全文获取类型
收费全文 | 6094篇 |
免费 | 283篇 |
国内免费 | 30篇 |
专业分类
化学 | 4286篇 |
晶体学 | 48篇 |
力学 | 118篇 |
数学 | 1046篇 |
物理学 | 909篇 |
出版年
2023年 | 46篇 |
2022年 | 116篇 |
2021年 | 161篇 |
2020年 | 123篇 |
2019年 | 137篇 |
2018年 | 117篇 |
2017年 | 108篇 |
2016年 | 268篇 |
2015年 | 215篇 |
2014年 | 214篇 |
2013年 | 498篇 |
2012年 | 485篇 |
2011年 | 522篇 |
2010年 | 293篇 |
2009年 | 237篇 |
2008年 | 377篇 |
2007年 | 339篇 |
2006年 | 303篇 |
2005年 | 322篇 |
2004年 | 230篇 |
2003年 | 208篇 |
2002年 | 180篇 |
2001年 | 73篇 |
2000年 | 69篇 |
1999年 | 42篇 |
1998年 | 51篇 |
1997年 | 47篇 |
1996年 | 79篇 |
1995年 | 36篇 |
1994年 | 36篇 |
1993年 | 44篇 |
1992年 | 31篇 |
1991年 | 35篇 |
1990年 | 38篇 |
1989年 | 31篇 |
1988年 | 29篇 |
1987年 | 30篇 |
1986年 | 18篇 |
1985年 | 37篇 |
1984年 | 29篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 19篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 6篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1974年 | 8篇 |
1970年 | 3篇 |
排序方式: 共有6407条查询结果,搜索用时 15 毫秒
71.
We use agarose gel electrophoresis to characterize how the monovalent catioinic surfactant cetyltrimethylammonium bromide (CTAB) compacts double-stranded DNA, which is detected as a reduction in electrophoretic DNA velocity. The velocity reaches a plateau at a ratio R = 1.8 of CTAB to DNA-phosphate charges, i.e., above the neutralization point, and the complexes retain a net negative charge at least up to R = 200. Condensation experiments on a mixture of two DNA sizes show that the complexes formed contain only one condensed DNA molecule each. These CTAB-DNA globules were further characterized by time-resolved measurements of their velocity inside the gel, which showed that CTAB does not dissociate during the migration but possibly upon entry into the gel. Using the Ogston-model for electrophoresis of spherical particles, the measured in-gel velocity of the globule is quantitatively consistent with CTAB having two opposite effects, reduction of both the electrophoretic charge and DNA coil size. In the case of CTAB the two effects nearly cancel, which can explain why opposite velocity shifts (globule faster than uncomplexed DNA) have been observed with some catioinic condensation agents. Dissociation of the complexes by addition of anionic surfactants was also studied. The DNA release from the globule was complete at a mixing ratio between anionic and cationic surfactants equal to 1, in agreement with equilibrium studies. Circular DNA retained its supercoiling, and this demonstrates a lack of DNA nicking in the compaction-release cycle which is important in DNA transfection and purification applications. 相似文献
72.
Epoxidation of methyl esters of resin acids with the pimarane skeleton with p-nitroperbenzoic acid has permitted the isolation of epoxides of the intracyclic double bond. Allylic diterpenic alcohols were obtained by acid-catalysed isomerisation of diterpene epoxides, and the action of Collins reagent on these allylic alcohols has been studied. 相似文献
73.
A new synthesis of stevastelin C3 (3), a [13]-membered ring component of the stevastelin family, whose structure was recently revised, is reported. Initially, a macrolactonization approach was attempted to generate the [13]-membered macrolactone but this met with failure, so a translactonization reaction was tried to obtain the targeted stevastelin C3 (3) from the corresponding [15]-membered ring counterpart. Unfortunately, this strategy did not prove successful, and, consequently, we opted to undertake a transesterification reaction from 23, as a means to accommodate the requisite aminoacid moiety at the correct position, to obtain 24. From 24, and through intermediates 25-28, the acyclic precursor of the [13]-membered ring macrolactone, compound 30, was efficiently prepared. By utilizing the synthetic course developed by Chida, we took 30 forward and completed the total synthesis of stevastelin C3 (3). 相似文献
74.
Raquel F. P. Nogueira Ronaldo A. Pilli Nelson durÁn 《Applied biochemistry and biotechnology》1992,33(3):169-176
Growth of the ascomyceteChrysonilia sitophila during degradation of lignin model dimers and monomers was compared to a glucose control. An inhibition of growth by Cα-carbonyl
monomers and stimulation by β-O-4 lignin model and vanillyl alcohol were observed. A comparison of the degradation by this
ascomycete with the basidiomycetePhanerochaete chrysosoporium showed similarities in relation to the type of degradation caused. 相似文献
75.
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
76.
Juan M. Quintana-Melgoza Antonio Gómez-cortés Miguel Avalos-Borja 《Reaction Kinetics and Catalysis Letters》2002,76(1):131-140
We present a comparative study of NiWO4, NiO, and WO3 catalysts for simultaneous conversion of NO and CO. Samples were synthesized by reacting ammonium metatungstate and/or nickel nitrate at high temperature (773 K to 903 K) under an oxygen stream. Catalysts were characterized by X-ray diffraction, surface area measurements, energy dispersive spectroscopy and scanning electron microscopy. The catalytic reduction of NO by CO took place in the temperature range (523 to 973) K under highly reductive conditions (NO:CO= 1:5) over NiWO4NiO, and WO3, respectively. The 100 % NO conversion at GHSV of 11460 h-1 was achieved at 773 K over NiWO4 and at 848 K over NiO. The WO3 was deactivated at 898 K. However, in the range (523 to 723) K NiO was more active than NiWO4 and WO3 catalysts. 相似文献
77.
Conejo-García A Campos JM Entrena A Sánchez-Martín RM Gallo MA Espinosa A 《The Journal of organic chemistry》2003,68(22):8697-8699
A complete study of the conformational behavior of 4,8-diaza-3(1,4),9(4,1)-dipyridina-1,6(1,4)-dibenzenacyclodecaphan-3(1),9(1)-bis(ilium) bishexafluorophosphate is described. This study allows us to conclude that the process observed by which the different chemical shifts of the pyridinium protons show coalescence at a high-temperature 1H NMR is the rotation around the C-N bond, whereas the conformational equilibrium between the four conformers is produced at low temperature. 相似文献
78.
A pressure-assisted capillary chromatography-mass spectrometry method based on the use of a monolithic octadecylsilica (ODS) capillary is proposed for the determination of aliphatic amines. A 25 mM citric acid buffer containing 10% methanol is used as running electrolyte. Separation is achieved by simultaneously applying a capillary electrophoresis (CE) voltage of 13 kV and an overimposed pressure of 8 bar. The use of pressure is required to ensure stable electrospray conditions. Analysis times are reduced by using a capillary column consisting of a 30 cm long monolithic silica capillary column bound with ODS and a fused-silica capillary column also 30 cm long. The proposed method was successfully applied to the determination of low-molecular-weight aliphatic amines in tap and river water. The analysis of real samples requires cleanup and preconcentration, which can be performed automatically by inserting a minicolumn in the replenishment system of the commercial instrument. 相似文献
79.
We present a quantum-mechanical investigation of the LiH depletion reaction LiH+H-->Li+H2 and of the H exchange reaction LiH+H'-->LiH'+H. We report product distributions, rate constant, and mechanism of the former, and rate constant and mechanism of the latter reaction. We use the potential-energy surface by Dunne et al. [Chem. Phys. Lett. 336, 1 (2001)], the real-wave-packet method by Gray and Balint-Kurti [J. Chem. Phys. 108, 950 (1998)], and the J-shifting approximation. The 1H2 nuclear-spin statistics and progressions of vib-rotational states (v',j') rule both initial-state-resolved and thermal product distributions, which have saw-toothed shapes with odd j' preferred with respect to even j'. At high collision energies and temperatures, we obtain a regular 3-to-1 intensity alternation of rotational states. At low collision energies and temperatures, the degeneracy and density of many H2 levels can, however, give more irregular distributions. During the collision, the energy flows from the reactant translational mode to the product vibration and recoil ones. The rate constants of both reactions are not Arrhenius type because the reactions are barrier-less. The low-temperature, LiH depletion rate constant is larger than the H exchange one, whereas the contrary holds at high temperature. The real-time mechanisms show the nuclear rearrangements of the nonreactive channel and of the reactive ones, and point out that the LiH depletion is preferred over the H exchange at short times. This confirms the rate-constant results. 相似文献
80.
On the basis of a detailed study of the pilocarpine-induced nickel(II) pre-wave using various polarographic techniques, an electrode process mechanism is proposed in which the formation of a catalytic complex between aquo-nickel(II) and veronalate-nickel(II) on the one hand and unprotonated pilocarpine adsorbed on the electrode surface on the other is followed by the reduction of nickel(II) in the complex and the release of the catalytic ligand. The pre-peak recorded by differential-pulse polarography in the system 1 × 10?3 M Ni(II)-1 × 10?2 M sodium veronal, nitric acid (pH 8.5) (with ionic strength maintained at 0.2 with sodium nitrate) can be used for quantitative determination of pilocarpine at concentrations in the range 2.5 × 10?7-8 × 10?6 M. 相似文献