全文获取类型
收费全文 | 13525篇 |
免费 | 1551篇 |
国内免费 | 2139篇 |
专业分类
化学 | 10208篇 |
晶体学 | 291篇 |
力学 | 597篇 |
综合类 | 172篇 |
数学 | 2179篇 |
物理学 | 3768篇 |
出版年
2024年 | 22篇 |
2023年 | 152篇 |
2022年 | 368篇 |
2021年 | 463篇 |
2020年 | 449篇 |
2019年 | 428篇 |
2018年 | 388篇 |
2017年 | 404篇 |
2016年 | 574篇 |
2015年 | 608篇 |
2014年 | 701篇 |
2013年 | 987篇 |
2012年 | 1149篇 |
2011年 | 1258篇 |
2010年 | 1021篇 |
2009年 | 905篇 |
2008年 | 1103篇 |
2007年 | 1076篇 |
2006年 | 988篇 |
2005年 | 807篇 |
2004年 | 701篇 |
2003年 | 445篇 |
2002年 | 395篇 |
2001年 | 271篇 |
2000年 | 267篇 |
1999年 | 220篇 |
1998年 | 110篇 |
1997年 | 91篇 |
1996年 | 100篇 |
1995年 | 88篇 |
1994年 | 85篇 |
1993年 | 71篇 |
1992年 | 73篇 |
1991年 | 55篇 |
1990年 | 55篇 |
1989年 | 39篇 |
1988年 | 26篇 |
1987年 | 37篇 |
1986年 | 29篇 |
1985年 | 33篇 |
1984年 | 30篇 |
1983年 | 23篇 |
1982年 | 9篇 |
1981年 | 10篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1975年 | 7篇 |
1974年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
García-Raso A Fiol JJ Tasada A Albertí FM Molins E Basallote MG Máñez MA Fernández-Trujillo MJ Sánchez D 《Dalton transactions (Cambridge, England : 2003)》2005,(23):3763-3772
Alkylidene-bis(2-aminopyrimidines) (pyr2Cx, x = 2-5) are useful ligands to interact with Ag(I) yielding discrete metallocycles. Crystal structures of the [(pyr2C2)Ag(NO3)]2 and [(H-pyr2C4)Ag(NO3)2]2 have been isolated where each macrocyclic moiety interacts with their surroundings through weak interactions, yielding 3D discrete structures, On the other hand, the solution study shows that the equilibrium constants for the formation of Ag(pyr2Cx)+ complexes are higher than the literature values for Ag(I) complexes with single pyrimidines, although the differences could be explained by invoking the solid-state structures of the Ag(I)-pyr2Cx complexes. 相似文献
82.
A new palladium-dipyridylmethylamine complex is an excellent catalyst for C-C bond-forming processes such as the Heck, Suzuki, and Sonogashira reactions in organic and aqueous solvents under homogeneous conditions. [reaction: see text] 相似文献
83.
本文利用单羧基二茂铁Fc-COOH和1,1′-二羧基二茂铁HOOC-Fc-COOH作为配体分别合成了双核铈配合物Ce2(FcCOO)6 (1)和二维层状配位聚合物Ce2(OOC-Fc-COO)3(2),测定了两种配合物的晶体结构。配合物1中的金属铈离子为九配位结构,分别与周围的羧基二茂铁上的氧原子和作为辅助配体的水分子配位,茂铁间的π-π相互作用将配合物1的二聚体单元连结在一起形成二维的网状结构。配合物2中的金属铈离子亦为九配位结构,分别与周围的羧基二茂铁上的氧原子,作为辅助配体的水分子和甲醇配位形成类似于配合物1的二聚体单元,1,1′-二羧基二茂铁HOOC-Fc-COOH作为桥基配体将二聚体单元连结在一起,形成二维网状的配位聚合物。 相似文献
84.
在干燥氮气气氛下,以无水乙醇为溶剂,制备了低水合氯化钕与二乙氨基荒酸二乙铵(D-DDC)配合物,确定其组成为Et2NH2[Nd(S2CNEt2)4].单晶结构分析表明,配合物中4个二乙氨基荒酸根各通过2个硫原子与钕离子成键形成八配位十二面体阴离子,并与二乙铵阳离子形成缔合型分子.晶体属单斜晶系,空间群P21/n,a=1.37517(14)nm,b=2.1146(2)nm,c=1.44641(15)nm,β=102.028(2)°,Z=4.用微量热法测定了298.15K下水合氯化钕和D-DDC在无水乙醇中的溶解焓及二乙氨基荒酸钕液相生成反应焓变分别为(-17.89±0.096),(50.280±0.151)和(-10.116±0.065)kJ/mol,求得固相生成反应焓变. 相似文献
85.
BIOLOGICAL UV-DOSES AND THE EFFECT OF AN OZONE LAYER DEPLETION 总被引:2,自引:0,他引:2
ARNE DAHLBACK THORMOD HENRIKSEN SøREN H. H. LARSEN KNUT STAMNES 《Photochemistry and photobiology》1989,49(5):621-625
Effective UV-doses were calculated based on the integrated product of the biological action spectrum (the one proposed by IEC, which extends to 400 nm, was adopted) and the spectral irradiance. The calculations include absorption and scattering of UV-radiation in the atmosphere, both for normal ozone conditions as well as for a depleted ozone layer. For Scandinavian latitudes the effective annual UV-dose increases by approximately 4% per degrees of latitude towards the Equator. An ozone depletion of one percent increases the annual UV-dose by approximately 1% at 60 degrees N (increases slightly at lower latitudes). A large depletion of 50% over Scandinavia (60 degrees N) would give these countries an effective UV-dose similar to that obtained, with normal ozone conditions, at a latitude of 40 degrees N (California or the Mediterranean countries). The Antarctic ozone hole increases the annual UV-dose by 20 to 25% which is a similar increase as that attained by moving 5 to 6 degrees of latitude nearer the Equator. The annual UV-dose at higher latitudes is mainly determined by the summer values of ozone. Both the ozone values and the effective UV-doses vary from one year to another (within +/- 4%). No positive or negative trend is observed for Scandinavia from 1978 to 1988. 相似文献
86.
87.
Juan Jos Berzas Nevado Rosa Carmen Rodríguez Martín-Doimeadis Francisco Javier Guzmn Bernardo Nuria Rodríguez Farias 《Microchemical Journal》2007,86(2):183-188
An analytical methodology was proposed and validated to be applied to the determination of p,p′-DDT and its metabolites p,p′-DDE and p,p′-DDD in fish oil. The analytical procedure presented in this paper involves a single-step clean up process prior to the analysis. A solution of 1,2,3,4-tetrachloronaphtalene was used as internal standard.The analytical technique used was gas chromatography coupled to an electron capture detector. Details on the validation process are provided.The limits of detection ranged from 2.6 to 4.7 pg μL− 1. The BCR 598 standard reference material (cod liver oil) was used to evaluate the performance of the methodology with satisfactory recoveries for all the compounds.The analytes were determined in three different fish oil pills sold in Spain as a supplementary vitamin support. The sum of p,p′-DDT and metabolites was from 13.2 to 51.3 ng g− 1, the dominant compound being p,p′-DDE. 相似文献
88.
Miguel A. Bañares Amador Angoso Juan L. Manzano Emilio Rodríguez 《Transition Metal Chemistry》1987,12(2):149-152
Summary Cobalt(II) chlorocomplexes with empirical formulae (bnH2)2CoCl6·2H2O (1) and (bnH2)2CoCl6 (2), where bnH
2
2+
represents the diprotonated species of putrescine (1,4-butanediamine), were prepared.The visible spectra in the solid state and measurement of the magnetic moments permit the assignation of an octahedral geometry for cobalt in (1) and a tetrahedral geometry for (2). 相似文献
89.
Victoria Bejar Concepcion Calvo Juan Moliz Francisco Diaz-Martinez Emilia Quesada 《Applied biochemistry and biotechnology》1996,59(1):77-86
The exopolysaccharide produced byVolcaniella eurihalina, an halophilic eubacterium, under different environmental and nutritional conditions, is studied.V. eurihalina synthesizes an acidic heteropolysaccharide, composed by rhamnose, glucose, and mannose, as well as amino sugars, uronic acids,
and acetyl and sulphate residues. This composition varies depending on the nutrients of culture medium. Viscosity and pseudoplasticity
of the polymer solutions are also influenced by the nutritional conditions in which the microorganism was grown. 相似文献
90.