全文获取类型
收费全文 | 472804篇 |
免费 | 4554篇 |
国内免费 | 1621篇 |
专业分类
化学 | 233140篇 |
晶体学 | 7605篇 |
力学 | 24072篇 |
综合类 | 12篇 |
数学 | 73274篇 |
物理学 | 140876篇 |
出版年
2020年 | 3251篇 |
2019年 | 3680篇 |
2018年 | 14437篇 |
2017年 | 14242篇 |
2016年 | 12573篇 |
2015年 | 5017篇 |
2014年 | 7163篇 |
2013年 | 17927篇 |
2012年 | 16380篇 |
2011年 | 25633篇 |
2010年 | 16554篇 |
2009年 | 16937篇 |
2008年 | 20645篇 |
2007年 | 22573篇 |
2006年 | 13761篇 |
2005年 | 13496篇 |
2004年 | 12688篇 |
2003年 | 12028篇 |
2002年 | 10898篇 |
2001年 | 12203篇 |
2000年 | 9407篇 |
1999年 | 7250篇 |
1998年 | 5970篇 |
1997年 | 5827篇 |
1996年 | 5653篇 |
1995年 | 5142篇 |
1994年 | 4965篇 |
1993年 | 4894篇 |
1992年 | 5574篇 |
1991年 | 5614篇 |
1990年 | 5357篇 |
1989年 | 5317篇 |
1988年 | 5380篇 |
1987年 | 5280篇 |
1986年 | 5002篇 |
1985年 | 6619篇 |
1984年 | 6820篇 |
1983年 | 5400篇 |
1982年 | 5675篇 |
1981年 | 5639篇 |
1980年 | 5354篇 |
1979年 | 5783篇 |
1978年 | 5847篇 |
1977年 | 5894篇 |
1976年 | 5777篇 |
1975年 | 5275篇 |
1974年 | 5294篇 |
1973年 | 5279篇 |
1972年 | 3695篇 |
1968年 | 3123篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
ABSTRACTQM(UB3LYP)/MM(AMBER) calculations were performed for the locations of the transition structure (TS) of the oxygen–oxygen (O–O) bond formation in the S4 state of the oxygen-evolving complex (OEC) of photosystem II (PSII). The natural orbital (NO) analysis of the broken-symmetry (BS) solutions was also performed to elucidate the nature of the chemical bonds at TS on the basis of several chemical indices defined by the occupation numbers of NO. The computational results revealed a concerted bond switching (CBS) mechanism for the oxygen–oxygen bond formation coupled with the one-electron transfer (OET) for water oxidation in OEC of PSII. The orbital interaction between the σ-HOMO of the Mn(IV)4–O(5) bond and the π*-LUMO of the Mn(V)1=O(6) bond plays an important role for the concerted O–O bond formation for water oxidation in the CaMn4O6 cluster of OEC of PSII. One electron transfer (OET) from the π-HOMO of the Mn(V)1=O(6) bond to the σ*-LUMO of the Mn(IV)4–O(5) bond occurs for the formation of electron transfer diradical, where the generated anion radical [Mn(IV)4–O(5)]-? part is relaxed to the ?Mn(III)4?…?O(5)- structure and the cation radical [O(6)=Mn(V)1]+ ? part is relaxed to the +O(6)–Mn(IV)1? structure because of the charge-spin separation for the electron-and hole-doped Mn–oxo bonds. Therefore, the local spins are responsible for the one-electron reductions of Mn(IV)4->Mn(III)4 and Mn(V)1->Mn(IV)1. On the other hand, the O(5)- and O(6)+ sites generated undergo the O–O bond formation in the CaMn4O6 cluster. The Ca(II) ion in the cubane- skeleton of the CaMn4O6 cluster assists the above orbital interactions by the lowering of the orbital energy levels of π*-LUMO of Mn(V)1=O(6) and σ*-LUMO of Mn(IV)4–O(5), indicating an important role of its Lewis acidity. Present CBS mechanism for the O–O bond formation coupled with one electron reductions of the high-valent Mn ions is different from the conventional radical coupling (RC) and acid-base (AB) mechanisms for water oxidation in artificial and native photosynthesis systems. The proton-coupled electron transfer (PC-OET) mechanism for the O–O bond formation is also touched in relation to the CBS-OET mechanism. 相似文献
72.
Anfimov N. V. Vorobel V. Kovalev Yu. S. Kodyš P. 《Physics of Particles and Nuclei Letters》2019,16(6):820-825
Physics of Particles and Nuclei Letters - The Silicon Photomultiplier (SiPM) is a novel semiconducting photodetector which can detect single photons. It consists of many microcells (pixels)... 相似文献
73.
Amphiphilic block copolymer self‐assemblies of poly(NVP)‐b‐poly(MDO‐co‐vinyl esters): Tunable dimensions and functionalities 下载免费PDF全文
Guillaume G. Hedir A. Pitto‐Barry Andrew P. Dove Rachel K. O'Reilly 《Journal of polymer science. Part A, Polymer chemistry》2015,53(23):2699-2710
Functional, degradable polymers were synthesized via the copolymerization of vinyl acetate (VAc) and 2‐methylene‐1,3‐dioxepane (MDO) using a macro‐xanthate CTA, poly(N‐vinylpyrrolidone), resulting in the formation of amphiphilic block copolymers of poly(NVP)‐b‐poly(MDO‐co‐VAc). The behavior of the block copolymers in water was investigated and resulted in the formation of self‐assembled nanoparticles containing a hydrophobic core and a hydrophilic corona. The size of the resultant nanoparticles was able to be tuned with variation of the hydrophilic and hydrophobic segments of the core and corona by changing the incorporation of the macro‐CTA as well as the monomer composition in the copolymers, as observed by Dynamic Light Scattering, Static Light Scattering, and Transmission Electron Microscopy analyses. The concept was further applied to a VAc derivative monomer, vinyl bromobutanoate, to incorporate further functionalities such as fluorescent dithiomaleimide groups throughout the polymer backbone using azidation and “click” chemistry as postpolymerization tools to create fluorescently labeled nanoparticles. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2699–2710 相似文献
74.
75.
76.
77.
78.
79.
Dr. Man-Bo Li Daniels Posevins Dr. Karl P. J. Gustafson Dr. Cheuk-Wai Tai Dr. Andrey Shchukarev Dr. Youai Qiu Dr. Jan-E. Bäckvall 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(1):210-215
A highly selective and efficient oxidative carbocyclization/borylation of enallenols catalyzed by palladium immobilized on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was developed for diastereoselective cyclobutenol synthesis. The heterogeneous palladium catalyst can be recovered and recycled without any observed loss of activity or selectivity. The high diastereoselectivity of the reaction is proposed to originate from a directing effect of the enallenol hydroxyl group. Optically pure cyclobutenol synthesis was achieved by the heterogeneous strategy by using chiral enallenol obtained from kinetic resolution. 相似文献
80.