全文获取类型
收费全文 | 4480篇 |
免费 | 91篇 |
国内免费 | 26篇 |
专业分类
化学 | 2286篇 |
晶体学 | 17篇 |
力学 | 318篇 |
数学 | 706篇 |
物理学 | 1270篇 |
出版年
2023年 | 25篇 |
2022年 | 89篇 |
2021年 | 142篇 |
2020年 | 93篇 |
2019年 | 141篇 |
2018年 | 154篇 |
2017年 | 116篇 |
2016年 | 138篇 |
2015年 | 126篇 |
2014年 | 136篇 |
2013年 | 214篇 |
2012年 | 212篇 |
2011年 | 249篇 |
2010年 | 211篇 |
2009年 | 203篇 |
2008年 | 241篇 |
2007年 | 255篇 |
2006年 | 207篇 |
2005年 | 188篇 |
2004年 | 175篇 |
2003年 | 129篇 |
2002年 | 129篇 |
2001年 | 82篇 |
2000年 | 61篇 |
1999年 | 44篇 |
1998年 | 52篇 |
1997年 | 39篇 |
1996年 | 45篇 |
1995年 | 36篇 |
1994年 | 32篇 |
1993年 | 38篇 |
1992年 | 34篇 |
1991年 | 27篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 37篇 |
1987年 | 21篇 |
1986年 | 21篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 26篇 |
1982年 | 33篇 |
1981年 | 20篇 |
1980年 | 21篇 |
1979年 | 35篇 |
1978年 | 23篇 |
1977年 | 27篇 |
1975年 | 27篇 |
1973年 | 23篇 |
1972年 | 20篇 |
排序方式: 共有4597条查询结果,搜索用时 0 毫秒
51.
Nezlin MV 《Chaos (Woodbury, N.Y.)》1994,4(2):109-111
In the context of planetary atmospheres and oceans, it is natural to define "coherent structures" as "long-lived," or "solitary," Rossby vortices. These can be described by the generalized Charney-Obukhov equation (in fluid dynamics) or the analogous generalized Hasegawa-Mima equation (in plasma physics). These two equations contain KdV-type nonlinearities which (together with the compensating dispersive spreading) determine the formation of the coherent structures and explain the clear-cut cyclonic/anticyclonic asymmetry observed experimentally in long-lived planetary Rossby vortices. Examples are given of natural vortices which are (and which are not) coherent structures. 相似文献
52.
I. A. Kyuntsel G. B. Soifer E. S. Kozlov A. D. Sinitsa 《Journal of Structural Chemistry》1993,34(3):402-405
The structure of chlorine-containing cyclic pentacoordinated phosphorus compounds has been studied by35Cl NQR. The distortion of molecular geometry from trigonal bipyramid to tetragonal pyramid in going from monocyclic phosphoranes to spiro phosphoranes and the corresponding changes in the NQR spectrum are discussed.Perm State University. Institute of Organic Chemistry, Ukrainian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 3, pp. 75–79, May–June 1993.Translated by L. Smolina 相似文献
53.
E. A. Dikusar N. G. Kozlov V. I. Potkin N. V. Kovganko 《Chemistry of Natural Compounds》2003,39(3):276-279
Esters 1b-15b were prepared from cetyl alcohol 1a, terpenols 2a-6a, sterols 7a-10a, plant phenols 11a-13a, and alcohols 14a-15a by reaction with 1-adamantanecarboxylic acid chloride in the presence of pyridine. 相似文献
54.
Gumerova D. F. Mashevskaya I. V. Maslivets A. N. Kozlov A. P. 《Russian Journal of Organic Chemistry》2003,39(7):995-997
The kinetics of the reaction of 3-aroyl-2,4-dihydro-1H-pyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones with aliphatic alcohols in toluene at 25°C were studied by spectrophotometry. The effects of electronic and steric factors in the nucleophile and substrate were estimated by treatment of the kinetic results by the Hammett and Taft equations. 相似文献
55.
Frampton MJ Akdas H Cowley AR Rogers JE Slagle JE Fleitz PA Drobizhev M Rebane A Anderson HL 《Organic letters》2005,7(24):5365-5368
[structure: see text] A vinylene-linked porphyrin dimer, with no substituents at the beta-positions, has been synthesized by CuI/CsF promoted Stille coupling. In the crystal structure of this dimer, the C(2)H(2) bridge is twisted by 45 degrees relative to the plane of the porphyrins. The absorption, emission spectra, and electrochemistry reveal substantial porphyrin-porphyrin pi-conjugation. The triplet excited-state absorption spectrum of this dimer makes it suitable for reverse saturable absorption at 710-900 nm. 相似文献
56.
A procedure has been developed for the selective preparation of 2-methylisoborneol by the action of methyllithium on camphane. It has been shown that the interaction of 2-methylisoborneol with aceto- and benzonitriles in the presence of sulfuric acid gives, as the result of a rearrangement of the carbon skeleton, high yields of the corresponding 1,4,7,7-tetramethylbicyclo[2.2.1]hept-exo-2-ylacylamines, which possess an anticataleptic action.Institute of Physical Organic Chemistry, Belarus Academy of Sciences, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 719–723, November–December, 1994. 相似文献
57.
A theory for the transfer function of an electrochemical cell under conditions of convective diffusion is constructed. The variational principle for the integration of the convective-diffusion equation is developed, which reduces the problem solution to selecting adequate trial functions. With the diffusion frequency exceeded, the decay in the frequency dependence of the transfer function is defined by the system geometry. At a certain balance between parameters of the electrode system the frequency dependence of the transfer function can be strictly analytical, i.e. of the –1 type. 相似文献
58.
Sautter A Kaletas BK Schmid DG Dobrawa R Zimine M Jung G van Stokkum IH De Cola L Williams RM Würthner F 《Journal of the American Chemical Society》2005,127(18):6719-6729
A molecular square with dimensions of about 4 nm, incorporating sixteen pyrene chromophores attached to four ditopic bay-functionalized perylene bisimide chromophores, has been synthesized by coordination to four Pt(II) phosphine corner units and fully characterized via NMR spectroscopy and ESI-FTICR mass spectrometry. Steady-state and time-resolved emission as well as femtosecond transient absorption studies reveal the presence of a highly efficient (>90%) and fast photoinduced energy transfer (k(en) approximately equal to 5.0 x 10(9) s(-1)) from the pyrene to the perylene bisimide chromophores and a very fast and efficient electron transfer (>94%, k(et) approximately equal to 5 x 10(11) up to 43 x 10(11) s(-1)). Spectrotemporal parametrization indicates upper excited-state electron-transfer processes, various energy and electron-transfer pathways, and chromophoric heterogeneity. Temperature-dependent time-resolved emission spectroscopy has shown that the acceptor emission lifetime increases with decreasing temperature from which an electron-transfer barrier is obtained. The extremely fast electron-transfer processes (substantially faster and more efficient than in the free ligand) that are normally only observed in solid materials, together with the closely packed structure of 20 chromophoric units, indicate that we can consider the molecular square as a monodisperse nanoaggregate: a molecularly defined ensemble of chromophores that partly behaves like a solid material. 相似文献
59.
Vnira R. Akhmetova Guzel R. Nadyrgulova Tat'yana V. Tyumkina Mikhail Yu. Antipin Usein M. Dzhemilev 《Tetrahedron》2007,63(47):11702-11709
Cyclocondensation of aliphatic diamines with CH2O and H2S (1:3:2 ratio, 0 °C) was carried out to give thiadiazabicyclanes and dithiadiazabicyclanes (1:6:4 ratio), which were previously difficult to synthesize. Symmetric α,ω-bis-1,3,5-dithiazinanes were synthesized at 80 °C by this reaction. The stereochemistry of thiadiazabicyclanes was assigned by 1H and 13C NMR spectroscopy and by theoretical DFT calculations, and of bis-dithiazinanes by X-ray diffraction study in the solid state. 相似文献
60.
E. V. Vashkevich V. I. Potkin N. G. Kozlov 《Russian Journal of Organic Chemistry》2006,42(10):1477-1480
2-(2-Naphthyl)thiophene was synthesized by condensation of 3-chloro-3-(2-naphthyl)prop-2-enal with sulfanylacetic acid. A modified procedure was proposed for the synthesis of 2-(2-naphthyl)-and 2-(2-naphthyl)-6-nitroquinolines from 3-chloro-3-(2-naphthyl)prop-2-enal. 相似文献