全文获取类型
收费全文 | 1623篇 |
免费 | 66篇 |
国内免费 | 8篇 |
专业分类
化学 | 934篇 |
晶体学 | 13篇 |
力学 | 84篇 |
数学 | 119篇 |
物理学 | 547篇 |
出版年
2023年 | 24篇 |
2022年 | 28篇 |
2021年 | 59篇 |
2020年 | 42篇 |
2019年 | 56篇 |
2018年 | 44篇 |
2017年 | 46篇 |
2016年 | 68篇 |
2015年 | 53篇 |
2014年 | 64篇 |
2013年 | 133篇 |
2012年 | 109篇 |
2011年 | 142篇 |
2010年 | 65篇 |
2009年 | 55篇 |
2008年 | 85篇 |
2007年 | 82篇 |
2006年 | 77篇 |
2005年 | 55篇 |
2004年 | 45篇 |
2003年 | 34篇 |
2002年 | 32篇 |
2001年 | 19篇 |
2000年 | 17篇 |
1999年 | 9篇 |
1998年 | 15篇 |
1997年 | 14篇 |
1996年 | 23篇 |
1995年 | 21篇 |
1994年 | 20篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1986年 | 9篇 |
1985年 | 13篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 3篇 |
1976年 | 5篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有1697条查询结果,搜索用时 171 毫秒
11.
A holographic image subtraction technique is proposed. The vector nature of light is utilized in introducing a phase difference of π between the two reconstructed object waves. The experimental results are also presented. 相似文献
12.
The micellization behavior of an anionic gemini surfactant, GA with nonionic surfactants C12E8 and C12E5 in presence of 0.1 M NaCl at 298 K temperature, has been studied tensiometrically in pure and mixed states, and the related
physicochemical parameters (cmc, γ
cmc, pC
20, Γ
max, and A
min) have been evaluated. Tensiometric profile (γ vs log [surfactant]), for conventional surfactants, generally consists of a single point of intersection; a gradually decreasing
line (normally linear, or with slight curvature) ultimately saturates in γ at a particular [surfactant], corresponding to complete monolayer saturation. The gemini, in this report, led to two unequivocal
breaks in the tensiometric isotherm. An attempt to the interpretation of the two breaks from molecular point of view is provided,
depending solely on the chemical structure of the surfactant. The gemini, even in mixed state with the conventional nonionic
surfactants C12E5 and C12E8, manifested the dual breaks; of course, the dominance of the feature decreases with increasing mole fraction of the nonionics
in the mixture. Theories of Clint, Rosen, Rubingh, Motomura, Georgiev, Maeda, and Nagarajan have been used to determine the
interaction between surfactants at the interface and micellar state of aggregation, the composition of the aggregates, the
theoretical cmc in pure and mixed states, and the structural parameters according to Tanford and Israelachvili. Several thermodynamic
parameters have also been predicted from those theories. 相似文献
13.
Himansu Sekhar Sahoo S. Mahalakshmi Md. Hedayetullah Mir R. Raghunathan 《Tetrahedron letters》2007,48(5):761-765
A remarkable upfield shift of the pyα protons of complexed 2,2′-bipyridine in [cis-Pd(bpy)(NO3)2] is observed which is considered to originate from the anisotropic influence of suitably positioned coordinated nitrate anions around the Pd(II) centre of the molecule. A typical complexation-induced downfield shift is observed for the NH2 protons in [cis-Pd(en)(NO3)2] where ‘en’ stands for ethylenediamine. 相似文献
14.
15.
. . 相似文献
16.
17.
A new tripodal ligand, tris[2-(((2-thiazolyl)methylidene)amino)ethyl]amine (Tatren), has been synthesized and characterized by NMR, IR, and UV-visible absorbance spectroscopy and elemental analysis. Tatren forms stable complexes with transition metal ions (Zn(2+), 1; Mn(2+), 2; Co(2+), 3) and the alkaline earth metal ions (Ca(2+), 4; Mg(2+), 5). Single-crystal X-ray structures of 1, 2, and 5 revealed six-coordinate chelate complexes with formula [M(Tatren)](ClO(4))(2) in which the metal centers are coordinated by three thiazolyl N atoms and three acyclic imine N atoms. Crystals of 1, 2, and 5 are monoclinic, P2(1)/c space group. Crystals of 4 are triclinic, P space group. The Ca(2+) complex is eight-coordinate with all N atoms of Tatren and one water molecule coordinated to the metal ion. Spectrophotometric titrations show that formation constants for the chelates of metal ions are >1 in methanol. Free Tatren inhibits the catalytic domain of matrix metalloproteinase-13 (MMP-13, collagenase-3) with K(i) = 3.5 +/- 0.6 microM. Molecular mechanics-based docking calculations suggest that one leg of Tatren coordinates to the catalytic Zn(2+) in MMPs-2, -9, and -13 with significant hydrogen bonding to backbone amide groups. High-level DFT calculations suggest that, in the absence of nonbonded interactions between Tatren and the enzyme, the most stable first coordination sphere of the catalytic Zn(2+) is achieved with three imidazolyl groups from His residues and two imine N atoms from one leg of Tatren. While complexes (1-3) do not inhibit MMP-13 to a significant extent, 4 does (K(i) = 30 +/- 10 microM). Hence, this study shows that tripodal chelating ligands of this class and their Ca(2+) complexes have potential as active-site inhibitors for MMPs. 相似文献
18.
As ab initio electronic structure calculations become more accurate, inherent sources of error in classical transition state theory such as barrier recrossing and tunneling may become major sources of error in calculating rate constants. This paper introduces a general method for diabatically constructing the transverse eigensystem of a reaction path Hamiltonian in systems with many degenerate transverse frequencies. The diabatically constructed reaction path Hamiltonian yields smoothly varying coupling constants that, in turn, facilitate reactive flux calculations. As an example we compute the dynamically corrected rate constant for the chair to boat interconversion of cyclohexane, a system with 48 degrees of freedom and a number of degenerate frequencies. The transmission coefficients obtained from the reactive flux simulations agree with previous results that have been calculated using an empirical potential. Furthermore, the calculated rate constants agree with experimental values. Comparison to variational transition state theory shows that, despite finding the true bottleneck along the reaction pathway, variational transition state theory only accounts for half of the rate constant reduction due to recrossing trajectories. 相似文献
19.
20.
Chakraborty R Das AK Cervera ML De La Guardia M 《Analytical and bioanalytical chemistry》1996,355(1):43-47
The determination of cadmium in different sample types has been carried out by electrothermal atomization atomic absorption spectrometry with D(2)-background correction using a unpyrocoated graphite tube, after pressurized microwave-assisted digestion. Five chemical modifiers [(NH(4))(2)HPO(4), Pd(NO)(3))(2), Ni(NO(3))(2), thiourea and Triton X-100] have been assayed and nickel nitrate has been found to be most effective for an accurate determination of cadmium in mussel tissue, pig kidney and sewage sludge. The characteristic mass of the method is of the order of 1 pg and the limit of detection is lower than 0.1 ng/ml. 相似文献