全文获取类型
收费全文 | 2689篇 |
免费 | 58篇 |
国内免费 | 23篇 |
专业分类
化学 | 2203篇 |
晶体学 | 24篇 |
力学 | 26篇 |
数学 | 120篇 |
物理学 | 397篇 |
出版年
2022年 | 13篇 |
2021年 | 14篇 |
2020年 | 22篇 |
2019年 | 18篇 |
2018年 | 21篇 |
2017年 | 22篇 |
2016年 | 34篇 |
2015年 | 24篇 |
2014年 | 30篇 |
2013年 | 108篇 |
2012年 | 87篇 |
2011年 | 126篇 |
2010年 | 76篇 |
2009年 | 67篇 |
2008年 | 126篇 |
2007年 | 149篇 |
2006年 | 130篇 |
2005年 | 140篇 |
2004年 | 156篇 |
2003年 | 122篇 |
2002年 | 131篇 |
2001年 | 60篇 |
2000年 | 61篇 |
1999年 | 45篇 |
1998年 | 32篇 |
1997年 | 30篇 |
1996年 | 31篇 |
1995年 | 34篇 |
1994年 | 27篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 38篇 |
1990年 | 32篇 |
1989年 | 29篇 |
1988年 | 42篇 |
1987年 | 31篇 |
1986年 | 34篇 |
1985年 | 46篇 |
1984年 | 47篇 |
1983年 | 26篇 |
1982年 | 45篇 |
1981年 | 59篇 |
1980年 | 44篇 |
1979年 | 34篇 |
1978年 | 38篇 |
1977年 | 30篇 |
1976年 | 32篇 |
1975年 | 27篇 |
1974年 | 35篇 |
1973年 | 30篇 |
排序方式: 共有2770条查询结果,搜索用时 15 毫秒
41.
Matsuda H Morikawa T Toguchida I Harima S Yoshikawa M 《Chemical & pharmaceutical bulletin》2002,50(7):972-975
Two new flavanone glycosides, (2S)- and (2R)-eriodictyol 7-O-beta-D-glucopyranosiduronic acids, and a new phenylbutanoid glycoside, (2S, 3S)-1-phenyl-2,3-butanediol 3-O-beta-D-glucopyranoside, were isolated from the flowers of Chrysanthemum indicum L. cultivated in China together with eight flavonoids. The absolute stereostructures of the new compounds were determined on the basis of chemical and physicochemical evidence. Both of the new flavanone glycosides were found to show inhibitory activity for rat lens aldose reductase. 相似文献
42.
Keiichiro Fukumoto Masatoshi Chihiro Yuichi Shiratori Masataka Ihara Tetsuji Kametani Toshio Honda 《Tetrahedron letters》1982,23(29):2973-2976
Thermolysis of a benzocyclobutene derivative () prepared from norcamphor produced stereoselectively a tetracyclic compound (), which was converted to a lactone () having correct stereochemistry at C7, C8, C9, C11, C13, and C14 positions of (±)-klaineanone. 相似文献
43.
Solid sampling technique for direct detection of condensed tannins in bark by matrix-assisted laser desorption/ionization mass spectrometry 总被引:2,自引:0,他引:2
Ishida Y Kitagawa K Goto K Ohtani H 《Rapid communications in mass spectrometry : RCM》2005,19(5):706-710
A novel method for the direct analysis of condensed tannin components in bark was developed on the basis of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) combined with a simple solid sampling technique. The MALDI mass spectra obtained from the wood (bark) powder sample clearly showed a series of peaks corresponding to the sodium ion adducts of condensed tannin oligomers up to around m/z 3000. The results indicate that the condensed tannins in the bark sample used in this work mostly consist of combinations of flavan-3-ol units such as profisetinidin (PF), prorobinetinidin (PR) and prodelphinidin (PD), at least up to 10-mers (m/z approximately 3000). 相似文献
44.
Novel synthesis of heterocyclic spirans by tandem cycloaddition to sulfur-containing heterocumulenes
Yoshiki Ohshiro Hitoshi Nanimoto Hiroyuki Tanaka Mitsuo Komatsu Toshio Agawa Noritake Yasuoka Yasushi Kai Nobutami Kasai 《Tetrahedron letters》1985,26(25):3015-3018
Novel Ni(C0)4-promoted tandem cycloadditions of diphenylcyclo- propenone to isothiocyanates and to CS2 were found to provide new heterocyclic spirans, pyrroline-2-one-5-spiro-5′-thiolene-4′-ones and a thiolene-2-one-5-spiro-5′-thiolene-4′-one, respectively, in moderate yields. 相似文献
45.
Toshio Ogawa Masakazu Sakai Wataru Ishitobi 《Journal of polymer science. Part A, Polymer chemistry》1986,24(1):109-118
Molecular weight distribution of Nylon 12 was determined with hexafluoroisopropanol/toluene mixture as eluent by gel permeation chromatography. Calibration curve for Nylon 12 was easily obtained from that of polystyrene because the method of universal calibration was applicable among these polymers. The molecular weight distributions of Nylon 12 were always broader than expected by the theory of polymerization, i.e., most probable distribution. This result was not caused by broadening effect in gel permeation chromatography, but by polymerization itself. 相似文献
46.
A flow-injection system is described for the simultaneous determination of calcium and magnesium with simultaneous injection of two sample plugs and a masking agent plug and with a single detector. The system utilizes the simultaneous injection of an ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetra-acetic acid plug and two small sample plugs into the same carrier stream and which are merged downstream with 3,3'-bis[N,N-bis(carboxymethyl)aminomethyl)]-o-cresolphthalein solution for spectrophotometric determination of calcium and magnesium. The results for the analysis of natural waters by the proposed method correspond well with those obtained by the conventional titration method with ethylenediaminetetra-acetic acid, and showed good reproducibility. The rate of analysis is about 15 samples/hr. 相似文献
47.
Structure of biluciferyl (luciferyl radical dimer), which is produced by chemical oxidation of . luciferin with such as ferricyanide, was determined to be the symmetric 5,5′-dimer of . luciferin. It gives light in the presence of . luciferase, although the bioluminescent rate is very low. We suggest that the biluciferyl is an intermediate in the oxidation of the luciferin to . luciferinol. 相似文献
48.
Two steroidal compounds BR-1 (I) and ?2 (II) which had been obtained from the rhizomes of Baker, were isolated also from those of Roth. They were identified, respectively, by X-ray analysis and chemical correlation as 1β,2β,3β,4β,5β,7α-hydroxy-spirost-25(27)-en-6-one and its corresponding 6β-hydroxy compound. 相似文献
49.
In the spectrophotometric determination of aluminium and iron with ferron (7-iodo-8-quinolinol-5-sulphonic acid, H(2)L), the addition of cationic surfactants greatly improves the linearity of the calibration curve and widens the useful pH range. The effect of cetyltrimethylammonium chloride (CTMAC) on the stepwise stability constants (K(1),K(2) and K(3)) of the ferron complexes of aluminium and iron (ML(+), ML(-)(2) and ML(3-)(3)) and on the acid-dissociation constants (K(a1) and K(a2)) of ferron has been studied in connection with the role of the surfactant. CTMAC greatly increases the value of K(3) while exerting little effect on K(1) and K(2), thus rendering ML(3-)(3) the predominant species even at very low concentration of free L(2-). It also has some effect on the acid-dissociation constants of ferron, but sometimes it acts to decrease the free L(2-) concentration. At is therefore concluded that the improvements due to addition of surfactant should be attributed to the increased K(3) value. The presence of surfactant micelles is not essential, because the surfactant has a favourable effect when present at well below its critical micelle concentration, and because the continuous variations plots show a peak at a point corresponding to the composition M: L: Q (Q = cationic surfactant) = 1:3:3. 相似文献
50.
Morikawa T Sun B Matsuda H Wu LJ Harima S Yoshikawa M 《Chemical & pharmaceutical bulletin》2004,52(10):1194-1199
A new beta-carboline-type alkaloidal glycoside, glucodichotomine B, four new neolignan glycosides, dichotomosides A, B, C, and D, and a new phenylpropanoid glycoside, dichotomoside E, were isolated from a Chinese natural medicine, the roots of Stellaria dichotoma L. var. lanceolata. The structures of the new glycosides were determined on the basis of chemical and physicochemical evidence. Among them, dichotomoside D inhibited the release of beta-hexosaminidase (IC(50)=64 microM) as well as tumor necrosis factor-alpha and interleukin-4 (IC(50)=16, 34 microM) in RBL-2H3 cells. These findings suggest that dichotomoside D is more effective against the late-phase reactions in type I allergy than in the immediate phase. 相似文献