全文获取类型
收费全文 | 5008篇 |
免费 | 113篇 |
国内免费 | 31篇 |
专业分类
化学 | 3432篇 |
晶体学 | 62篇 |
力学 | 66篇 |
数学 | 772篇 |
物理学 | 820篇 |
出版年
2021年 | 29篇 |
2020年 | 34篇 |
2019年 | 59篇 |
2018年 | 39篇 |
2017年 | 40篇 |
2016年 | 75篇 |
2015年 | 72篇 |
2014年 | 77篇 |
2013年 | 247篇 |
2012年 | 220篇 |
2011年 | 278篇 |
2010年 | 132篇 |
2009年 | 140篇 |
2008年 | 263篇 |
2007年 | 255篇 |
2006年 | 260篇 |
2005年 | 269篇 |
2004年 | 230篇 |
2003年 | 191篇 |
2002年 | 182篇 |
2001年 | 91篇 |
2000年 | 59篇 |
1999年 | 56篇 |
1998年 | 71篇 |
1997年 | 60篇 |
1996年 | 102篇 |
1995年 | 84篇 |
1994年 | 81篇 |
1993年 | 69篇 |
1992年 | 66篇 |
1991年 | 60篇 |
1990年 | 47篇 |
1989年 | 40篇 |
1988年 | 52篇 |
1987年 | 52篇 |
1986年 | 33篇 |
1985年 | 52篇 |
1984年 | 65篇 |
1983年 | 48篇 |
1982年 | 54篇 |
1981年 | 64篇 |
1980年 | 64篇 |
1979年 | 65篇 |
1978年 | 60篇 |
1977年 | 45篇 |
1976年 | 42篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1973年 | 36篇 |
1969年 | 22篇 |
排序方式: 共有5152条查询结果,搜索用时 234 毫秒
41.
Kohji Chihara Takeshi Takemura Tsuguo Yamaoka N. Yamamoto Arnold Schaffer Ralph S. Becker 《Photochemistry and photobiology》1979,29(5):1001-1008
Abstract— The absorption and emission spectra, lifetimes and quantum yields of all-trans retinol have been examined as a function of solvent and temperature. In addition, the spectroscopy of retinyl ether and 2 other polyene alcohols have been determined. Based on the results obtained, we conclude that the singlet excited state of retinol and retinyl cther is of a forbidden character and of the type loosely called 1 A- g . Retinol forms a dimer in an alkane solvent when cooling from 298 to 77 K. A general structure for the dimer is proposed. 相似文献
42.
Jocelyne Hellou Raymond J. Andersen Shahin Rafii Edward Arnold Jon Clardy 《Tetrahedron letters》1981,22(42):4173-4176
The structure of luteone (), a twenty three carbon terpenoid, has been solved by performing a single crystal x-ray analysis on a 3,4-dinitrophenylhydrazone derivative. 相似文献
43.
44.
45.
46.
47.
Triflate abstraction from the complex [Re(OTf)(CO)(3)(bipy)] (1) using the salt NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane solution in the presence of L = PPh(3), NCMe, NCPh, imines, ketones, Et(2)O, THF, MeOH, and MeI affords cationic complexes [Re(L)(CO)(3)(bipy)](+) as their BAr'(4)(-) salts. The new complexes have been characterized spectroscopically and, for [Re(eta(1)-O=C(Me)R)(CO)(3)(bipy)]BAr'(4) (R = CH(3), 6a; R = Ph, 6b), and [Re(THF)(CO)(3)(bipy)]BAr'(4) (9), also by single-crystal X-ray diffraction. Compared with conventional methodologies, the route reported here allows the coordination of a broader range of weakly coordinating ligands and requires considerably milder conditions. On the other hand, the reactions of lithium acetylides with [Re(THF)(CO)(3)(bipy)]BAr'(4) (9) can be used for the high-yield syntheses of rhenium alkynyls [Re(Ctbd1;CR)(CO)(3)(bipy)] (R = Ph, 12; R = SiMe(3), 13). Complex 9 was found to catalyze the aziridination of benzylideneaniline with ethyl diazoacetate. 相似文献
48.
The subject of this report is the determination of lysophospholipids; lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylserine and lysophosphatidic acid, by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS). The mean signal-to-noise ratio (S/N) was used for the first time as a measure of lysophospholipid concentration. Two different sample preparation procedures were applied, the 'standard' procedure and the 'premix' in order to check to what extent these methods influence the results of the lysophospholipid quantification. Results can be summarised as follows: (a) All classes of lysophospholipids can be easily and sensitively analysed by MALDI-TOF MS. The smallest detectable amount of lysophospholipids was 0.09 pmol on the sample plate. That is about two orders of magnitude lower than the amount detectable by standard chromatographic methods. (b) The mean S/N of all peaks detected in the positive ion mass spectra can be used as a measure of the lysophospholipid concentration. Whereas the S/N for neutral lysophospholipids correlated with the applied concentrations only when the samples were analysed as 'premix', the sample preparation and application procedure did not influence the quantification of acidic lysophospholipids. The standard deviations were not higher than 10% of the mean value. (c) All spectra were additionally analysed in the presence of CsCl. The addition of caesium ions makes the peak identification unambiguous in phospholipid mixtures, but the Cs adducts of lysophospholipids do not properly reflect their concentration and, therefore, they were not useful for quantification. (d) The applicability of the method was demonstrated on the organic extract of human neutrophils. 相似文献
49.
I. L. Dalinger T. K. Shkineva S. A. Shevelev V. Kral Z. Arnold 《Russian Chemical Bulletin》1993,42(7):1215-1218
The double Vilsmeier formylation of theC-methyl group in pyrazole derivatives has been shown to occur when a nitro-group is in the adjacent position of the ring. A method for the synthesis ofC-(diformylmethyl)nitropyrazoles based on this reaction has been developed.For the previous communication, see Ref. 1; for Part 2, see Ref.2
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1273–1276, July, 1993. 相似文献
50.
Robert F. Kovar F. E. Arnold 《Journal of polymer science. Part A, Polymer chemistry》1976,14(11):2807-2817
A series of novel AB monomers such as 2-[p-carboxyphenyl]-5,6-diaminobenzimidazole hydrochloride have been synthesized. In addition, a new aromatic monomer, 1,3-diamino-4,6-bis(p-toluenesulfonamido)benzene has been prepared in high purity and substituted for 1,2,4,5-tetraaminobenzene in a polymerization with terephthalic acid. Homopolymerization of the AB monomers, and polycondensation of monomer with terephthalic acid in polyphosphoric acid, produced the rod-like para-oriented polymer, poly[1,7-dihydrobenzo(1,2-d: 4,5-d)diimidazole-2,6-diyl-1,4-phenylene]. The yellow polymer was completely soluble in methanesulfonic acid (MSA) and PPA, exhibiting intrinsic viscosities as high as 5 dl/g in MSA, and a blue opalescence in solution. Polymerization at temperatures above 225°C caused crystallization and subsequent precipitation to occur. Polymer thus obtained was completely insoluble in MSA and possessed a high degree of crystallinity as demonstrated by x-ray analysis. 相似文献