全文获取类型
收费全文 | 1251篇 |
免费 | 42篇 |
国内免费 | 6篇 |
专业分类
化学 | 1079篇 |
晶体学 | 5篇 |
力学 | 11篇 |
数学 | 72篇 |
物理学 | 132篇 |
出版年
2024年 | 2篇 |
2023年 | 9篇 |
2022年 | 55篇 |
2021年 | 51篇 |
2020年 | 52篇 |
2019年 | 37篇 |
2018年 | 45篇 |
2017年 | 24篇 |
2016年 | 53篇 |
2015年 | 52篇 |
2014年 | 43篇 |
2013年 | 86篇 |
2012年 | 89篇 |
2011年 | 135篇 |
2010年 | 60篇 |
2009年 | 67篇 |
2008年 | 67篇 |
2007年 | 71篇 |
2006年 | 68篇 |
2005年 | 53篇 |
2004年 | 43篇 |
2003年 | 26篇 |
2002年 | 21篇 |
2001年 | 13篇 |
2000年 | 11篇 |
1999年 | 8篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 2篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1972年 | 1篇 |
排序方式: 共有1299条查询结果,搜索用时 15 毫秒
931.
Katsyuba S Kovalenko V Chernova A Vandyukova E Zverev V Shagidullin R Antipin I Solovieva S Stoikov I Konovalov A 《Organic & biomolecular chemistry》2005,3(14):2558-2565
The IR and Raman spectra and conformations of calix[4]arene, thiacalix[4]arene and their p-tert-butyl derivatives have been analysed within the framework of scaled quantum mechanics (SQM). It is shown that the introduction of four p-tert-Bu groups into the calixarene molecules influences the relative energies of their conformers and the enthalpy of the cooperative intramolecular H-bonding (DeltaH(intra)) almost negligibly. DeltaH(intra), evaluated from Iogansen's rule, amounts to approximately 26-28 kcal mol(-1) for the calixarenes and approximately 20-21 kcal mol(-1) for the thiacalixarenes, which essentially exceeds the enthalpies of non-cooperative H-bonds formed by related phenols. As a result of this strong bonding, bands of stretching, bending and torsion vibrations of an eight-membered cyclic system (OH...)4 arise, e.g., two delta(OH)4 bands are observed in the IR spectra of the most highly symmetric C4 cone conformations of calix[4]arene and thiacalix[4]arene. The "duplication" of the number of OH infrared bands is a good new indicator of cooperativity of intramolecular H-bonding of the calixarenes. 相似文献
932.
Sviridenko FB Stass DV Kobzeva TV Tretyakov EV Klyatskaya SV Mshvidobadze EV Vasilevsky SF Molin YN 《Journal of the American Chemical Society》2004,126(9):2807-2819
This contribution reports the design and synthesis of a series of spin-labeled charge acceptors to produce three-spin systems of "radical ion/biradical ion" type in X-irradiated alkane liquids. This opens the way to study spin triads in experimental conditions, in which short-lived radical ion pairs are conventionally studied, thus offering optically detected techniques such as magneto-resonance OD ESR and level-crossing MARY spectroscopy. The structure of the synthesized 2-imidazoline-1-oxyl derivatives is A-Sp-R, where A is a positive or negative charge acceptor, R is a stable radical, and Sp is a hydrocarbon bridge. The set of 20+ compounds represent a convenient tool to construct experimental three-spin systems with various properties, e.g. with the "third" spin introduced into one or the other partner of the radical ion pair. The degree of exchange coupling between the two paramagnetic fragments in the biradical ion has been demonstrated to strongly depend on the type of the radical fragment R and the structure of the bridge Sp. As a result, a series of acceptors with systematically reduced exchange interaction has been synthesized, and optimal systems for the observation of low magnetic field effect have been found. In the most favorable case, an OD ESR signal from a spin triad living as short as ca. 100 ns has been registered as a single unresolved line. The exchange integral for this biradical anion (9) was estimated from OD ESR and ESR experiments to be ca. 10(3) G by the order of magnitude, which is much greater than the hyperfine couplings in the biradical ion but much smaller than the thermal energy kT. 相似文献
933.
The reaction of copper with benzyl bromides in dipolar aprotic solvents has been studied. There are no linear or other simple relations between ε, 1/ε, Y, P, n, and the rate of reaction. The activity of the solvent is determined by donor number (DN) in reaction under consideration. The kinetic and thermodynamic parameters of the reaction of copper with benzyl bromide in dimethyl sulfoxide (DMSO) have been clarified. Hammett plots of log (k/k°) vs. the substituent constant σ gave good correlations (ρ = 0.18, Sρ = 0.02, r = 0.961 in dimethyl sulfoxide and ρ = 0.21, Sρ = 0.02, r = 0.947 in dimethylacetamide (DMAA)). The structure of the organic group has little effect on the rate of reaction of benzyl bromide with copper. The Hammett ρ value also depends on DN. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 496–501, 2005 相似文献
934.
Svetlana Knežević R. Milačič Marjan Veber 《Analytical and bioanalytical chemistry》1998,362(1):162-166
A procedure was developed for the preconcentration and determination of aluminium and copper in dialysis concentrates at
the ng cm–3 level. The preconcentration was achieved on microcolumns filled with Chelex-100 resin adjusted to a pH of 4.0. Five repetitive
cycles of the sample through the column ensured a sufficient contact time for quantitative retention of aluminium and copper
ions. The retained ions were eluted with HNO3 (0.5 mol dm–3). Aluminium and copper were determined in the eluate by Zeeman ETAAS using the standard addition technique. The procedure
was performed under clean room conditions (class 10,000), The reliability of the results was evaluated by recovery tests,
using dialysis concentrates spiked with aluminium and copper. The recoveries obtained ranged from 86 to 106% for aluminium
and from 92 to 97% for copper. Using the recommended procedure, the LOD of aluminium and copper in dialysis concentrates (preconcentration
factor 2) was found to be 0.5 ng cm–3 and 0.2 ng cm–3, respectively.
Received: 19 December 1997 / Revised: 10 March 1998 / Accepted: 28 March 1998 相似文献
935.
Farrell S Ronkainen-Matsuno NJ Halsall HB Heineman WR 《Analytical and bioanalytical chemistry》2004,379(3):358-367
The suitability of a microelectrode as the detector for a small-volume, bead-based enzyme-labeled immunoassay for later use in a microfluidic device was investigated. The microelectrode helps to overcome consumption of the electroactive species by the electrode (depletion) that is encountered with macroelectrodes such as the rotating disk electrode (RDE) and allows the volume of the detection cell to be reduced. Microelectrodes also allow the chemical reactions to be monitored in real time due to the electrodes close proximity to the assay site. A bead-based sandwich immunoassay for mouse IgG was developed with alkaline phosphatase (AP) as the enzyme label, p-aminophenyl phosphate (PAPP) as the enzyme substrate, and microelectrode detection. The diffusion coefficient of the product of enzymatic hydrolysis, p-aminophenol (PAP), was determined to be 7.2±0.9×10–6 cm2 s–1. The detection limits were determined for free (0.52 ng mL–1) and bead-bound AP (10 ng mL–1). The number of binding sites for AP per bead was calculated to be 9.6×104 molecules/bead, and under saturation conditions the minimum detectable number of beads was 2500. Lower detection limits could be achieved with the microelectrode than the RDE while maintaining similar reproducibility. The microelectrode also made it possible to work with lower sample volumes (down to 10 L) than with the RDE (minimum volume of 40 L). Depletion of PAP was not observed with the microelectrode. The results obtained here with a microelectrode showed great promise for later use of microelectrodes in microfluidic devices with limited sample volumes. RDE detection cannot be used in a microfluidic system due to its complex set-up that includes a motor for rotation.Abbreviations: Ab
Antibody - Ag
Antigen - AP
Alkaline phosphatase - NSA
Nonspecific adsorption - PAP
p-aminophenol - PAPP
p-aminophenyl phosphate - RDE
Rotating disk electrode - NSA
Nonspecific adsorption - PBS
Phosphate-buffered saline 相似文献
936.
Novikov Vladimir P. Tarasenko Svetlana A. Samdal Svein Vilkov Lev V. 《Structural chemistry》2000,11(2-3):111-120
Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V
0[(/
e
)2 – 1]2 with V
0 = 1.0 ± 0.5 kcal/mol and
e
= 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r
a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds. 相似文献
937.
Svetlana Dratch Tanya Charnikhova Florence C.E SarabèrBen J.M Jansen Aede de Groot 《Tetrahedron》2003,59(24):4287-4295
A short and efficient procedure has been developed for the synthesis of C,D-cis coupled steroid and d-homo steroid skeletons. A Mukaiyama reaction with transfer of the silyl group of the starting silyl enol ether to the enol of the adduct followed by addition of vinyl magnesium bromide to the unprotected carbonyl group leads to adducts which can be cyclized with ZnBr2. The synthesis of functionalized steroid skeletons in overall yields of about 50% in four steps can be achieved in this way. 相似文献
938.
Sokolov MN Gushchin AL Abramov PA Virovets AV Peresypkina EV Kozlova SG Kolesov BA Vicent C Fedin VP 《Inorganic chemistry》2005,44(24):8756-8761
Single crystals of Ta4S9Br8 are obtained by heating Ta, S, and Br2 at 400 degrees C in a 4.0:9.0:4.0 molar ratio in a 44% yield. The structure was determined by X-ray analysis and consists of molecular clusters [Ta4(mu4-S)(mu-S2)4Br8]. The tantalum atoms form a square with long Ta...Ta distances (3.30 angstroms), with four S2 ligands bridging the Ta-Ta edges and one capping the square. Each Ta atom has two terminal bromine atoms. The compound is diamagnetic and has only two electrons for metal-metal bonding. IR and Raman spectral studies with the use of 34S allow to identify characteristic vibrations S-S (537 cm(-1)) and Ta4-mu4-S (407 cm(-1)). The compound is soluble in CH3CN, giving a dark-red solution with a characteristic electronic spectrum, which was assigned on the base of DFT calculations. ESI-MS spectra of the solutions show formation of [[Ta4S9Br8]Br]- associates. 相似文献
939.
940.
Baca SG Malaestean IL Keene TD Adams H Ward MD Hauser J Neels A Decurtins S 《Inorganic chemistry》2008,47(23):11108-11119
The synthesis, crystal structures and magnetic properties of five new manganese compounds are reported. These include a linear trinuclear cluster [Mn(II)(3)(O(2)CCHMe(2))(6)(dpa)(2)].2MeCN (1) (dpa = 2,2'-dipyridylamine), a tetranuclear cluster [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCMe(3))(6)(bpy)(2)] (3) (bpy = 2,2'-bipyridine), and chain coordination polymers composed of cluster blocks such as Mn(3), Mn(3)O, and Mn(4)O(2) bridged by 2,2'-bipyrimidine (bpm) or hexamethylentetramine (hmta) ligands to give ([Mn(II)(3)(O(2)CCHMe(2))(6)(bpm)].2EtOH)(n) (2), [Mn(II)(2)Mn(III)(2)O(2)(O(2)CCHMe(2))(6)(bpm)(EtOH)(4)](n) (4), and (([Mn(II)Mn(III)(2)O(O(2)CCHMe(2))(6)(hmta)(2)].EtOH)(n) (5). The magnetic analysis of the compounds was achieved using a combination of vector coupling and full-matrix diagonalization methods. Susceptibility data for compound 1 was fitted using a vector coupling model to give g = 2.02(1) and 2J/k(B) = -5.38(2) K. To model the trimer chain, we used vector coupling for initial values of J(1) and then diagonalization techniques to estimate J(2) to give g = 1.98(1), 2J(1)/k(B) = -3.3(1) K and 2J(2)/k(B) = -1.0(1) K by approximating the system to a dimer of trimers. The analysis of 3 was made difficult by the mixture of polymorphs and the difficulties of a three-J model, while for 4 an analysis was not possible because of the size of the computation and the relative magnitudes of the three couplings. Compound 5 was modeled using the same techniques as 2 to give g = 1.99(1), 2J(1)/k(B) = +32.5(2) K, 2J(2)/k(B) = -16.8(1) K, and 2J(3)/k(B) = +0.4(1) K. The combination of techniques has worked well for compounds 2 and 5 and thus opens up a method of modeling complex chains. 相似文献