全文获取类型
收费全文 | 18708篇 |
免费 | 1267篇 |
国内免费 | 1292篇 |
专业分类
化学 | 16711篇 |
晶体学 | 203篇 |
力学 | 217篇 |
综合类 | 14篇 |
数学 | 50篇 |
物理学 | 4072篇 |
出版年
2024年 | 9篇 |
2023年 | 170篇 |
2022年 | 249篇 |
2021年 | 324篇 |
2020年 | 434篇 |
2019年 | 428篇 |
2018年 | 275篇 |
2017年 | 366篇 |
2016年 | 678篇 |
2015年 | 685篇 |
2014年 | 672篇 |
2013年 | 1426篇 |
2012年 | 952篇 |
2011年 | 1028篇 |
2010年 | 887篇 |
2009年 | 1127篇 |
2008年 | 1134篇 |
2007年 | 1239篇 |
2006年 | 1115篇 |
2005年 | 932篇 |
2004年 | 937篇 |
2003年 | 810篇 |
2002年 | 607篇 |
2001年 | 449篇 |
2000年 | 489篇 |
1999年 | 412篇 |
1998年 | 407篇 |
1997年 | 348篇 |
1996年 | 344篇 |
1995年 | 350篇 |
1994年 | 283篇 |
1993年 | 235篇 |
1992年 | 235篇 |
1991年 | 168篇 |
1990年 | 135篇 |
1989年 | 116篇 |
1988年 | 131篇 |
1987年 | 102篇 |
1986年 | 86篇 |
1985年 | 76篇 |
1984年 | 89篇 |
1983年 | 47篇 |
1982年 | 88篇 |
1981年 | 45篇 |
1980年 | 39篇 |
1979年 | 47篇 |
1978年 | 21篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
951.
Veronika Pilařová Tomáš Gottvald Pavel Svoboda Ondřej Novák Karolína Benešová Sylvie Běláková Lucie Nováková 《Analytica chimica acta》2016
The goal of this study was to develop an effective supercritical fluid chromatography method using single quadrupole MS for analysis of all isomeric forms of vitamin E. Finally, two fast and effective methods, the high resolution one and the high speed one, for the determination of 8 vitamin E isomers in human serum were developed. 相似文献
952.
Swantje Mohr Dr. Jagannath Jana Yoanes Maria Vianney Prof. Dr. Klaus Weisz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(40):10437-10447
Canonical G-quadruplexes can adopt a variety of different topologies depending on the arrangement of propeller, lateral, or diagonal loops connecting the four G-columns. A novel intramolecular G-quadruplex structure is derived through inversion of the last G-tract of a three-layered parallel fold, associated with the transition of a single propeller into a lateral loop. The resulting (3+1) hybrid fold features three syn⋅anti⋅anti⋅anti G-tetrads with a 3’-terminal all-syn G-column. Although the ability of forming a duplex stem-loop between G-tracts seems beneficial for a propeller-to-lateral loop rearrangement, unmodified G-rich sequences resist folding into the new (3+1) topology. However, refolding can be driven by the incorporation of syn-favoring guanosine analogues into positions of the fourth G-stretch. The presented hybrid-type G-quadruplex structure as determined by NMR spectroscopy may provide for an additional scaffold in quadruplex-based technologies. 相似文献
953.
Atomic levels population measurements constitute an essential contribution to various aspects of plasma research: balance and cross-section of inelastic processes, admixtures effects on level excitation mechanism, purposeful optimisation of plasma parameters etc. Atomic level populations in the negative glow of a hollow cathode discharge are of utmost interest in view of the peculiarities of the spectral source: a rich spectrum consisting of gas lines, lines of the cathode material and also lines of the substances introduced in the discharge; intensive atomic and ionic lines of high excitation potentials; small spectral linewidth, convenient for analytical purposes. 相似文献
954.
Sibel Topuz Buket Alpertunga 《International journal of environmental analytical chemistry》2013,93(9):787-795
A rapid and selective method for the simultaneous determination of triazine herbicides (atrazine, its degradation product desethylatrazine, simazine, prometryn, terbutryn) and N-methylcarbamate insecticides (propoxur, carbaryl and methiocarb) in surface water has been developed. A 0.5 L of the water sample was preconcentrated by passage through a 1 g C18 solid-phase extraction cartridge. The retained compounds were eluted with 5 mL of methanol from the cartridge. The pesticides were separated and quantified by reversed-phase high-performance liquid chromatography with UV diode-array detection. Analytical separation was performed using a concave gradient elution with acetonitrile and water on a C18 column. Prometryn and terbutryn were determined at 240 nm; propoxur, methiocarb at 204 nm and the others at 220 nm. Recoveries varied from 85 to 102% over concentrations at 0.025 and 0.2 µg L?1. The limits of detection for the compounds investigated are in the range of 0.005-0.012 µg L?1. 相似文献
955.
956.
I. Kraicheva P. Finocchiaro S. Failla 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2345-2354
The synthesis of three novel 5-methylfuryl-containing Schiff bases: N,N′-bis(5-methylfurfurylidene)-4,4′-diaminodiphenylmethane, N,N′-bis(5-methylfurfurylidene)-1,4-phenylenediamine, and N,N′-bis(5methylfurfurylidene)benzidine and the corresponding bis(aminophosphonates) derived from them, 4,4′-bis{N-methyl(diethoxyphosphonyl)-1-[(5-methyl)-2-furyl]} diaminodiphenylmethane, 1,4-bis { N-methyl(diethoxyphosphonyl)-1-[(5-methyl)-2-furyl]} diaminobenzene, and bis{N-methyl(diethoxyphosphonyl)-1-[(5-methyl)-2-furyl]}- benzidine, is reported. The compounds have been characterized by elemental analysis, TLC, IR, and NMR ( 1 H, 13 C, and 31 P) spectra. For comparison, new 13 C and 31 P NMR data of three furyl-containing analogues of the above bis(aminophosphonates) are also regarded. The NMR studies of the two series of bis(aminophosphonates) reveal the presence of one diastereomer (meso or racemic form). 相似文献
957.
958.
SpecPad: device‐independent NMR data visualization and processing based on the novel DART programming language and Html5 Web technology 下载免费PDF全文
Bruno Guigas 《Magnetic resonance in chemistry : MRC》2017,55(9):821-827
SpecPad is a new device‐independent software program for the visualization and processing of one‐dimensional and two‐dimensional nuclear magnetic resonance (NMR) time domain (FID) and frequency domain (spectrum) data. It is the result of a project to investigate whether the novel programming language DART, in combination with Html5 Web technology, forms a suitable base to write an NMR data evaluation software which runs on modern computing devices such as Android, iOS, and Windows tablets as well as on Windows, Linux, and Mac OS X desktop PCs and notebooks. Another topic of interest is whether this technique also effectively supports the required sophisticated graphical and computational algorithms. SpecPad is device‐independent because DART's compiled executable code is JavaScript and can, therefore, be run by the browsers of PCs and tablets. Because of Html5 browser cache technology, SpecPad may be operated off‐line. Network access is only required during data import or export, e.g. via a Cloud service, or for software updates. A professional and easy to use graphical user interface consistent across all hardware platforms supports touch screen features on mobile devices for zooming and panning and for NMR‐related interactive operations such as phasing, integration, peak picking, or atom assignment. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
959.
Effect of methoxyl groups on the NMR spectra: configuration and conformation of natural and synthetic indanic and tetralinic structures 下载免费PDF全文
Beatriz Lantaño José M. Aguirre Eleonora V. Drago Diego J. de la Faba Nicolás Pomilio Jorge D. Mufato 《Magnetic resonance in chemistry : MRC》2017,55(7):619-633
Here, we studied the influence of the methoxyl groups attached at C‐7 and C‐2′ of natural and synthetic 1‐arylindanes on the chemical shift of the signal of bibenzylic hydrogen and carbon atoms and J1,2 coupling constants. This influence was also analysed in natural 1‐aryltetralins and related compounds that possess methoxyl and/or hydroxyl groups bound at C‐8 and C‐2′. The methoxyl groups attached at C‐7 in indanes or at C‐8 in tetralins produce a deshielding signal at H‐1 and shield at C‐1 and a strong decrease in the value of J1,2 due to the pseudoequatorial location adopted by the aryl group bound at C‐1, avoiding an ‘A1,3 strain’. Furthermore, compounds with hydroxyl or methoxyl groups in C‐2′, in the absence of substituents of C‐7 or C‐8, present a strong deshielding signal at H‐1, strong shield of the C‐1 signal and a decrease in the value of J1,2. This is attributed to the stereoelectronic effects of the methoxyl or hydroxyl groups, which we have called ‘Asarone effect’. NOESY experiments were conducted to confirm the configuration and conformation of some of the compounds included in this work. This study shows that both effects, A1,3 strain and Asarone effect, must be taken into account when the structure of natural indanes and tetralins is analysed by using 1H‐NMR and 13C‐NMR spectra. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
960.