全文获取类型
收费全文 | 66027篇 |
免费 | 14901篇 |
国内免费 | 6765篇 |
专业分类
化学 | 62947篇 |
晶体学 | 939篇 |
力学 | 2349篇 |
综合类 | 464篇 |
数学 | 5763篇 |
物理学 | 15231篇 |
出版年
2024年 | 48篇 |
2023年 | 547篇 |
2022年 | 735篇 |
2021年 | 1164篇 |
2020年 | 2347篇 |
2019年 | 3607篇 |
2018年 | 1997篇 |
2017年 | 1667篇 |
2016年 | 4661篇 |
2015年 | 4847篇 |
2014年 | 5070篇 |
2013年 | 6207篇 |
2012年 | 5802篇 |
2011年 | 5185篇 |
2010年 | 5090篇 |
2009年 | 5011篇 |
2008年 | 4824篇 |
2007年 | 3929篇 |
2006年 | 3579篇 |
2005年 | 3415篇 |
2004年 | 2919篇 |
2003年 | 2452篇 |
2002年 | 3293篇 |
2001年 | 2409篇 |
2000年 | 2167篇 |
1999年 | 1081篇 |
1998年 | 549篇 |
1997年 | 417篇 |
1996年 | 420篇 |
1995年 | 355篇 |
1994年 | 317篇 |
1993年 | 268篇 |
1992年 | 190篇 |
1991年 | 193篇 |
1990年 | 157篇 |
1989年 | 120篇 |
1988年 | 99篇 |
1987年 | 66篇 |
1986年 | 65篇 |
1985年 | 66篇 |
1984年 | 43篇 |
1983年 | 44篇 |
1982年 | 30篇 |
1981年 | 34篇 |
1980年 | 21篇 |
1979年 | 41篇 |
1978年 | 28篇 |
1977年 | 23篇 |
1975年 | 17篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
Controlling chemical reactivity has been the central theme in chemistry. Herein, we review the recent progress on the development of genetically encoded protein coupling reactions and their potential applications. The chemical reactivity is encoded in the protein sequences. The information is read out by folding and molecular recognition between two reactive components and subsequently translated into chemical bonding via autocatalysis. It has emerged as a unique way to tune the chemical reactivity and is regarded as one type of information‐coded reactions. Not only has it received many applications such as protein topology engineering, bioconjugation, biomaterials and synthetic biology, but also its principle may be extended beyond protein chemistry to enable new modes of supramolecular interactions that promote chemical bonding and that are simultaneously reinforced by covalent bonds. 相似文献
992.
993.
M. G. Fan Y. C. Liang Y. F. Ming J. X. Chen T. Ye Q. Y. Zhang B. A. Xu S. Jin 《Research on Chemical Intermediates》1998,24(9):961-971
Photophysical behavior of spiro[1,3,3-trimethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP1) and spiro[1,3,3,2′-tetramethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP2) were studied. The fluorescent species and their spectra of SP1 and SP2 in polar solvents (acetonitrile and methanol) and non-polar solvent (cyclohexane) were investigated. The fluorescence decay
in polar solvents was studied by picosecond time-correlated single photon counting. In most cases, fluorescence decay obeyed
triexponential decay kinetics. The major fluorescent species is an excited intermediate which has similar conformation as
its precursor (1
SP
*) formed after the bond cleavage between spirocarbon and oxygen in oxazine ring. The effects of molecular structure and solvent
polarity on fluorescence spectra and fluorescence decay lifetime were studied. 相似文献
994.
四(三苯基膦)化钯催化的若干菲啶类化合物的合成 总被引:1,自引:0,他引:1
邻甲酰基芳基硼酸与取代邻溴苯胺经四(三苯基膦)化钯催化偶联缩合, 合成了9个菲啶衍生物。邻溴苯胺的取代基无论是吸电子或供电子基团, 反应均可顺利进行。 相似文献
995.
顺磁性聚酯金属配合物的合成及其驰豫性能的研究 总被引:2,自引:0,他引:2
本文通过二乙三胺五乙酸(DTPA)或乙二胺四乙酸(EDTA)的双酸酐与二元醇或二元酚进行聚合反应,制备了两个系列共15种新的聚酯型大分子配体及其顺磁性金属配合物,用核磁、红外光谱以及元素分析表征了配体和配合物的结构。初步试验结果表明,与相应的小分子金属配合物相比,聚酯金属配合物具有较高的弛豫性能。 相似文献
996.
997.
998.
A simple and sensitive kinetic method was developed for the determination of ritodrine hydrochloride in pharmaceutical preparations. The method is based upon a kinetic investigation of the oxidation reaction of the drug with alkaline potassium permenganate at room temperature for a fixed 12.5 min; the absorbance of the colored permenganate ion was measured at 609 nm. The absorbance concentration plots were rectilinear over the range of 1.6–11.2 μg mL?1 (r = 0.9992) with a minimum detectability of 0.096 μgmL?1 (2.88 × 10?7 M). The different experimental parameters affecting the development and stability of color were carefully studied and optimized. The determination of ritodrine hydrochloride by the fixed concentration and rate constant methods is also feasible with the calibration equations obtained, but the fixed time method has been found to be more applicable. The proposed method was applied successfully to the determination of ritodrine hydrochloride in tablets and ampoules with average recoveries of 100.37 ± 0.93% and 100.42 ± 0.87%, respectively. The results obtained were in good agreement with those obtained using a reference method for comparison. A proposal of the reaction pathway is also presented. 相似文献
999.
Sm(NO_3)_3·6H_2O热分解过程的研究工作虽有文献报道,但结果不尽相同。文献[2]不同于文献[1,3],认为分解过程中存在无水盐阶段;文献[3]不同于文献[1,2],认为分解过程中存在低水合物。而且文献尚缺低水合物热分解机理的报道。我们也曾做过一些工作,但脱水阶段也不详尽。为此,我们详细地研究了Sm(NO_3)_3·nH_2O(n= 相似文献
1000.
不饱和链烃沸点的拓扑研究 总被引:4,自引:0,他引:4
基于邻接矩阵与边价 (fi)定义边价连接性指数 ( mF) ,其中的0 F ,1 F与 2 94种不饱和链烃 (包括烯烃、炔烃及烯炔烃 )的沸点 (Tb)关联 ,得到良好的数学模型 :ln( 70 0 -Tb) =6 5 2 3 3 1-0 0 0 4880 F2 -0 3 46771 F0 5(n =2 94,R =0 9979,R2 =0 995 8,F =3 490 4,S =5 12 ) ,该回归模型经Jackknife法检验具有总体稳健性 .可以预示 ,该指数将在定量构效关系研究中成为重要参数 . 相似文献