全文获取类型
收费全文 | 1335篇 |
免费 | 150篇 |
国内免费 | 223篇 |
专业分类
化学 | 1561篇 |
晶体学 | 5篇 |
力学 | 4篇 |
综合类 | 4篇 |
数学 | 17篇 |
物理学 | 117篇 |
出版年
2024年 | 3篇 |
2023年 | 23篇 |
2022年 | 47篇 |
2021年 | 59篇 |
2020年 | 72篇 |
2019年 | 51篇 |
2018年 | 52篇 |
2017年 | 42篇 |
2016年 | 81篇 |
2015年 | 63篇 |
2014年 | 66篇 |
2013年 | 77篇 |
2012年 | 89篇 |
2011年 | 75篇 |
2010年 | 63篇 |
2009年 | 77篇 |
2008年 | 65篇 |
2007年 | 78篇 |
2006年 | 59篇 |
2005年 | 63篇 |
2004年 | 47篇 |
2003年 | 45篇 |
2002年 | 90篇 |
2001年 | 41篇 |
2000年 | 31篇 |
1999年 | 40篇 |
1998年 | 35篇 |
1997年 | 20篇 |
1996年 | 17篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 17篇 |
1992年 | 14篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1977年 | 1篇 |
排序方式: 共有1708条查询结果,搜索用时 31 毫秒
1.
Jerry Ray Dias 《Molecular physics》2020,118(12)
Cycloparaphenylene ([r]CPP) and cyclacene ([r]CA) series are models for short carbon nanotubes. It is shown that armchair edges in model cycloparaphenylenes possess greater aromaticity and cyclic conjugation than do zigzag edges in model cyclacenes. According to Aihara’s bond resonance energy (BRE) and Bosanac and Gutman energy effect (ef) measurements, cycloparaphenylenes are twice as aromatic as cyclacenes. The general solution of all eigenvalues of all members of the cycloparaphenylene series is given. The origin of the recurrence of some eigenvalues are determined. 相似文献
2.
Amine boranes bearing hydrophobic substituents were used to reduce aryl ketones in competition with a methyl ketone. Their high stability in protic solvents combined with their ease of preparation made amine boranes useful compounds in the study of hydrophobically directed selective reductions. Several characteristics of the reducing agent were found to be important in determining the reaction selectivity, including available hydrocarbon surface area, degree of fluorination, and proximity of the hydrophobic group to the active hydrides. 相似文献
3.
Various antimonate compounds are well known as important inorganic ion exchangers, since they resist radiation and chemical degradation and also exhibit selectivities towards different cations. Ceric, silicon, titanium and ferric antimonates were prepared as inorganic ion exchangers. Characterization of these materials has been described using different techniques, including thermal analysis, surface area measurements, X-ray diffraction and IR-spectroscopy. In batch distribution experiments the influence of HNO3 molarity and Mo concentration for Mo sorption on different matrices is described in terms of their retention capacities and distribution coefficients.The selectivities of these exchangers towards molybdenum are in the order: CeSb > SiSb > FeSb > TiSb. 相似文献
4.
S. Mohri D. Amutha Rani Y. Yamamoto Y. Tsujita H. Yoshimizu 《Journal of Polymer Science.Polymer Physics》2004,42(2):238-245
Syndiotactic polystyrene (sPS) has various crystalline forms such as α, β, γ, and δ forms, and a mesophase depending on the preparation method. In this study, we focused on the mesophase with the molecular cavity of sPS, which is obtained by step‐wise extraction of the guest molecules from the sPS δ form. To prepare the mesophase containing different shapes and sizes of the cavity, two kinds of the sPS δ form membrane cast from either toluene or chloroform solution were first prepared and then the guest molecules were removed by a step‐wise extraction method using acetone and methanol. We could succeed in the preparation of two kinds of mesophase with different shapes and sizes of the molecular cavity. Either toluene or chloroform vapor sorption to the sPS mesophase membranes was examined at 25 °C. Sorption analysis indicates that the mesophase with large molecular cavities can mainly sorb large molecules; on the other hand, the mesophase with small cavities can sorb only the small molecules, and is unable to sorb a large amount of large molecule because the cavity was too small to sorb the large molecules. Therefore, the sPS mesophase membrane has sorption selectivity based on the size of the molecular cavity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 238–245, 2004 相似文献
5.
A highly selective solid phase extraction sorbent for pre-concentration of sameridine made by molecular imprinting 总被引:5,自引:0,他引:5
Summary A novel approach to solid phase extraction, based on the use of a highly selective molecularly imprinted polymer, is presented.
The versatility of this type of sorbent for solid phase extraction was demonstrated in a model batch-wise pre-concentration
of sameridine prior to gas chromatography. Problems associated with leakage of remaining imprint molecules during the desorption
phase could be eliminated by the use of a close structural analogue of sameridine as the imprint species. It was found that
a major benefit of the imprinted polymer was its specificity, which lead to distinctly cleaner chromatographic traces and
ability to improve sensitivity by extracting sameridine from larger sample volumes. 相似文献
6.
Sarah E. Reybuck Robert M. Waymouth 《Journal of polymer science. Part A, Polymer chemistry》2004,42(13):3323-3331
Ethylene/1‐hexene copolymerizations with disiloxane‐bridged metallocenes, rac‐ and meso‐1,1,3,3‐tetramethyldisiloxanediyl‐bis(1‐indenyl)zirconium dichloride (rac‐ 1 , meso‐ 1 ) activated by modified methylaluminoxane were performed to investigate the influence of conformational dynamics on comonomer selectivity. Although 1H NOESY (nuclear Overhauser and exchange spectroscopy) analysis indicated that the most stable conformation for the meso isomer in solution was that in which both indenes project over the metal coordination site, this isomer showed higher 1‐hexene selectivity in copolymerization (re = 140 ± 30, rh = 0.024 ± 0.004) than the rac isomer with only one indene over the coordination site (re = 240 ± 20, rh = 0.005 ± 0.001). The meso isomer showed high 1‐hexene selectivity, a high product of reactivity ratios (rerh = 3.3 ± 0.5) and produced copolymers that could be separated into fractions with different ethylene content suggesting that the active species exhibited multisite behavior and populated conformations with different comonomer selectivities during the copolymerization. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3323–3331, 2004 相似文献
7.
Metal ion uptake properties of polystyrene-supported chelating polymer resins functionalized with (i) glycine, (ii) hydroxy
benzoic acid, (iii) Schiff base and (iv) diethanol amine have been investigated. The effects of pH, time and initial concentration
on the uptake of metal ions have been studied. The uptake of metal ion depends on pH. The resins are more selective at pH
10 for Pb(II) and Hg(II), whereas at pH 6 they are found to be Cd(II) and Cr(VI) selective. Metal ion uptake properties of
resins follow Freundlich’s equation. The resins are recyclable and are therefore employed for the removal of heavy metal pollutants
from industrial waste water. 相似文献
8.
9.
Timofei S. Zatsepin Dmitry A. Stetsenko Tatiana S. Oretskaya 《Tetrahedron letters》2004,45(39):7327-7330
Oligodeoxyribonucleotides were synthesized that contain a novel nucleoside, 2′-O-(2-oxoethyl)arabinouridine. Whereas such oligonucleotides showed only a slight reduction in the TM values of their complexes with complementary DNA, a significant destabilization was observed in the case of duplexes formed with RNA. This may be explained by the C2′-endo conformation of 2′-O-(2,3-dihydroxypropyl)arabinouridine as demonstrated by NMR experiments in D2O. The modified oligonucleotides were used to synthesize a number of conjugates with dyes, biotin and a N-modified laminin peptide, by hydrazone and oxime formation. We suggest that the 2′-arabinoaldehyde-containing DNA duplexes may be valuable tools for affinity modification of DNA-binding proteins. 相似文献
10.
甲苯定向硝化的理论研究 总被引:7,自引:0,他引:7
运用密度泛函理论(DFT)B3LYP方法,在6-31G~(* *)基组水平上,全优化计算 了硝酰阳离子NO_2~+对苯和从0-,m-,p-位进攻甲苯亲电取代硝化后应,求得4条反 应途径上包括反应物、过渡态和Wheland中间体共(4×3)12个反应驻点σ络合物的 分子几何、电子结构、能量和IR光谱等性质,阐明了反应中无同位素效应的实验事 实,求得各反应途径的活化能排序:p->o->PhH>m-和σ络合物(R,TSak或INT)的 相对稳定化能排序:p-ArCH_3-NO_2~+>o-ArCH_3-NO_2~+>m-ArCH_3-NO_2~+> PhH-NO_2~+,从而阐明了甲基对苯环致活(或致钝)以及增加甲苯硝化络合物稳定性 的双重功能,对甲苯定向硝化的理论预示与实验结果相吻合。 相似文献