全文获取类型
收费全文 | 1301篇 |
免费 | 32篇 |
国内免费 | 6篇 |
专业分类
化学 | 869篇 |
晶体学 | 8篇 |
力学 | 3篇 |
数学 | 198篇 |
物理学 | 261篇 |
出版年
2022年 | 17篇 |
2021年 | 28篇 |
2020年 | 14篇 |
2019年 | 9篇 |
2018年 | 15篇 |
2017年 | 11篇 |
2016年 | 37篇 |
2015年 | 28篇 |
2014年 | 42篇 |
2013年 | 67篇 |
2012年 | 62篇 |
2011年 | 66篇 |
2010年 | 40篇 |
2009年 | 44篇 |
2008年 | 75篇 |
2007年 | 60篇 |
2006年 | 56篇 |
2005年 | 59篇 |
2004年 | 54篇 |
2003年 | 45篇 |
2002年 | 40篇 |
2001年 | 26篇 |
2000年 | 29篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 7篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 18篇 |
1993年 | 7篇 |
1992年 | 19篇 |
1991年 | 19篇 |
1990年 | 19篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 15篇 |
1986年 | 9篇 |
1985年 | 18篇 |
1984年 | 14篇 |
1983年 | 11篇 |
1982年 | 14篇 |
1981年 | 10篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1977年 | 13篇 |
1976年 | 15篇 |
1973年 | 9篇 |
1970年 | 5篇 |
1936年 | 6篇 |
排序方式: 共有1339条查询结果,搜索用时 31 毫秒
31.
32.
István Szilágyi Krisztina Imrik Dariusz Sarzisky Sándor Dóbé Tibor Bérces 《Reaction Kinetics and Catalysis Letters》2002,77(2):341-345
The title reactions have been studied at room temperature by applying the discharge flow method coupled with laser induced fluorescence detection of methoxy radicals and resonance fluorescence detection of bromine atoms. The following rate constants were determined: CH3O + Br Õ products (1) k
1 (298 K) = (3.4 ± 0.4 (1)) × 1013 cm3 mol-1 s-1, CH3O + Br2 Õ products (2) k
2 (298 K) £ 5 × 108 cm3 mol-1 s-1. 相似文献
33.
Based on the method of SAMSAHL a programmed, semi-automated processing system has been developed and tested for the neutron
activation analysis of the following elements: As, Se, Sb, Br, Sn and Te. The main characteristics of the procedure are the
following: wet ashing of the sample (by means of digestion in a mixture of concentrated sulphuric acid and hydrogen peroxide),
bromination, and chlorination. The procedure is controlled via the appropriate dosage of the reagents and a carefully managed
thermal balance. The chemical yield and its reproducibility were determined by tracer techniques using exactly the same parameters
as for the active runs. According to the results the method can be used for the routine determination of the given elements
except antimony.
Presented at the 4th Symposium on the Recent Developments in Neutron Activation Analysis, 1975, Cambridge. 相似文献
34.
L. Turi Nagy P. Pelikn M. Lika M. Haring M. eppan M. Breza 《International journal of quantum chemistry》1979,16(3):485-500
The model systems of molecular nitrogen fixation [N2 + H]?, [N2 + H]+, [N2 + H]?, [N2 + H2], [N2 + H2]+, and [N2 + H2]? were studied by the semiempirical INDO method. The study was based on the formal analogy between the catalytic reactions and the photochemical, radical, and ionic reactions on the other side. Symmetrical and donor-acceptor properties of necessary catalytic systems were proposed using the dependence of energy characteristics and electron structure on reaction coordinate. On the basis of this MO study we have proposed the appropriate symmetry types of catalysts for each of acceptable models of nitrogen fixation. For one of the proposed systems there was realised a model MO computation with explicit inclusion of atoms of transition metals (Fe, V). 相似文献
35.
36.
György Schultz Tamás Nagy Gustavo Portalone Fabio Ramondo István Hargittai Aldo Domenicano 《Structural chemistry》1993,4(3):183-190
The molecular structure and benzene ring distortions of ethynylbenzene have been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and 6-3G** levels. Least-squares refinement of a model withC
2v, symmetry, with constraints from the MO calculations, yielded the following important bond distances and angles:r
g(C
i
-C
o
)=1.407±0.003 Å,r
g(C
o
-C
m
)=1.397±0.003 Å,r
g(C
m
-C
p
)=1.400±0.003 Å,r
g(Cr
i
-CCH)=1.436 ±0.004 Å,r
g(C=C)=1.205±0.005 Å, C
o
-C
i
-C
o
=119.8±0.4°. The deformation of the benzene ring of ethynylbenzene given by the MO calculations, including o-Ci-Co=119.4°, is insensitive to the basis set used and agrees with that obtained by low-temperature X-ray crystallography for the phenylethynyl fragment, C6H5-CC-, in two different crystal environments. The partial substitution structure of ethynylbenzene from microwave spectroscopy is shown to be inaccurate in the ipso region of the benzene ring. 相似文献
37.
R. Garg V. K. Garg I. Czakó-Nagy S. Nagy E. Kuzmann A. Vértes 《Journal of Radioanalytical and Nuclear Chemistry》1994,186(6):455-462
Mössbauer spectroscopy and X-ray diffractometry have been used to study Sm–Fe mixed oxides (with different SmFe atomic ratios) annealed at 550, 850, 1000 and 1250 °C. The room temperature Mössbauer spectra can be interpreted in terms of one, two or three sextets and in some cases by an additional doublet depending on the composition and the heat treatment. The sextets have been associated with SmFeO3 perovskite, Sm3Fe5O12 garnet and -Fe2O3 hematite. These results are in agreement with those of X-ray diffractometric measurements, which give a clear, evidence of the presence of these phases. 相似文献
38.
Monte Carlo simulations have been performed to explore the solution structure of ethyl, isopropyl, isobutyl, and tertiary butyl alcohols in pure water, pure acetonitrile, and different mixtures of the two solvents. The explicit solvent studies in NpT ensembles at T = 298 K illustrate that the solute "discriminates" the solvent's components and that the composition of the first solvation shell differs from that of the bulk solution. Since the polarizable continuum dielectric method (PCM) does not presently model the solvation of molecules with both polar and apolar sites in mixed protic solvents, we suggest a direction for further program development wherein a continuum dielectric method would accept more than one solvent and the solute sites would be solvated by user-defined solvent components. The prevailing solvation model will be determined upon the lowest free energy calculated for a particular solvation pattern of the solute having a specific conformational/tautomeric state. Characterization of equilibrium hydrogen-bond formation becomes a complicated problem that depends on the chemical properties of the solute and its conformation, as well as upon the varying nature of the first solvation shell. For example, while the number of hydrogen bonds to secondary and tertiary alcohol solutes are nearly constant in pure water and in water-acetonitrile mixtures with at least 50% water content, the number of hydrogen bonds to primary alcohols gradually decreases for most of their conformations when acetonitrile content is increased. Nonetheless, the calculations indicate that O-H...O(water) hydrogen bonds are still possible in a small fraction of the arrangements for the solution models with water content of 30% or less. The isopentene solute does not form any observable hydrogen bonds, despite having an electron-rich, double-bond site. 相似文献
39.
New ionization potential values are reported for sixteen aromatic molecules. They are obtained from CT spectra of a new electron acceptor, 3,5-dinitro-phthalic anhydride with the aromatic electron donors in 1,2-dichloroethane. The results are correlated with molecular electronic structure using perturbation theory. 相似文献
40.
-Al2O3-supported Bi–Pt and Pd–Pt bimetallic catalysts were prepared by electrochemical metal adsorption. Isomerization of n-pentane was studied over these catalysts in a pulse reactor. It can be concluded from the results that the properties of these catalysts differ substantially from the base Pt/-Al2O3 catalyst. On the modified catalysts conversion and the yield of i-pentane was found to be lower although selectivity was better. The ratio of cracking reactions was also lower than over the base catalyst.It is very probable that the adsorbed bismuth was slowly desorbing from the catalyst surface during the experiments.
Bi–Pt Pd–Pt -Al2O3. -, . , Pt/Al2O3. , - , . , . , .相似文献