全文获取类型
收费全文 | 4595篇 |
免费 | 108篇 |
国内免费 | 18篇 |
专业分类
化学 | 2992篇 |
晶体学 | 65篇 |
力学 | 119篇 |
数学 | 417篇 |
物理学 | 1128篇 |
出版年
2021年 | 33篇 |
2020年 | 55篇 |
2019年 | 61篇 |
2018年 | 37篇 |
2017年 | 38篇 |
2016年 | 102篇 |
2015年 | 73篇 |
2014年 | 92篇 |
2013年 | 233篇 |
2012年 | 194篇 |
2011年 | 299篇 |
2010年 | 189篇 |
2009年 | 171篇 |
2008年 | 229篇 |
2007年 | 265篇 |
2006年 | 224篇 |
2005年 | 203篇 |
2004年 | 152篇 |
2003年 | 147篇 |
2002年 | 129篇 |
2001年 | 85篇 |
2000年 | 74篇 |
1999年 | 72篇 |
1998年 | 50篇 |
1997年 | 48篇 |
1996年 | 52篇 |
1995年 | 47篇 |
1994年 | 47篇 |
1993年 | 46篇 |
1992年 | 59篇 |
1991年 | 39篇 |
1990年 | 40篇 |
1989年 | 40篇 |
1988年 | 36篇 |
1987年 | 42篇 |
1986年 | 58篇 |
1985年 | 49篇 |
1984年 | 42篇 |
1983年 | 36篇 |
1982年 | 38篇 |
1981年 | 51篇 |
1980年 | 42篇 |
1979年 | 50篇 |
1978年 | 47篇 |
1977年 | 30篇 |
1976年 | 43篇 |
1975年 | 42篇 |
1974年 | 35篇 |
1973年 | 36篇 |
1972年 | 32篇 |
排序方式: 共有4721条查询结果,搜索用时 25 毫秒
31.
32.
A novel linear trinuclear mu-O-bridging 2-phosphinophenolate nickel(II) complex with fac-tris(P(intersection)O- chelates) in the terminal positions and the three oxygen atoms each facing the central nickel(II) cation was synthesized and structurally characterized by X-ray crystallography. To the best of our knowledge, this is the first example of an octahedral Ni(II) tris(P(intersection)O- chelate). 相似文献
33.
The reaction of B2H4 with acetylene has been studied by the MNDO method. It is shown that the reaction is exothermic and proceeds in two steps. The first step is the formation of a three-center -complex and this is the rate-determining step of the reaction. The second step is the rearrangement of the -complex to the product and this step requires a very small amount of activation energy. The activation barrier for the diboration reaction is 12.8 kcal/mol.The proposed mechanism is significantly different from those proposed earlier and explains all experimental data relating to this reaction. 相似文献
34.
The redox properties of the system Fe(tmphen)3(II/III) (tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) have been studied in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate, dimethylformamide, dimethylacetamide, dimethylsulfoxide and of the systems Fe(phen)3(II/III) (phen=1,10-phenanthroline) and Fe(niphen)3(II/III) (niphen=5-nitro-1,10-phenanthroline) in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate and acetone. The redox potentials of Fe(tmphen)3(II/III) are nearly independent of the solvent suggesting that the system might be used as a reference redox couple similar to the systems ferrocene/ferricinium or bisbiphenylchromium(0/I). In contrast the redox potentials of Fe(niphen)3(II/III) show a significant decrease with increasing donor number of the solvent which can be explained by nucleophilic attack of solvent molecules at the iron. It is shown that such a mechanism is consistent with the known solvent and salt effects on the kinetics of dissociation of ferroin and ferriin type complexes. 相似文献
35.
36.
Ray K Nakahara H Sakamoto A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(1-2):103-107
The fluorescence lifetimes decays and picosecond time-resolved fluorescence spectra were measured to investigate the dynamics of the excited state of sulforhodamine B (SRB) molecules adsorbed in the mono- and multilayered Langmuir-Blodgett (LB) films of octadecylamine. Steady-state and time-resolved fluorescence spectroscopy reveals that the fluorescence lifetimes and contents of the monomer and dimers in the molecular organizates depend upon the concentration of the dye in the solution and the adsorption process. SRB dye molecules adsorbed in LB films have been imaged with scanning near-field optical microscopy (SNOM). This information is exploited to map the distribution with molecular spatial resolution. SNOM provide the visual evidence of the monomers and dimers of SRB in cationic LB films. 相似文献
37.
Deuterium labeling in the acetate derivatives of cholic acid demonstrates that electron impact induced consecutive loss of HOAc from the molecular ion occurs in the following order of decreasing preference: 12α-OAc > 7α-OAc > 3α-OAc. Numerous examples are presented showing that consecutive loss of HOAc from a common ion generally increases progressively with relative intensity. In the acetate pregnan-20-one derivatives of cholic acid, loss of 43 mass units from the molecular ion comes from the 12α-OAc and not the 17β-acetyl sidechain. Ketene is ejected from an acetoxy group located in an α position to a carbonyl on which an ion-radical charge is localized. 相似文献
38.
Jerry Ray Dias 《Journal of mathematical chemistry》1990,4(1):17-29
The genesis of the aufbau and excised internal structure concepts is traced. These concepts were pivotal to the first published enumerations of the strictly peri-condensed, non-Kekuléan, and essentially strain-free total resonant sextet benzenoid groups. Aperiodic table set is defined as a partially ordered set forming a two-dimensional array which complies with thetriad principle where a central element has a metric property that is the arithmetic mean of two adjacent surrounding member elements. 相似文献
39.
Jerry Ray Dias 《Journal of mass spectrometry : JMS》1976,11(4):333-346
Mass spectra of various functionalized lanostanes and their deuterated analogs are compared. It is proposed that the transfer of a hydrogen from the 18-methyl group to the 11-oxo group via a McLafferty rearrangement is made geometrically possible by prior ionization of the 13,14 bond which allows an optimum interatomic distance between the oxygen and hydrogen to be acquired; the 8,9 double bond is a requisite for electronic induction of this specific process. Transfer of the hydrogen from the 7β-hydroxy group to position 14 is implicated in the mass spectrum of 3β-acetoxy-7β-hydroxy-5α-lanostan-11-one. 相似文献
40.
Paital AR Mitra T Ray D Wong WT Ribas-Ariño J Novoa JJ Ribas J Aromí G 《Chemical communications (Cambridge, England)》2005,(41):5172-5174
Based on a combined theoretical-experimental study, we propose that substituted m-phenylene ligands (m-N-Phi-N) can act as tuneable strong ferromagnetic couplers connecting Cu(II) ions; a new complex presenting that bridge with J close to +15 cm(-1) has been suggested and synthesized. 相似文献