全文获取类型
收费全文 | 3888篇 |
免费 | 118篇 |
国内免费 | 80篇 |
专业分类
化学 | 2742篇 |
晶体学 | 28篇 |
力学 | 54篇 |
综合类 | 3篇 |
数学 | 699篇 |
物理学 | 560篇 |
出版年
2023年 | 19篇 |
2021年 | 37篇 |
2020年 | 23篇 |
2019年 | 45篇 |
2018年 | 27篇 |
2017年 | 32篇 |
2016年 | 77篇 |
2015年 | 59篇 |
2014年 | 68篇 |
2013年 | 198篇 |
2012年 | 173篇 |
2011年 | 199篇 |
2010年 | 104篇 |
2009年 | 89篇 |
2008年 | 251篇 |
2007年 | 266篇 |
2006年 | 268篇 |
2005年 | 278篇 |
2004年 | 211篇 |
2003年 | 142篇 |
2002年 | 138篇 |
2001年 | 57篇 |
2000年 | 68篇 |
1999年 | 63篇 |
1998年 | 45篇 |
1997年 | 50篇 |
1996年 | 73篇 |
1995年 | 50篇 |
1994年 | 54篇 |
1993年 | 40篇 |
1992年 | 43篇 |
1991年 | 32篇 |
1990年 | 34篇 |
1989年 | 28篇 |
1988年 | 32篇 |
1987年 | 37篇 |
1986年 | 46篇 |
1985年 | 61篇 |
1984年 | 63篇 |
1983年 | 51篇 |
1982年 | 43篇 |
1981年 | 47篇 |
1980年 | 53篇 |
1979年 | 33篇 |
1978年 | 46篇 |
1977年 | 37篇 |
1976年 | 30篇 |
1975年 | 26篇 |
1974年 | 24篇 |
1973年 | 31篇 |
排序方式: 共有4086条查询结果,搜索用时 312 毫秒
51.
Abstract— The binding of DNA to protein mediated by U V (254 nm) radiation has been investigated using binding of the complex to Millipore membrane filters as an assay technique. The reaction proceeds through an activated protein intermediate which then reacts with the DNA. The activated protein has a half-life of about 75 min at 0°C and about 18 min at 37°C. Short wavelengths are more efficient in forming the complex than wavelengths in the 250–280 nm range. N-ethyl maleimide treatment of protein before irradiation markedly inhibits the reaction. 相似文献
52.
Ian R. Gould Jacques E. Moser Bruce Armitage Samir Farid 《Research on Chemical Intermediates》1995,21(7):793-806
The overall efficiencies of photoinduced electron transfer reactions in polar solvents are usually determined by the efficiency with which separated radical ions are formed from the initially formed geminate radical-ion pairs. These separation efficiencies are determined by the competition between retum electron transfer and separation within the geminate pairs. A method is described for determining whether variations in the quantum yields for formation of separated radical ions are due to changes in the reorganization parameters for the return electron transfer reactions, or to other factors. The use of the method is illustrated in studies of the effects of varying steric bulk and molecular size of the donors, and also in studies of the effect of using a charged sensitizer. 相似文献
53.
Robert L Waterland Kerwin D Dobbs Amy M Rinehart Andrew E Feiring Robert C Wheland Bruce E Smart 《Journal of fluorine chemistry》2003,122(1):37-46
In its continuing quest for smaller length scales, the electronics industry plans to introduce 157 nm as the next lithographic wavelength. Accordingly, there is a pressing need to develop photoresists that are more transparent, and pellicles that are both more transparent and more durable. With the advent and popularization of time-dependent density functional theory (TD-DFT), we now have a practical quantum chemical method for calculating excitation energies and transition moments in the vacuum ultraviolet (VUV) which can greatly assist in the scouting of highly transparent materials. We have performed TD-DFT calculations for a broad variety of fluorinated molecules and we will report calculated VUV photoabsorption spectra for a large family of model fluorohexanes. These calculations, which span a range from 1-fluorohexane to CH3CF2CF2CF2CF2CH3, illustrate some of the principles one may use to design low absorption polymeric materials. 相似文献
54.
Michael J. Costanzo Harold R. AlmondJr. A. Diane Gauthier Bruce E. Maryanoff 《Tetrahedron: Asymmetry》1994,5(12)
The conformational preferences for 2,3-O-isopropylidene-α-
-sorbopyranose derivatives 3–6 were determined by using 1H NMR data and empirical force field calculations. Proton NMR studies of 3–6 indicate that a twist-boat (or skew) conformation (3S0) prevails over possible chair forms for each compound. Force-field calculations (MM2, MNDO, AM1) on a model 2,3-O-isopropylidene-α-
-sorbopyranose system (18) indicate that the 3S0 conformation is among the low-energy structures. X-Ray crystallographic analysis of α-
-sorbopyranose sulfamate 3, a compound with potent anticonvulsant activity, demonstrates that the 3S0 skew conformation is manifested in the solid state, as well. 相似文献
55.
The most prominent ion in the mass spectra of C6F5CH2X (X ? H, Br, CH:CH2, COCl, and CH2C6F5) is C7F5H2+, formulated as the pentafluorotropylium cation. This ion is also found, in an amount comparable to the parent ion, in the spectrum of (C6F5)2CH2. The heptafluorotropylium cation is found similarly in the spectrum of C6F5CF3. The mass spectra of (C6F5)2CHBr and [(C6H5)2CH]2 exhibit an ion C13F10H+ as the base peak, which is probably a pentafluorophenylpentafluorotropylium cation. The alcohol (C6F5)2CHOH shows loss of C6F5, followed by 2H, as a major breakdown pathway. The mode of formation, and the subsequent fragmentation, of the major ions in these spectra, are discussed. 相似文献
56.
The glucuronic acid analog 5 of 1-deoxynojirimycin has been synthesized in good overall yield from D-glucose. 相似文献
57.
In an approach to SRS-A and analogues thereof, the key (5, 6)-epoxy alcohol and its 6-epimer were prepared starting from -araboascorbic acid and -diethyl tartrate, respectively. 相似文献
58.
King NC Dickinson C Zhou W Bruce DW 《Dalton transactions (Cambridge, England : 2003)》2005,(6):1027-1032
Mesoporous silicates are prepared by templating on the hexagonal (H1) mesophase of surfactant bipyridine complexes of ruthenium(II) using a true liquid-crystal templating approach. On calcination, the surfactant template is removed except for the central metal ion that is oxidised, forming nanoparticles of RuO2 that deposit within the pores. RuO2 is a known oxidation catalyst and, despite its anhydrous nature in these silicates, is found to be very active in catalyzing the oxidation of water by acidic CeIV. 相似文献
59.
This article reports in detail on the discovery that zirconium(IV) salts of secondary amides and lactams are transformed by Cp(2)ZrHCl to N-substituted imines in one step. The method represents the first controlled reduction of amides and lactams to the corresponding imines, a transformation that is otherwise very difficult to achieve because imines are reduced more rapidly than carboxamides by most metal hydride reagents. No products of reductive cleavage of the carboxamides are observed. Efforts to replace 2 mol equiv of Cp(2)ZrHCl with simpler, less costly alternatives led to the finding that a diisobutylaluminum enolate can be substituted for the initial zirconium enolate. Such aluminum amides are smoothly reduced to the corresponding imine using Cp(2)ZrHCl in good yield. Moreover, aluminum amides are also reduced to imines using either low-valent titanium species or triethylsilane. In these alternative procedures, use of Cp(2)ZrHCl is eliminated altogether in the title transformation. 相似文献
60.
King RB 《Inorganic chemistry》2003,42(26):8755-8761
The bismuth polyhedra in ternary transition metal-centered bismuth cluster halides may form discrete molecules or ions, infinite chains, and/or infinite layers. The chemical bonding in many of these diverse structures is related to that in deltahedral boranes exhibiting three-dimensional aromaticity by replacing the multicenter core bond in the boranes with two-center two-electron (2c-2e) bonds from the central transition metal to the nearest neighbor bismuth vertices. Examples of discrete molecules or ions include octahedral MBi(6)(micro-X)(12)(z)()(-) (X = Br, I; M = Rh, Ir, z = 3; M = Ru, z = 4) with exclusively 2c-2e bonds and pentagonal bipyramidal RhBi(7)Br(8) with a 5c-4e bond in the equatorial pentagonal plane indicative of M?bius aromaticity. The compound Ru(3)Bi(24)Br(20) contains a more complicated discrete bismuth cluster ion Ru(2)Bi(17)(micro-Br)(4)(5+), which can be dissected into a RuBi(5) closo octahedron and a RuBi(8) nido capped square antiprism bridged by a Ru(2)Bi(4)(micro-Br)(4) structural unit. In RuBi(4)X(2) (X = Br, I), the same Ru(2)Bi(4)(micro-Br)(4) structural unit bridges Bi(4) squares similar to those found in the known Zintl ion Bi(4)(2)(-) to give infinite chains of Ru(2)Bi(4) octahedra. The electron counts of the RuBi(5), RuBi(8), and Ru(2)Bi(4) polyhedra in these structures follow the Wade-Mingos rules. A different infinite chain structure is constructed from fused RhBi(7/2)Bi bicapped trigonal prisms in Rh(2)Bi(9)Br(3). This Rh(2)Bi(9)Br(3) structure can alternatively be derived from alternating Rh(2/2)Bi(4) octahedra and Rh(2/)(2)Bi(5) pentagonal bipyramids with electron counts obeying the Wade-Mingos rules. Related chemical bonding principles appear to apply to more complicated layer structures such as Pt(3)Bi(13)I(7) containing Kagomé nets of PtBi(8/2) cubes and Ni(4)Bi(12)X(3) containing linked chains of NiBi(6/3)Bi capped trigonal prisms. 相似文献