全文获取类型
收费全文 | 3886篇 |
免费 | 281篇 |
国内免费 | 4篇 |
专业分类
化学 | 2773篇 |
晶体学 | 21篇 |
力学 | 143篇 |
数学 | 585篇 |
物理学 | 649篇 |
出版年
2023年 | 15篇 |
2022年 | 25篇 |
2021年 | 37篇 |
2020年 | 70篇 |
2019年 | 76篇 |
2018年 | 72篇 |
2017年 | 39篇 |
2016年 | 148篇 |
2015年 | 119篇 |
2014年 | 119篇 |
2013年 | 243篇 |
2012年 | 276篇 |
2011年 | 288篇 |
2010年 | 152篇 |
2009年 | 168篇 |
2008年 | 233篇 |
2007年 | 242篇 |
2006年 | 235篇 |
2005年 | 241篇 |
2004年 | 190篇 |
2003年 | 173篇 |
2002年 | 132篇 |
2001年 | 39篇 |
2000年 | 37篇 |
1999年 | 41篇 |
1998年 | 15篇 |
1997年 | 18篇 |
1996年 | 30篇 |
1995年 | 22篇 |
1993年 | 24篇 |
1992年 | 22篇 |
1991年 | 19篇 |
1990年 | 23篇 |
1989年 | 21篇 |
1987年 | 20篇 |
1986年 | 13篇 |
1985年 | 32篇 |
1984年 | 33篇 |
1983年 | 20篇 |
1982年 | 37篇 |
1981年 | 22篇 |
1980年 | 32篇 |
1979年 | 37篇 |
1978年 | 25篇 |
1977年 | 29篇 |
1976年 | 34篇 |
1975年 | 24篇 |
1974年 | 27篇 |
1973年 | 26篇 |
1968年 | 12篇 |
排序方式: 共有4171条查询结果,搜索用时 15 毫秒
81.
L De Boni L T Fran?a H P Grieneisen M Janowicz T B Kist A R Consiglio J R Schoffen V Stefani C Termignoni 《Electrophoresis》1999,20(12):2493-2500
The phenomenon of electrophoresis in free solution has been studied theoretically down to the molecular level for decades. In addition, intermolecular photo-induced proton transfer reactions, which occur in a wide class of molecules (phenols and aminoarenes) as well as proteins (green fluorescent protein), were also studied extensively. However, the study of the effect of light-induced electrophoretic mobility changes of the analytes in electrophoresis was begun only recently. In the present work, capillary zone electrophoresis was chosen as the environment to measure the magnitude of these electrophoretic mobility shifts induced by light. Background electrolytes (running electrolytes) with high refractive indices were developed, allowing the capillary to work like an optical fiber. The experimental conditions for obtaining stable coupling and guided laser light along the liquid core are discussed. Experimental evidence of band compression is observed, leading to a solitary wave behavior of the analyte band (2-naphthol). These solitary waves result from competition between thermal diffusion (dispersion mechanism) and a nonlinear (band compression) effect due to the combined electrophoresis phenomenon and absorption of guided light by the molecules of the band (which are subjected to a "reversible intermolecular proton transfer reaction" as one of their decay routes). The possibilities of applying this effect to different methods and techniques are also discussed. 相似文献
82.
H. -J. Stöckmann D. Dubbers M. Grupp H. Grupp H. Ackermann P. Heitjans 《Zeitschrift für Physik B Condensed Matter》1978,30(1):19-27
Neutron activated20F nuclei (T
1/2=11 s) in CaF2 serve for two purposes: first they produce well defined point defects by their (n, )-production mechanism, second, they act as probes to detect the properties of these defects in a- and-ray detected nuclear magnetic resonance experiment. Below about 70 K an interstitial defect is observed due to a quadrupolar splitting of the20F-NMR line. The defect induced electric field gradient is oriented along the 111-axis of the crystal and causes a quadrupole coupling constante
2
qQ/h=–2.0(1) MHz at the20F site. The defect anneals above 70 K with an Arrhenius like temperature dependence corresponding to an activation energyE
M
=0.17(4) eV. 相似文献
83.
Turbiez M Frère P Allain M Videlot C Ackermann J Roncali J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(12):3742-3752
Hybrid oligothiophenes based on a various combinations of thiophene and 3,4-ethylenedioxythiophene (EDOT) groups have been synthesized. UV/Vis absorption spectra show that the number and relative positions of the EDOT groups considerably affect the width of the HOMO-LUMO gap and the rigidity of the conjugated system. Analysis of the crystallographic structure of two hybrid quaterthiophenes confirms that insertion of two adjacent EDOT units in the middle of the molecule leads to a self-rigidification of the conjugated systems by intramolecular SO interactions. Cyclic voltammetry data shows that the first oxidation potential of the oligomers decreases with increasing chain length and increasing number of EDOT groups for a given chain length. Electrochemical studies and theoretical calculations show that the positions of the EDOT units in the conjugated chain control the potential difference (DeltaE(p)) between the first and second oxidation steps. Moving the EDOT groups from the outer to the inner positions of the conjugated system increases DeltaE(p). Theoretical calculations confirm that this phenomenon reflects an increase of the intramolecular coulombic repulsion between positive charges in the dication. A thin-film field-effect transistor was fabricated by vacuum sublimation of a pentamer with alternating thiophene-EDOT structure, and the hole mobility was determined. 相似文献
84.
The metastable ion supported fragmentation process in the mass spectra of the cyclohexadienyl derivative C6H7Mn(CO)3, the cycloheptadienyl derivative C7H9Mn(CO)3, the 1,2,3,4,5-and 1,2,3,5,6-pentahaptocyclootadienyl derivatives C8H11Mn(CO)3, the cyclooctatrienyl derivative C8H9Mn(CO)3 and the substituted cyclopentadienyl derivative (CH3)2NCH2C5H4Mn(CO)3, are described. Losses of carbonyl groups, generally stepwise, from the molecular ions to give the corresponding [M – 3CO]+· ions are first observed. Further fragmentation of the carbonyl-free [M – 3CO]+· ions can involve a variety of processes such as the following: (a) elimination of a neutral manganese atom to give a hydrocarbon fragment; (b) elimination of a neutral hydrocarbon fragment to give an [MnH]+· ion; (c) dehydrogenation; (d) elimination of a 2-carbon C2H2 or C2H4 fragment; (e) elimination of a C3H4 or C3H6 fragment as a neutral species when it is bridging two carbon atoms bonded to manganese, as in C8H9Mn(CO)3 and 1,2,3,4,5,h5-C8H11Mn(CO)3, respectively. Fragmentation of the [M – 3CO]+· ion in (CH3)2NCH2C5H4Mn(CO)3 presents the following additional features: (a) elimination of C6H6 with a nitrogen shift from carbon to manganese; (b) elimination of a neutral dimethylamino fragment to give [C6H6Mn]+·, which then loses neutral C6H6, C6H5 or Mn fragments and thus is formulated tentatively as [(fulvene)Mn]+· or [C6H5MnH]+· rather than [(benzene)Mn]+·. 相似文献
85.
Pd/C-mediated allylic substitution in water is described as an interesting alternative to classical homogeneous conditions. The reaction applied to allylic acetates showed a wide range of compatibility with various nitrogen, sulfur, oxygen, and carbon nucleophiles. Notably, the method features inexpensive reagents and a nontoxic solvent. Moreover, measurement of the palladium content in water by ICP-MS shows low palladium contamination (4 ppm) of the solvent, rendering this method safer for the environment compared to homogeneous conditions. The first asymmetric example of Pd/C-mediated allylic substitution is also disclosed. 相似文献
86.
Christiaens FJ Chardon A Fourtanier A Frederick JE 《Photochemistry and photobiology》2005,81(4):874-878
The skin is exposed to ultraviolet radiation (UVR) from natural or artificial sources on a daily basis. The effects of chronic low dose exposure merit investigation, even when these effects are neither conspicuous nor clinically assessable. The purpose of the present study was to define a relative spectral UV irradiance that is representative of frequent nonextreme sun exposure conditions and therefore more appropriate for studies of the long-term and daily effects of solar UV on the skin. Solar spectral UV irradiance values were calculated for different dates and locations by using a radiative transfer model. The spectral irradiance values obtained when the solar elevation is lower than 45 degrees were averaged. An important feature is the dUVA (320-400 nm) to dUVB (290-320 nm) irradiance values ratio, which was found to be 27.3 for the overall average. When the months corresponding to extreme irradiance values (low or high) were excluded from the calculations, the dUVA to dUVB ratio ranged from 27.2 to 27.5. The mean spectral irradiance of the model presented here represents environmental UV exposure conditions and can be used both as a standard to investigate the biological effects of a nonextreme UVR and to assess the effectiveness of products for daily skin protection. 相似文献
87.
Knippenberg S Nixon KL Mackenzie-Ross H Brunger MJ Wang F Deleuze MS François JP Winkler DA 《The journal of physical chemistry. A》2005,109(41):9324-9340
Results of a study of the valence electronic structure of norbornene (C(7)H(10)), up to binding energies of 30 eV, are reported. Experimental electron momentum spectroscopy (EMS) and theoretical Green's function and density functional theory approaches were utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all 19 valence orbitals of norbornene. This experimentally validated model was then used to extract other molecular properties of norbornene (geometry, infrared spectrum). When these calculated properties are compared to corresponding results from independent measurements, reasonable agreement is typically found. Due to the improved energy resolution, EMS is now at a stage to very finely image the effective topology of molecular orbitals at varying distances from the molecular center, and the way the individual atomic components interact with each other, often in excellent agreement with theory. This will be demonstrated here. Green's Function calculations employing the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than about 22 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet emission and newly presented (e,2e) ionization spectra. Finally, limitations inherent to calculations of momentum distributions based on Kohn-Sham orbitals and employing the vertical depiction of ionization processes are emphasized, in a formal discussion of EMS cross sections employing Dyson orbitals. 相似文献
88.
Over the past few years, a large number of studies have been prepared that describe the analysis of peptides and proteins using capillary electrophoresis (CE) and laser-induced fluorescence (LIF). These studies have focused on two general goals: (i) development of automatic, selective and quick separation and detection of mixtures of peptides or proteins; (ii) generation of new methods of quantitation for very low concentrations (nm and subnanomolar) of peptides. These two goals are attained with the use of covalent labelling reactions using a variety of dyes that can be readily excited by the radiation from a commonly available laser or via the use of noncovalent labelling (immunoassay using a labelled antibody or antigen or noncovalent dye interactions). In this review article, we summarize the works which were performed for protein and peptide analysis via CE-LIF. 相似文献
89.
Michael Aizenman FranÇois Delyon Bernard Souillard 《Journal of statistical physics》1980,23(3):267-280
We rigorously prove that the probabilityP
n
that the origin of ad-dimensional lattice belongs to a cluster of exactlyn sites satisfiesP
n
> exp(–n
(d–1)/d
) whenever percolation occurs. This holds for the usual (noninteracting) percolation models for any concentrationp > p
c
, as well as for the equilibrium states of lattice spin systems with quite general interactions. Such a lower bound applies also if no percolation occurs, but if it appears in some other phase of the system. 相似文献
90.
Molecular chaperones, folding catalysts, and the recovery of active recombinant proteins fromE. coli
Jeffrey G. Thomas Amanda Ayling François Baneyx 《Applied biochemistry and biotechnology》1997,66(3):197-238
The high-level expression of recombinant gene products in the gramnegative bacteriumEscherichia coli often results in the misfolding of the protein of interest and its subsequent degradation by cellular proteases or its deposition
into biologically inactive aggregates known as inclusion bodies. It has recently become clear that in vivo protein folding
is an energy-dependent process mediated by two classes of folding modulators. Molecular chaperones, such as the DnaK-DnaJ-GrpE
and GroEL-GroES systems, suppress off-pathway aggregation reactions and facilitate proper folding through ATP-coordinated
cycles of binding and release of folding intermediates. On the other hand, folding catalysts (foldases) accelerate rate-limiting
steps along the protein folding pathway such as thecis/trans isomerization of peptidyl-prolyl bonds and the formation and reshuffling of disulfide bridges. Manipulating the cytoplasmic
folding environment by increasing the intracellular concentration of all or specific folding modulators, or by inactivating
genes encoding these proteins, holds great promise in facilitating the production and purification of heterologous proteins.
Purified folding modulators and artificial systems that mimic their mode of action have also proven useful in improving the
in vitro refolding yields of chemically denatured polypeptides. This review examines the usefulness and limitations of molecular
chaperones and folding catalysts in both in vivo and in vitro folding processes. 相似文献