全文获取类型
收费全文 | 379篇 |
免费 | 19篇 |
国内免费 | 2篇 |
专业分类
化学 | 325篇 |
力学 | 1篇 |
数学 | 26篇 |
物理学 | 48篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 7篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 13篇 |
2015年 | 6篇 |
2014年 | 11篇 |
2013年 | 17篇 |
2012年 | 20篇 |
2011年 | 34篇 |
2010年 | 22篇 |
2009年 | 5篇 |
2008年 | 31篇 |
2007年 | 20篇 |
2006年 | 29篇 |
2005年 | 24篇 |
2004年 | 20篇 |
2003年 | 22篇 |
2002年 | 16篇 |
2001年 | 11篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 4篇 |
1994年 | 2篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1934年 | 1篇 |
排序方式: 共有400条查询结果,搜索用时 312 毫秒
101.
102.
Kwan JC Ratnayake R Abboud KA Paul VJ Luesch H 《The Journal of organic chemistry》2010,75(23):8012-8023
Grassypeptolides A-C (1-3), a group of closely related bis-thiazoline containing cyclic depsipeptides, have been isolated from extracts of the marine cyanobacterium Lyngbya confervoides. Although structural differences between the analogues are minimal, comparison of the in vitro cytotoxicity of the series revealed a structure-activity relationship. When the ethyl substituent of 1 is changed to a methyl substituent in 2, activity is only slightly reduced (3-4-fold), whereas inversion of the Phe unit flanking the bis-thiazoline moiety results in 16-23-fold greater potency. We show that both 1 and 3 cause G1 phase cell cycle arrest at lower concentrations, followed at higher concentrations by G2/M phase arrest, and that these compounds bind Cu(2+) and Zn(2+). The three-dimensional structure of 2 was determined by MS, NMR, and X-ray crystallography, and the structure of 3 was established by MS, NMR, and chemical degradation. The structure of 3 was explored by in silico molecular modeling, revealing subtle differences in overall conformation between 1 and 3. Attempts to interconvert 1 and 3 with base were unsuccessful, but enzymatic conversion may be possible and could be a novel form of activation for chemical defense. 相似文献
103.
Valerie R. Kempf Kazumasa Wakamatsu Shosuke Ito John D. Simon 《Photochemistry and photobiology》2010,86(4):765-771
The morphological and chemical changes associated with the exposure of melanosomes to methyl iodide are assessed by a variety of analytical, imaging and spectroscopic methods. Scanning electron microscopy, light scattering and N2 adsorption measurements all indicate significant changes in the morphology of the pigment following methylation. Solid-state nuclear magnetic resonance (SS-NMR) spectroscopy and chemical degradation analysis reveals the methylation results in the introduction of ester groups into the pigment structures. Amino acid analysis further reveals that Arg, Cys, His, Ser and Tyr undergo methylation; the SS-NMR data provide additional evidence for the methylation of the sulfur of Cys. Methylation results in increased solubility of the melanosome; the absorption properties of the dissolved material are characterized by an absorption maximum at 225 nm, with a long tail throughout the UV-A and UV-B, indicating that the solubilized material is a combination of protein and pigment. The methylation-induced decomposition of the melanosomes provides new insights into both the observed increase in O-methyl derivatives of the indolic precursor to eumelanin in the urine of melanoma patients and how increased levels of biologic methylating agents in the brain induce symptoms that resemble Parkinson’s disease. 相似文献
104.
105.
Sebastien Gibanel Valerie Heroguez Jacqueline Forcada 《Journal of polymer science. Part A, Polymer chemistry》2001,39(16):2767-2776
Macromonomers were synthesized by anionic “living” polymerization. They comprised a poly(ethylene oxide) hydrophilic block and a hydrophobic block or sequence terminated with an unsaturation. The surface activity properties of these materials (critical micelle concentration and parking area) were determined, and the values were compared and discussed in terms of the molecular structure of these new surfactants. Some of the macromonomers were employed as emulsifiers in two‐step emulsion polymerizations. The data obtained were discussed while taking into account the different chemical structures of the macromonomers and the efficiency of these species as emulsifiers in the polymerization recipes. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2767–2776, 2001 相似文献
106.
Hideo Tomioka Naoki Takase Yohichi Maeyama Keiji Hida Valerie Lemee Jean Pierre Fouassier Dominique Burget 《Research on Chemical Intermediates》2001,27(1-2):47-59
The title benzoins are prepared and their photochemistry has been investigated. Product analysis studies indicate that the benzoin undergoes Norrish Type I photocleavage to generate benzoyl and benzyloxybenzyl radical, followed by thermal fragmentation to produce deoxybenzoin thereby generating benzyl radical. All three transient species are actually detected and characterized by laser flash photolysis of the benzoin in solution at room temperature. The efficiency of the benzoins as photoinitiator has been also investigated. Effects of ring substituents and polycyclic aromatic rings on the photoreactivity have been revealed. 相似文献
107.
Graziani EI Ritacco FV Summers MY Zabriskie TM Yu K Bernan VS Greenstein M Carter GT 《Organic letters》2003,5(14):2385-2388
[reaction: see text] Two novel sulfur-containing analogs of the immunosuppressive natural product rapamycin (1) were obtained by feeding cultures of Streptomyces hygroscopicus with l-nipecotic acid (4) and either (S)-1,3-thiazane-4-carboxylic acid (5) or (S)-1,4-thiazane-3-carboxylic acid (6). The structures of the two new compounds, 20-thiarapamycin (2) and 15-deoxo-19-sulfoxylrapamycin (3), were determined by spectroscopic methods. 相似文献
108.
Harry Adams Neil A. Bailey Valerie A. Osborn Mark J. Winter 《Journal of organometallic chemistry》1985,284(1):C1-C4
Treatment of Mo(CNMe)(CO)2(η-C5H5)? with I[CH2]3I in tetrahydrofuran affords the carbene complex cis-MoI{C(NMe)[CH2[CH2} (CO)2 (η-C5H5), which has been characterised by X-ray crystallography. This complex does not isomerise to the corresponding trans isomer, as might have been expected by analogy with related 2-oxacyclopentylidene systems. 相似文献
109.
Wang W Cuyckens F Van den Heuvel H Apers S Pieters L Steenkamp V Stewart MJ Luyckx VA Claeys M 《Rapid communications in mass spectrometry : RCM》2003,17(1):49-55
Two novel compounds, 8-C-D-glucopyranosyl-7-hydroxy-5-methylchromone-2-carboxylic acid and a 2-O'-p-coumaroyl derivative thereof, were identified in a herbal tea that caused severe vomiting in a South African patient who had taken the traditional remedy to clean his stomach. For structural characterization, electrospray (ES) ionization in combination with collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS) were used, as well as UV and nuclear magnetic resonance (NMR) spectroscopy. Specific ions or neutral losses generated under conditions of ES-MS/CID/MS permitted the establishment of structural features such as the free carboxyl group, the C-hexosidic part and the p-coumaroyl group. NMR spectroscopy was necessary to support the structure of the chromone-type aglycone and the glucosidic parts. Since the compounds are structurally related to aloesin and aloeresin A, which are chemotaxonomic markers of Aloe species, and have not been previously reported, we propose that they were formed by oxidative degradation during preparation of the herbal tea from an Aloe species or during its storage. 相似文献
110.
Summary The recent upsurge of interest in chromium(V)(1–8) has highlighted two features of especial interest in its chemistry. Firstly, the complexes of chromium(V) are considerably more stable than has generally been assumed in the past, as exemplified by the recent characterisation of a water soluble complex, potassium bis(2-hydroxy-2-methylbutyrato)oxochromate(V)(1). Secondly, the d1 electronic configuration of the complexes makes them ideal model systems for study by such techniques as electron paramagnetic resonance (2) and electronic absorption spectroscopy(3,4) . Although chromium(V) oxide trichloride was first prepared in 1962, by the reaction between chromium(VI) oxide and SOCl2 or SO2Cl2
(9 10), and subsequently by the reaction between CrO2Cl2 and BCl3
(11), its chemistry and physical properties have remained virtually unstudied. We report here the first series of reactions of CrOCl3, along with details of its i.r. and photoelectron spectra. 相似文献