全文获取类型
收费全文 | 1686篇 |
免费 | 11篇 |
国内免费 | 4篇 |
专业分类
化学 | 1154篇 |
晶体学 | 17篇 |
力学 | 11篇 |
数学 | 304篇 |
物理学 | 215篇 |
出版年
2021年 | 27篇 |
2020年 | 11篇 |
2019年 | 15篇 |
2018年 | 21篇 |
2017年 | 24篇 |
2016年 | 31篇 |
2015年 | 39篇 |
2014年 | 35篇 |
2013年 | 109篇 |
2012年 | 59篇 |
2011年 | 80篇 |
2010年 | 44篇 |
2009年 | 39篇 |
2008年 | 68篇 |
2007年 | 55篇 |
2006年 | 69篇 |
2005年 | 65篇 |
2004年 | 60篇 |
2003年 | 61篇 |
2002年 | 51篇 |
2001年 | 39篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 33篇 |
1997年 | 21篇 |
1996年 | 28篇 |
1995年 | 15篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 19篇 |
1991年 | 28篇 |
1990年 | 26篇 |
1989年 | 16篇 |
1988年 | 27篇 |
1987年 | 23篇 |
1986年 | 16篇 |
1985年 | 34篇 |
1984年 | 35篇 |
1983年 | 15篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 15篇 |
1977年 | 15篇 |
1976年 | 18篇 |
1975年 | 18篇 |
1974年 | 17篇 |
1973年 | 18篇 |
1972年 | 8篇 |
排序方式: 共有1701条查询结果,搜索用时 15 毫秒
31.
Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock 6-31G level for 1-methylphospholane showed that, as is known for nitrogen, planarization at phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the unsaturated and saturated systems could be used to detect the presence of electron delocalization in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)phosphole showed that the P-substituent adopted an angle of 55.7 degrees (DFT/6-31G level; 57.6 degrees at the HF/6-31 level) with respect to the C(2)-P-C(5) plane (for P-phenyl, 67.1 degrees and 68.3 degrees, respectively). 相似文献
32.
Summary Gas chromatographic chiral separation of several chiral 2-and 3-alkanols and diols was studied both in their free hydroxyl
and in their trimethylsilyl ether forms. First, the derivatization procedure was verified through the identification of the
trimethylsilyl ethers formed on the basis of their mass spectra and optimized to obtain quantitative reaction. The optimized
procedure was applied to the trimethylsilylation of racemic mixtures of various hydroxyl compounds. The silylation was found
to be highly effective in the improvement of the separation of the individual enantiomers. The major advantages of the derivatization
process can be summarized as: (i) excellent baseline separation of the enantiomers of the silyl ethers was achieved in contrast
to the parent OH-containing compounds, (ii) the sensitivity of detection highly increased, (iii) the separations do not show
any significant concentration dependence and finally (iv) the analysis time needed decreased significantly.
Presented at: Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 3–5, 1997 相似文献
33.
Summary The paper deals with the behaviour of the so-called algorithms with respect to interval filling sequences A connection is established between the uniquely representable points and the continuity points of the algorithms; also strong continuity properties on monotonic algorithms are proved. Finally the results are applied to additive functions. The theorems extend some former results by the authors, by I. Kátai and by A. Járai.Dedicated to the memory of Alexander M. Ostrowski on the occasion of the 100th anniversary of his birth 相似文献
34.
Given two graphsH andG, letH(G) denote the number of subgraphs ofG isomorphic toH. We prove that ifH is a bipartite graph with a one-factor, then for every triangle-free graphG withn verticesH(G) H(T
2(n)), whereT
2(n) denotes the complete bipartite graph ofn vertices whose colour classes are as equal as possible. We also prove that ifK is a completet-partite graph ofm vertices,r > t, n max(m, r – 1), then there exists a complete (r – 1)-partite graphG* withn vertices such thatK(G) K(G*) holds for everyK
r
-free graphG withn vertices. In particular, in the class of allK
r
-free graphs withn vertices the complete balanced (r – 1)-partite graphT
r–1(n) has the largest number of subgraphs isomorphic toK
t
(t < r),C
4,K
2,3. These generalize some theorems of Turán, Erdös and Sauer.Dedicated to Paul Turán on his 80th Birthday 相似文献
35.
Iron(II) complexes containing CNBX?3 or CNBX2NC? ligands were prepared from the reaction of K[(π-C5H5)Fe(CO)(CN)2] with boranes (BX3; X = F, Cl, Br H, Ph). Stable, twelve-membered ring compounds containing Fe, C, N, and B atoms were formed involving CNBF2NC? and CNBBr2NC? ligands. The reaction of K[(π-C5H5)Fe(CO)(CN)2] with AlCl3 gave a four-center complex with two Fe and two Al atoms. The compounds were studied by infrared and mass spectroscopic methods. 相似文献
36.
Dalma Czett Katalin Bddi Veronika Nagy Anik Taktsy Jzsef Deli Paul Tone Gyrgy T. Balogh Anna Vincze Attila Agcs 《Molecules (Basel, Switzerland)》2022,27(15)
Carotenoid succinates were synthesized from hydroxy carotenoids and were coupled to a commercially available derivative of melatonin via amide bond for producing more powerful anti-oxidants and yet new hybrid lipophilic bifunctional molecules with additional therapeutic effects. The coupling reactions produced conjugates in acceptable to good yields. Succinylation increased the water solubility of the carotenoids, while the conjugation with melatonin resulted in more lipophilic derivatives. The conjugates showed self-assembly in aqueous medium and yielded relatively stable colloidal solutions in phosphate-buffered saline. Antioxidant behavior was measured with ABTS and the FRAP methods for the carotenoids, the carotenoid succinates, and the conjugates with melatonin. A strong dependence on the quality of the solvent was observed. TEAC values of the new derivatives in phosphate-buffered saline were found to be comparable to or higher than those of parent carotenoids, however, synergism was observed only in FRAP assays. 相似文献
37.
In this paper a proposal is put forward for the calculation of the shearing factor NS linked to the well known, demagnetization coefficient ND, from the data of a sheared saturation loop, obtained by a non-toroidal, open circuit hysteresis measurement. Following that, the paper illustrates the way to recover the real static loop data from an open circuit, measured data. The proposal is based on the hyperbolic model. The proposed method was illustrated on the tests carried out on two closed and open toroidal samples, mimicking the demagnetization effect of the open circuit VSM measurement, made of NO Fe–Si electrical steel sheets. These test results, presented here, agree very well with the calculated results, based on the hyperbolic modeling. 相似文献
38.
Gy. Keglevich I. Petnehazy L. Tuke 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4)
Abstract The ring enlargement of ready available phospholenes by dichlorocarbene addition to the double bond and subsequent opening of the cyclopropane ring is discussed. Adductcs of phospholene wih dichlorocarbene (2)can be synthetised under liquid-liquid phase transfer circumstances. The simplest bray to open the cyclopropane ring in the adducts is thermal transformation giving the two regioisomers of the appropriate dihydrophosphorin (3). Electrophilic reagents can also be used for -the opening of the cyclopropane ring. On heating with silver nitrate in water or in alcohols the adducts are transformed to the -two rcgioisomers of the hydroxy- or the alkoxy -tetrahydrophosphorins, respectively (4, Y=H, CH3 C2H5) In the reaction of adducts with mercury acetate-acetic acid the corresponding acetoxy-derivatives (4, Y=CH3CO) are formed. The Friedl Crofts reaction of the adducts with benzene iii the presence of aluminium trichloride yields diarylated product. 相似文献
39.
DIHALOCARBENE ADDITION TO ARYLSUBSTTTUTED VINYL PHOSPHATES UNDER PHASE TRANSFER CATALYTIC CONDITIONS
Imre Petneházy György Keglevich László Tőke 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2):77-82
Abstract Dihalocarbene addition to aryl-substituted vinyl phosphates was carried out both in solid-liquid and in liquid-liquid two phase systems. The dihalocyclopropane adducts of vinyl phosphates could be obtained in better yield using dihalocarbenes generated by the latter method and no hydrolysis of the vinyl phosphate and of the adduct occurred under these circumstances. Eighteen new vinyl phosphate-dihalocarbene adducts were synthetized and characterized. 相似文献
40.
I. Petneházy Zs. M. Jászay Gy. Keglevich L. Toke 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4)
Abstract Reaction of trialkyl phosphites 1 with α,β-unsaturated oxo-compounds 2 gives oxaphospholenes 3 or/and two types of Arbuzov products, γ-keto-phosphonates 4 and alkyl-enol-ethers 5. 相似文献