全文获取类型
收费全文 | 2189篇 |
免费 | 128篇 |
国内免费 | 7篇 |
专业分类
化学 | 1866篇 |
晶体学 | 14篇 |
力学 | 28篇 |
数学 | 232篇 |
物理学 | 184篇 |
出版年
2024年 | 4篇 |
2023年 | 35篇 |
2022年 | 75篇 |
2021年 | 84篇 |
2020年 | 93篇 |
2019年 | 67篇 |
2018年 | 35篇 |
2017年 | 43篇 |
2016年 | 85篇 |
2015年 | 93篇 |
2014年 | 88篇 |
2013年 | 125篇 |
2012年 | 164篇 |
2011年 | 200篇 |
2010年 | 112篇 |
2009年 | 90篇 |
2008年 | 142篇 |
2007年 | 120篇 |
2006年 | 105篇 |
2005年 | 86篇 |
2004年 | 80篇 |
2003年 | 56篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 11篇 |
1999年 | 14篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 15篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1988年 | 10篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 14篇 |
1984年 | 11篇 |
1982年 | 18篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 14篇 |
1975年 | 6篇 |
1974年 | 8篇 |
1973年 | 9篇 |
排序方式: 共有2324条查询结果,搜索用时 15 毫秒
991.
A robust, accurate and sensitive analytical procedure for the determination of Se in plant and peat samples by hydride generation-atomic fluorescence spectrometry (HG-AFS) was developed. Aliquots (200 mg) of dried samples were digested with 3 mL nitric acid and 0.5 mL hydrogen peroxide in closed, pressurized PTFE vessels in a microwave oven at 220 °C. Addition of HBF4 or HF to the digestion mixture was not required because experiments demonstrated that Se was not hosted in the silicate fraction of the investigated sample matrices. Selenium(VI) was directly reduced to Se(IV) in the undiluted digestion solutions after addition of 3.8 mL of 4 M HCl in a microwave oven at 103 °C for 3 min. Other reduction reagents, such as hydroxylamine hydrochloride or urea, were not necessary to cope with potential interferences from nitrogen oxides that could hamper the reliable determination of Se by HG-AFS. Optimum hydride generation of Se was achieved by using 0.9% NaBH4 and 4.5 M HCl. A solution detection limit of 11 ng L−1 was obtained under the optimized experimental conditions which corresponds to a method detection limit of 2.8 ng g−1 in solid peat and plant materials. The precision of replicate measurements was better than 3% at Se concentrations of 50 ng L−1. The analytical procedure was critically evaluated by analysing two certified plant reference materials (SRM 1515 Apple Leaves and SRM 1547 Peach Leaves) as well as three peat reference materials. Excellent agreement between the experimental values ranging from 50 ng g−1 to ∼2 μg g−1 and the certified concentrations was obtained. 相似文献
992.
Julia S. Lock Bruce L. May Philip Clements Stephen F. Lincoln Christopher J. Easton 《Journal of inclusion phenomena and macrocyclic chemistry》2004,50(1-2):13-18
E-4-(2-(4- tert - butylphenyl) ethen-1- yl)benzoate, E-1–, photoisomerizes to the Z-1– isomer and vice versa in the free state and in the binary complexes 2·E-1–, 2·Z-1–, 3·E-1– and 3·Z-1– where 2 is the urea-linked cyclodextrin N-(6
A
-deoxy--cyclodextrin-6
A
- yl)-N-(6
A
-deoxy--cyclodextrin-6
A
- yl)urea and 3 is N,N- bis(6
A
-deoxy--cyclodextrin-6
A
- yl)urea. In 2·E-1–and 3·E-1– the stilbene occupies both cyclodextrin (CD) components of 2 and 3, whereas in 2·Z-1– and 3·Z-1– it only occupies one CD component while the other CD component is unoccupied. 4- tert - Butylphenolate, 4–, and its carboxylate, 5–, and sulfonate, 6–, analogues form the ternary complex 2·Z-1–·4– and its analogues and also 3·Z-1–·4– and its analogues. These photoisomerize to 2·E-1–and 3·E-1– and either free 4–, 57– or 6– and thereby function as molecular devices. 相似文献
993.
Julia Hitzbleck 《Journal of organometallic chemistry》2007,692(21):4702-4707
Half-sandwich dibenzyl complexes of scandium have been prepared by stepwise treatment of scandium trichloride with lithium derivatives of silyl-functionalized tetramethylcyclopentadienes (C5Me4H)SiMe2R (R = Me, Ph) and benzyl magnesium chloride. The resulting complexes [Sc(η5-C5Me4SiMe3)(CH2Ph)2(THF)] and [Sc(η5-C5Me4SiMe2Ph)(CH2Ph)2(1,4-dioxane)] show structure related to that of the corresponding bis(trimethylsilylmethyl) compounds [Sc(η5-C5Me4SiMe2R)(CH2SiMe3)2(THF)]. The four-coordinate complexes display η1-coordinated benzyl ligands without significant interaction of the ipso-carbon of the phenyl moiety. Conversion of [Sc(η5-C5Me4SiMe3)(CH2Ph)2(THF)] into the cationic species by treatment with triphenylborane in THF led to the formation of a stable charge separated complex [Sc(η5-C5Me4SiMe3)(CH2Ph)(THF)x][BPh3(CH2Ph)]. Benzyl cation formed using [Ph3C][B(C6F5)4] in toluene resulted in a moderately active syndiospecific styrene polymerization catalyst. 相似文献
994.
995.
Ivachtchenko AV Kobak VV Ilyn AP Khvat AV Kysil VM Williams CT Kuzovkova JA Kravchenko DV 《Journal of combinatorial chemistry》2005,7(2):227-235
Two efficient strategies for solution-phase parallel synthesis of libraries of quinoline derivatives are described. The first synthetic pathway features the Pfitzinger reaction of isatin with diethyl malonate and sulfochlorination of the resulting 2-oxo-1,2-dihydroquinoline-4-carboxylate followed by generation of sulfonamide library. The second strategy employs the unusual behavior of 5-sulfamoylisatins in Pfitzinger reactions, which results in formation of 6-sulfamoyl-4-carboxyquinolines instead of the anticipated 2-oxo-1,2-dihydroquinoline structures. The obtained carboxylates appeared to be convenient synthetic intermediates for the generation of the corresponding carboxamide libraries. Using these reagents, the parallel solution-phase synthesis of more than 500 substituted quinoline and 2-oxo-1,2-dihydroquinoline derivatives has been accomplished on the 50-100-mg scale. Simple manual techniques for parallel reactions using special CombiSyn synthesizers were coupled with easy purification procedures to give high-purity final products. The scope and limitations of the developed approaches are discussed. 相似文献
996.
Theresa Julia Zielinski Masayuki Shibata Robert Rein 《International journal of quantum chemistry》1981,19(1):171-177
Ab initio STO -3G geometries and relative energies for uracil (U) and the tautomer 4-hydroxyuracil (U*) were obtained with the HONDO program utilizing the rapidly convergent method of Murtaugh and Sargent for geometry optimization. ΔE for U?U* is 6.61 kcal/mole. The reaction field continuum model for solvent effect indicates a preferential stabilization of U* by 1.0 kcal/mole. The calculated gas phase Kt and solution Kt for U?U* are 1.44×10?5 and 1.3×10?4, respectively. 相似文献
997.
We develop a hybrid computational approach for simulating mixtures of binary fluids and mobile, submicron particles. The model couples a lattice Boltzmann method for the binary fluid with a Brownian dynamics model for the particles. The particles can exhibit preferential wetting interactions with the different components of the fluid. As an illustration of the method, we carry out simulations in two dimensions to compare the spinodal decomposition of a pure binary fluid with the phase separation of binary blends that contain either fixed or mobile particles. We then isolate conditions where the flow of a binary fluid/particle mixture past surfaces with well-defined asperities leads to the formation of monodisperse droplets, which encapsulate the nanoparticles. The findings provide guidelines for creating multiphase emulsions with well-controlled morphologies. 相似文献
998.
Hausmann J Klingele MH Lozan V Steinfeld G Siebert D Journaux Y Girerd JJ Kersting B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1716-1728
A series of dinickel(II) complexes with the 24-membered macrocyclic hexaazadithiophenol ligand H(2)L(Me) was prepared and examined. The doubly deprotonated form (L(Me))(2-) forms complexes of the type [(L(Me))Ni2II(mu-L')](n+) with a bioctahedral N(3)Ni(II)(mu-SR)(2)(mu-L')Ni(II)N(3) core and an overall calixarene-like structure. The bridging coordination site L' is accessible for a wide range of exogenous coligands. In this study L'=NO(3)(-), NO(2)(-), N(3)(-), N(2)H(4), pyrazolate (pz), pyridazine (pydz), phthalazine (phtz), and benzoate (OBz). Crystallographic studies reveal that each substrate binds in a distinct fashion to the [(L(Me))Ni(2)](2+) portion: NO(2)(-), N(2)H(4), pz, pydz, and phtz form mu(1,2)-bridges, whereas NO(3)(-), N(3)(-), and OBz(-) are mu(1,3)-bridging. These distinctive binding motifs and the fact that some of the coligands adopt unusual conformations is discussed in terms of complementary host-guest interactions and the size and form of the binding pocket of the [(L(Me))Ni(2)](2+) fragment. UV/Vis and electrochemical studies reveal that the solid-state structures are retained in the solution state. The relative stabilities of the complexes indicate that the [(L(Me))Ni(2)](2+) fragment binds anionic coligands preferentially over neutral ones and strong-field ligands over weak-field ligands. Secondary van der Waals interactions also contribute to the stability of the complexes. Intramolecular ferromagnetic exchange interactions are present in the nitrito-, pyridazine-, and the benzoato-bridged complexes where J=+6.7, +3.5, and +5.8 cm(-1) (H=-2 JS(1)S(2), S(1)=S(2)=1) as indicated by magnetic susceptibility data taken from 300 to 2 K. In contrast, the azido bridge in [(L(Me))Ni(2)(mu(1,3)-N(3))](+) results in an antiferromagnetic exchange interaction J=-46.7 cm(-1). An explanation for this difference is qualitatively discussed in terms of bonding differences. 相似文献
999.
Strong bases can abstract a proton α to double bond. A phenylthio group ψ to that double bond can be eliminated at the same time leading to a cyclopropane ring. The procedure is illustrated by a conversion of limonene into car-2-ene. 相似文献
1000.
We present a new method (fFLASH) for the virtual screening of compound databases that is based on explicit three-dimensional molecular superpositions. fFLASH takes the torsional flexibility of the database molecules fully into account, and can deal with an arbitrary number of conformation-dependent molecular features. The method utilizes a fragmentation-reassembly approach which allows for an efficient sampling of the conformational space. A fast clique-based pattern matching algorithm generates alignments of pairs of adjacent molecular fragments on the rigid query molecule that are subsequently reassembled to complete database molecules. Using conventional molecular features (hydrogen bond donors and acceptors, charges, and hydrophobic groups) we show that fFLASH is able to rapidly produce accurate alignments of medium-sized drug-like molecules. Experiments with a test database containing a diverse set of 1780 drug-like molecules (including all conformers) have shown that average query processing times of the order of 0.1 seconds per molecule can be achieved on a PC. 相似文献