全文获取类型
收费全文 | 5734篇 |
免费 | 131篇 |
国内免费 | 19篇 |
专业分类
化学 | 4080篇 |
晶体学 | 21篇 |
力学 | 71篇 |
数学 | 993篇 |
物理学 | 719篇 |
出版年
2020年 | 50篇 |
2019年 | 54篇 |
2018年 | 40篇 |
2016年 | 93篇 |
2015年 | 94篇 |
2014年 | 104篇 |
2013年 | 167篇 |
2012年 | 161篇 |
2011年 | 229篇 |
2010年 | 115篇 |
2009年 | 106篇 |
2008年 | 217篇 |
2007年 | 214篇 |
2006年 | 218篇 |
2005年 | 252篇 |
2004年 | 190篇 |
2003年 | 171篇 |
2002年 | 168篇 |
2001年 | 88篇 |
2000年 | 81篇 |
1999年 | 70篇 |
1998年 | 53篇 |
1997年 | 76篇 |
1996年 | 76篇 |
1995年 | 79篇 |
1994年 | 54篇 |
1993年 | 53篇 |
1992年 | 71篇 |
1991年 | 54篇 |
1990年 | 75篇 |
1989年 | 45篇 |
1988年 | 66篇 |
1987年 | 70篇 |
1986年 | 45篇 |
1985年 | 105篇 |
1984年 | 61篇 |
1983年 | 67篇 |
1982年 | 74篇 |
1981年 | 67篇 |
1980年 | 56篇 |
1979年 | 69篇 |
1978年 | 74篇 |
1977年 | 54篇 |
1976年 | 81篇 |
1975年 | 80篇 |
1974年 | 54篇 |
1973年 | 49篇 |
1972年 | 42篇 |
1970年 | 48篇 |
1966年 | 42篇 |
排序方式: 共有5884条查询结果,搜索用时 15 毫秒
191.
Donzello MP De Mori G Viola E Ercolani C Bodo E Mannina L Capitani D Rizzoli C Gontrani L Aquilanti G Kadish KM D'Angelo P 《Inorganic chemistry》2011,50(23):12116-12125
The solid state and solution structure of 2,3-dicyano-5,6-di(2-thienyl)-1,4-pyrazine, [(CN)(2)Th(2)Pyz], and its Pd(II) derivative, [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, formed by reaction of [(CN)(2)Th(2)Pyz] with [(C(6)H(5)CN)(2)PdCl(2)] were characterized by X-ray, UV-visible, (1)H and (13)C NMR, and extended X-ray absorption fine structure (EXAFS) spectral measurements. The X-ray crystal structure of [(CN)(2)Th(2)Pyz] shows the presence of one thienyl ring positioned orthogonal to the rest of the molecule, with the two vicinal thienyl rings lying orthogonal to each other in a rare arrangement. NMR studies of [(CN)(2)Th(2)Pyz] in the solid state and in solutions of dimethylformamide or dimethyl sulfoxide confirm a nonequivalence of the thienyl rings in the solid state and also in solution. EXAFS results indicate that two distinct Pd(II) coordination sites are formed at the di(2-thienyl)pyrazino moiety of [(CN)(2)Th(2)Pyz(PdCl(2))(2)]·H(2)O, with identical Pd-N(pyz) (2.03(3) ?) and Pd-Cl (2.36(3) ?) bond lengths but with different Pd-S1 (2.25(4) ?) and Pd-S2 (3.21(5) ?) bond distances in an overall asymmetric molecular framework. Density functional theory (DFT) and time-dependent DFT (TDDFT) theoretical studies also provide information about the structure and spectral behavior of the precursor and its metalated Pd(II) derivative. (1)H/(13)C NMR and UV-visible spectral measurements were also carried out on two heteropentametallic porphyrazine macrocycles which were prepared by a reaction of PdCl(2) with [Th(8)TPyzPzM] where Th(8)TPyzPz = tetrakis-2,3-[5,6-di-(2-thienyl)-pyrazino]porphyrazinato dianion and M = Mg(II)(H(2)O) or Zn(II). Spectroscopic data on the newly synthesized [(PdCl(2))(4)Th(8)TPyzPzM] compounds suggest that the binding of PdCl(2) involves coordination sites of the type S(2(th))PdCl(2) with the two thienyl rings of each di(2-thienyl)pyrazino fragment bound to Pd(II) in an equivalent manner ("th-th" coordination). This is similar to what was found for the corresponding octapyridinated analogues ("py-py" coordination). 相似文献
192.
Petra Hoffman Siegfried Hünig Leonhard Walz Karl Peters Hans-Georg von Schnering 《Tetrahedron》1995,51(48):13197-13216
Azo compounds 1 and 3 containing a CC-double bond in a parallel but distant position are quatemized by Me3OBF4 to 1,MeBF4 and 3,MeBF4, whereas MeI produces the cage compounds 2,HX and 4,HX. These [3+2] cycloadducts also are quantitatively formed from 1,MeBF4 and 3,MeBF4 with catalytic amounts of azo compounds. Intermolecular [3+2] cycloadditions occur with a mixture of DBH (5) or DBO (8), MeI and a variety of alkenes (→ HI) salts of (6, 7, 9 — 12). The intermediate azomethineimines, if stabilized by a fluorenylidene group, can be isolated (20, 22,24), but not, however, in the presence of a close parallel CC-bond (25 → 26). 相似文献
193.
194.
195.
Jobst KJ Trikoupis MA 《European journal of mass spectrometry (Chichester, England)》2012,18(2):183-194
Hydrogen-bridged radical cations (HBRCs) are an intriguing subclass of ion-molecule complexes. They may act as key intermediates of remarkable stability in both association and dissociation reactions of heteroatom-containing molecular ions. The H-bridge of such an HBRC can promote isomerization of its ionic component by H-transfer. Proton-transport catalysis (PTC) is a prime example. Here, a neutral molecule promotes the smooth transformation of an ion into its H-shift isomer by consecutive proton-transfer reactions. A celebrated case is the water catalyzed isomerization of CH(3)OH(?+) into its more stable distonic isomer (?)CH(2)OH(2)(+). Other early studies of PTC also deal with catalyzing 1,2-H shifts in association reactions. This short review focuses on more recent combined experimental and computational studies of catalysis in HBRCs. Mechanisms involving both proton and H atom transfers have been proposed for a variety of systems of H-shift isomers. It has also become clear that PTC is not confined to bimolecular reactions. It also features in the unimolecular chemistry of heteroatom- containing ions, which have a tendency to isomerize to HBRCs en route to their dissociation. 相似文献
196.
Sebastian O. Klemm Sascha E. Pust Achim Walter Hassel Jürgen Hüpkes Karl J. J. Mayrhofer 《Journal of Solid State Electrochemistry》2012,16(1):283-290
The processes during chemical and electrochemical etching of Al-doped ZnO are investigated utilizing a scanning flow cell
setup with online detection of dissolved Zn ions. The rate of chemical dissolution was found to be a linear function of buffer
and proton concentration in near neutral pH solutions according to a transport limited reaction. In contrast, electrochemical
etching is limited by the kinetics of the reaction and increases linearly with the imposed current density. Due to this fundamental
difference, the dissolution of Zn can be either uniform over the whole surface or highly localized at active sites like grain
boundaries. A combined approach of chemical etching and the well-controllable galvanostatic dissolution thus allows a fine
adjustment of the ZnO:Al surface texture for applications in silicon thin film photovoltaic cells in order to improve their
overall energy conversion efficiency. 相似文献
197.
Synthesis and Crystal Structure Determination of Dimethyldi-(N-Methylimidazolium)silicon Bromide The reaction of dibromodimethylsilane with n-methylimidazole (NMI) leads to a 1:2 compound stable at room temperature. The reaction was carried out at room temperature and colorless, moisture sensitive crystals were obtained by sublimation. The addition compound crystallizes in the orthorhombic space group Pcam (No. 57) with lattice constants a = 1 110 96(8) pm, b = 1 142.3(2) pm and c = 1 238.9(3) pm. For 1 317 independent reflections, measured at 21°C, the structure could be refined to R = 0.040 and Rw = 0.041. 相似文献
198.
Localized molecular orbitals, two- and three-center bond indices of 1,5-dicarba-closo-pentaborane ( 5 ) and 1,5-dicarba-closo-pentaborane ( 3 ) and their hydrocarbon analogs bicyclo [1,1,1]-pentane and [1,1,1] propellane, respectively, have been calculated using a suitable basis set. No previously reported three-center CBC bond has been found. However, a new three-center BCB bond has been observed in 1,5-dicarba-closo-pentaborane ( 5 ). © 1995 John Wiley & Sons, Inc. 相似文献
199.
200.