全文获取类型
收费全文 | 2514篇 |
免费 | 30篇 |
国内免费 | 12篇 |
专业分类
化学 | 1958篇 |
晶体学 | 6篇 |
力学 | 30篇 |
数学 | 250篇 |
物理学 | 312篇 |
出版年
2016年 | 21篇 |
2015年 | 18篇 |
2014年 | 22篇 |
2013年 | 40篇 |
2012年 | 52篇 |
2011年 | 64篇 |
2010年 | 32篇 |
2009年 | 34篇 |
2008年 | 49篇 |
2007年 | 50篇 |
2006年 | 72篇 |
2005年 | 67篇 |
2004年 | 56篇 |
2003年 | 43篇 |
2002年 | 46篇 |
2001年 | 30篇 |
2000年 | 45篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 28篇 |
1996年 | 34篇 |
1994年 | 38篇 |
1993年 | 42篇 |
1992年 | 54篇 |
1991年 | 33篇 |
1990年 | 28篇 |
1989年 | 30篇 |
1988年 | 43篇 |
1987年 | 65篇 |
1986年 | 39篇 |
1985年 | 49篇 |
1984年 | 52篇 |
1983年 | 39篇 |
1982年 | 49篇 |
1981年 | 43篇 |
1980年 | 41篇 |
1979年 | 45篇 |
1978年 | 46篇 |
1977年 | 55篇 |
1976年 | 58篇 |
1975年 | 36篇 |
1974年 | 43篇 |
1973年 | 29篇 |
1972年 | 30篇 |
1971年 | 23篇 |
1970年 | 31篇 |
1967年 | 22篇 |
1966年 | 17篇 |
1932年 | 16篇 |
1909年 | 24篇 |
排序方式: 共有2556条查询结果,搜索用时 140 毫秒
31.
Tanaka K Ding MY Helaleh MI Taoda H Takahashi H Hu W Hasebe K Haddad PR Fritz JS Sarzanini C 《Journal of chromatography. A》2002,956(1-2):209-214
In this preliminary study, a new approach to ion-exclusion chromatography is proposed to overcome the relatively poor conductivity detection response which occurs in ion-exclusion chromatography when acids are added to the eluent in order to improve peak shape. This approach, termed vacancy ion-exclusion chromatography, requires the sample to be used as eluent and a sample of water to be injected onto a weakly acidic cation-exchange column (TSKgel OApak-A). Vacancy peaks for each of the analytes appear at the retention times of these analytes. Highly sensitive conductivity detection is possible and sharp, well-shaped peaks are produced, leading to efficient separations. Retention times were found to be affected by the concentration of the analytes in the eluent, and also by the presence of an organic modifier such as methanol in the eluent. Detection limits for oxalic, formic, acetic, propionic, butyric and valeric acids were 0.1, 0.2, 0.3, 0.3, 0.4 and 0.5 microM, respectively, and linear ranges for some acids extended over two orders of magnitude. Precision values for retention times were 0.21% and for peak areas were <1.90%. The vacancy ion-exclusion chromatography method was found to give detection responses four to 10 times higher than conventional ion-exclusion chromatography using sulfuric acid eluent and two to five times higher than conventional ion-exclusion chromatography using benzoic acid eluent. 相似文献
32.
Distribution coefficients and capacities have been determined for many different organic compounds on a new molecular sieve called silicalite, which does not adsorb water but does adsorb small organic molecules with diameters up to 6 A, from both liquid and gaseous streams. The characteristics of silicalite have been examined closely and new applications for it are briefly described. 相似文献
33.
The condensation of 2.4-ethylidene-erythrose with the sodium salt of diethyl-phosphono-piperidino-ethylacetate yields an enamino-lacton V which is converted under mild conditions into an α-keto ester, 2-oxo-3-desoxy-gluconic acid, and its quinoxaline derivative. 相似文献
34.
Coulometric titrations of mineral acids, sulphonic acids, carboxylic acids, enols, imides and phenols have been carried out in t-butanol or in acetone with electrically generated tetrabutylammonium hydroxide. Either a potentiometric titration or a visual indicator end-point may be used. The amount of acid titrated ranges from 10 to 60 muequiv, and the precision and accuracy of the method are excellent. 相似文献
35.
A determination of ethanol is described, which is based on a purging system in conjunction with a photoionization detector. With that system a fast and reliable determination of ethanol in aqueous solutions is possible. The system has been used for the analysis of wine. The 3delta-detection limit has been 0.005% ethanol, the relative standard deviation 4.8 to 6.0% and the time constant of the entire analytical system 20 s. The photoionization detector has been also applied to the analysis of artificial and genuine human breath. A comparison with gas-chromatography and non-dispersive IR-detection has been proven the reliability of results. 相似文献
36.
Monodisperse Linear and Cyclic Oligo[(R)-3-hydroxybutanoates] Containing up to 128 Monomeric Units Using benzyl ester/(tert-butyl)diphenylsilyl ether protection, (COCl)2/pyridine esterification conditions, and a fragment-coupling strategy (with H2/Pd-C debenzylation and HF · pyridine desilylation), linear oligomers of (R)-3-hydroxybutanoic acid (3-HB) containing up to 128 3-HB building blocks (mol. weight > 11 000 Da) are assembled (Schemes 1,2,5, and 6). In contrast to the previously employed protecting-group combination, and due to the low-temperature esterifying conditions, this procedure leads to monodisperse oligomers: all steps occur without loss of single 3-HB units. The product oligomers with two, one, and no terminal protecting groups (mostly prepared in multi-gram amounts) are characterized by all standard spectroscopic methods, especially by mass spectroscopy (Figs. 2 and 3), by their optical activity, and by elemental analyses. Cyclization of the oligo[(R)-3-hydroxybutanoic acids] with up to 32 3-HB units, using thiopyridine activation and CuBr2 for the ring closure, produces oligolides consisting of up to 128 ring atoms (Scheme 7). Mixed oligolides containing 3-HB and (R)-3-hydroxypentanoic units are prepared from the corresponding linear trimers, using Yamaguchi's method for the ring closure (Scheme 8 and Fig.4 (X-ray crystal structures of two folded conformers)). Comparisons of melting points (Table 1), of [α] values (Tables 2 and 3), of 1H-NMR coupling constants (Table 3), and of molecular volume/hydroxyalkanoate unit (Table 4) of linear and cyclic oligomer derivatives and of the high-molecular-weigh polymer show that the monodisperse oligomers appear to be surprisingly good models for the polymer. Besides this insight, our synthesis is supplying the samples to further test the role of P(3-HB) (ca. 140 units) as a component of complexes forming channels through cell-wall phospholipid bilayers. 相似文献
37.
Investigations on Lithiation and Substitution of HP[Si(t-Bu)2]2PH HP[Si(t-Bu)2]2PH 1 is monolithiated by reaction with LiPH2 · DME or LiBu in toluene. The crystalline compound HP[Si(t-Bu)2]2PLi · 2 DME 2 can be isolated in DME. Reaction of 2 with Me2SiCl2 leads to HP[Si(t-Bu)2]2P? SiMe2Cl 4 , ClMe2Si? P[Si(t-Bu)2]2P? SiMe2Cl 5 , HP[Si(t-Bu)2]2P? SiMe2? P[Si(t-Bu)2] 2PH 6 . Isomerization by Li/H migration between 4 and 2 leads to the formation of 5 . Reaction of Li(t-Bu) with 1 or 2 yields LiP[Si(t-Bu)2]2PLi 3 by further lithiation. 3 could not be obtained purely, only in a mixture with 2 . These compounds favourably generate with t-BuPCl2 in hexane Cl(t-Bu)P? P[Si(t-Bu)2]2P? P(t-Bu)Cl 9 , in THF HP[Si(t-Bu)2]2P? P(t-Bu)? P[Si(t-Bu)2]2 PH 12 (main product), 9 , H(t-Bu)P? P[Si(t-Bu)2]2P? P(t-Bu)Cl 10 , H(t-Bu)P? P[Si(t-Bu)2]2P? P(t-Bu)H 11 as well as HP[Si(t-Bu)2]2P? P(t-Bu)H 13 and HP[Si(t-Bu)2]2P? P(t-Bu)2 14 . 相似文献
38.
Title compound 1 was synthesized by a published route which had to be modified (seven steps from readily obtainable starting materials). Characterization of 1 was achieved by spectroscopic means (FAB-MS, 1H-NMR, including 2D-COSY). Furthermore, commercially available reference material purchased for comparison, was unequivocally established to be 10 , i.e. incompletely deprotected 1 . 相似文献
39.
Lukin O Müller WM Müller U Kaufmann A Schmidt C Leszczynski J Vögtle F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(15):3507-3517
The readily available in gram quantities tris(allyloxy)knot of the amide-type 5 (knotane) can be completely and partially deprotected with nBu(3)SnH in the presence of a palladium catalyst resulting in hydroxyknotanes 7-9. These, in turn, react with diethylchlorophosphate giving rise to knotanes equipped with between one and three phosphoryl groups. Sulfonylation of bis(allyloxy)monohydroxyknotane 8 with p-toluenesulfonyl chloride and, following removal of one or two allyl groups from the intermediate monosulfonate 13, give rise to sulfonyloxy-allyloxy-hydroxy- and sulfonyloxy-dihydroxy-knotanes 15 and 14, respectively. This provides a convenient method for the preparation of knotanes with any substitution pattern. All new knotanes have been isolated in preparative amounts and as highly pure substances with an exception of allyloxy-dihydroxyknotane 9. This compound could only be obtained as a mixture with the corresponding monohydroxy-derivative 8. The structures of all synthesized compounds were established by means of FAB and MALDI TOF mass spectrometry, (1)H and (31)P NMR spectroscopy. The triphosphorylated knotane 10 exhibits high solubility in alcohols, allowing its complete enantiomeric resolution with a commercially available chiral HPLC column. (1)H,(1)H DQF-COSY correlation spectroscopy along with H/D exchange experiments and ab initio calculations provided the first detailed (1)H NMR signal assignments of knotanes in [D(6)]DMSO solution. The combination of variable temperature (1)H and (31)P NMR spectroscopy and molecular modeling has been applied to study the conformational behavior of the new knotanes in different solvents. It has been shown that in DMSO solution at room temperature knotanes exist in a relatively rigid nonsymmetrical conformation similar to that found in the solid state while faster conformational exchange leading to the average D(3) symmetrical structure was detected in a number of other solvents. 相似文献
40.
Transition Metal Complexes of the Hexamethyl-trisila-tetraphospha-nortricyclene P4 (Sime 2) 3 P4(Sime2)3 1 reacts with Mo(CO)6, Cr(CO)5THF, and Mn(η-C5H5)(CO)2THF to give crystalline complexes in which 1 functions as a monodentate ligand. In each compound one phophorus atom of the cyclotriphosphane ring coordinates to the metal atom. Using Mn(η-C5H5)(CO)2THF, two different P atoms of the P3 Cr(CO)4 norbornadiene and 1 react, yielding the dimeric, red, crystalline compound (CO) 4Cr[μ-P4(Sime2)3]2Cr(CO)4. In this complex the two molecules of 1 are both bonded by two P atoms of the P 3 ring to the two Cr(CO)4 Units, forming a six-memered (CrP2)2ring. 相似文献