | 355篇 |
2006年 | 374篇 |
2005年 | 318篇 |
2004年 | 255篇 |
2003年 | 242篇 |
2002年 | 209篇 |
2001年 | 175篇 |
2000年 | 173篇 |
1999年 | 117篇 |
1998年 | 86篇 |
1997年 | 84篇 |
1996年 | 102篇 |
1995年 | 115篇 |
1994年 | 104篇 |
1993年 | 120篇 |
1992年 | 109篇 |
1991年 | 81篇 |
1990年 | 101篇 |
1989年 | 98篇 |
1988年 | 99篇 |
1987年 | 85篇 |
1986年 | 67篇 |
1985年 | 107篇 |
1984年 | 88篇 |
1983年 | 91篇 |
1982年 | 110篇 |
1981年 | 85篇 |
1980年 | 94篇 |
1979年 | 75篇 |
1978年 | 85篇 |
1977年 | 70篇 |
1976年 | 86篇 |
1975年 | 92篇 |
1974年 | 72篇 |
1973年 | 82篇 |
排序方式: 共有8011条查询结果,搜索用时 15 毫秒
121.
Determination of benzydamine and its N-oxide in biological fluids by high-performance liquid chromatography 总被引:1,自引:0,他引:1
A simple, sensitive and selective method for the determination of benzydamine in human plasma and urine, and for benzydamine N-oxide in urine, has been developed using high-performance liquid chromatography in the reversed-phase mode. The limit of reliable determination of benzydamine in plasma was 0.5 ng/ml and that in urine 1 ng/ml; the limit of reliable determination of benzydamine N-oxide in urine was 50 ng/ml. The method has been successfully applied to the analysis of these compounds in biological fluids after administration of intravenous and oral doses of benzydamine to human volunteers. 相似文献
122.
An efficient and practical one-pot method for the synthesis of novel diversified sugar derived dihydro-quinazolino[4,3-b]quinazolin-8-ones has been reported. Various protected sugar hemiacetals were used to synthesize the hybrid tetracyclic ring system. The one-step reductive transformation of 2-(2-nitrophenyl)-3H-quinazolin-4-one with different sugar hemiacetals furnished the desired tetracyclic product in good yields and with high purity. 相似文献
123.
Summary N-(orthovanillidene)L-alanine (OVAlaH2), N-(salicylidene)L-valine (SValH2), N-(orthovanillidene)L-valine (OVValH2), N-(orthovanillidene)L-tyrosine (OVTyrH3) and N-(salicylidene)L-glutamic acid (SGluH3) react with LaIII, PrIII, NdIII and SmIII to yield new chiral complexes. Their dominant conformers were determined from c.d. spectra supported by1H n.m.r. data, except for the L-glutamic acid derived complexes which were insoluble. 相似文献
124.
The short-time nuclear dynamics of Cu(H(2)O) is investigated using femtosecond photodetachment-photoionization spectroscopy and time-dependent quantum wave packet calculations. The Cu(H(2)O) dynamics is initiated in the electronic ground state of the complex by electron photodetachment from the Cu(-)(H(2)O) complex, where hydrogen atoms are oriented toward Cu. Several time-resolved resonant multiphoton ionization schemes are used to probe the ensuing reorientation and dissociation. Immediately following photodetachment, the neutral complex is far from its minimum energy geometry and possesses an internal energy comparable to the Cu-H(2)O dissociation energy and undergoes both large-amplitude H(2)O motion and dissociation. Dissociation is observed to occur on three distinct time scales: 0.6, 8, and 100 ps. These results are compared to the results of time-dependent J=0 wave packet calculations, propagating the initial anion vibrational wave functions on the ground-state potential of the neutral complex. An excellent agreement is obtained between the experimental results and the ionization signals derived from the calculated probability amplitudes. Related experiments and calculations are carried out on the Cu(D(2)O) complex, with results very similar to those of Cu(H(2)O). 相似文献
125.
The regio– and stereoselective synthesis of the keto-esters 20 and 22 are described, the latter being a potential intermediate to several diterpenes. The key steps involve the Diels-Alder cycloaddition between the benzocycloheptenone 5 and cyclopentadiene followed by a regioselective functionalisation of the adduct 6. A remarkable reversal of regioselectivity was observed during oxymercuration of the unsymmetric double bond in 6 and its reduced product 11 leading to 8 and 12 which were subsequently transformed to 20 and 22 respectively. 相似文献
126.
The selenium analogues of anti-thyroid drugs exhibit their anti-thyroid action by a mechanism different from that of MMI. The selenium analogue of MMI and related selenium compounds exhibit high GPx activity, providing a novel method for the reversible inhibition of thyroid hormone biosynthesis. 相似文献
127.
128.
J. C. Salamone P. Taylor B. Snider S. C. Israel 《Journal of polymer science. Part A, Polymer chemistry》1975,13(1):161-170
Copolymerization studies of cationic monomers have been reported in the literature to yield wide variations in reactivity ratios and Q–e values, depending on the comonomer and the nature of the solvent. In this work are presented the copolymerization characteristics of a variety of vinylimidazolium salts in both water and ethanol solution. From these studies, the effect of solvent polarity, of substitution at the imidazolium 2-position, of the type of counterion, and of the hydrophilic–hydrophobic character of the monomeric salts could be ascertained. The results of the study are consistent with other related investigations, in that solvent polarity and comonomer both strongly affected copolymerization. 相似文献
129.
Plieger PG Ehler DS Duran BL Taylor TP John KD Keizer TS McCleskey TM Burrell AK Kampf JW Haase T Rasmussen PG Karr J 《Inorganic chemistry》2005,44(16):5761-5769
The ligand 4,5-dicarboxyimidazole (H(2)DCI) and its methyl derivative 1-methyl-4,5-dicarboxyimidazole (H(2)MDCI) have been shown to bind to Be(II) forming a zwitterionic species that has been structurally characterized. A new dicarboxyimidazole-based polymer has been prepared and its Be-binding properties have been studied using NMR ((1)H and (9)Be) and fluorescence spectroscopy; it represents a rare example of beryllium binding to a polymer. Models of the mononuclear and polymeric Be(II)-binding sites have been studied using density functional theory (DFT), and the (9)Be NMR chemical shifts of these model materials have been calculated for the purpose of direct comparison to experimentally observed values. Differences in the binding modes of the mononuclear and polymeric species are discussed. 相似文献
130.
Alpha-hydroxyketones undergo MnO2-mediated oxidation followed by in situ trapping with aromatic or aliphatic 1,2-diamines to give quinoxalines or dihydropyrazines, respectively, in a one pot procedure which avoids the need to isolate the highly reactive 1,2-dicarbonyl intermediates. Modifications of the procedure allow the formation of pyrazines and piperazines. 相似文献