全文获取类型
收费全文 | 346篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
化学 | 204篇 |
晶体学 | 9篇 |
力学 | 8篇 |
数学 | 12篇 |
物理学 | 118篇 |
出版年
2024年 | 2篇 |
2021年 | 4篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 7篇 |
2016年 | 5篇 |
2015年 | 6篇 |
2014年 | 9篇 |
2013年 | 29篇 |
2012年 | 14篇 |
2011年 | 23篇 |
2010年 | 2篇 |
2009年 | 14篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 17篇 |
2005年 | 17篇 |
2004年 | 18篇 |
2003年 | 10篇 |
2002年 | 16篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 8篇 |
1998年 | 4篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 12篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 4篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1977年 | 4篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 4篇 |
1968年 | 3篇 |
1959年 | 2篇 |
1956年 | 1篇 |
1935年 | 1篇 |
1934年 | 3篇 |
1933年 | 1篇 |
1932年 | 1篇 |
排序方式: 共有351条查询结果,搜索用时 15 毫秒
61.
Vishal A. Mahajan 《Tetrahedron letters》2005,46(6):1009-1012
A synthesis of novel 4-(substituted)benzyl-5-methylene-2(5H)-furanones involving Stobbe condensation of substituted aldehydes with ethyl levulinate followed by treatment with acetic anhydride in the presence of sodium acetate, has been developed. 相似文献
62.
The stability constants of the fluoride complexes of cerium(IV) in 1 M (HClO(4), NaClO(4)) medium have been measured potentiometrically using a fluoride ion-selective electrode. Quantitative oxidation of cerium to its tetravalent state and its stabilisation in the perchlorate medium were accomplished by oxidation with AgO followed by quick addition of a known amount of fluoride ion. This procedure ensures stability of the oxidation state and prevents hydrolysis and polymerisation of Ce(IV). Logarithms of the average values of beta(1), beta(2), beta(3) and beta(4) were estimated to be 7.57+/-0.04, 14.50+/-0.03, 20.13+/-0.37 and 24.14+/-0.10 respectively. 相似文献
63.
Chemoselective hydrogenation of substituted nitroaromatics using novel water-soluble iron complex catalysts 总被引:2,自引:0,他引:2
Deshpande RM Mahajan AN Diwakar MM Ozarde PS Chaudhari RV 《The Journal of organic chemistry》2004,69(14):4835-4838
Chemoselective hydrogenation of substituted nitroaromatic compounds by water-soluble iron complex catalysts with molecular hydrogen has been reported for the first time. This biphasic catalyst presents an opportunity for a solvent-free hydrogenation. This catalyst system provides a low-cost, efficient alternative to the selective but environmentally unacceptable stoichiometric reductions as well as the supported noble metal catalysts used for hydrogenation. An efficient recycling strategy has resulted in a cumulative turnover number above 6000. 相似文献
64.
Determination of fluoride by spectrophotometry of Arsenazo(III) complex of the back-extracted Th(IV)
R. K. Rastogi M. A. Mahajan N. K. Chaudhuri 《Journal of Radioanalytical and Nuclear Chemistry》1994,188(4):291-298
A method based on the back-extraction of Th(IV) from its TTA complex in benzene by aqueous F– followed by spectrophotometric measurement of Th(IV), for the determination of fluoride has been developed. The coefficients of variation obtained are 2.4% and 1.4% in 11 determinations at F– concentration levels of 1.0 g ml–1, respectively. 相似文献
65.
Penicillin acylase ofE. coli NCIM 2400 has been purified to homogeneity using a combination of hydrophobic interaction chromatography and DEAE-cellulose
treatment. A variety of substituted matrices were synthesized using D- or DL-phenylglycine, norleucine, ampicillin, or amoxycillin
as ligands, all of which retained penicillin acylase at high concentrations of ammonium sulfate or sodium sulfate. The enzyme
could be eluted nonbiospecifically by buffer of lower ionic strength with over 95% recovery of the activity. Ammonium chloride,
ammonium nitrate, sodium chloride, sodium nitrate, and potassium chloride were ineffective in either adsorption or elution
of the enzyme on these columns. Further purification of this partially pure enzyme with DEAE-cellulose at pH 7.0–7.2 yielded
an enzyme preparation of very high purity according to electrophoretic and ultracentrifugal analyses, its specific activity
being as high as 37 U/mg protein. The purifiedf enzyme has a molecular weight of 67,000 a sedimentation coefficient of 4.0S,
and resolves into two forms upon isoelectric focusing. Overall recoveries ranged between 75 and 85%. Ease of operation, high
recoveries, high purity of the enzyme and prolonged reuse of the conjugates make the process economically feasible and possibly
of great commercial importance. 相似文献
66.
Renu Mahajan Himani Nandedkar Sheeba Daniel 《Molecular Crystals and Liquid Crystals》2013,570(1):697-707
Abstract The present work deals with synthesis of two homologous series possessing ester and amide linkage and a large lateral aryl substituent. The large lateral group is attached to the main molecule through an amide linkage. Mesogenic amides are rare even when the amide linkage is present in the main molecular framework. However, in the present work the compounds containing amide linkage in lateral moiety also exhibit mesophases of high thermal stability. 相似文献
67.
Mandeep Singh Bakshi Poonam Bhandari Shweta Sachar Rakesh Kumar Mahajan 《Colloid and polymer science》2006,284(12):1363-1370
This is the first report on the mixed micelles of binary triblock polymer (TBP) mixtures. The steady-state fluorescence and viscosity measurements have been carried out for the various binary combinations of TBP (i.e., P103 + F127/P84/L64/P104/P123) in pure water at 30 °C. All the TBP components selected for the study show clear micelle formation process. The pyrene fluorescence has been used to determine the critical micelle concentration. As micelle-forming TBP can also be termed as nonionic surfactants; therefore, their mixed micelle formation has been evaluated by applying the regular solution theory. It has been observed that this theory very well predicts the nature of mixed micelles. The P103 + F127/P84/L64 binary mixtures undergo mixed micelle formation due to the synergistic interactions while P103 + P104/P123 show antagonistic behavior. The results clearly show that the mixtures with greater number of poly(ethylene oxide) units undergo favorable mixing. 相似文献
68.
The cloud point (CP) studies on aqueous solutions of two ethylene oxide-propylene oxide triblock copolymers (EO)2.5(PO)31(EO)2.5 and (EO)13(PO)30(EO)13 with varying number of ethylene oxide (EO) units were carried out in the presence of series of additives, such as alkali,
acids, ionic surfactants, alcohols, salts, and hydrotropes. The results of this study show that sodium hydroxide decreases
the CP of the two copolymers. Acids increase the CP in the order hydrochloric acid > acetic acid > formic acid for both the
triblock copolymers. Hydrotropes increase the CP, whereas salts decrease or increase the CP based on their salting-out/salting-in
nature. Alcohols, which are polar organic additives, affect the CP of the two copolymers differently. The change in the CP
of the triblock copolymers depends upon the structure and concentration of the additives and on the number of EO units of
the two triblock copolymers. 相似文献
69.
Parameterizing above Guaranteed Values: MaxSat and MaxCut 总被引:1,自引:0,他引:1
Meena Mahajan Venkatesh Raman 《Journal of Algorithms in Cognition, Informatics and Logic》1999,31(2):335
In this paper we investigate the parameterized complexity of the problems MaxSat and MaxCut using the framework developed by Downey and Fellows. LetGbe an arbitrary graph havingnvertices andmedges, and letfbe an arbitrary CNF formula withmclauses onnvariables. We improve Cai and Chen'sO(22ckcm) time algorithm for determining if at leastkclauses of ac-CNF formulafcan be satisfied; our algorithm runs inO(|f| + k2φk) time for arbitrary formulae and inO(cm + ckφk) time forc-CNF formulae, where φ is the golden ratio
. We also give an algorithm for finding a cut of size at leastk; our algorithm runs inO(m + n + k4k) time. We then argue that the standard parameterization of these problems is unsuitable, because nontrivial situations arise only for large parameter values (k ≥ m/2), in which range the fixed-parameter tractable algorithms are infeasible. A more meaningful question in the parameterized setting is to ask whether m/2 + kclauses can be satisfied, or m/2 + kedges can be placed in a cut. We show that these problems remain fixed-parameter tractable even under this parameterization. Furthermore, for up to logarithmic values of the parameter, our algorithms for these versions also run in polynomial time. 相似文献
70.
The synthesis of chiral (2R) 2,5‐diaryl‐2,3‐dihydropyrano[2,3‐b]quinolin‐4‐ones, was achieved, at ambient temperature, by the reaction of 3‐acetyl‐4‐aryl‐carbostyril and an aldehyde, in the presence of bismuth triflate–L(?)‐proline complex, formed in situ. The products were obtained in 62–78% yield with high enantioselectivity (72–96% ee). J. Heterocyclic Chem., (2011). 相似文献