全文获取类型
收费全文 | 57679篇 |
免费 | 7695篇 |
国内免费 | 10038篇 |
专业分类
化学 | 44706篇 |
晶体学 | 2141篇 |
力学 | 3714篇 |
综合类 | 473篇 |
数学 | 3689篇 |
物理学 | 20689篇 |
出版年
2024年 | 147篇 |
2023年 | 556篇 |
2022年 | 1182篇 |
2021年 | 1404篇 |
2020年 | 1881篇 |
2019年 | 1627篇 |
2018年 | 1508篇 |
2017年 | 1797篇 |
2016年 | 2348篇 |
2015年 | 2192篇 |
2014年 | 2835篇 |
2013年 | 4439篇 |
2012年 | 4397篇 |
2011年 | 3469篇 |
2010年 | 2979篇 |
2009年 | 3690篇 |
2008年 | 3855篇 |
2007年 | 3998篇 |
2006年 | 3746篇 |
2005年 | 3308篇 |
2004年 | 3189篇 |
2003年 | 2758篇 |
2002年 | 2790篇 |
2001年 | 1919篇 |
2000年 | 1929篇 |
1999年 | 1607篇 |
1998年 | 1408篇 |
1997年 | 1136篇 |
1996年 | 1094篇 |
1995年 | 956篇 |
1994年 | 896篇 |
1993年 | 668篇 |
1992年 | 670篇 |
1991年 | 452篇 |
1990年 | 352篇 |
1989年 | 287篇 |
1988年 | 274篇 |
1987年 | 201篇 |
1986年 | 183篇 |
1985年 | 193篇 |
1984年 | 153篇 |
1983年 | 109篇 |
1982年 | 119篇 |
1981年 | 109篇 |
1980年 | 98篇 |
1979年 | 105篇 |
1978年 | 66篇 |
1977年 | 77篇 |
1976年 | 55篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Yoshihiko Saito 《中国化学会会志》1991,38(5):405-424
A historical overview will be given on the structural studies on transition metal compounds and their interaction with other fields of coordination chemistry. About three decades have passed away since the structure and absolute configuration of tris(ethylenediamine)cobalt(III) complex ion were determined. At present accumulation of the structural data for isomers has enabled us to understand structural principles of chelate complexes in considerable detail. The energy minimization calculations can predict the detailed geometries of the complexes. Differences in thermodynamic properties between different conformers are well reproduced. Aspherical distribution of 3d electrons in transition metal complexes was detected for the first time in crystals of [Co(NH3)6][Co(CN)6] in 1973. Such an accurate electron density study provides important information on the d electrons placed in a ligand field. The high-spin and low-spin states can be distinguished unequivocally. In spite of a very small valence/total electron ratio, the asphericity of 4d and 5d electrons in a ligand field can be detected. The crystal structures of a series of dimeric copper(II) carboxylate adducts of the general formula [Cu(RCOO)2L]2 have been determined or redetermined as accurately as possible. The temperature dependent magnetic susceptibility of these crystals indicated that the isolated pairs of Cu(II) ions interact strongly through exchange forces. Molecular orbital calculations revealed that the electron population in the carbon atom of the bridging OCO group plays an important role in determining the strength of the spin superexchange interaction. In the crystals of some cobaloxime complexes, racemization of chiral groups bonded to Co proceeds on X-ray exposure without degradation of crystallinity. Several intermediate stages could be determined by X-ray analysis. Various reaction pathways were recognized and the reaction rate could be correlated with the atomic arrangement in the crystal. 相似文献
992.
研究了对象基苯甲酸(ABA)/聚对苯二甲酸乙二酯(PET)液晶共聚酯酰胺的晶体结构,以及热性能、流变学和力学性质。广角X射线衍射研究结果表明ABA链节的引入使PET的晶态结构发生畸变,但大部分结晶衍射峰仍然存在。随着ABA含量的增加,各晶面间距基本不变,微晶尺寸显著增大,结晶度急剧降低。差示扫描量热分析和热失重结果表明共聚酯胺具有优良的热稳定性,并且随着ABA含量的增加,共聚物的热稳定性提高,共聚 相似文献
993.
The title compound ist the first example of an adduct between telluric acid and the twenty four membered ring anion of a cyclododecaphosphate. [C(NH2)3]12P12O36 · 12 Te(OH)6 · 24 H2O crystallizes trigonal (rhomboedral: R3) with Z = 3 and the unit-cell dimensions a = 15.854(9), c = 51.26(2) Å in the hexagonal setting. The crystal structure was solved by direct methods and refined to a final R value of 0.031. It is characterized by a succession of three different typs of alternating layers perpendicular to the c direction. This layers are connected only by hydrogen bonds. The individual layers are built up of A: P12O36 anions, guanidinium cations and water of crystallisation, B: hexagonal arranged Te(OH)6 groups and guanidinium cations and C: water of crystallization. 相似文献
994.
New Rhenium Complexes Containing Trichalcogenido and Tetrachalcogenido Chelate Ligands The reactions of Cp*ReCl4 with polychalcogenide salts such as Na2S4 or (NEt4)2Se6 lead initially to the violet trichalcogenido chelate complexes Cp*ReCl2(E3) (E = S ( 3a ), Se ( 3b )) which, due to their functional chloro ligands, can be used as intermediates for further reactions. Upon hydrolysis in moist solvents or aminolysis with tert. butylamine 3a, b are converted into the tetrachalcogenido chelate complexes Cp*Re(O)(E4) (E = S ( 4a ), Se ( 4b )) and Cp*Re(NtBu)(E4) (E = S ( 5a ), Se ( 5b )), respectively. X-Ray structure analyses were carried out for the three mononuclear cyclo-oligoselenido compounds 3b–5b . It appears that the size of the Se2?n chelate ring (n = 3 or 4) essentially depends on steric factors within the coordination sphere of rhenium. 相似文献
995.
The title compounds were prepared by reaction of CaSb2, SrSb2, or BaSb3 with the transition metals and antimony in sealed silica tubes. They crystallize with the cubic LaFe4P12-structure, which was refined from single-crystal X-ray data of CaFe4Sb12, SrRu4Sb12, and BaRu4Sb12 to residuals of R = 0.014, 0.016, and 0.014, respectively. The thermal parameters of the alkaline earth ions increase with decreasing ionic size. The Sb? Sb distances are greater in the iron compound than they are in the two ruthenium compounds. This is rationalized to be due to a larger portion of electrons in antibonding Sb? Sb states in the iron compound. 相似文献
996.
核/壳结构增韧剂对环氧树脂的抗冲改性 总被引:7,自引:0,他引:7
核/壳结构增韧剂对环氧树脂的抗冲改性张明耀,张会轩,杨海东,冯之榴(吉林工学院化学工程系长春130012)(中国科学院长春应用化学研究所长春)关键词核/壳结构,增韧剂,环氧树脂,共混核/壳结构增韧剂是用分步乳液聚合技术制造的一种复合乳胶粒子,其核为橡... 相似文献
997.
5,7,8位取代的喹诺酮类化合物定量构效关系的量子化学研究 总被引:2,自引:0,他引:2
5,7,8位取代的喹诺酮类化合物定量构效关系的量子化学研究俞庆森,朱龙观,林瑞森,蔡国强(浙江大学化学系杭州310027)关键词:氟喹喏酮类,定量构效关系,CNDO/2氟喹诺酮类是一类重要的抗感染化疗药物 ̄[1],近十年对其研究进展神速 ̄[2],结构... 相似文献
998.
Crystal Structures of KNdTe4, RbPrTe4, and RbNdTe4 — Investigations concerning the Thermal Stability of KNdTe4 as well as some Remarks concerning Additional Representatives of the Composition ALnTe4 (A = K, Rb, Cs and Ln = Rare Earth Metal) Of the compounds ALnQ4 (A = Na, K, Rb, Cs; Ln = Lanthanoid; Q = S, Se and Te) the crystal structures of the three new tellurides KNdTe4, RbPrTe4 and RbNdTe4 were determined by X‐ray single‐crystal structure analysis and of the three additional new ones KCeTe4, KPrTe4 and CsNdTe4 by X‐ray powder diffraction experiments. All six new compounds are isotypic with KCeSe4. Characteristic for the crystal structure of the compounds mentioned above are layers built from (Q2)2— dumbbells in form of 4.32.4.3 nets with embedded cations A+ and Ln3+ between them, which are coordinated eightfold in form of square‐shaped antiprisms by Q ions. The distances Te‐Te within the dumbbells were found to be 277.8(2) pm for all investigated tellurides. By combination of X‐ray diffraction and DTA measurements it was shown that the compound KNdTe4 is metastable at ambient temperature with a limited existence range between the temperatures 260 and 498 °C. 相似文献
999.
Lead(II) 2,2'-bipyridine hexachloroplumba tetetrahydrate was synthesized and investigated by DTA, TG and DTG. IR spectroscopy
and other methods enabled the identification of some of the decomposition products. Comparative studies on the corresponding
chlorides: [Pb(bipy)]Cl2 and [Pb(bipy)3]Cl2, which can be considered as precursors of the hexachloroplumbate, were also undertaken. X-ray measurements enabled the tentative
determination of the crystal structure of [Pb(bipy)]Cl2. Hexachloroplumbate decomposes with the liberation of chlorine, water and organic ligands, and the process is accompanied
by the simultaneous transition of Pb(IV)→Pb(II). Chlorides release only ligands upon heating. Residues comprised always PbCl2.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
1000.
HeI photoelectron spectra have been recorded for the reaction of atomic fluorine with ethyl chloride at different reaction times. A structured band associated with a short-lived primary reaction product has been recorded with adiabatic and vertical ionization energies of (7.84±0.02) and (8.18±0.02) eV respectively. An average vibrational separation of (680±30) cm–1was observed in this band. Comparison between the experimental vertical and adiabatic ionization energies and ionization energies computed for CH3CHCl (X2A) and CH2CH2Cl (X2A) at different levels of theory led to the assignment of the observed first photoelectron band to the ionization of CH3CHCl (X2A). The observed vibrational structure was assigned to excitation of the C–Cl stretching mode in CH3CHCl+(X1A).Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail 相似文献