全文获取类型
收费全文 | 1946篇 |
免费 | 67篇 |
国内免费 | 13篇 |
专业分类
化学 | 1316篇 |
晶体学 | 54篇 |
力学 | 48篇 |
数学 | 233篇 |
物理学 | 375篇 |
出版年
2023年 | 12篇 |
2022年 | 46篇 |
2021年 | 48篇 |
2020年 | 48篇 |
2019年 | 38篇 |
2018年 | 40篇 |
2017年 | 47篇 |
2016年 | 67篇 |
2015年 | 42篇 |
2014年 | 62篇 |
2013年 | 137篇 |
2012年 | 110篇 |
2011年 | 122篇 |
2010年 | 77篇 |
2009年 | 60篇 |
2008年 | 83篇 |
2007年 | 93篇 |
2006年 | 97篇 |
2005年 | 82篇 |
2004年 | 80篇 |
2003年 | 59篇 |
2002年 | 49篇 |
2001年 | 20篇 |
2000年 | 21篇 |
1999年 | 19篇 |
1998年 | 13篇 |
1997年 | 11篇 |
1996年 | 19篇 |
1995年 | 13篇 |
1994年 | 15篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 14篇 |
1990年 | 19篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 18篇 |
1985年 | 17篇 |
1984年 | 26篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 14篇 |
1980年 | 27篇 |
1979年 | 20篇 |
1978年 | 18篇 |
1977年 | 17篇 |
1976年 | 18篇 |
1974年 | 8篇 |
1971年 | 7篇 |
排序方式: 共有2026条查询结果,搜索用时 234 毫秒
51.
Won Sick Woo Sam Sik Kang Otto Seligmann V. Mohan Chari Hildebert Wagner 《Tetrahedron letters》1980,21(44):4255-4258
From the seeds of Phytolacca americana two new lignans, Americanin B and D, have been isolated and their structures elucidated mainly by spectroscopic methods 相似文献
52.
Venkatapuram Padmavathi Pinnu Thriveni Boggu Jagan Mohan Reddy Adivireddy Padmaja 《Journal of heterocyclic chemistry》2005,42(1):113-116
A new class of five membered heterocycles, thiadiazoles, triazoles and oxadiazoles were prepared from sulfonyl acetic acids via acid hydrazides. 相似文献
53.
T. L. Luke H. Mohan V. M. Manoj P. Manoj J. P. Mittal C. T. Aravindakumar 《Research on Chemical Intermediates》2003,29(4):379-391
Reactions of sulphate radical anion (SO·4
-) with 4,6-dihydroxy-2-methyl pyrimidine (DHMP), 2,4-dimethyl-6-hydroxy pyrimidine (DMHP), 6-methyl uracil (MU) and 5,6-dimethyl uracil (DMU) have been studied by pulse radiolysis at pH 3 and at pH 10. The transient intermediate spectra were compared with those from the reaction of hydroxyl radical (·OH). It is proposed that SO·4
- produces radical cations of these pyrimidines in the initial stage. These radical cations are short-lived except in the case of DMHP where a relatively longer lived radical cation is proposed to be formed. When there is a hydrogen atom attached to the N(1) or N(3) position, a deprotonation from these sites is highly favored. When there is no hydrogen attached to these sites, deprotonation from a substituted methyl group is favored. At acidic pH, deprotonation from nitrogen is observed for DHMP, MU and DMU. At basic pH, the radical cation reacts with OH- leading to the formation of OH adducts. 相似文献
54.
[reaction: see text] The 1,4-dipole derived from isoquinoline and DMAD has been shown to react readily with N-tosylimines resulting in the diastereoselective synthesis of 2H-pyrimido[2,1-a]isoquinoline derivatives. 相似文献
55.
Mandal S Das A Srivastava R Sastry M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(6):2408-2413
Development of simple and reliable protocols for the synthesis of organically soluble catalytically active metal nanoparticles is an important aspect of research in nanomaterials. We demonstrate herein the formation of Pd nanoparticles by reduction of aqueous Pd(NO(3))(2) by photoexcited Keggin ions (phosphotungstate anions). This results in the formation of Pd nanoparticles capped with with Keggin ions that render the particles negatively charged. The Keggin ion capped Pd nanoparticles may then be phase transferred into nonpolar organic solvents such as toluene by electrostatic complexation with cationic surfactants such as octadecylamine at the liquid-liquid interface. This results in a new class of catalyst wherein both the Pd core and Keggin ion shell may be used in a range of catalytic reactions leading to a truly multifunctional catalyst dispersible in organic solvents. 相似文献
56.
R. Krishna Mohan Rao Kallury Pramod V. Upadhyaya Tumbalam G. Surendra Nath Vankipuram R. Srinivasan 《Journal of mass spectrometry : JMS》1977,12(5):307-312
The mass spectra of eight 1,2,4-triazole derivaties have been recorded and found tao reveal extensive hydrogen and skeletal migrations. The structures of the fragments have been confirmed by deuterium labelling and exact mass measurement. The compounds revealed striking differences in their spectra depending on the nature of the substituents. 相似文献
57.
K. P. Madhusudanan Mohan Prasad Shri Nivas Rastogi D. Fraisse 《Journal of mass spectrometry : JMS》1985,20(11):680-684
The nucleophilic substitution reaction under NH3 chemical ionization (CI) conditions in cis- and trans-1,2-dihydroxybenzosuberans (1–4) has been studied with the help of ND3 CI and metastable data. The results indicate that in the parent diols 1 (cis) and 2 (trans), the substitution ion [MsH]+, is produced mainly by the loss of H2O from the [MNH4]+ ion (SNi reaction) while in their 7-methoxy derivatives 3 and 4, the ion-molecule reaction between [M? OH]+ and NH3 seems to be the major pathway for the formation of [MsH]+. The substitution ion from 1 and 2 and the [MH]+ ion from trans-1-amino-2-hydroxybenzosuberan give similar collision-induced dissociation mass-analysed ion kinetic energy spectra. Interestingly, their diacetates do not undergo the substitution reaction. 相似文献
58.
Sanyal A Rautaray D Bansal V Ahmad A Sastry M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7220-7224
We show here that reaction of the fungus, Fusarium oxysporum, with the aqueous heavy-metal ions Pb2+ and Cd2+ results in the one-step formation of the corresponding metal carbonates. The metal carbonates are formed by reaction of the heavy-metal ions with CO2 produced by the fungus during metabolism and thus provide a completely biological method for production of crystals of metal carbonates. The PbCO3 and CdCO3 crystals thus produced have interesting morphologies that are shown to arise because of interaction of the growing crystals with specific proteins secreted by the fungus during reaction. An additional advantage of this approach is that the reaction leads to detoxification of the aqueous solution and could have immense potential for bioremediation of heavy metals. Under conditions of this study, the metal ions are not toxic to the fungus, which readily grows after exposure to the metal ions. 相似文献
59.
A new and convenient synthesis of 2-isopropylfuro-(2,3-b)quinolines () from 3-(3-methylbut-1-enyl)-2-quinolones () is described. A neat synthesis of the alkaloids (±)-lunacrine (), (±)-lunasine () and (±)-demethoxylunacrine () is also reported. 相似文献
60.
Ulrich Vgeli Wolfgang von Philipsborn Kuppuswamy Nagarajan Mohan D. Nair 《Helvetica chimica acta》1978,61(2):607-617
Heterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by 13C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C,H-spin coupling is again demonstrated in the present series. 相似文献