全文获取类型
收费全文 | 2746篇 |
免费 | 96篇 |
国内免费 | 9篇 |
专业分类
化学 | 1934篇 |
晶体学 | 30篇 |
力学 | 52篇 |
数学 | 403篇 |
物理学 | 432篇 |
出版年
2022年 | 20篇 |
2021年 | 31篇 |
2020年 | 46篇 |
2019年 | 49篇 |
2018年 | 24篇 |
2017年 | 33篇 |
2016年 | 66篇 |
2015年 | 57篇 |
2014年 | 67篇 |
2013年 | 111篇 |
2012年 | 164篇 |
2011年 | 159篇 |
2010年 | 98篇 |
2009年 | 77篇 |
2008年 | 112篇 |
2007年 | 142篇 |
2006年 | 161篇 |
2005年 | 154篇 |
2004年 | 128篇 |
2003年 | 109篇 |
2002年 | 110篇 |
2001年 | 43篇 |
2000年 | 16篇 |
1999年 | 12篇 |
1998年 | 28篇 |
1997年 | 33篇 |
1996年 | 35篇 |
1995年 | 23篇 |
1994年 | 26篇 |
1993年 | 36篇 |
1992年 | 33篇 |
1991年 | 24篇 |
1990年 | 24篇 |
1989年 | 33篇 |
1988年 | 25篇 |
1987年 | 18篇 |
1986年 | 21篇 |
1985年 | 47篇 |
1984年 | 22篇 |
1983年 | 22篇 |
1982年 | 38篇 |
1981年 | 43篇 |
1980年 | 36篇 |
1979年 | 41篇 |
1978年 | 47篇 |
1977年 | 25篇 |
1976年 | 46篇 |
1975年 | 28篇 |
1974年 | 20篇 |
1973年 | 24篇 |
排序方式: 共有2851条查询结果,搜索用时 187 毫秒
111.
Anthony Fratiello Vicki Kubo-Anderson Soheil Azimi Thomas Flores Eric Marinez Dennis Matejka Richard Perrigan Michael Vigil 《Journal of solution chemistry》1990,19(8):811-829
A coordination study of Lu(III) has been carried out for the nitrate and perchlorate salts in aqueous mixtures of acetone-d6 and Freon-12 by1H,15N and35Cl NMR spectroscopy. At temperatures lower than –90°C, proton and ligand exchange are slow enough to permit the direct observation of1H resonance signals for coordinated and free water molecules, leading to an accurate measure of the Lu(III) hydration number. In perchlorate solution, in the absence of inner-shell ion-pairing, Lu(III) exhibits a maximum coordination number of six over the allowable concentration range of study, contrasting markedly with the report of values of six to nine or greater as determined by a similar NMR method. The absence of contact ion-pairing was confirmed by35Cl NMR chemical shift and linewidth measurements. Extensive ion-pairing was observed in the nitrate solutions as reflected by the lower Lu(III) hydration numbers of two to three in these systems, the observation of two coordinated water signals, and15N NMR signals for two complexes. The1H and15N NMR spectra and the hydration number could be accounted for by the presence of (H2O)4Lu(NO3)2+ and (H2O)2Lu(NO3)
2
1+
. 相似文献
112.
The electron distributions and bonding in Ru3(CO)9(
3-
2,
2,
2-C6H6) and Ru3(CO)9(
3-
2,
2,
2-C60) are examined via electronic structure calculations in order to compare the nature of ligation of benzene and buckminsterfullerene to the common Ru3(CO)9 inorganic cluster. A fragment orbital approach, which is aided by the relatively high symmetry that these molecules possess, reveals important features of the electronic structures of these two systems. Reported crystal structures show that both benzene and C60 are geometrically distorted when bound to the metal cluster fragment, and our ab initio calculations indicate that the energies of these distortions are similar. The experimental Ru–Cfullerene bond lengths are shorter than the corresponding Ru–Cbenzene distances and the Ru–Ru bond lengths are longer in the fullerene-bound cluster than for the benzene-ligated cluster. Also, the carbonyl stretching frequencies are slightly higher for Ru3(CO)9(
3-
2,
2,
2-C60) than for Ru3(CO)9(
3-
2,
2,
2-C6H6). As a whole, these observations suggest that electron density is being pulled away from the metal centers and CO ligands to form stronger Ru–Cfullerene than Ru–Cbenzene bonds. Fenske-Hall molecular orbital calculations show that an important interaction is donation of electron density in the metal–metal bonds to empty orbitals of C60 and C6H6. Bonds to the metal cluster that result from this interaction are the second highest occupied orbitals of both systems. A larger amount of density is donated to C60 than to C6H6, thus accounting for the longer metal–metal bonds in the fullerene-bound cluster. The principal metal–arene bonding modes are the same in both systems, but the more band-like electronic structure of the fullerene (i.e., the greater number density of donor and acceptor orbitals in a given energy region) as compared to C6H6 permits a greater degree of electron flow and stronger bonding between the Ru3(CO)9 and C60 fragments. Of significance to the reduction chemistry of M3(CO)9(
3-
2,
2,
2-C60) molecules, the HOMO is largely localized on the metal–carbonyl fragment and the LUMO is largely localized on the C60 portion of the molecule. The localized C60 character of the LUMO is consistent with the similarity of the first two reductions of this class of molecules to the first two reductions of free C60. The set of orbitals above the LUMO shows partial delocalization (in an antibonding sense) to the metal fragment, thus accounting for the relative ease of the third reduction of this class of molecules compared to the third reduction of free C60. 相似文献
113.
114.
The homogeneity of the sample load across the radial cross section of analytical scale columns was determined when operating under overload conditions. The study was performed using active flow technology columns operating in parallel segmentation mode. The outlet segmentation ratio was varied to enable different volume fractions of mobile phase, and thus sample, to elute from the peripheral and central flow regions of the column. The amount of solute exiting the peripheral and radial central exit ports was determined as a function of the flow segmentation ratio. The experimental data using an analytical scale column with dimensions, 100?×?4.6?mm, indicated that the sample load distribution was essentially uniform as a function of the column radial cross section. 相似文献
115.
Michelle Galgoci Janel E. Davidson Clinton K. Harrington Dennis L. Hasha Christian T. Goralski 《合成通讯》2013,43(17):2477-2483
A simple method for the preparation of the title compounds via the reaction of dimethyl 2-ethenylcyclopropane-1,1-dicarboxylate with 6M ammonia in methanol is described. 相似文献
116.
Jiang Bolong Muddemann Thorben Kunz Ulrich Haupt Dennis Bormann Hinnerk Niedermeiser Michael Schläfer Ottmar Sievers Michael 《Research on Chemical Intermediates》2018,44(1):639-655
Research on Chemical Intermediates - Effective methods for graphite felt (GF) treatment based on Fenton’s reagent treatment and thermal modification have been used to improve microbial fuel... 相似文献
117.
The reaction of substituted arylboronic acids with 5-bromo-2-methoxytropone catalyzed by palladium(0) complex furnished 5-arylated tropone derivatives in good yield. 相似文献
118.
Shawna M. Liff William T. Pennington Dennis W. Smith Jr. 《Journal of polymer science. Part A, Polymer chemistry》2014,52(2):232-238
A novel class of semifluorinated perfluorocyclohexenyl (PFCH) aryl ether homo/copolymers was successfully synthesized with high yield through the step‐growth polymerization of commercially available bisphenols and decafluorocyclohexene in the presence of a triethylamine base. The synthesized polymers exhibit variable thermal properties depending on the functional spacer group (R). PFCH aryl ether copolymers with random and alternating architectures were also prepared from versatile bis‐perfluorocyclohexenyl aryl ether monomers. The PFCH polymers show high thermal stabilities with a 5% decomposition temperature ranging from 359 to 444 °C in air and nitrogen atmosphere. These semifluorinated PFCH aromatic ether polymers contain intact enchained PFCH olefin moieties, making further reactions such as crosslinking and application specific functionalization possible. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 232–238 相似文献
119.
120.