全文获取类型
收费全文 | 10428篇 |
免费 | 2158篇 |
国内免费 | 3689篇 |
专业分类
化学 | 9095篇 |
晶体学 | 333篇 |
力学 | 662篇 |
综合类 | 428篇 |
数学 | 1367篇 |
物理学 | 4390篇 |
出版年
2024年 | 24篇 |
2023年 | 175篇 |
2022年 | 324篇 |
2021年 | 338篇 |
2020年 | 405篇 |
2019年 | 416篇 |
2018年 | 329篇 |
2017年 | 483篇 |
2016年 | 454篇 |
2015年 | 555篇 |
2014年 | 630篇 |
2013年 | 846篇 |
2012年 | 884篇 |
2011年 | 938篇 |
2010年 | 785篇 |
2009年 | 866篇 |
2008年 | 944篇 |
2007年 | 770篇 |
2006年 | 800篇 |
2005年 | 764篇 |
2004年 | 540篇 |
2003年 | 512篇 |
2002年 | 538篇 |
2001年 | 528篇 |
2000年 | 486篇 |
1999年 | 307篇 |
1998年 | 179篇 |
1997年 | 138篇 |
1996年 | 141篇 |
1995年 | 136篇 |
1994年 | 150篇 |
1993年 | 146篇 |
1992年 | 102篇 |
1991年 | 86篇 |
1990年 | 87篇 |
1989年 | 75篇 |
1988年 | 61篇 |
1987年 | 55篇 |
1986年 | 70篇 |
1985年 | 38篇 |
1984年 | 41篇 |
1983年 | 28篇 |
1982年 | 23篇 |
1981年 | 17篇 |
1980年 | 5篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1965年 | 5篇 |
1959年 | 5篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
301.
标题配合物 [Cu(C8H4 F3O2 S) 2 (C12 H8N2 ) ]·C3H6 O属于三斜晶系 ,空间群为P1,并测得如下晶胞参数a =10 .5 77(3) ,b =15 .72 2 (4) ,c =10 .133(2 ) ,α =94 .5 3(2 ) ,β =10 0 .81(2 ) ,γ =96 .18(2 )° ,V =16 37.0 3 3,Z =2 ,Mr =74 4.18,Dx =1.5 1× 10 6 g·m- 3,F(0 0 0 ) =5 2 6 ,μ =8 2 4cm- 1,最终偏差因子为R =0 .0 83,Rw=0 .0 74。Cu(II)与 2个 4 ,4 ,4 三氟 1 (2 噻吩基 )丁二酮 1,3中的四个氧原子和菲咯啉中的两个氮原子 ,组成了一个畸变的八面体构型。 相似文献
302.
Ji‐Jiang Wang Meng‐Lin Yang Huai‐Ming Hu Prof. Dr. Gang‐Lin Xue Dong‐Sheng Li Qi‐Zhen Shi 《无机化学与普通化学杂志》2007,633(2):341-345
To determine the influence of the size of the aromatic chelate ligands on the frameworks of metal tretracarboxylate polymers, two new coordination polymers [Cd(btc)0.5 (2,2′‐bpy)] ( 1 ) and [Cd(btc)0.5(phen)]·H2O ( 2 ) (H4btc = biphenyl‐3,3′,4,4′‐tetracarboxylic acid, 2,2′‐bpy = 2,2′‐bipyridine, phen = 1,10‐phenanthroline) have been synthesized under similar hydrothermal conditions. In complex 1 , the dimeric Cd2 units are linked by bridging btc4? ligand to form a 2D layered network, whereas complex 2 possesses a 3D metal‐organic framework consisting of the dimeric Cd2 units. The differences of two metal‐organic frameworks demonstrate that the size of the rigid aromatic chelate ligands have an important effect on the structures of their complexes. Additionally, the two complexes show strong fluorescence in the solid state at room temperature. 相似文献
303.
AnLIU ZhongMeiZOU LiZhenXU ShiLinYANG 《中国化学快报》2003,14(11):1144-1145
A new compound, subglain B, was isolated from the stems of Uvaria tonkinensis var.subglabra and its structure was identified as 1S, 2R, 3S, 6R-1-benzoyloxymethylene-1,2-dihydroxy-3-benzoyloxy-6-chlorocyclohex-4-ene (1), by spectral evidences. 相似文献
304.
甘氨酸与氯化碱金属在水中的焓相互作用参数 总被引:2,自引:0,他引:2
氨基酸在混合溶液中的热力学性质及相互作用的研究不论对溶液化学还是生命科学都是十分重要的.对此国内已有不少的研究h,’1.利用偿效应的方法研究氨基酸与电解质间的相互作用是很有效的卜一句,但大多数的工作都是在低浓度下研究一个氨基酸分子和一个电解质离子对间的相互作用烩.本文测定了甘氨酸在水中和在LICI、NaCI、KCI的水溶液中的溶解偿,计算出甘氨酸与各个公在水中的偿对相互作用参数和三相互作用参数,并由此讨论了甘氨酸与这些盐的对分子相互作用和三分子相互作用.1实验部分试剂:甘氨酸为ARfa,K甲醇一水重结晶.LI… 相似文献
305.
稀土化合物催化内酯开环聚合(Ⅲ)──氯化钕-环氧化合物体系催化ε-己内酯本体聚合申有青,沈之荃,沈建良,张富尧,张一烽(浙江大学高分子科学与工程学系,杭州,310027)关键词氯化钕,环氧化合物,ε-己内酯,本体聚合聚己内酯(PCL)和聚丙交酯(PL... 相似文献
306.
Short DNA analogues with bridging dimethylene sulfide, sulfoxide, and sulfone groups replacing the phosphate diesters (S-DNAs) were synthesized from building blocks prepared via two routes, both starting from D-glucose. Building blocks for RNA analogues were prepared by stereoselective introduction of nucleobase into a 2'-acylated ribose analogue. The ribose analogues were converted to deoxyribose analogues by replacement of a 3'-OH group by a thioacetyl unit, followed by photolytic deoxygenation or radical-based 2'-deoxygenation. DNA analogues joined via CH(2)(-)S-CH(2) units were prepared by S(N)2 displacement of a 6'-mesyl group on one building block using a thiolate nucleophile of another. 4,4'-Dimethoxytrityl protection and deprotection schemes were established for both the thiol and hydroxyl groups. The corresponding sulfoxide DNA analogues were obtained by oxidation with hydrogen peroxide. Sulfone DNA analogues were obtained by oxidation of the sulfide DNA with persulfate or hydrogen peroxide in the presence of a titanium silicate catalyst. The physical properties of several representative oligonucleotide analogues were examined, and interpreted in light of a "second-generation" model for DNA strand-strand recognition, a model that emphasizes the role of the polyanionic backbone in diminishing unwanted tendencies of highly functionalized molecules to form "structure" in solution. Even short sulfide-linked DNA analogues displayed association properties different from those displayed by standard DNA molecules. Complex formation observed with sulfide-linked tetramers by HPLC study in different solvents suggested that the complex is formed using hydrogen bonding. Sulfone-linked dinucleotides display Watson-Crick behavior; the tetramer, however, displayed self-structure. Self-structure and self-aggregation become more prominent as the length of the oligonucleotide analogues increases. The tendency to self-aggregate can be decreased by adding a charged sulfonate group to the 3'-end of the DNA analogue. Features of the second-generation model are important for many areas of nucleic acid chemistry, from the design of nucleic acid therapeutic agents to the search for life on other planets. 相似文献
307.
The title compound C20H30N4O6, Mr = 422.48)was synthesized and its crystal structure was determined by X-ray diffraction method. It crystallizes in the triclinic system, space group P ī with cell parameters: a = 8.330(2), b = 8.468(2), c = 16.017(3) A, α = 97.30(3), β = 92.33(3), γ = 103.94(3)o, V = 1084.7(4) A3, Dc = 1.294 g/cm3, Z = 2, F(000) = 452 and μ = 0.096 mm-1. The structure was refined to R = 0.0483 for 3732 observed reflections with I>2(I) and wR = 0.1335 for 4828 unique reflections. The hydantoin rings are planar and the two ring planes of one molecule are paralleled to each other. 相似文献
308.
309.
A homodinuclear complex (NH3CH2CH2CH2NH3)2 {Na2[(C6H4O2)2] (C6H4O2H)2} (1) has been synthesized by a solution-based self-assembly route. It crystallized in monoclinic system with space group P21/c. Every sodium ion coordinates in a tetragonal prism fashion with two O atoms of a terminal chelating catecholate ligand and three O atoms from two bridging catecholate ligands. Two neighboring NaO5 tetragonal prisms are edge-shared and centrosymmetric with regard to the inversion center to form a binuclear cluster {Na2[(C6H4O2)2](C6H4OOH)2}^4- anion. The complex anions were aligned parallelly by n-n interaction and linked with the protonated 1,3-propylenediamine through hydrogen bonds which were assembled into a multi-lamellar structure with channels. The crystal exhibits rectangular geometry with an interior triangle hollow structure under optical microscopy. And the scanning electron microscopy (SEM) indicates that the wall of the tubes shows multi-lamella morphologies. Further, the transmission electron microscopy (TEM) reveals that the crystal is composed of multi-lamellar nano-tubes with diameters less than 100 nm. The molecular structure of the complex was compared with that of its isomer complex 2. 相似文献
310.
ZHENG Yan YANG Bing ZHANG Hai-quan LU Ping SHEN Fang-zhong LIU Lin-lin XU Hai YANG Guang-di MA Yu-guang 《高等学校化学研究》2007,23(6):720-725
A soluble poly(meta-phenylene)derivative with rigid twisted biphenyl unit was synthesized by the Yamamoto coupling reaction.The polymer is soluble in common organic solvents,and the number-average molecular weight is about 6500.The UV-Vis and quantum chemical calculation indicate that the different conformation segments named "conformers" exist in the polymer backbones;it was also further confirmed by the single crystal X-ray diffraction study of the dimeric model compound.The π-π transition of biphenyl segments of twisted and planar conformations made the polymer exhibit a strong absorption around 256 nm and a weak absorption at about 300 nm.Furthermore,the polymer exhibits a strong UV photoluminescence at 372 nm when the excitation wavelengths are longer than 300 nm.The ultraviolet-emitting electroluminescence(EL)device with the single layer structure shows EL λmax of the derivative at 370 nm. 相似文献