全文获取类型
收费全文 | 3159篇 |
免费 | 541篇 |
国内免费 | 648篇 |
专业分类
化学 | 2129篇 |
晶体学 | 25篇 |
力学 | 62篇 |
综合类 | 34篇 |
数学 | 1077篇 |
物理学 | 1021篇 |
出版年
2024年 | 8篇 |
2023年 | 25篇 |
2022年 | 86篇 |
2021年 | 102篇 |
2020年 | 97篇 |
2019年 | 104篇 |
2018年 | 87篇 |
2017年 | 107篇 |
2016年 | 102篇 |
2015年 | 162篇 |
2014年 | 187篇 |
2013年 | 262篇 |
2012年 | 209篇 |
2011年 | 232篇 |
2010年 | 217篇 |
2009年 | 237篇 |
2008年 | 224篇 |
2007年 | 277篇 |
2006年 | 204篇 |
2005年 | 193篇 |
2004年 | 195篇 |
2003年 | 178篇 |
2002年 | 147篇 |
2001年 | 89篇 |
2000年 | 92篇 |
1999年 | 95篇 |
1998年 | 79篇 |
1997年 | 63篇 |
1996年 | 58篇 |
1995年 | 31篇 |
1994年 | 31篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 14篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有4348条查询结果,搜索用时 15 毫秒
71.
T. Hondoh H. Anzai A. Goto S. Mae A. Higashi C. C. Langway Jr. 《Journal of inclusion phenomena and macrocyclic chemistry》1990,8(1-2):17-24
We have carried out X-ray diffraction studies on single crystals of natural air-hydrate in deep ice cores recovered at Dye-3 Greenland. Integrated intensities for 470 diffracting planes were measured by an automated four-circle diffractometer. The space group determined is cubicFd3m and the lattice constant is 17.21(3) Å. These results indicate that the crystallographic structure is the Stackelberg's structure II, in contrast to the previously anticipated structure. This finding agrees with the recent results on the synthetic air-hydrate by Davidsonet al. It was also found by difference Fourier synthesis for guest molecules that electron density in a 16-hedral cage has multiple maxima displaced from the center of the cage while that in the 12-hedron was approximately spherical.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena. 相似文献
72.
许树声 《应用数学学报(英文版)》1994,10(3):262-273
ALTERNATIONTHEORYINAPPROXIMATIONBYPOLYNOMIALSHAVINGBOUNDEDCOEFFICIENTSXUSHUSHENG(许树声)(JiangnanUniversity,Wuxi214063,China)Abs... 相似文献
73.
Srdjan Petrovic 《Proceedings of the American Mathematical Society》1996,124(9):2807-2811
In this paper we study the Foias-Williams operator
where , and is a Hankel operator with symbol . We exhibit a relationship between the similarity of to a contraction and the rate of decay of , the absolute values of the Fourier coefficients of the symbol .
74.
LetR be a ring with unit and invariant basis property. In [1], the authors define a functorK(_;R):TOP/LIP
c-LPEP by combining the open cone construction of [7] with a geometric module construction and show this functor is a homology theory. This paper shows that if attention is restricted to objects TOP/LIP
c with a homotopy colimit structure, then the functorK(_;R) is a Quinn homology theory, In particular, for each having a homotopy colimit structure,K(;R) is a homotopy colimit in the category of -spectra. Furthermore, the constituent spectra of this homotopy colimit are obtained naturally from the fibres of .Partially supported by the National Science Foundation under grant number DMS88-03148.Partially supported by the SNF (Denmark) under grant number 11-7792. 相似文献
75.
John D. Watts Miroslav Urban Rodney J. Bartlett 《Theoretical chemistry accounts》1995,90(5-6):341-355
Summary This paper reports a series of coupled-cluster (CC) calculations through CCSDT on the theoretically challenging ground state of the BeO molecule. Along with CC methods, quadratic configuration interaction (QCI) approximations to CC theory have been used (QCISD and QCISD(T)), which show several dramatic failings. Equilibrium electrical properties (,
xx
, and
zz
) and basic spectroscopic properties (r
e, e,D
e, and infrared intensity (I)) have been computed. Basis set and electron correlation effects are analyzed in order to arrive at accurate values of the dipole moment and polarizability, which are not known experimentally. For the dipole moment, we obtain a value of 6.25 D, with an uncertainty of about 0.1 D. For
xx
and
zz
, we suggest respective values of 32 and 36 atomic units (a.u.) and error bars of about 1 and 2 a.u. With extended basis sets, the spectroscopic propertiesr
e, e, andD
e are reproduced to high accuracy, which is the first time this has been achieved for this species byab initio methods. At the highest calculation levels,I is predicted to be very small. AlthoughI has not been measured, some support for this prediction comes from a recent infrared study of BeO-rare gas complexes. The QCI methods are shown to be much more sensitive to basis set, and even with large basis sets yield values of
zz
andI which differ from CC results by an order of magnitude and three orders of magnitude, respectively. These differences doubtless arise from the importance of single excitations (T
1) for this molecule, as several terms involvingT
1 are neglected in the QCISD approximation compared with CCSD. We also report CC calculations with Brueckner orbitals, which yield results similar to those obtained with restricted Hartree-Fock orbitals. 相似文献
76.
浓度,温度,压力,体系组分及不同分子间相互作用等因素对双亲性化合物分子在水溶液体系中的聚集状态及相变过程均有影响。本文综述了富里叶变换红外光谱方法在这一研究领域内应用十余年间的研究进展。 相似文献
77.
Andrzej Olczak Marek L. Gwka Jolanta Goka Magorzata Szczesio Joanna Bojarska Krystyna Kozowska Henryk Foks Czesawa Orlewska 《Journal of Molecular Structure》2007,830(1-3):171-175
Crystal structures of two title compounds and several their relatives known earlier reveal conservative and characteristic features, which may be related to their tuberculostatic activity. The molecules are predominantly planar due to conjugation through five successive bonds in the zwitterionic fragment S−–C(sp2)–N–NH+–C(sp2)–NH2 and intramolecular hydrogen bonds, which prevent rotation of the adjacent pyrazine (or pyridine) ring. It has been suggested that in spatial sense such planar molecules resemble acridines intercalating with nucleic acids and that similar process may be responsible for tuberculostatic activity of the title pyrazine-2-carboxamide-N′-carbonothioyl-hydrazones. 相似文献
78.
对于NO分子轨道的能级次序现有两种不同的说法,不少人根据NO与O_2~+是等电子体,由O_2~+分子的能级次序确定NO分子组态。本文用NO和O_2~(+)从头计算结果以及NO的紫外光电子能谱相结合的方法说明NO的5σ轨道是弱成键轨道,5σ的轨道能稍高于1π轨道能,也就是说NO的能级次序是与N_2分子相同的;NO与O_2~+虽是等电子体,但是能级次序并不相同,因此由O_2~+的能级次序确定NO分子的电子组态是不妥的。 相似文献
79.
Fullerenes分子是科学家在研究碳原子簇过程中发现的一类崭新的有机分子,简单情形是C_(60)。Fullerenes及其衍生物为碳化学打开了—个广阔天地,同时也为固体物理学提供了一个新的研究领域。它还将为材料科学带来新的机会。尽管化学家和物理学家为这类新奇的原子簇勾画了许多理论图象,但关于其键构的—般性规则的研究仍然太少。本文将在 相似文献
80.
The ab initio Neglect of Differential Diatomic Overlap (N.D.D.O.) method of Roby is tested numerically for an extensive series of molecules. Agreement with the full ab initio molecular orbital method is poor. Total energies are more negative and dipole moments are overestimated. The failings of the N.D.D.O. method are accounted for using multipole-multipole expansions. 相似文献