全文获取类型
收费全文 | 1245篇 |
免费 | 44篇 |
国内免费 | 3篇 |
专业分类
化学 | 835篇 |
晶体学 | 17篇 |
力学 | 27篇 |
数学 | 129篇 |
物理学 | 284篇 |
出版年
2023年 | 9篇 |
2022年 | 6篇 |
2021年 | 18篇 |
2020年 | 20篇 |
2019年 | 23篇 |
2018年 | 25篇 |
2017年 | 23篇 |
2016年 | 32篇 |
2015年 | 34篇 |
2014年 | 35篇 |
2013年 | 64篇 |
2012年 | 71篇 |
2011年 | 82篇 |
2010年 | 43篇 |
2009年 | 42篇 |
2008年 | 71篇 |
2007年 | 49篇 |
2006年 | 49篇 |
2005年 | 68篇 |
2004年 | 54篇 |
2003年 | 40篇 |
2002年 | 51篇 |
2001年 | 32篇 |
2000年 | 26篇 |
1999年 | 13篇 |
1998年 | 17篇 |
1997年 | 9篇 |
1996年 | 13篇 |
1995年 | 18篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 15篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 11篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 11篇 |
1975年 | 10篇 |
1974年 | 6篇 |
排序方式: 共有1292条查询结果,搜索用时 776 毫秒
51.
Shimazawa R Sano H Tanatani A Miyachi H Hashimoto Y 《Chemical & pharmaceutical bulletin》2004,52(4):498-499
Thalidomide has been found to exhibit weak nitric oxide synthase (NOS)-inhibitory activity. Structural development studies of thalidomide showed that some N-2,6-dimethylphenylhomophthalimide analogs possess NOS-inhibiting activity. 相似文献
52.
Summary For visual analysis of the density reorganization and distortion, the one-dimensional cut (x, y
0,z
0) and the two-dimensional cut (x, y, z
0) of the three-dimensional electron density difference function (x, y, z) are frequently employed. However, these cut functions do not satisfy any sum rules in contrast to the original difference function (x, y, z). To avoid this difficulty, the use of the marginal electron density functions
x
(x) and
xy
(x, y) and their difference functions
x
(x) and
xy
(x, y) is proposed. The marginal densities are condensation of the three-dimensional density onto a particular plane or line of our interest, and they satisfy the sum rule (i.e., the conservation of the number of electrons) exactly. Some basic properties of the marginal electron density are clarified for typical diatomic molecular orbitals. An illustrative application is given for the bonding and antibonding processes in the H2 system. 相似文献
53.
The Pictet-Spengler cyclization of the imines (3) prepared by the condensation of L-tryptophan methyl ester (1) and aryl methyl ketones (2), using titanium(IV) isopropoxide as an iminating reagent, quantitatively proceeded, when treated with trifluoroacetic acid (TFA) or formic acid, to provide two diastereomers, that is (1S,3S)-1-aryl-3-isopropoxycarbonyl-1-methyl-1,2,3,4-tetrahydro-beta-carbolines (4) and their (1R,3S)-diastereomers (5), of which the diastereomer ratios varied from 1 to 5 depending on the reaction conditions. The (1R,3S)-diastereomers (5) are thermodynamically more stable than their (1S,3S)-congeners (4), as shown by equilibration experiments in TFA. The conversion of 4 to 5 (also 5 to 4) should occur under acidic conditions by cleavage of the C(1)-N(2) bond with complete retention of configuration at the C-3 chiral center. The low diastereo-selectivity observed in the Pictet-Spengler reaction of 1 and 2 is concluded to be a stereochemical outcome under conditions of kinetic control (lower temperature, shorter reaction time), while the high diastereo selectivity with preferential formation of the more stable isomer (5) is the result of thermodynamically controlled experiments (higher temperature, longer reaction time). 相似文献
54.
T. Sato K. Ishishita M. Katada M. Nakada K. Endo H. Sano 《Hyperfine Interactions》1992,70(1-4):1237-1240
Mixed-valence states of57Fe atoms produced after EC-decay of57Co in a series of trinuclear cobalt-iron halogenoacetate complexes, [CoIIFe 2 III O(CH3?nXnCO2)6(H2O)3] (0≤n≤3, X=Cl, Br, and I), were studied by comparing the results obtained by emission Mössbauer spectroscopy with those observed in absorption Mössbauer spectra of analogous trinuclear iron complexes, [FeIIFe 2 III O(CH3?nXnCO2)6(H2O)3]. Some of the emission Mössbauer spectra show a temperature-dependent mixed-valence state as found in the absorption Mössbauer spectra. Others show a somewhat different temperature dependence compared with the absorption Mössbauer spectra. The results were interpreted in terms of after-effects of the EC-decay. 相似文献
55.
The methyl transfer occurring in the production of methyl N,N-dimethyl-γ-aminobutyrate by pyrolysis of carpronium chloride was examined by means of pyrolysis gas chromatography mass spectrometry with the aid of some deuterated compounds. The mass spectra of methyl N,N-dimethyl-γ-aminobutyrate, produced from deuterated derivatives of carpronium chloride, showed inter alia, characteristic molecular ion peaks which indicated that the methyl of the trimethylammonium group transfers and displaces the methyl of the carbomethoxy group of the tertiary amino compound. The results show that an intermolecular methyl transfer occurs in part on pyrolysis of carpronium chloride, to form methyl N,N-dimethyl-γ-aminobutyrate in which the methyl oxygen is replaced by a methyl from the nitrogen of the original compound. The mechanism presented involves the bimolecular reaction between zwitterionic intermediates formed by ionic O-demethylation of carpronium chloride. 相似文献
56.
Raquel Simancas Anand Chokkalingam Shanmugam P. Elangovan Zhendong Liu Tsuneji Sano Kenta Iyoki Toru Wakihara Tatsuya Okubo 《Chemical science》2021,12(22):7677
Zeolites have been successfully employed in many catalytic reactions of industrial relevance. The severe conditions required in some processes, where high temperatures are frequently combined with the presence of steam, highlight the need of considering the evolution of the catalyst structure during the reaction. This review attempts to summarize the recently developed strategies to improve the hydrothermal framework stability of zeolites.This review attempts to summarize the recently developed strategies to improve the hydrothermal framework stability of zeolites. 相似文献
57.
Hiroyoshi Nakajima Katsuhiro Yamada Yuki Iseki Satoru Hosoda Ayako Hanai Yasunori Oumi Toshiharu Teranishi Tsuneji Sano 《Journal of polymer science. Part A, Polymer chemistry》2003,41(24):3324-3332
Polypropylene (PP)/Ti-MCM-41 nanocomposites were prepared by isospecific propylene polymerization with Ti-MCM-41/Al(i-C4H9)3 catalyst. The cross polarization/magic angle spinning (CP/MAS) 13C NMR spectrum of the composite was similar to that of the conventional isotactic PP, and the decrease in the pore volume of Ti-MCM-41 in the nanocomposites, as measured by N2 adsorption, was consistent with the value calculated from the weight loss in the thermogravimetric analysis (TGA) curve; both these facts attest to propylene polymerization within the mesopores of Ti-MCM-41. Alkali treatment followed by extraction with o-dichlorobenzene allows us to extract the confined PP out of the Ti-MCM-41 mesopores. Although the PP/Ti-MCM-41 nanocomposites do not exhibit a crystalline melting point, the same PP when extracted from the mesopores showed a clear melting point at 154.7 °C; this indicates that the crystallization of PP confined in mesopores is strongly hindered. For the PP polymerized within the confinement, the molecular weight (Mw) and molecular weight distribution (Mw/Mn) were 84,000 and 4.3, respectively; these values were considerably smaller than those of the PP polymerized concurrently outside the Ti-MCM-41 mesopores (Mw = 200,000–450,000, Mw/Mn = 40–75). Therefore, the confinement also has a marked effect on the molecular weight of the PP. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3324–3332, 2003 相似文献
58.
Emission Mössbauer spectra of mixedvalence trinuclear complexes,57Co-labelled [CoFe2O(CH3CO2)6(H2O)3] and [CoFe2O(CH3CO2)6 (py)3]py were compared with that of [CoCr2O(CH3CO2)6 (py)3]py. It was found that the57Fe atoms produced in57Co-labelled [CoFe2O(CH3CO2)6(H2O)3] and [CoFe2O(CH3CO2)6 (py)3]py showed a temperature-dependent mixed-valence state as observed in the absorption Mössbauer spectra of [Fe3O(CH3CO2)6 (H2O)3] and [Fe3O(CH3CO2)6 (py)3]py, respectively, while57Co-labelled [CoCr2O(CH3CO2)6 (py)3]py showed a temperature-independent divalent iron/II/ state. The results indicate that the decayed atoms keep their position at the sites of original57Co atoms and that the valence electrons are transferred through the intramolcular bondings. 相似文献
59.
Kazumi Ohya Hiroaki Kitaoka Yasuhiko Yotsui Mitsuji Sano 《Journal of mass spectrometry : JMS》1983,18(4):139-146
The mechanism of the pyrolysis reaction of carpronium chloride [(CH3)3N+? (CH2)3? COOCH3CI?] leading to γ-butyrolactone and tetramethylammonium chloride was investigated by means of thermal analysis, pyrolysis gas chromatography mass spectrometry and field desorption mass spectrometry, using deuterium labelling. The results indicated that carpronium chloride pyrolysed to yield equimolar amounts of γ-butyrolactone and tetramethylammonium chloride, methyl transfer occurred between N and O during the pyrolysis process. The mechanism is discussed on the basis of the experimental results, and with the aid of the theoretical results calculated by the CNDO/2 method. The mechanism presented is as follows. γ-Butyrolactone is formed by the intramolecular migration of the π-orbital of C?O to the carbon adjacent to [(CH3)3N]+ via a 5-membered ring transition state, accompanied by a bimolecular reaction between [(CH3)3N]+ and the CH3 of O? CH3, resulting in the formation of tetramethylammonium chloride in an amount equimolar with γ-butyrolactone. 相似文献
60.
Mária Hvastijová Jiri Kohout Marián Okruhlica Jerzy Mroziński Lothar Jäger 《Transition Metal Chemistry》1993,18(6):579-582
Summary Dicyanamide complexes of CuII, NiII and CoII of the type M[N(CN)2]2L2, where L = benzimidazole, 2-methyl- or 2-ethylbenzimidazole, have been prepared and studied by spectroscopy and magnetochemistry. The complexes, except for Co[N(CN)2]2 (benzimidazole)2, are six-coordinate, involving bidentate bridging dicyanamide groups. While the NiII complexes have practically octahedral structures, the CuII complexes are pseudooctahedral with similar tetragonal distortion. The ligand field strength in these complexes depends mainly on the steric effect of the benzimidazole ligands. The CoII complex of benzimidazole is monomeric tetrahedral, but that of 2-ethylbenzimidazole is tetragonal octahedral. The oridging function of dicyanamide in the six-coordinate complexes is realized either through both cyanide or through amide and cyanide nitrogens. The complex Cu[N(CN)2]2 (2-methylbenzimidazole)2 is a weak antiferromagnet (J = -0.1 cm–1), exhibiting under ca. 15 K a long-range antiferromagnetic ordering. 相似文献