全文获取类型
收费全文 | 746篇 |
免费 | 45篇 |
专业分类
化学 | 445篇 |
晶体学 | 3篇 |
力学 | 24篇 |
数学 | 128篇 |
物理学 | 191篇 |
出版年
2023年 | 4篇 |
2022年 | 10篇 |
2021年 | 7篇 |
2020年 | 8篇 |
2019年 | 14篇 |
2018年 | 19篇 |
2017年 | 8篇 |
2016年 | 56篇 |
2015年 | 36篇 |
2014年 | 44篇 |
2013年 | 90篇 |
2012年 | 36篇 |
2011年 | 40篇 |
2010年 | 27篇 |
2009年 | 31篇 |
2008年 | 32篇 |
2007年 | 20篇 |
2006年 | 35篇 |
2005年 | 18篇 |
2004年 | 17篇 |
2003年 | 11篇 |
2002年 | 8篇 |
2001年 | 6篇 |
2000年 | 8篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1992年 | 3篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 3篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 20篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1973年 | 4篇 |
1972年 | 5篇 |
排序方式: 共有791条查询结果,搜索用时 15 毫秒
61.
The comparative analysis of IR and Raman spectra of peramine and its four derivatives in solid state was carried out. The harmonic vibrational frequencies, infrared intensities, and Raman scattering activities were calculated at density functional B3LYP methods with 6-311++G(d,p) basis set. For the predicted spectra, a potential energy distribution of normal modes was also calculated. For peramine derivatives the conjugation effect of pyrrole with pyrazinone ring was observed as a result of introduction of double bond. Moreover, 1H NMR analysis indicated that pyrrole protons are deshielded in comparison with the pyrrolopyrazinone model ring system. 相似文献
62.
I. Fuks-Janczarek Jean Ebothe R. Miedziński R. Gabański A. H. Reshak M. Łapkowski R. Motyka I. V. Kityk J. Suwiński 《Laser Physics》2008,18(9):1056-1069
A new promising organic chromophore for two-photon laser absorption at 1064 nm with 1,4-diethoxy-2,5-bis[2-(5-methylthien-2-yl)ethenyl]benzene (A-C) was synthesized. We have performed evaluations of the two-photon absorption for these chromophores incorporated into the polymer matrices. Following the obtained quantum chemical data, we have performed quantum chemical simulations of the third-order susceptibilities for the investigated chrompophore incorporated into the PMMA matrices. The calculations were done within the three-level model. We have established that the experimentally calculated data are a bit less than theoretically calculated; however, the general tendency of their changes shows a good coincidence. The maximally achieved value of the TPA is equal to about 59.2 cm/GW at wavelength 1064 nm, which, together with their high photothermal stabilities, make them good candidates for optical-limiting processes. 相似文献
63.
E. Łodyga-Chruścińska S. Ołdziej D. Sanna G. Micera L. Chruściński K. Kaczmarek R.J. Nachman J. Zabrocki A. Sykuła 《Polyhedron》2009
Spectroscopic data, including electronic absorption, CD and EPR results, as well as theoretical calculations have shown that the insertion of 4-aminopyroglutamate, a novel cis-peptide bond mimic, in the insect kinin peptide leads to an effective ligand towards Cu(II) ions at basic pH ranges. The 4-aminopyroglutamate motif induces a specific peptide conformation which favors the formation of one or two five-membered chelating rings stabilizing a bent structure, with the coordination of 3N-type or 4N-type in the metallopeptide molecule involving in metal bonding C-terminal of modified peptide. It is worth noting that the copper(II) bonding does not undergo hydrolysis even at a very high pH range. 相似文献
64.
Albrecht Ł Ransborg LK Gschwend B Jørgensen KA 《Journal of the American Chemical Society》2010,132(50):17886-17893
The first enantioselective methodology for the synthesis of electron-poor 2-hydroxyalkyl- and 2-aminoalkyl furanes is demonstrated in this study. It utilizes a highly stereoselective organocatalytic one-pot reaction cascade: epoxidation or aziridination of α,β-unsaturated aldehydes followed by Feist-Be?nary reaction of various 1,3-dicarbonyl compounds to give the target furanes. This efficient multibond forming reaction cascade benefits from low catalyst loadings and readily available starting materials. Furthermore, the possibility to interrupt the reaction sequence at the stage of the corresponding optically active 2-hydroxyalkyl- and 2-aminoalkyl 2,3-dihydrofuranes with three stereogenic centers is also presented. Finally, models which account for the formation of the optically active 2,3-dihydrofuranes are proposed. 相似文献
65.
Angelika Baranowska Krzysztof Z. Ła̧czkowski Andrzej J. Sadlej 《Journal of computational chemistry》2010,31(6):1176-1181
We have carried out extensive studies on the basis set dependence of the calculated specific optical rotation (OR) in molecules at the level of the time–dependent Hartree–Fock and density functional approximations. To reach the limits of the basis set saturation, we have devised an artificial model, the asymmetrically deformed (chiral) methane (CM) molecule. This small system permits to use basis sets which are prohibitively large for real chiral molecules and yet shows all the important features of the basis set dependence of the OR values. The convergence of the OR has been studied with n‐aug‐cc‐pVXZ basis sets of Dunning up to the 6–ζ. In a parallel series of calculations, we have used the recently developed large polarized (LPolX) basis sets. The relatively small LPolX sets have been shown to be competitive to very large n‐aug‐cc‐pVXZ basis sets. The conclusions reached in calculations of OR in CM concerning the usefulness of LPolX basis sets have been further tested on (S)‐methyloxirane and (S)‐fluoro‐oxirane. The smallest set of the LPolX family (LPol–ds) has been found to yield OR values of similar quality as those obtained with much larger Dunning's aug‐cc‐pVQZ basis set. These results have encouraged us to carry out the OR calculations with LPol–ds basis sets for systems as large as β‐pinene and trans‐pinane. In both cases, our calculations have lead to the correct sign of the OR value in these molecules. This makes the relatively small LPol–ds basis sets likely to be useful in OR calculations for large molecules. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 相似文献
66.
We show that the equality m1(f(x))=m2(g(x)) for x in a neighborhood of a point a remains valid for all x provided that f and g are open holomorphic maps, f(a)=g(a)=0 and m1,m2 are Minkowski functionals of bounded balanced domains. Moreover, a polynomial relation between f and g is obtained. 相似文献
67.
68.
Łukasz Dębowski 《Journal of Theoretical Probability》2014,27(2):539-551
We investigate stationary hidden Markov processes for which mutual information between the past and the future is infinite. It is assumed that the number of observable states is finite and the number of hidden states is countably infinite. Under this assumption, we show that the block mutual information of a hidden Markov process is upper bounded by a power law determined by the tail index of the hidden state distribution. Moreover, we exhibit three examples of processes. The first example, considered previously, is nonergodic and the mutual information between the blocks is bounded by the logarithm of the block length. The second example is also nonergodic but the mutual information between the blocks obeys a power law. The third example obeys the power law and is ergodic. 相似文献
69.
Łukasz Janczewski Małgorzata Walczak Justyna Frączyk Zbigniew J. Kamiński 《合成通讯》2013,43(23):3290-3300
The microwave-assisted Cannizzaro reaction was studied in order to develop fully reproducible synthetic protocols for transformation of aldehydes to carboxylic acid and alcohols. Optimised were the following process parameters: power, temperature, and time. Aromatic, heteroaromatic and aliphatic aldehydes were used in the studies. It was found that furfural, thiophene-2-carbaldehyde, pyridinecarboxaldehyde and aromatic aldehydes react under mild conditions, while 1-methyl-pyrrole-2-carboxaldehyde derivatives and aliphatic aldehydes require more drastic reaction conditions and a longer exposure time to microwave radiation. 相似文献
70.
Sajewicz Mieczysław Wojtal Łukasz Hajnos Michał Waksmundzka-Hajnos Monika Kowalska Teresa 《平面色谱法杂志一现代薄层色谱法》2010,23(4):270-276
JPC – Journal of Planar Chromatography – Modern TLC - In a previous paper we discussed the possibility of fractionating the essential oils of different sage species by low-temperature... 相似文献