全文获取类型
收费全文 | 74828篇 |
免费 | 7380篇 |
国内免费 | 5379篇 |
专业分类
化学 | 42801篇 |
晶体学 | 793篇 |
力学 | 4226篇 |
综合类 | 401篇 |
数学 | 14538篇 |
物理学 | 24828篇 |
出版年
2024年 | 92篇 |
2023年 | 917篇 |
2022年 | 1529篇 |
2021年 | 1594篇 |
2020年 | 1874篇 |
2019年 | 1715篇 |
2018年 | 2629篇 |
2017年 | 2779篇 |
2016年 | 2719篇 |
2015年 | 2660篇 |
2014年 | 2992篇 |
2013年 | 3947篇 |
2012年 | 7247篇 |
2011年 | 6432篇 |
2010年 | 4726篇 |
2009年 | 4304篇 |
2008年 | 3548篇 |
2007年 | 3231篇 |
2006年 | 3016篇 |
2005年 | 6423篇 |
2004年 | 5536篇 |
2003年 | 3545篇 |
2002年 | 1655篇 |
2001年 | 1226篇 |
2000年 | 992篇 |
1999年 | 1146篇 |
1998年 | 879篇 |
1997年 | 784篇 |
1996年 | 818篇 |
1995年 | 712篇 |
1994年 | 643篇 |
1993年 | 547篇 |
1992年 | 653篇 |
1991年 | 552篇 |
1990年 | 491篇 |
1989年 | 367篇 |
1988年 | 351篇 |
1987年 | 263篇 |
1986年 | 201篇 |
1985年 | 204篇 |
1984年 | 173篇 |
1983年 | 156篇 |
1982年 | 112篇 |
1981年 | 85篇 |
1980年 | 76篇 |
1979年 | 77篇 |
1976年 | 112篇 |
1975年 | 73篇 |
1974年 | 70篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Yang Cao Liangjing Chen Bo Chen Jianwen Fen Wenjian Chen Yuh-Kang Pan 《International journal of quantum chemistry》1995,54(4):265-270
The three-dimensional EHMO crystal orbital calculations for crystalline C60, Ca3C60, and Ca5C60 are reported. The ground states of both undoped solid C60 and partially doped Ca3C60 are found to be insulating with an indirect energy gap of 1.2 and 0.5 eV, respectively. In contrast, Ca5C60 forms a metallic conducting phase with a set of three half-filled bands crossing the Fermi level, which is found to be located close to a peak of the density of state. The character of crystal orbitals near the Fermi level for both Ca3C60 and Ca5C60 is completely carbonlike. In both cases, the Ca atoms are almost fully ionized and C60 molecules form a stable negative charge state with six to 10 additional electrons. © 1995 John Wiley & Sons, Inc. 相似文献
992.
Chisholm MH Macintosh AM Huffman JC Wu D Davidson ER Clark RJ Firth S 《Inorganic chemistry》2000,39(16):3544-3550
Mo2(OtBu)6 and Mo2(NMe2)6 each react with (S,S,S)-triisopropanolamine (2 equiv) in benzene to yield dimolybdenum bis((S,S,S)-isopropanolaminate(3-)), Mo2[(OC-(S)-HMeCH2)3N]2 (M identical to M), as a blue crystalline solid. Cell parameters at -160 degrees C: a = 17.389(6) A, b = 10.843(3) A, c = 10.463(3) A, beta = 125.28(1) degrees, Z = 2 in space group C2. The molecular structure involves an Mo2 unit inside an O6N2 distorted cubic box. The Mo2 axis is disordered about three positions with occupancy factors of ca. 45%, 45%, and 10%. Despite this disorder, the molecular structure is shown to contain a central Mo identical to Mo unit of distance 2.15(3) A coordinated to two triolate ligands which each have two chelating arms and one that spans the Mo identical to Mo bond. The local Mo2O6N2 moiety has approximate C2h symmetry, and the Mo-N distances are long, 2.4 A. The 1H and 13C(1H) NMR spectra recorded in benzene-d6 are consistent with the geometry found in the solid-state structure. The blue color arises from weak absorptions, epsilon approximately 150 dm3 mol-1 cm-1, at 580 and 450 nm in the visible region of the electronic absorption spectrum. Raman spectra recorded in KCl reveal pronounced resonance effects with excitation wavelengths of 488.0, 514.5, and 568.2 nm, particularly for the 322 cm-1 band, which can probably be assigned to nu(Mo identical to Mo). The electronic structure of this compound is investigated by B3LYP DFT calculations, and a comparison is made with the more typical ethane-like (D3d) Mo2(OR)6 compounds is presented. The distortion imposed on the molecule by the triisopropanolaminate(3-) ligands removes the degeneracy of the M-M pi molecular orbitals. The HOMO and SHOMO are both M-M pi and M-O sigma* in character, while the LUMO is M-M pi* and the SLUMO is predominantly M-O sigma* with metal sp character. The calculated singlet-singlet transition energies are compared with those implicit in the observed electronic spectrum. 相似文献
993.
A modification of the procedure reported byJohnson andWalseth
1 for the preparation of (-32P)ATP:(-32P)GTP with a specific activity of more than 5000 Ci/mmol was made firstly, then it was used as the phosphate donor in the reaction catalyzed by the polynucleotide kinase, to transfer its radioactive phosphate to the 5-position of the 3-AMP, resulting (5-32P)3-ADP. After that, (5-32P)3-ADP was hydrolysed to (5-32P)AMP under the catalysis of Nuclease P1, the late was then phosphorylated to produce (-32P)ATP by the action of myokinase and pyruvate kinase. By this method, we firstly prepared (-32P)ATP with a specific activity of more than 4000 Ci/mmol which was measured by HPLC analysis. 相似文献
994.
Chen Xiao-Ming Wu Yu-Luan Tong Ye-Xiang Sun Ziming David N. Hendrickson 《Polyhedron》1997,16(24):1041
A novel tetranuclear terbium(III) complex [Tb4(OH)4(pybet)6(H2O)8][Tb4(OH)4(pybet)6(H2O)7 (NO3)](ClO4)14·6H2O has been synthesized and shown by X-ray crystallography to have a cubane-like Tb4(μ3-OH)4(μ2-carboxylato-O,O′)6 core. The ligand pybet is pyridinoacetate, C5H5+N-CH2CO2−. Magnetic susceptibility data were measured for this Tb4 complex in the range of 2.0–320 K and in fields of 1.0 G to 50.0 kG. It is concluded that either there is very weak antiferromagnetic exchange interaction (J = −0.015 cm−1) or there is a small crystal-field splitting of the 7F6 TbIII ground state. 相似文献
995.
叙述了同步辐射白光全反射X射线荧光分析的实验装置,给出了几种标准物质TXRF实验的检出限,并对实验结果进行了讨论。 相似文献
996.
Yang Jie Wu Shou Quan Huo Li Yang Chen Xia Du 《Journal of organometallic chemistry》1995,490(1-2):249-254
Three series of cyclomercurated ferrocenylimines (2-chloromercurio-ferrocenylimines) have been studied using 13C NMR spectroscopy. Good to excellent linear relationships have been found to exist between the chemical shifts of the carbon atoms in the ferrocenyl moiety and normal Hammett substituent constants σm and σp. The δ values of the iminyl carbon atoms show excellent linear correlations with the σ values. In three series of the ferrocenes, the sensitivity of the carbon atoms to the substituent effect is different, which is discussed in terms of the twist angle between the N-phenyl ring and the C---C=N---C plane. The relative sensitivity of the chemical shifts on different positions in ferrocenyl moiety to the substituent effect has also been presented. 相似文献
997.
998.
R. G?tz P. Enge P. Friesel K. Roch B. Schilling B. Hartmann H. Wunsch 《Fresenius' Journal of Analytical Chemistry》1994,348(10):694-695
Sampling and determination of dissolved and particlebound PCBs and HCB in the river Elbe are described. A new filtration/adsorption unit for sampling of dissolved lipophilic contaminants was developed. The detection limit for dissolved PCBs and HCB was 5 pg/l. The data show that more than 98% of the PCBs and of HCB in the water of the river Elbe are particle-bound. 相似文献
999.
A series of heat energy storage microcapsules was prepared using melamine-formaldehyde resin as the shell material and the mechanical properties of the shell were investigated. A phase change material whose melting point was 24 °C was used as core and the quantity of heat involved in phase transition was 225.5 J/g. Average diameter of the microcapsules varied from 5 to 10 μm, and the globular surface was smooth and compact. The mechanical properties of the shell were evaluated by observing the surface morphological structure change after application of pressure by means of scanning electron microscopy. When the mass ratio of the core and shell material is 3:1, a yield point of about 1.1×105 Pa was found and when the compression was increased beyond this point the microcapsules showed plastic behavior. This has been attributed to the cross-link density and to the high degree of reaction of the shell material. Different yield points subsequently reflected differences in the mechanical behavior. It was also found that the mechanical intensity of double-shell microcapsules was better than that of single shelled ones. 相似文献
1000.
Single crystals of U3+:LaBr3 were grown by the Bridgman-Stockbarger technique. High-resolution polarized absorption spectra of the crystals were recorded at 4.2 K in the 4000-50,000 cm−1 range. Sixty-four experimental crystal-field energy levels of the U3+ ion were fitted to a semiempirical Hamiltonian employing free-ion, one-electron crystal-field as well as two-particle correlation crystal-field (CCF) operators with an r.m.s. deviation of 28 cm−1. The performed analysis of the spectra enabled the determination of crystal-field parameters and assignment of the observed 5f3→5f3 transitions. The effects of selected CCF operators on the splitting of some specific U3+ multiplets have been investigated and the obtained values of Hamiltonian parameters are discussed and compared with those reported in previous analyses. 相似文献