全文获取类型
收费全文 | 4030篇 |
免费 | 176篇 |
国内免费 | 454篇 |
专业分类
化学 | 2184篇 |
晶体学 | 22篇 |
力学 | 29篇 |
综合类 | 1篇 |
数学 | 10篇 |
物理学 | 2414篇 |
出版年
2024年 | 6篇 |
2023年 | 83篇 |
2022年 | 53篇 |
2021年 | 76篇 |
2020年 | 60篇 |
2019年 | 71篇 |
2018年 | 81篇 |
2017年 | 76篇 |
2016年 | 106篇 |
2015年 | 106篇 |
2014年 | 132篇 |
2013年 | 235篇 |
2012年 | 150篇 |
2011年 | 277篇 |
2010年 | 179篇 |
2009年 | 270篇 |
2008年 | 251篇 |
2007年 | 339篇 |
2006年 | 305篇 |
2005年 | 196篇 |
2004年 | 187篇 |
2003年 | 147篇 |
2002年 | 131篇 |
2001年 | 124篇 |
2000年 | 138篇 |
1999年 | 129篇 |
1998年 | 131篇 |
1997年 | 88篇 |
1996年 | 58篇 |
1995年 | 62篇 |
1994年 | 59篇 |
1993年 | 55篇 |
1992年 | 34篇 |
1991年 | 34篇 |
1990年 | 30篇 |
1989年 | 16篇 |
1988年 | 22篇 |
1987年 | 25篇 |
1986年 | 23篇 |
1985年 | 22篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有4660条查询结果,搜索用时 31 毫秒
31.
采用特制的Fe-Si、Fe-Mn合金系列标样,用电子探针测定了钢或铁基合金中低含量的Si和Mn元素、与用纯元素作标样、经ZAF修正的结果相比较,提高了分析准确度. 相似文献
32.
研究了阿富汗青金石矿石、敦煌石窟青金石颜料、合成群青的呈色机理。实验表明三者均有g=2.029峰,前两者多一个g=2.047峰,400℃时,g=2.047峰变平,600℃时消失。g=2.029峰随温度升高而增强,800℃时增强放慢,颜色也随温度变深。在M0O3/SiO2和M0O3/Al2O3表面上,S2^-的EPR有g1=2.047,g2=2.208,g3=1.998三个分量。因此,群青、青金石和 相似文献
33.
Cl···O intramolecular contacts are analysed here. Hence the Cambridge Structural Database (CSD) search was performed to find
five-membered Cl–C=C–C=O pseudo-rings closed through Cl···O intramolecular contacts. Such rings were analysed and it was found
that some of Cl···O distances are smaller than the corresponding sum of van der Waals radii. Additionally ab initio MP2/6-311++G(d,p)
calculations and AIM analyses were carried out here on 3-chloropropenal and its fluoro derivatives to deepen the nature of
intramolecular Cl···O interactions, some of findings may indicate that they are attractive and stabilizing ones.
This paper is dedicated to Prof. T. M. Krygowski on the occasion of his 70th birthday 相似文献
34.
Hiroki Higuchi Hidetoshi Kawai Masakazu Ohkita Takanori Suzuki 《Tetrahedron letters》2004,45(15):3027-3030
The newly prepared tetracyanoanthraquinodimethane (TCNAQ) derivatives 1a,b with a chiral auxiliary are good electron acceptors and exhibit weak circular dichroism (CD) based on the absorption of TCNAQ. The twin-type electron acceptor 1c with two TCNAQ units shows larger ellipticity by exciton coupling. UV-vis, CD, and fluorescence spectra were changed drastically upon electrochemical reduction of 1c, which demonstrates the unprecedented three-way-output response system. 相似文献
35.
The process of electron transport plays an essential role in the fundamental phenomena of life like photosynthesis, respiration and vision as well as in photoelectronic devices. However, the molecular mechanisms of the electron way and factors governing the transport rate in such systems are still unclear. Several groups have reported theoretical approaches for searching the mechanisms by using statistical mechanics, coherent dynamics and quantum mechanics. The current density vector inside the semiconducting layer is determined. In this paper we consider the problem of transport of electron promoted in the electrochemical cell constructed of two electrodes with the dye molecules immersed in. We describe the process of electron promotion by refractive light wave on the vacuum–semiconductor boundary as well as on the semiconducting electrode and the dye molecule layer in terms of extended phenomenological electrodynamics formalism. The results of our theoretical model show that such a theoretical approach will give more information on the mechanism of electron transport and will give insight in the determination of some electric features of materials. 相似文献
36.
Ludwik Adamowicz Rodney J. Bartlett Józef S. Kwiatkowski Willis B. Person 《Theoretical chemistry accounts》1988,73(2-3):135-145
The diatomic systems, PO and PO– are studied, using numerical Hartree-Fock (NHF) and coupled-cluster calculations. The latter employs a hybrid NHF and Slater orbital basis set. Highly accurate CCSD methods predict bond lengths accurate to <0.004 Å and frequencies to 60 cm–1. In addition the electron affinity of PO is computed to be 0.89 eV compared to an experimental value of 1.09±0.01. Comparisons are made with SCF and MBPT(2) results for PO+ using conventional basis sets.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayGuggenheim Fellow 相似文献
37.
The rates of photoinduced electron transfer (ET) reactions across two oligo-norbornyl spacer groups (S), that is, structure 1 fused by two norbornadiene (NBD) units and structure 2 fused by three NBD units, are examined. Substituted naphthalene acted as an electron donor (D), whilst ethylene-1,2-dicarboxylate as an electron acceptor (A). ET rates were measured by fluorescence quenching experiments on these D-S-A dyads, and the results were correlated with reaction free energies according to the Marcus relationship. It was found that naphthalene with phenyl substituents showed relatively slower ET rates. The conformational flexibility of phenyl substituents may cause a hindrance on the electronic coupling between D and A. Another salient feature was the abnormally high quenching rates observed in nonpolar solvents such as cyclohexane, the results of which may be ascribed to a competing energy transfer process. 相似文献
38.
The cation influence on the water molecule in the Li+·H2O, Be2+·H2O, Mg2+·H2O and A13+·H2O complexes has been studied by means of quantum-mechanical ab initio calculations. A number of general trends are noted. (1) The calculated equilibrium water O-H distances increase with increasing binding energies, i.e. in the order Li+, Mg2+, Be2+, Al3+. The H-O-H angles differ by about ±1 ° from the calculated equilibrium angle for the free H2O molecule; the variation has no systematic trend. (2) The electron density redistribution accompanying the change in the internal H2O geometry in these complexes is considerably smaller than the redistribution brought about by the direct influence of the external field. (3) The harmonic O-H stretching force constant decreases with increased cation-water bonding. (4) The qualitative features of the density changes are very similar for the four complexes. The magnitudes of the interactions follow the relation Li+ < Mg2+ < Be2+ Al3+. An increased polarization of the H2O molecule occurs with electron migration from the H atoms towards the O atom and an accumulation of electron charge approximately at the centre of the Men+—O bond, especially in Be2+·H2O and A13+·H2O. An electron deficiency is found in the lone-pair region. 相似文献
39.
J. C. Evans K. R. N. Rao S. K. Jackson C. C. Rowlands M. D. Barratt 《Journal of separation science》1985,8(12):829-830
Radicals generated in linoleic acid and deuterated linoleic acid have been trapped by the spin trap 2-methyl-2-nitroso-propane (MNP) and identified by electron spin resonance (ESR) and ENDOR spectroscopy. The formation of two distinct secondary alkyl radical adducts (one conjugated, the other non-conjugated, as shown by their UV absorption spectra) was demonstrated by 11, 11-dideuterio-9-cis-12-cis-linoleic acid using HPLC and ESR spectroscopy. 相似文献
40.
Summary The results of various quantum chemical calculations, the Hartree-Fock (HF) method, the Møller-Plesset perturbation theory (MP2), and the Hartree-Fock-Slater (HFS) method are compared. Atomic charges, dipole moments, topological properties of the electron density distribution and polarizabilities, and first hyperpolarizabilities are calculated. Atomic charges obtained with the HFS method are found to be very close to those calculated with the MP2 method, from which we conclude that the HFS method describes to some extent electron correlation effects. Performing an MP2 calculation after an HF calculation improves the molecular dipole moments considerably, yielding values close to the experimental ones. HFS calculations are computationally less demanding than MP2 and yield comparable results for the electron density distributions, dipole moments and polarizabilities. 相似文献