全文获取类型
收费全文 | 25867篇 |
免费 | 678篇 |
国内免费 | 144篇 |
专业分类
化学 | 17650篇 |
晶体学 | 131篇 |
力学 | 633篇 |
数学 | 4409篇 |
物理学 | 3866篇 |
出版年
2022年 | 183篇 |
2021年 | 266篇 |
2020年 | 318篇 |
2019年 | 328篇 |
2018年 | 240篇 |
2017年 | 200篇 |
2016年 | 499篇 |
2015年 | 468篇 |
2014年 | 520篇 |
2013年 | 1363篇 |
2012年 | 1179篇 |
2011年 | 1495篇 |
2010年 | 774篇 |
2009年 | 737篇 |
2008年 | 1313篇 |
2007年 | 1317篇 |
2006年 | 1300篇 |
2005年 | 1235篇 |
2004年 | 1088篇 |
2003年 | 932篇 |
2002年 | 861篇 |
2001年 | 387篇 |
2000年 | 315篇 |
1999年 | 282篇 |
1998年 | 301篇 |
1997年 | 333篇 |
1996年 | 391篇 |
1995年 | 273篇 |
1994年 | 306篇 |
1993年 | 293篇 |
1992年 | 264篇 |
1991年 | 269篇 |
1990年 | 212篇 |
1989年 | 255篇 |
1988年 | 257篇 |
1987年 | 227篇 |
1986年 | 209篇 |
1985年 | 316篇 |
1984年 | 358篇 |
1983年 | 242篇 |
1982年 | 386篇 |
1981年 | 360篇 |
1980年 | 347篇 |
1979年 | 339篇 |
1978年 | 339篇 |
1977年 | 307篇 |
1976年 | 298篇 |
1975年 | 265篇 |
1974年 | 268篇 |
1973年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Robert Skibiński 《Central European Journal of Chemistry》2012,10(1):232-240
Photodegradation of quetiapine under UVC irradiation in methanol solution was investigated and structural elucidation of its
photodegradation products was performed with the use of the reversed phase UHPLC system coupled with accurate mass hybrid
ESI-Q-TOF mass spectrometer. During one run all essential data for the determination of photodegradation kinetics and for
the structural elucidation of the products was collected with the use of auto MS/MS mode. Five degradation products were found
and their masses and formulas were obtained with high accuracy (0.26–5.02 ppm). For all the analyzed compounds, MS/MS fragmentation
spectra were also obtained allowing structural elucidation of the unknown degradation products and indicating photodegradation
pathways of quetiapine. The main photodegradation product was identified as 2-[2-[4-(5-oxidodibenzo[b,f][1,4]thiazepin-11-yl)-1-piperazinyl]ethoxy]-ethanol
and the photodegradation reaction yields the first-order kinetics with the rate constant k = 0.1094 h−1. 相似文献
982.
Solid-liquid phase transfer of cyanide ion by 18-cro'wn-6 increases the yield of the Reissert reaction and eliminates the undesirable pseudo-base formation. 相似文献
983.
An overall three step preparation of the bis-diimine bridging ligand 4,7-phenanthrolino-5,6:5′,6′-pyrazine (ppz, 1) from 2-methoxy-1,4-phenylenediamine hydrosulfate 4 is reported. 相似文献
984.
The aerobic epoxidation of terminal or electron deficient alkenes with an aldehyde does not proceed with cobalt(II) acetylacetonate but goes to completion with the cobalt(II) benzoylacetonate and cobalt(II) 1,1,1,5,5,5-hexafluoroacetylacetonate complexes. 相似文献
985.
Ann M. Tickner G. Kris Huang Kerry Gombatz Robert J. Mills Vance Novack Kevin S. Webb 《合成通讯》2013,43(16):2497-2505
Two efficient syntheses of methyl 4-mercaptobenzoate are described, one utilizing the dianion of 4-bromothiophenol, the other a SNAr reaction starting with 4-fluorobenzonitrile. 相似文献
986.
Dr. Robert J. Gale 《光谱学快报》2013,46(5):597-604
We are not aware of any techniques that have been developed to utilize the rotational magnetic properties of either ions or molecular dipoles in solution for analytical applications. Nor have any recent fundamental studies attempted to detect specific permeability dispersion of solutions in the upper-UHF region. The electromagnetic spectral regions which extend from high frequencies (MHz) to microwaves (THz) are presently used for liquid/solid nmr studies, esr studies, microwave spectroscopy of gases, and dielectric spectroscopy of electrolytes and solids. However, the effects of a magnetic field on the rotational behaviour of certain molecules, functional groups, and ions in solution also could be investigated using methodologies based on inductive coupled measurements from cross-field excitation. Some preliminary data are presented which indicate that a 1T magnetic field can cause what appears to be inductive peaks in concentrated aqueous lithium nitrate at approximately 0.84 and 2.14 GHz. Further studies are necessary to ascertain the origin of such phenomena, which are perhaps atomic nuclei relaxation and/or molecular effects. 相似文献
987.
Milan Jakubek Zdeněk Kejík Robert Kaplánek Hana Veselá David Sýkora Pavel Martásek 《Supramolecular chemistry》2018,30(3):218-226
Perimidine-based chelators 1 and 2 were prepared, and their structures were confirmed by 1H and 13C NMR, MS spectroscopy and elemental analysis. These compounds were studied as specific synthetic receptors for the recognition of transition metal ions. They exhibited high affinity and selectivity towards Cu(II) ions. The conditional binding constants, linear dynamic range and detection limit were determined by UV–vis spectroscopy. These parameters demonstrated high potential of the prepared synthetic receptors for the recognition and determination of Cu(II) ions. The minimum detectable concentrations of Cu(II) ions for the synthetic receptors 1 and 2 were 270 and 75 nM (R 2 = 0.9915 and 0.9964) in aqueous medium (water/DMSO; 99:1 (v/v)), respectively. 相似文献
988.
989.
990.
The richly varied reactivity of the aldehyde group frequently imparts pivotal importance to this functionality in organic synthesis. This fact has resulted in the development of several methods for the elaboration of this structural unit from a variety of precursors.2 The report delineates the feasibility of two new couplementary approaches which proceed under mild conditions and demonstrate the utility of “halothiation” as applied to the oxidation of primary halides and terminal olefins. We were led to investigate this approach as a direct consequence of our interest in the Ramberg-Bäcklund rearrangement3 where the preliminary step often involves α-chlorination of the sulfide substrate.4 Halothiation is defined by us as a three-step transformation involving introduction of an ArS moiety, directed α-chlorination of the resulting sulfide, and hydrolysis. In principle, of course, the ArSCH2-unit is uniquely an aldehyde synthon and attention is therefore focused specifically on it at this time. 相似文献