全文获取类型
收费全文 | 28674篇 |
免费 | 4466篇 |
国内免费 | 4163篇 |
专业分类
化学 | 21530篇 |
晶体学 | 425篇 |
力学 | 1520篇 |
综合类 | 359篇 |
数学 | 3196篇 |
物理学 | 10273篇 |
出版年
2024年 | 12篇 |
2023年 | 416篇 |
2022年 | 556篇 |
2021年 | 863篇 |
2020年 | 941篇 |
2019年 | 1017篇 |
2018年 | 838篇 |
2017年 | 769篇 |
2016年 | 1277篇 |
2015年 | 1289篇 |
2014年 | 1554篇 |
2013年 | 2080篇 |
2012年 | 2485篇 |
2011年 | 2518篇 |
2010年 | 1940篇 |
2009年 | 1845篇 |
2008年 | 2081篇 |
2007年 | 1891篇 |
2006年 | 1783篇 |
2005年 | 1521篇 |
2004年 | 1273篇 |
2003年 | 1127篇 |
2002年 | 1225篇 |
2001年 | 963篇 |
2000年 | 775篇 |
1999年 | 641篇 |
1998年 | 451篇 |
1997年 | 477篇 |
1996年 | 396篇 |
1995年 | 364篇 |
1994年 | 317篇 |
1993年 | 266篇 |
1992年 | 241篇 |
1991年 | 189篇 |
1990年 | 176篇 |
1989年 | 154篇 |
1988年 | 100篇 |
1987年 | 81篇 |
1986年 | 84篇 |
1985年 | 68篇 |
1984年 | 44篇 |
1983年 | 30篇 |
1982年 | 27篇 |
1981年 | 21篇 |
1980年 | 18篇 |
1978年 | 14篇 |
1976年 | 9篇 |
1975年 | 12篇 |
1973年 | 11篇 |
1972年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
991.
乙胺嘧啶(商品名:达拉匹林)是一种抗寄生虫药物,改进了合成方法,并设计转化为创新型综合教学实验。从对氯苯乙腈出发,使用丙酸乙酯为酰化试剂,在叔丁醇钾作用下得到烯醇前体。在三甲基氧鎓四氟硼酸盐(Me3O·BF4,Meerwein盐)和碱作用下进行选择性甲基化反应得到烯醇甲醚后,与胍在碱性条件下环化得到最终产物达拉匹林,并通过红外光谱、核磁共振对其结构进行表征。改进后的实验方法具有重现性好、毒性低、操作方便、实验微型化等优点。实验过程中不仅提升了学生的实验技能和科研思维,更提高了学生的创新能力和绿色环保意识。 相似文献
992.
应用原子吸收分光光度法对济南市健康老年人血清中铜、锌、镁含量进行了测定。结果表明,健康老年人血液铜、锌水平低,但铜、锌比值正常,而镁的含量则较高。提示这组健康老年人未患心、脑血管疾病及糖尿病的原因可能与铜、锌、镁等微量元素有关。 相似文献
993.
Seven chiral thiazolidines bound rhodium complexes were synthesized and their catalytic asymmetric hydrosilation properties were investigated It was found through investigation that the configuration of newly formed chiral centre C2 of substituted chiral thiazolidines prepared from L-cysteine or its esters has no effect on the final results of catalytic asymmetric hydrosilation.The direct reason for causing this phenomenon is reported by the present quantitative results for the first time:the rapid racemation of chiral center C2 of chiral thiazolidine ligands takes place under the catalysis of rhodium(Ⅰ) complex [Rh(COD)CI]2 相似文献
994.
Zheng Z Xu X Fan X Lau WM Kwok RW 《Journal of the American Chemical Society》2004,126(39):12336-12342
A new synthetic approach for the formation of ultrathin polymer films with customizable properties was developed. In this approach, the kinematic nature of proton collisions with simple organic molecules condensed on a substrate is exploited to break C-H bonds preferentially. The subsequent recombination of carbon radicals gives a cross-linked polymer thin film, and the selectivity of C-H cleavage preserves the chemical functionalities of the precursor molecules. The nature and validity of the method are exemplified with theoretical results from ab initio molecular dynamics calculations and experimental evidence from a variety of characterization techniques. Its applicability is demonstrated by the synthesis of ultrathin polymer films with precursor molecules such as dotriacontane, docosanoic acid, poly(acrylic acid) oligomer, and polyisoprene. The approach is fundamentally different from conventional chemical synthesis as it involves an unusual mix of physical and chemical processes including charge exchange, projectile penetration, kinematics, collision-induced dissociation, inelastic energy transfer, chain transfer, and chain cross-linking. 相似文献
995.
配体的空间构型及疏水性对钌(Ⅱ)多吡啶配合物与DNA作用的影响 总被引:19,自引:2,他引:19
合成了4-氰基苯基咪唑并[5,6-f]邻菲咯啉(CYIP)和2-羧基苯基咪唑并[5, 6-f]邻菲咯啉(COIP)两种新配体及它们的钌混配配合物[Ru(bpy)2CYIP](ClO4)2 ·H2O(Rul)(bpy=2,2′-联吡啶),[Ru(phen)2CYIP](ClO4)2·H2O(Ru2) (phen=1,10-邻菲咯啉),[Ru(bpy)2COIP](ClO4)2·3H2O(Ru3)和[Ru(phen)2COIP] (ClO4)2·H2O(Ru4),并用红外光谱、紫外光谱、核磁和质谱对它们进行了表征。 通过循环伏安法研究了这些配合物的电化学性质。采用电子吸收光谱、稳态荧光、 圆二色谱和粘度测定研究了配合物与小牛胸腺DNA的相互作用。结果表明配合物 Rul和Ru2通过CYIP配体以插入的方式与DNA结合,而配合物Ru3和Ru4则通过COIP配 体以部分插入的方式与DNA结合。 相似文献
996.
Theoretical study of the unimolecular decomposition mechanisms of energetic TNAD and TNAZ explosives
Min‐Hsien Liu Cheng Chen Yaw‐Shun Hong 《International journal of quantum chemistry》2005,102(4):398-408
Calculation methods based on hybrid Density Functional Theory (DFT) with the basis sets of the B3LYP/6‐31+G(d)//B3LYP/4‐31G(d) method and the differential overlap (INDO) program were used to derive reasonable decomposition mechanisms of 1,4,5,8‐tetranitro‐1,4,5,8‐tetraazadecalin (TNAD) and 1,3,3‐trinitroazetidine (TNAZ) explosives. All possible decomposition species and transition states, including intermediates and products, were identified and their corresponding enthalpy of formation and Gibbs free energy of formation were obtained using polyparametric modification equations. INDO bond energy calculation results reveal the weakest bonding site for reference and determine where cleavage can occur easily. This work is concerned mainly with eliminating HONO (cis or trans form). The activation energy for trans‐form HONO elimination is lower than that of cis‐form HONO elimination in the initial steps of both TNAD and TNAZ decomposition, being 18.5 kJ/mol and 33.3 kJ/mol, respectively. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
997.
998.
以苯重氮硼氟酸盐与亚铁盐在微量铜离子存在时能引发丙烯腈聚合,聚合速率服从:R∝[φN_2~+]~(1/2)[Fe~(++)]~(1/4)[Cu~(++)]~(1/4),氧化还原引发历程为: Fe~(++)+Cu~(++) Fe~(+++)+Cu~+ φN_2~++Cu~+→φ·+N_2+Cu~(++) φ·+M→φM·→φMM·…该历程符合上述动力学关系。但使用对硝基苯重氮硼氟酸盐时,微量铜离子对聚合反应速率影响不大,与亚铁盐浓度在一定范围呈R∝[Fe~(++)]~(1/2)的关系,因此例的引发历程为: φN_2~++Fe~(++)→φ·+N_2+Fe~(+++) φ·+M→φM·→φMM·…苯环上推电子取代基降低了重氮基的氧化势,如对位取代甲氧基、甲基或没有取代基都不能用纯亚铁盐引发丙烯腈聚合,需有微量铜盐存在才能聚合。而拉电子取代基如对位取代硝基、羧基可用纯亚铁盐引发聚合。 相似文献
999.
Three novel heteropolytungstates, [Cu(phen)2]4[α-SiW12O40] (1), [Cu4(4,4′-bpy)3(2,2′-bpy)4][α-SiW12O40] · H2O (2) and [Cu(4,4′-bpy)(4,4′-Hbpy)0.5]2[PW12O40] (3) (phen = 1,10-phenanthroline, 4,4′-bpy = 4,4′-bipyridine, 2,2′-bpy = 2,2′-bipyridine), have been synthesized and characterized
by elemental analyses, IR, TG analyses and single-crystal X-ray diffraction. Compound (1) exhibits interesting chiral layer constructed from interperpendicular helical chains running along a crystallographic 21 axis in the c and a directions. Furthermore, the chiral layers are connected by the [α-SiW12O40]4− anions via hydrogen bonding interactions to form a 3D superamolecular structure. The [Cu4(4,4′-bpy)3(2,2′-bpy)4]4+ coordinated complexes in compound (2) are packed together via the aromatic π–π stacking interactions and exhibit an interesting 3D sandglasslike “host” network with 1D channels, in which
[α-SiW12O40]4− anions “guests” reside. Compound (3) has a unique 2D superamolecular network, which is composed of cationic CuI coordination polymer chains and discrete [PW12O40]3− polyoxoanions as linkers. It is noteworthy that the monprotonated 4,4′-bpy ligands of (3) act as arms and connect the adjacent 2D network, generating a 3D interpenetrating superamolecular structure. 相似文献
1000.
Zhen‐Yu Li Ping Chen Hong‐Gui Xu Shu‐Ying Peng Yi‐Ming Yang Zhong‐Zhen Zhao Yue‐Wei Guo 《Helvetica chimica acta》2007,90(7):1353-1359
Five new polycyclic Daphniphyllum alkaloids, macropodumines F ( 1 ) and G ( 2 ), 17‐oxoyuzurimine ( 3 ), and macropodumines H ( 4 ) and I ( 5 ), were isolated from the leaves of D. macropodum Miq ., collected in Sichuan Province, China. The structures and relative configurations of the new compounds – as well as of four known, related alkaloids – were elucidated on the basis of in‐depth spectroscopic and mass‐spectrometric analyses, by chemical derivatization, and by comparison of spectroscopic data with those of known compounds. 相似文献