全文获取类型
收费全文 | 68173篇 |
免费 | 10380篇 |
国内免费 | 3028篇 |
专业分类
化学 | 65400篇 |
晶体学 | 997篇 |
力学 | 1574篇 |
综合类 | 31篇 |
数学 | 8234篇 |
物理学 | 5345篇 |
出版年
2024年 | 16篇 |
2023年 | 115篇 |
2022年 | 176篇 |
2021年 | 377篇 |
2020年 | 779篇 |
2019年 | 2577篇 |
2018年 | 2423篇 |
2017年 | 2842篇 |
2016年 | 3142篇 |
2015年 | 5446篇 |
2014年 | 5212篇 |
2013年 | 7323篇 |
2012年 | 5830篇 |
2011年 | 5424篇 |
2010年 | 4509篇 |
2009年 | 4357篇 |
2008年 | 4763篇 |
2007年 | 4016篇 |
2006年 | 3666篇 |
2005年 | 3513篇 |
2004年 | 2970篇 |
2003年 | 2694篇 |
2002年 | 3355篇 |
2001年 | 1742篇 |
2000年 | 1601篇 |
1999年 | 794篇 |
1998年 | 261篇 |
1997年 | 210篇 |
1996年 | 172篇 |
1995年 | 136篇 |
1994年 | 143篇 |
1993年 | 105篇 |
1992年 | 101篇 |
1991年 | 78篇 |
1990年 | 78篇 |
1989年 | 91篇 |
1988年 | 64篇 |
1987年 | 47篇 |
1986年 | 45篇 |
1985年 | 54篇 |
1984年 | 47篇 |
1983年 | 37篇 |
1982年 | 36篇 |
1981年 | 34篇 |
1980年 | 35篇 |
1979年 | 30篇 |
1978年 | 21篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1971年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Katarzyna Paradowska Micha Wolniak Maciej Pisklak Jan A. Gliski Matthew H. Davey Iwona Wawer 《Solid state nuclear magnetic resonance》2008,34(4):202-209
Oxindole alkaloids, isolated from the bark of Uncaria tomentosa [Willd. ex Schult.] Rubiaceae, are considered to be responsible for the biological activity of this herb. Five pentacyclic and two tetracyclic alkaloids were studied by solid-state NMR and theoretical GIAO DFT methods. The 13C and 15N CPMAS NMR spectra were recorded for mitraphylline, isomitraphylline, pteropodine (uncarine C), isopteropodine (uncarine E), speciophylline (uncarine D), rhynchophylline and isorhynchophylline. Theoretical GIAO DFT calculations of shielding constants provide arguments for identification of asymmetric centers and proper assignment of NMR spectra. These alkaloids are 7R/7S and 20R/20S stereoisomeric pairs. Based on the 13C CP MAS chemical shifts the 7S alkaloids (δ C3 70–71 ppm) can be easily and conveniently distinguished from 7R (δC3 74.5–74.9 ppm), also 20R (δC20 41.3–41.7 ppm) from the 20S (δC20 36.3–38.3 ppm). The epiallo-type isomer (3R, 20S) of speciophylline is characterized by a larger 15N MAS chemical shift of N4 (64.6 ppm) than the allo-type (3S, 20S) of isopteropodine (δN4 53.3 ppm). 15N MAS chemical shifts of N1–H in pentacyclic alkaloids are within 131.9–140.4 ppm. 相似文献
992.
《Journal of separation science》2018,41(18):3553-3560
The combination of sofosbuvir and daclatasvir or sofosbuvir and ledipasvir is now widely used as an ideal treatment for hepatitis C virus infection. For this purpose, a simple, sensitive, accurate, economic, and precise high‐performance thin‐layer chromatography was developed and validated for the determination of sofosbuvir, daclatasvir, and ledipasvir in their pure form as well as their different pharmaceutical products. The method used Merck high‐performance thin‐layer chromatography aluminum plates precoated with silica gel 60 F254 as a stationary phase and mobile phase consisting of methylene chloride/methanol/ethyl acetate/ammonia (25%) (6:1:4:1, v/v/v/v). This system was found to give compact symmetric peaks of sofosbuvir, daclatasvir, and ledipasvir with retardation factors of 0.27 ± 0.01, 0.50 ± 0.007, and 0.68 ± 0.008, respectively. The densitometric scanner was set at 275 nm using a deuterium lamp. The calibration curves were linear over the range of 100–3000 ng/spot for sofosbuvir, and daclatasvir, and range of 50–3000 ng/spot for ledipasvir. The detection limits were 22.5, 31.90, and 15.80 for sofosbuvir, daclatasvir, and ledipasvir. The quantitation limits were 67.50, 95.60, and 47.50 for sofosbuvir, daclatasvir, and ledipasvir. The proposed method was validated according to International Conference on Harmonization (ICH) guidelines and the results were acceptable. 相似文献
993.
《Macromolecular bioscience》2018,18(7)
Herein, a new series of non‐ionic dendritic and carbohydrate based amphiphiles is synthesized employing biocompatible starting materials and studied for supramolecular aggregate formation in aqueous solution. The dendritic amphiphiles 12 and 13 possessing poly(glycerol) [G2.0] as hydrophilic unit and C‐10 and C‐18 hydrophobic alkyl chains, respectively, exhibit low critical aggregation concentration (CAC) in the order of 10−5m and hydrodynamic diameters in the 8–10 nm range and supplemented by cryogenic transmission electron microscopy. Ultraviolet‐visible (UV‐Vis) and fluorescence spectroscopy suggests the effective solubilization of hydrophobic guests by the self‐assembled architectures, with the nanotransporters 12 and 13 possessing the highest encapsulation efficiency of 80.74 and 98.03% for curcumin. Efficient uptake of encapsulated curcumin in adenocarcinomic human alveolar basal epithelial (A549) cells is observed by confocal laser scanning microscopy. Amphiphiles 12 and 13 are non‐cytotoxic at the concentrations studied, however, curcumin encapsulated samples efficiently reduce the viability of A549 cells in vitro. Experimental studies indicate the ability of amphiphile 13 to encapsulate 1‐anilinonaphthalene‐8‐sulfonic acid (ANS) and curcumin with binding constant of 1.16 × 1055m −1 and 1.43 × 106m −1, respectively. Overall, our findings demonstrate the potential of these dendritic amphiphiles for the development of prospective nanocarriers for the solubilization of hydrophobic drugs. 相似文献
994.
Elimination of vinyl alcohol from 5‐vinyloxypyrrolines in the presence of superbases (final step of domino 3H‐pyrroles synthesis from ketoximes and acetylene) is studied computationally at different levels of theory in DMSO solution (PCM). The sequences of transformations starting from nucleophilic addition of hydroxide ion to carbon‐carbon or carbon‐nitrogen double bonds are proposed as possible mechanisms. Unusual reactivity of 2,5‐dimethylphenyl substituted 5‐vinyloxypyrroline is explained within these mechanisms. 相似文献
995.
996.
997.
998.
999.
1000.