全文获取类型
收费全文 | 2479篇 |
免费 | 40篇 |
国内免费 | 13篇 |
专业分类
化学 | 1908篇 |
晶体学 | 32篇 |
力学 | 42篇 |
数学 | 92篇 |
物理学 | 458篇 |
出版年
2023年 | 9篇 |
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 20篇 |
2018年 | 16篇 |
2017年 | 16篇 |
2016年 | 33篇 |
2015年 | 22篇 |
2014年 | 38篇 |
2013年 | 94篇 |
2012年 | 140篇 |
2011年 | 121篇 |
2010年 | 72篇 |
2009年 | 73篇 |
2008年 | 127篇 |
2007年 | 122篇 |
2006年 | 153篇 |
2005年 | 146篇 |
2004年 | 129篇 |
2003年 | 129篇 |
2002年 | 108篇 |
2001年 | 62篇 |
2000年 | 68篇 |
1999年 | 42篇 |
1998年 | 34篇 |
1997年 | 50篇 |
1996年 | 31篇 |
1995年 | 28篇 |
1994年 | 28篇 |
1993年 | 34篇 |
1992年 | 49篇 |
1991年 | 31篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 35篇 |
1987年 | 31篇 |
1986年 | 35篇 |
1985年 | 46篇 |
1984年 | 34篇 |
1983年 | 25篇 |
1982年 | 24篇 |
1981年 | 21篇 |
1980年 | 13篇 |
1979年 | 29篇 |
1978年 | 19篇 |
1977年 | 17篇 |
1975年 | 17篇 |
1974年 | 17篇 |
1973年 | 10篇 |
排序方式: 共有2532条查询结果,搜索用时 0 毫秒
51.
The structural analysis of amylose tris(3,5-dimethylphenylcarbamate) (ADMPC) was performed by NMR spectroscopy using a sample with a lower degree of polymerization in order to understand the chiral recognition mechanism when it was used as a chiral stationary phase (CSP) in high-performance liquid chromatography (HPLC). ADMPC exhibited chiral discrimination for many enantiomers, including 1-(9-anthryl)-2,2,2-trifluoroethanol (1) and 1,1'-bi-2-naphthol (2) in both NMR and HPLC. A good agreement was observed between the HPLC and NMR results when chloroform was employed as the common solvent. The structure of ADMPC in solution was investigated by NMR using the 2D NOESY technique coupled with computer modeling, and a left-handed 4/3 helical structure was obtained as the most probable one. The binding geometry between ADMPC and the enantiomers of 1 was also investigated by (1)H NMR titration. On the basis of these results combined with molecular modeling, a rational model to explain the chiral discrimination mechanism of 1 on ADMPC was proposed. 相似文献
52.
A novel lupane-triterpene glycoside, called wujiapioside B (1), was isolated from the leaves of Acanthopanax gracilistylus (Araliaceae) together with three known lupane-triterpene glycosides, acankoreoside C (2), acantrifoside A (3) and 3-epibetulinic acid 28-O-alpha-L-rhamnopyranosyl-(1-->4)-beta-D-glucopyranosyl-(1-->6)-beta-D-glucopyranosyl ester (4). Based on spectroscopic data, the chemical structure of 1 was determined as 3alpha,23-dihydroxy-lup-20(29)-en-28-oic acid 28-O-alpha-L-rhamnopyranosyl-(1-->4)-beta-D-glucopyranosyl-(1-->6)-beta-D-glucopyranosyl ester. Compounds 2-3 were obtained for the first time from this plant and compound 4 has not been isolated from Acanthopanax genus yet. 相似文献
53.
Okihiro Sugihara Tomoyuki Hirama Hisashi Fujimura Naomichi Okamoto 《Optical Review》1996,3(3):150-152
Thin film waveguides of germanosilicate glass are fabricated byrf-sputtering. Second-order nonlinearity ofd 33=3.2 pm/V is obtained from electrically poled film, and the lifetime of the nonlinearity is found to be310 years. The poled channel-waveguide is prepared and the Cerenkov-type phase-matched frequency doubling is successfully observed. 相似文献
54.
55.
56.
Jacques E. Desnoyers Gérald Perron Sylvain Léger Byron Y. Okamoto Terrence H. Lilley Robert H. Wood 《Journal of solution chemistry》1978,7(3):165-178
The freezing-point depression of the ternary systems tetraalkylammonium bromides-t-butanol-water for the first five homologs of R4NBr was measured. In the case of Bu4NBr, the effect of size of the alcohol (methahol ton-butanol) was also investigated. From the corresponding freezing-point data for the binary systems the apparent salting constants
were calculated. The true salting constantsk
s were obtained by extrapolation to infinite dilution. These are all very close to zero at the freezing temperature. From the
corresponding thermochemical data the temperature dependence ofk
s was calculated, and above 5°C all the R4NBr salts int-butanol; the salting-in increases with temperature and with the size of the hydrophobic cations. The scaled-particle theory
is at present the only one which can account semiquantitatively for the temperature dependence of the salting-in effect.
On leave of absence from Chemistry Department, The University, Sheffield S3 7HF, England
To whom correspondence should be addressed. 相似文献
57.
Treatment of 3,4-alkadienyl carbonates 2a-i with a low-valent titanium reagent diisopropoxy(eta(2)-propene)titanium (1), readily generated by the reaction of Ti(O-i-Pr)(4) with 2 i-PrMgCl, resulted in an intramolecular nucleophilic acyl substitution (INAS) reaction to afford vinyltitanium compounds 3 which, in turn, reacted with H(3)O(+), D(2)O, or iodine to give alpha-substituted beta,gamma-unsaturated esters 4 in good to excellent yields. The olefin moiety of the hydrolysis product 4 has (Z)-geometry mainly except for 4h. Starting from chiral 2f or 2g, the reaction proceeded stereospecifically to give optically active alpha-substituted beta,gamma-unsaturated ester 4f or 4g having (Z)-olefin geometry exclusively. 相似文献
58.
Simple syntheses of syncarpurea (1) from syncarpic acid and the amino acids ornithine or citrulline are presented. 相似文献
59.
Yoshihisa Kurasawa Tomoyoshi Hosaka Kazue Ikeda Yuko Matsumoto Aiko Ishikura Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1994,31(2):527-533
The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 9–11 were synthesized by the reaction of the quinoxalines 6–8 with various p-substituted benzenediazonium salts. Compounds 9–11 showed the tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effect on the tautomer ratios of A to B was studied by the nmr spectroscopy to clarify that the presence of the ester group R2 on the hydrazone carbon and electron-donating p-substituent R1 on the side chain benzene ring exhibited a tendency to increase the ratios of the tautomer A . 相似文献
60.