全文获取类型
收费全文 | 5613篇 |
免费 | 203篇 |
国内免费 | 21篇 |
专业分类
化学 | 4107篇 |
晶体学 | 36篇 |
力学 | 112篇 |
数学 | 696篇 |
物理学 | 886篇 |
出版年
2022年 | 55篇 |
2021年 | 80篇 |
2020年 | 108篇 |
2019年 | 112篇 |
2018年 | 71篇 |
2017年 | 65篇 |
2016年 | 163篇 |
2015年 | 111篇 |
2014年 | 146篇 |
2013年 | 216篇 |
2012年 | 307篇 |
2011年 | 377篇 |
2010年 | 153篇 |
2009年 | 130篇 |
2008年 | 303篇 |
2007年 | 260篇 |
2006年 | 273篇 |
2005年 | 243篇 |
2004年 | 190篇 |
2003年 | 165篇 |
2002年 | 156篇 |
2001年 | 77篇 |
2000年 | 88篇 |
1999年 | 47篇 |
1998年 | 57篇 |
1997年 | 41篇 |
1996年 | 72篇 |
1995年 | 89篇 |
1994年 | 53篇 |
1993年 | 76篇 |
1992年 | 71篇 |
1991年 | 58篇 |
1990年 | 56篇 |
1989年 | 56篇 |
1988年 | 45篇 |
1987年 | 49篇 |
1985年 | 49篇 |
1984年 | 63篇 |
1983年 | 43篇 |
1982年 | 74篇 |
1981年 | 67篇 |
1980年 | 54篇 |
1979年 | 57篇 |
1978年 | 49篇 |
1977年 | 55篇 |
1976年 | 50篇 |
1974年 | 51篇 |
1973年 | 41篇 |
1966年 | 42篇 |
1964年 | 41篇 |
排序方式: 共有5837条查询结果,搜索用时 15 毫秒
91.
Emil Pop Marcus E. Brewster J. J. Kaminski Nicholas Bodor 《International journal of quantum chemistry》1989,35(2):315-324
9-Amino-1,2,3,4-tetrahydroacridine (THA ), a potent cholinesterase inhibitor, was recently used in the treatment of Alzheimer's disease. On attempting to prepare a dihydropyridine ? pyridinium salt-based redox chemical delivery system (CDS ) to enhance brain delivery of THA , several of the practical synthetic challenges were examined by using a theoretical MO approach. The structures, reactivities and stability of THA , derivatives of THA and a model compound, 4-aminopyridine, a simple dibasic heterocyclic amine, were studied in the framework of the AM -1 approximation. The study included the possible protonated forms of THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine. The calculated heats of formation showed that ring nitrogen protonated forms are more stable for both THA and 4-aminopyridine, consistent with experimental results. Electron delocalization is responsible for the remarkable stability of these molecules and for the observed lack of reactivity of the amino group, both in the basic and protonated forms. The site of N-alkylation of the 9-nicotinamide derivative of THA (an intermediate in the synthesis of THA -CDS ) is controlled by electronic, thermodynamic, and steric factors. 相似文献
92.
Using recently developed theoretical techniques it has been possible to achieve an exact variational solution of the Schrödinger equation within a modest basis set or one-eleltron functions. The full configuration interaction for this system included a total of 256473 1A1 spin-and space-adapted configurations. Comparison with many-body perturbation theory proves to be quite interesting. 相似文献
93.
Two-dimensional on-line detection of brominated and iodinated volatile organic compounds by ECD and ICP-MS after GC separation 总被引:1,自引:0,他引:1
Inductively coupled plasma-mass spectrometry (ICP-MS) was coupled to a gas chromatographic (GC) system with electron capture detector (ECD), which enables relatively easy characterization and quantification of brominated and iodinated (halogenated) volatile organic compounds (HVOCs) in aquatic and air samples. The GC-ECD system is connected in series with an ICP-MS by a directly heated transfer line and an outlet port-hole for elimination of the ECD make-up gas during ignition of the plasma. The hyphenated GC-ECD/ICP-MS system provides high selectivity and sensitivity for monitoring individual HVOCs under fast chromatographic conditions. The ECD is most sensitive for the detection of chlorinated and brominated but the ICP-MS for iodinated compounds. The greatest advantage of the use of an ICP-MS is its element-specific detection, which allows clear identification of compounds in most cases. The absolute detection limits for ICP-MS are 0.5 pg for iodinated, 10 pg for brominated, and 50 pg for chlorinated HVOCs with the additional advantage that calibration is almost independent on different compounds of the same halogen. In contrast to that detection limits for ECD vary for the different halogenated compounds and lie in the range of 0.03-11 pg. The two-dimensional GC-ECD/ICP-MS instrumentation is compared with electron impact mass spectrometry (EI-MS) and microwave induced plasma atomic emission detection (MIP-AED). Even if EI-MS has additional power in identifying unknown peaks by its scan mode, the detection limits are much higher compared with GC-ECD/ICP-MS, whereas the selective ion monitoring mode (SIM) reaches similar detection limits. The MIP-AED detection limits are at the same level as EI-MS in the scan mode. 相似文献
94.
We report on phototriggered Ru-S --> Ru-O and thermal Ru-O --> Ru-S intramolecular linkage isomerizations in cis- and trans-[Ru(bpy)2(dmso)2]2+. The cis complex features only S-bonded sulfoxides (cis-[S,S]), whereas the trans isomer is characterized by S- and O-bonded dmso ligands. Both cis-[S,S] and trans-[S,O] exhibit photochromism at room temperature in dmso solution and ionic liquid (IL). Rates of reaction in IL were monitored by UV-visible spectroscopy and are similar to those reported in dmso solution (k(O-->S) ranges from approximately 10(-3) to 10(-4) s(-1)). Cyclic voltammetric measurements of cis-[S,S] and trans-[S,O] are consistent with an electrochemically triggered linkage isomerism mechanism. While both cis-[S,S] and trans-[S,O] are photochromic at room temperature, neither complex is emissive. However, upon cooling to 77 K, cis-[S,S] exhibits LMCT (ligand-to-metal charge transfer) emission typical of many ruthenium polypyridine complexes. In contrast to cis-[S,S], trans-[S,O] does not show any detectable emission even at 77 K. 相似文献
95.
Regulation of polar cap formation in the life cycle of Escherichia coli 总被引:48,自引:0,他引:48
96.
97.
Helmut Schwarz Chrysostomos Wesdemiotis Ferdinand Bohlmann 《Journal of mass spectrometry : JMS》1974,9(12):1223-1225
The electron-impact-induced elimination of piperidine from the title compound could be established by investigation of the unsymmetrically 2H-labelled compound. The mechanism of a 1,1-elimination can be excluded be unequivocally. 相似文献
98.
Multivalent cations can cause DNA to condense from its extended state in solution into high-density toroid-shaped particles. Developing methods to control the size and size distribution of DNA toroids is an important goal for the development of artificial gene delivery systems. Here we demonstrate that changes in salt conditions, prior to condensation by multivalent cations, can significantly affect DNA condensation. Specifically, millimolar concentrations of MgCl2 are shown to cause the formation of toroid clusters, whereas NaCl at the same ionic strength does not. 相似文献
99.
100.
Xudong?Hu Gui-Bing?Zhao S.?V.?B.?Janardhan Garikipati Kim?Nicholas Stanislaw?F.?Legowski Maciej?RadoszEmail author 《Plasma Chemistry and Plasma Processing》2005,25(4):351-370
Laser-induced fluorescence (LIF) is an effective in-situ probe for NO concentrations below 300 ppm in a non-thermal plasma reactor. A new method has been developed to measure in-situ NO concentration in the reactor discharge region using a long-time—on the order of seconds—averaged fluorescence detection. This method, for quantifying NO concentration in a nonthermal plasma reactor, is simpler than a short-time—on the order of nanoseconds—fluorescence detection. For accurate measurement based on the new method, the LIF intensity must be close to the corona-induced fluorescence (CIF) intensity; the CIF intensity serves as a guide in selecting the LIF intensity. We find that a kinetic model proposed earlier works for two-tube reactors and represents the NO concentration in the middle of the reactor, which verifies the assumption of gas plug flow. 相似文献