全文获取类型
收费全文 | 3960篇 |
免费 | 129篇 |
国内免费 | 18篇 |
专业分类
化学 | 2875篇 |
晶体学 | 15篇 |
力学 | 58篇 |
数学 | 620篇 |
物理学 | 407篇 |
无线电 | 132篇 |
出版年
2023年 | 22篇 |
2022年 | 21篇 |
2021年 | 45篇 |
2020年 | 65篇 |
2019年 | 60篇 |
2018年 | 33篇 |
2017年 | 29篇 |
2016年 | 121篇 |
2015年 | 102篇 |
2014年 | 101篇 |
2013年 | 185篇 |
2012年 | 165篇 |
2011年 | 196篇 |
2010年 | 135篇 |
2009年 | 120篇 |
2008年 | 186篇 |
2007年 | 199篇 |
2006年 | 181篇 |
2005年 | 178篇 |
2004年 | 185篇 |
2003年 | 132篇 |
2002年 | 135篇 |
2001年 | 77篇 |
2000年 | 58篇 |
1999年 | 67篇 |
1998年 | 68篇 |
1997年 | 65篇 |
1996年 | 71篇 |
1995年 | 61篇 |
1994年 | 64篇 |
1993年 | 55篇 |
1992年 | 49篇 |
1991年 | 40篇 |
1990年 | 28篇 |
1989年 | 66篇 |
1988年 | 57篇 |
1987年 | 64篇 |
1986年 | 43篇 |
1985年 | 42篇 |
1984年 | 54篇 |
1983年 | 43篇 |
1982年 | 57篇 |
1981年 | 49篇 |
1980年 | 50篇 |
1979年 | 40篇 |
1978年 | 28篇 |
1977年 | 38篇 |
1976年 | 22篇 |
1975年 | 30篇 |
1973年 | 25篇 |
排序方式: 共有4107条查询结果,搜索用时 31 毫秒
91.
92.
Bernd M. Rode 《Chemical physics letters》1974,26(3):350-355
A typical “chelate” effect is demonstrated by means of all-electron ab initio LCGO SCF calculations on the [Li(HCONH2)2]+ complex in different geometrical arrangements. Energetic factors and the electronic structure of the complexes are discussed, showing the chelate effect to be connected with striking changes in bonding, density distribution and electronic energy. 相似文献
93.
Achim Häußermann Dr. Frank Rominger Prof. Dr. Bernd F. Straub 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(44):14174-14185
A sterically shielded 3‐substituted zwitterionic N,N‐dimethylisotryptammonium carboxylate has been synthesized by consecutive chemoselective double alkylation of indole. The carboxylate undergoes a quantitative and unusually facile decarboxylation in dimethyl sulfoxide (DMSO) or dimethyl formamide (DMF) at room temperature. The breaking of a nearly equidistant hydrogen bond by solvent molecules initiates heterolytic C? C cleavage. The decarboxylation rate decreases with increasing CO2 partial pressure, proving the competitiveness of protonation and re‐carboxylation of the carbanionic intermediate. Corresponding spiro compounds containing silylene and stannylene moieties show high thermal stability. Addition of an excess of methyllithium to the sodium salt triggers a reaction sequence comprising a deprotonation, carboxylate transfer, and nucleophilic trapping of the rearranged carboxylate by another equivalent of methyllithium. Hydrolytic work‐up of the geminal diolate leads to an acetyl product. The role of the sodium counterion and the mechanism of the rearrangement have been unraveled by deuteration experiments. 相似文献
94.
Latest environmental regulations require a very deep desulfurization to meet the ultra-low sulfur diesel (ULSD, 15 ppm sulfur) specifications. Due to the disadvantages of hydrotreating technology on the slashing production conditions, costs and safety as well as environmental protection, the ultrasound-assisted oxidative desulfurization (UAOD) as an alternative technology has been developed. UAOD process selectively oxidizes sulfur in common thiophenes in diesel to sulfoxides and sulfones which can be removed via selective adsorption or extractant. SulphCo has successfully used a 5000 barrel/day mobile “Sonocracking” unit to duplicate on a commercial scale its proprietary process that applies ultrasonics at relatively low temperatures and pressures. The UAOD technology estimate capital costs less than half the cost of a new high-pressure hydrotreater. The physical and chemical mechanisms of UAOD process are illustrated, and the effective factors, such as ultrasonic frequency and power, oxidants, catalysts, phase-transfer agent, extractant and adsorbent, on reaction kinetics and product recovery are discussed in this review. 相似文献
95.
96.
Christos Apostolidis Dr. Bernd Schimmelpfennig Dr. Nicola Magnani Dr. Patric Lindqvist‐Reis Dr. Olaf Walter Dr. Richard Sykora Dr. Alfred Morgenstern Dr. Eric Colineau Dr. Roberto Caciuffo Prof. Dr. Reinhardt Klenze Dr. Richard G. Haire Dr. Jean Rebizant Dr. Frank Bruchertseifer Dr. Thomas Fanghänel Prof. Dr. 《Angewandte Chemie (International ed. in English)》2010,49(36):6229-6229
97.
The preparation of luminescent hydrogels based on the electrostatic self-assembly of 1-pyrenesulfonate (PyS) anions in a positively charged N-isopropylacrylamide (NiPAAm)/surfmer copolymer gel is described. The hydrogels were prepared from a micellar aqueous solution of 11-acryloyloxyundecyltrimethylammonium bromide and NiPAAm upon 60Co-gamma irradiation. For assembly of PyS, the gel was shrunken at 50 °C and re-swollen at 20 °C in an aqueous solution of sodium 1-pyrenesulfonate. Re-swelling was accompanied by electrostatic assembly of PyS anions within the gel. Subsequently, the excess non-bound PyS ions were removed by repeatedly swelling and shrinking the gel in pure water at 20 °C and 50 °C, respectively. Incorporation of PyS ions in the hydrogel was studied using UV/Vis spectroscopy and energy dispersive x-ray (EDX) measurements. The amount of electrostatically adsorbed PyS was found to be proportional to the amount of copolymerized surfmer. EDX studies indicated that 20% of the bromide counterions were replaced for PyS. The PyS counterions could be released again if the functionalized hydrogel was immersed in acidified water. At a pH of 1, an almost complete release of PyS was found after 250 h. The preparation method can be used to introduce a variety of functional properties in thermoresponsive charged copolymer gels. 相似文献
98.
The use of 1,2‐diselenolato‐1,2‐dicarba‐closo‐dodecaborane(12) dianions [1,2‐(1,2‐C2B10H10)Se2]2? prepared in situ as the dilithium salt may lead to irreproducible results. This is shown by the straightforward synthesis of silanes using the purified and isolated dianions, in contrast with previous less successful attempts. Thus, the reactions of the dianions with dichlorosilanes afford the five‐membered diselenasila cycles containing the SiMe2 or the SiPh2 units, and with 1,2‐dichloro‐tetramethyldisilane the six‐membered cycle containing the Si2Me4 unit. The latter was studied by X‐ray diffraction, and all products were characterized by multinuclear magnetic resonance (1H, 13C, 29Si, 77Se NMR) in solution. Novel isotope effects were detected in 13C and 77Se NMR spectra. Exchange reactions of the five‐ and six‐membered diselanasila cycles with chlorosilanes were studied. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
99.
Hubertus Appler Lutz W. Gross Bernd Mayer Wilhelm P. Neumann 《Journal of organometallic chemistry》1985,291(1):9-23
Factors governing the ease and mechanism of 7-silabicyclo[2.2.1]heptadienes thermolysis in order to generate free silylenes and the corresponding benzene derivatives are investigated. For this purpose, 29 new compounds of the types VII–X have been prepared. No indications for a polar mechanism or an intermediate biradical could be found. The degradation is exactly of first order in all cases investigated sofar, and is enhanced by phenyl groups at the bridgehead C atoms, if a conformation coplanar with the basic ring is allowed by the neighbouring substituents, but is not enhanced by phenyl groups at the Si. The X-ray structure of two typical derivatives is discussed with this respect. A special mechanism is operating in the easy thermolysis of carbomethoxy-substituted compounds leading to cyclic sila enolether intermediates. 相似文献
100.
The diastereoselectivity of the mercuration of acyclic alkenes can be reversed by changing the substituent in the allylic position; with alcohols the erythro isomers and with esters or hemiacetals the threo isomers and are formed predominantly (Table I). 相似文献