全文获取类型
收费全文 | 32883篇 |
免费 | 1941篇 |
国内免费 | 1667篇 |
专业分类
化学 | 20118篇 |
晶体学 | 135篇 |
力学 | 709篇 |
综合类 | 618篇 |
数学 | 6541篇 |
物理学 | 8370篇 |
出版年
2023年 | 208篇 |
2022年 | 649篇 |
2021年 | 1350篇 |
2020年 | 616篇 |
2019年 | 724篇 |
2018年 | 489篇 |
2017年 | 530篇 |
2016年 | 695篇 |
2015年 | 776篇 |
2014年 | 1015篇 |
2013年 | 1803篇 |
2012年 | 1182篇 |
2011年 | 1378篇 |
2010年 | 1307篇 |
2009年 | 1711篇 |
2008年 | 1804篇 |
2007年 | 1966篇 |
2006年 | 1445篇 |
2005年 | 867篇 |
2004年 | 828篇 |
2003年 | 907篇 |
2002年 | 3414篇 |
2001年 | 1149篇 |
2000年 | 699篇 |
1999年 | 571篇 |
1998年 | 580篇 |
1997年 | 409篇 |
1996年 | 496篇 |
1995年 | 417篇 |
1994年 | 432篇 |
1993年 | 480篇 |
1992年 | 477篇 |
1991年 | 308篇 |
1990年 | 281篇 |
1989年 | 244篇 |
1988年 | 263篇 |
1987年 | 187篇 |
1986年 | 199篇 |
1985年 | 308篇 |
1984年 | 228篇 |
1983年 | 139篇 |
1982年 | 292篇 |
1981年 | 464篇 |
1980年 | 423篇 |
1979年 | 463篇 |
1978年 | 368篇 |
1977年 | 278篇 |
1976年 | 237篇 |
1974年 | 74篇 |
1973年 | 149篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
N. V. Gorbulenko T. M. Tkachuk T. V. Shokol V. V. Semeniuchenko A. V. Turov V. P. Khilya 《Chemistry of Heterocyclic Compounds》2007,43(5):569-575
The interaction has been studied of a series of substituted 3-hetaryl-7-hydroxychromones with amino acids and formaldehyde
(reactants ratio 1: 1: 2 respectively). In the case of glycine and Het = 3-isoxazolyl the product of aminomethylation at position
8 of the chromone was obtained, and with other Het (including Het = 4-phenyl-1,2,4-triazol-3-yl) 2-[6-alkyl-3-hetaryl-4-oxo-9,10-dihydro-4H,8H-chromeno[8,7-e][1,3]-oxazin-9-yl]acetic
acids were formed. With β-alanine and Het = 4-phenyl-1,2,4-triazol-3-yl the corresponding β-substituted propionic acid was
synthesized, but proline did not participate in the reaction, leading to bis(6-ethyl-3-hetaryl-7-hydroxychromon-8-yl)methane.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 683–689, May, 2007. 相似文献
992.
AES depth profiles on ceramic powders (untreated/hydrolyzed/oxidized/ (Al, Y)2O3 coated Si3N4, [BaO, SiO2] coated Al2O3) are feasible on thin, homogeneous layers or m sized agglomerations prepared on an Au foil. By means of the depth profiles one can qualitatively characterize the coating around the particles. Factor analysis of the depth profiles on the differently treated Si3N4 powders suggests the existence of an Si2N2O phase on the oxidized sample.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday 相似文献
993.
I. Ya. Mainagashev L. S. Klimenko V. P. Vetchinov V. I. Mamatyuk 《Russian Chemical Bulletin》1993,42(5):899-903
Irradiation of a mixture of 1-aryloxy-2-amino-9,10-anthraquinone and amines in benzene results in formation of the derivatives of 2-amino-1-hydroxy-9,10-anthraquinone-9-alkyl(aryl)imines in a high yield. 2-Amino-1-hydroxy-9,10-anthraquinone-9-aikylimines were shown by13C and15N NMR to exist predominantly in the enaminoquinoid form whereas 2-amino-1-hydroxy-9,10-anthraquinone-9-arylimines exist in the oxyimine form. Sunlight irradiation of the derivatives of 2-benzoylamino-9,10-anthraqumone-9-alkylimines results in formation of the photocyclization products — 4-benzoylaminoanthra-[9,1-d,e]-1,3-oxasinones-7.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 940–944, May, 1993. 相似文献
994.
Fu-Sheng S 《Talanta》1983,30(6):446-448
The conditions for the reaction between phenylarsenazo (PAA) (2-[(2-arsonophenyl)azoj-7-(phenylazo)-1, 8-dihydroxynaphthalene-3,6-disulphonic acid) and chromium(III) have been studied. A blue 1:1 complex is formed at pH 2.2 on heating the reactants at 100 degrees for 15 min. It has maximum absorption at 635 nm and is stable for at least 24 hr. The molar absorptivity is 3.3 x 10(4) 1.mole(-1).cm(-1). Beer's law is obeyed in the chromium concentration range 0-1.4 mug ml . The reaction has been successfully applied to determination of chromium in alloy steel. 相似文献
995.
The effect of 1,1,2,2-tetrachloroethane (TCE) on the IR spectra of HCl complexes withN,N-dimethylformamide (DMF) and 1-methyl-2-pyrrolidone (N-MP) with strong quasisymmetric hydrogen bonds was studied using Multiple Attenuated Total Reflection (MATR) IR spectroscopy.
The addition of TCE does not change the background absorption spectra, but results in a change in the extinction coefficients
of some bands of these complexes. The analysis of the spectra shows that the HCl−DMF complexes interact only with one molecule
of TCE, and the HCl−N-MP complexes interact with two molecules of TCE. It is shown that the neutral component of the system (TCE) has no effect
on the parameters of the strong quasi-symmetric H-bond in the complexes studied.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 956–960, May, 1997. 相似文献
996.
F. A. Selimov O. A. Ptashko A. A. Fatykhov N. R. Khalikova U. M. Dzhemilev 《Russian Chemical Bulletin》1993,42(5):872-878
The linear and cyclic cooligomerization of 2-vinyl-, 2-methyl-5-vinyl-, and 4-vinylpyridines with 1,3-dienes and trienes catalyzed by complexes of transition metals (Fe, Co, Ni, Mn, Cr, Pd, Ru, Rh, and Zr) was carried out to give unsaturated pyridines containing alkenyl and cycloalkenyl substituents.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 916–921, May, 1993. 相似文献
997.
von Niessen Wolfgang Cederbaum Lorenz S. Kraemer Wolfgang P. 《Theoretical chemistry accounts》1977,44(1):85-93
The vertical valence ionization potentials of cyclopropane, ethylene oxide and ethylene imine are calculated by a many-body Green's function method. For C3H6 the ordering of the ionization potentials is 2e(), 1e(), 2a1(), 1a2(), 1e(). The assignment of the 2a1 and the 1a2 ionization potentials which has been controversial is thus clarified. The ordering is in agreement with the result obtained via Koopmans' theorem. For ethylene oxide and ethylene imine Koopmans' theorem fails in predicting the correct order of ionic states. For C2H4O the ordering of the ionization potentials is 2b
1(), 4a
1, 1a
2(), 2b
2,3a
1, 1b
1(), 1b
2, 2a
1 and for C2H5N 6a, 5a, 3a, 2a, 4a, 3a, 1a, 2a. The agreement of the computed ionization potentials with the experimental values is very satisfactory. 相似文献
998.
H. J. Cortes C. D. Pfeiffer B. E. Richter T. S. Stevens 《Journal of separation science》1987,10(8):446-448
Porous ceramic bed supports for fused silica packed capillary columns utilized in liquid chromatography were prepared by polymerizing solutions containing potassium silicate in-situ within a column to create a mechanically stable, rugged, and easily constructed termination. The effect of the bed support length on efficiency, and comparisons to glass wool bed supports, were considered in terms of column efficiencies and hydrodynamic variables. Results obtained indicate better performance for the ceramic bed support. 相似文献
999.
Volker Staemmler 《Theoretical chemistry accounts》1982,62(1):69-80
Ab initio calculations for the 2 ground states of SF and SCl have been performed on Hartree-Fock level and with inclusion of valence shell correlation effects by means of the CI and CEPA approaches. The calculated properties are: Equilibrium distances, vibration frequencies, and dipole moment curves in the vicinity of the respective equilibrium geometries. Our best estimates for the 0 1 infrared absorption frequencies o for SF and SCl are 786 cm–1 and 520 cm–1, respectively, both with an uncertainty of about 10 cm–1. This confirms a recent experimental value obtained by Willner for SF (791 cm–1), but indicates that for SCl both experimental values reported previously in the literature (617 cm–1 and 574 cm–1) are wrong. The S—F and S—Cl bonds in SF and SCl are very similar to the ones in SF2 and SCl2, being essentially single p-bonds in either case. In the analogous oxygen-halogen molecules the situation is different, the O—F and O—Cl bonds in the diatomic radicals OF and OCl have partial double bond character and are much stronger than those in OF2 and OCl2 or in HOF and HOCl. 相似文献
1000.
John C. Schug 《Theoretical chemistry accounts》1979,53(2):197-201
Ab initio projected-unrestricted Hartree-Fock calculations have been carried out on a number of excited and ionic states of the water molecule. Results have been compared with large-scale CI calculations, with IVO calculations, and with those of Mrozek and Golebiewski obtained by the 2 × 2 rotation method applied to orbitals. It is concluded that the PUHF method may provide the most useful alternative to large-scale CI for calculating properties of open-shell systems. But it will not be generally useful for calculating spectral transition energies. 相似文献