全文获取类型
收费全文 | 311866篇 |
免费 | 2340篇 |
国内免费 | 980篇 |
专业分类
化学 | 146091篇 |
晶体学 | 4795篇 |
力学 | 16670篇 |
综合类 | 4篇 |
数学 | 57901篇 |
物理学 | 89725篇 |
出版年
2021年 | 1967篇 |
2020年 | 2215篇 |
2019年 | 2512篇 |
2018年 | 13475篇 |
2017年 | 13447篇 |
2016年 | 10867篇 |
2015年 | 3566篇 |
2014年 | 4834篇 |
2013年 | 10713篇 |
2012年 | 11745篇 |
2011年 | 19653篇 |
2010年 | 12606篇 |
2009年 | 13032篇 |
2008年 | 15335篇 |
2007年 | 17247篇 |
2006年 | 8598篇 |
2005年 | 8686篇 |
2004年 | 8418篇 |
2003年 | 8203篇 |
2002年 | 7186篇 |
2001年 | 7287篇 |
2000年 | 5528篇 |
1999年 | 4161篇 |
1998年 | 3642篇 |
1997年 | 3413篇 |
1996年 | 3189篇 |
1995年 | 2934篇 |
1994年 | 2856篇 |
1993年 | 2791篇 |
1992年 | 3100篇 |
1991年 | 3097篇 |
1990年 | 2980篇 |
1989年 | 2927篇 |
1988年 | 2859篇 |
1987年 | 2820篇 |
1986年 | 2670篇 |
1985年 | 3415篇 |
1984年 | 3543篇 |
1983年 | 3112篇 |
1982年 | 3234篇 |
1981年 | 2960篇 |
1980年 | 2931篇 |
1979年 | 3098篇 |
1978年 | 3236篇 |
1977年 | 3182篇 |
1976年 | 3146篇 |
1975年 | 3044篇 |
1974年 | 3002篇 |
1973年 | 3163篇 |
1972年 | 2142篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
D. Charalambidis B. H. Feng C. Fotakis 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1989,14(3):223-227
Photoelectron angular distributions have been measured for the three-photon resonant one-photon ionization, (3+1), of Xe via the 7s[1 1/2]0 1 and 8s[1 1/2]0 1 states. The results are in good agreement with the theoretical predictions of Tang and Lambropoulos [13] for the 8s[1 1/2]0 1, but not for the 7s[1 1/2]0 1 state. Furthermore, the results are compared to those which have been obtained by Blazewicz et al. [1] for the three-photon resonant two-photon ionization, (3+2), of Xe via the 6s[1 1/2]0 1 state. 相似文献
992.
The negative-ion mass spectra of a variety of substituted nitroaryl benzoates and aryl nitrobenzoates have been studied. Pronounced proximity effects are observed in the spectra of ortho-substituted compounds. The meta and para pairs fragment in a similar manner. Linear plots are observed for many cleavages using either the McLafferty or Harrison methods. Evidence is provided which suggests that the fragmentations of meta and para isomers may proceed by the same mechanistic pathway. The magnitude of ρ does not appear to provide precise information concerning the nature of the transition state for a particular reaction of a negative ion. 相似文献
993.
Oxidation of cysteine, glutathione and ascorbate by photoexcited proteins from normal and cataractous lenses was investigated using electron paramagnetic resonance in combination with spin trapping. We report that illumination of these proteins in pH 7 buffer with light > 300 nm in the presence of thiols (RSH) and a spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), afforded DMPO/S-cysteine and DMPO/SG adducts, suggesting the formation of the corresponding thiyl radicals. In a nonbuffered aqueous solution, illumination of the proteins and glutathione also produced superoxide detected as a DMPO/O2H adduct. Irradiation of these proteins in the presence of ascorbate generated ascorbate radical. We conclude that chromophores present in the natural normal and cataractous lenses are capable of initiating photooxidative processes involving endogenous thiols and ascorbic acid. This observation may be pertinent to UV-induced development of cataract. 相似文献
994.
B. B. Tanganov 《Russian Journal of General Chemistry》2007,77(8):1319-1323
A model is developed for estimating thermodynamic dissociation constants of tribasic acids in organic solvents, with an example of ethanol solution of citric acid in which the functional groups are neutralized jointly. 相似文献
995.
N. B. Morozova A. E. Turgambaeva I. A. Baidina V. V. Krysyuk I. K. Igumenov 《Journal of Structural Chemistry》2005,46(6):1047-1051
The structure of titanyl dipivalylmethanate TiO(dpm)2 has been studied by X-ray diffraction analysis. The compound has a molecular structure formed by isolated centrosymmetrical dimers [TiO(dpm)2]2; the unit cell contains two structurally related, crystallographically unique binuclear molecules. The Ti...Ti distance in the dimer is 2.73 Å. Crystal data for Ti2C36H76O10: a = 32.477(6) Å, b = 14.409(3) Å, c = 25.630(5) Å; β = 107.82(3)°, space group C2/c, Z = 8, d calc = 1.002 g/cm3. 相似文献
996.
V. A. Tartakovskii A. S. Ermakov Yu. A. Strelenko D. B. Vinogradov E. Yu. Petrov 《Russian Journal of Organic Chemistry》2005,41(1):120-123
A practical procedure has been proposed for the synthesis of functionally substituted 4,5-dihydro-1,2,3-oxadiazole 2-oxides on the basis ofsulfamic acid derivatives.__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 1, 2005, pp. 121–123.Original Russian Text Copyright © 2005 by Tartakovskii, Ermakov, Strelenko, Vinogradov, Petrov. 相似文献
997.
V. Kalcheva Kh. Lozanova D. Simov P. B. Terent'ev 《Chemistry of Heterocyclic Compounds》1985,21(9):985-987
3-Acetylaminomethyl-2-benzoxazolones are obtained by Beckmann rearrangement of the anti-oximes of 3-acetonyl-2-benzoxazolones. The isomeric 3-(N-methylcarbamoylmethyl)-2-benzoxazolones are synthesized by alkylation of 2-benzoxazolone with chloroacetic acid, followed by reaction of the 3-carboxymethyl-2-benzoxazolones obtained with thionyl chloride and methylamine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1185–1188, September, 1985. 相似文献
998.
C. FRESNEAU B. ARRIO B. LECUYER A. DUPAIX N. LESCURE P. VOLFIN 《Photochemistry and photobiology》1984,39(2):255-261
Abstract— The fluorescence of scaleworms has been attributed by Harvey (1952) to a product of the bioluminescent reaction confined in the scales of these worms. We have purified this fluorescent molecule by gel filtration. This compound has an apparent low molecular weight as shown by polyacrylamide gel clectrophoresis in the presence of SDS. The yield of the fluorescent product, after gel filtration, varies with the stimulation of the bioluminescence, triggered either chemically or enzymatically. The fluorescence spectrum of the purified product is similar to the one observed in vivo , with a maximum centered at 520 nm. Consequently, the fluorescent compound isolated is likely the reaction product of the bioluminescent reaction. 相似文献
999.
John P. Priestle Jr Richard H. Rhyne Jr Jessie B. Salmon Marvin L. Hackerit 《Photochemistry and photobiology》1982,35(6):827-834
Abstract— Trimeric and hexameric solution forms of C-phycocyanin (CPC) from the cyanophyte Agme-nellum quadruplicatum have been isolated and their spectral properties compared to those obtained from single crystals. Although the absorbance peak of a suspension of small C-phycocyanin crystals is red-shifted only 7 nm relative to the solution forms, the single crystal fluorescence is red-shifted 60 nm relative to the solution forms. The crystal fluorescence spectrum exhibits a single peak at LDmax= 708 nm when excited at 514.5 or 530.9 nm and two peaks (LDmax= 661 and 708 nm) when excitation occurs at 568.2 nm. Fluorescence depolarization measurements indicate that extensive energy transfer could occur for both solution and crystal forms with the latter being dependent upon the relative orientation of the crystal with respect to the excitation dipole. Similar results were obtained with B-phycoerythrin (BPE) from the red alga Porphyridium cruentum where the single crystal fluorescence is red-shifted =50nm relative to the solution spectra with two peaks (LDmax= 583 and 617 nm) observed whose relative intensities are dependent on the excitation wavelength (LDmax– 514.5 and 530.9 nm). Single crystal fluorescent lifetimes exhibited considerable shortening relative to that observed for the solution forms. The implications of these results are discussed with respect to the possible relationships of the crystalline structures to the assembly forms present within phycobilisomes. 相似文献
1000.
We have compared the experimental results of study of the preferential solvation of polyvinylpyrrolidone in halogenated solvent/proton-donor solvent mixtures, obtained by light scattering and i.r. spectroscopy and described in the two preceding papers. The preferential adsorption of the proton-donor solvent is explained by the formation of a hydrogen-bond complex of polymer with proton-donor solvent, on which other proton-donor molecules are bound by induced autoassociation. This complexing occurs even in mixtures where the “inert” solvent (1,2-dichloroethane) is preferentially adsorbed; this is experimental proof of a differing solvation of the chains in their close neighbourhood and within short range of them. 相似文献