全文获取类型
收费全文 | 1468篇 |
免费 | 42篇 |
国内免费 | 12篇 |
专业分类
化学 | 963篇 |
晶体学 | 9篇 |
力学 | 35篇 |
数学 | 207篇 |
物理学 | 308篇 |
出版年
2023年 | 15篇 |
2022年 | 9篇 |
2021年 | 24篇 |
2020年 | 32篇 |
2019年 | 23篇 |
2018年 | 21篇 |
2016年 | 29篇 |
2015年 | 23篇 |
2014年 | 25篇 |
2013年 | 69篇 |
2012年 | 88篇 |
2011年 | 92篇 |
2010年 | 52篇 |
2009年 | 59篇 |
2008年 | 74篇 |
2007年 | 91篇 |
2006年 | 86篇 |
2005年 | 77篇 |
2004年 | 51篇 |
2003年 | 37篇 |
2002年 | 32篇 |
2001年 | 16篇 |
2000年 | 20篇 |
1999年 | 22篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 19篇 |
1994年 | 15篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 21篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 23篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 15篇 |
1977年 | 13篇 |
1976年 | 20篇 |
1975年 | 8篇 |
1974年 | 18篇 |
1973年 | 6篇 |
排序方式: 共有1522条查询结果,搜索用时 15 毫秒
51.
Yi‐Xun Zhang Shauntina H. Jackson Mohamed S. Rajab Frank R. Fronczek Steven F. Watkins 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(4):o219-o221
3β‐Hydroxy‐7‐drimen‐12,11‐olide hemihydrate, C15H22O3·0.5H2O, (I), has two sesquiterpene molecules and one water molecule in the asymmetric unit. The OH groups of both molecules and both H atoms of the water molecule are involved in near‐linear intermolecular hydrogen bonds, having O⋯O distances in the range 2.632 (3)–2.791 (2) Å. 3β‐Acetoxy‐7‐drimen‐12,11‐olide, C17H24O4, (II), has its ring system in very nearly the same conformation as the two molecules of (I). 相似文献
52.
Ruth Freitag Thomas Baltes Martin Eggert 《Journal of polymer science. Part A, Polymer chemistry》1994,32(16):3019-3030
Several batches of poly-N,N-diethylacrylamide were synthesized by anionic and by group transfer polymerization (GTP). A radical poly-N,N-diethylacrylamide prepared from the same monomer was also included in the comparison. According to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) both types of living polymerization resulted in narrow molecular weight distributions with Mw/Mn values below 1.5. Average molecular weights (Mn) between 888 and 4678 g/mol were calculated in these cases. The radical polymer had an average molecular weight (Mn) of approximately 130,000 g/mol. The dry anionic and GTP polymers were investigated by differential scanning calorimetry (DSC) and x-ray diffraction spectrometry. Evidence for partial crystallinity in the solid state was found. The conformation of all polymers was examined by high resolution (600 MHz) NMR. According to these measurements, 75% of the ? CHR? groups of the anionic poly-N,N-diethylacrylamide were located in an isotactic triade. The remaining 25% had heterotactic structure, while no indication for the presence of syndiotactic protons was found. Poly-N,N-diethylacrylamide prepared by GTP, on the other hand, had mainly syndiotactic structure. The aqueous solutions of the polymers showed phase separation upon heating. Whereas the lower critical solution temperature (LCST) was approximately 30°C in the case of the poly-N,N-diethylacrylamide prepared by GTP and by radical polymerization, uncommonly high LCSTs of more than 40°C were observed for the anionic poly-N,N-diethylacrylamide. © 1994 John Wiley & Sons, Inc. 相似文献
53.
Barro R Ares S Garcia-Jares C Llompart M Cela R 《Analytical and bioanalytical chemistry》2005,381(1):255-260
A combination of sorbent enrichment and ultrasound-assisted solvent extraction has been used to determine polychlorinated biphenyls in air. Analytes were sampled by pumping a known volume of air through a porous polymer (Tenax TA). The enriched adsorbent was transferred into a glass vial, and ultrasound-assisted extraction of the analytes was then performed in n-hexane. Quantification was carried out by using gas chromatography coupled to tandem mass spectrometry. Breakthrough volume of the sampling step was studied, indicating that 10 m3 of air could be processed without losses of the most volatile compounds. Good recoveries (75–96%) were obtained, and limits of detection at the sub ng m–3 were achieved for all the analytes. The proposed method is very simple and fast, avoiding the use of large solvent volumes and time-consuming preconcentration steps. 相似文献
54.
55.
56.
57.
58.
59.
Stephen Pleasance Joseph F. Anacleto M. Ruth Bailey David H. North 《Journal of the American Society for Mass Spectrometry》1992,3(4):378-397
Atmospheric pressure ionization (API) techniques are evaluated for the mass spectral analysis of N-methyl carbamate pesticides. Atmospheric pressure chemical ionization (APCI) using a heated nebulizer interface provided both protonated molecules and abundant, characteristic fragment ions. With ion spray (ISP; pneumatically assisted electrospray ionization), which utilizes a milder “ion evaporation” process, primarily protonated molecules were obtained, although fragment ions similar to those observed in APCI could be induced by variation of the API orifice voltage. Product ion spectra of ISP-derived protonated molecules, generated by tandem mass spectrometry using collision-induced dissociation, are also presented. The APCI and ISP spectra of the carbamates are compared to those obtained with a thermospray interface and also to their electron ionization and methane CI spectra obtained with a particle beam interface. For all four interfaces, combined liquid chromatography mass spectrometry methods using conventional (4.6 mm i.d.) columns are described for the separation and detection of pesticide mixtures. These methods are applied to the confirmatory analysis of three representative carbamate pesticides, spiked at the 0.1-ppm level in green peppers. For those carbamates amenable to gas chromatography mass spectrometry, comparative results are presented. 相似文献
60.
Dong Xiao Brian J. Lavey Anandan Palani Robert G. Aslanian Neng-Yang Shih Gerard P. Randolph Ruth A. Duffy 《Tetrahedron letters》2005,46(44):7653-7656
Unlike the lithiation of N-Boc-2-alkylpiperidines, which occurs at the 6-position, N-Boc-2-phenylpiperidine and N-Boc-2-phenylpyrrolidine can be lithiated exclusively at the 2-position. The tertiary carbanions can be trapped with a variety of electrophiles. This chemistry was used for the synthesis of a potent NK1 ligand (Ki = 0.3 nM). The bioactive configuration at the piperidine quaternary center was determined by X-ray analysis to be (S). 相似文献