全文获取类型
收费全文 | 1000篇 |
免费 | 26篇 |
国内免费 | 6篇 |
专业分类
化学 | 796篇 |
晶体学 | 11篇 |
力学 | 15篇 |
数学 | 63篇 |
物理学 | 147篇 |
出版年
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 15篇 |
2017年 | 5篇 |
2016年 | 16篇 |
2015年 | 11篇 |
2014年 | 19篇 |
2013年 | 43篇 |
2012年 | 43篇 |
2011年 | 50篇 |
2010年 | 33篇 |
2009年 | 25篇 |
2008年 | 48篇 |
2007年 | 51篇 |
2006年 | 35篇 |
2005年 | 47篇 |
2004年 | 41篇 |
2003年 | 36篇 |
2002年 | 32篇 |
2001年 | 24篇 |
2000年 | 30篇 |
1999年 | 14篇 |
1998年 | 12篇 |
1997年 | 11篇 |
1996年 | 5篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 7篇 |
1992年 | 19篇 |
1991年 | 15篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 16篇 |
1986年 | 9篇 |
1985年 | 24篇 |
1984年 | 23篇 |
1983年 | 12篇 |
1982年 | 23篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 11篇 |
1975年 | 7篇 |
1974年 | 12篇 |
1973年 | 12篇 |
1970年 | 4篇 |
1968年 | 8篇 |
排序方式: 共有1032条查询结果,搜索用时 31 毫秒
11.
Homogeneous palladium catalyst suppressing Pd black formation in air oxidation of alcohols 总被引:1,自引:0,他引:1
Iwasawa T Tokunaga M Obora Y Tsuji Y 《Journal of the American Chemical Society》2004,126(21):6554-6555
In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 degrees C. Comparison with structurally related pyridine ligands revealed that introduction of the 2,3,4,5-tetraphenylphenyl substituent at the 3-position of pyridine ring effectively suppresses the Pd black formation, maintaining the catalytic activity for a long time to give aldehydes or ketones as products in high yields. 相似文献
12.
A bicyclic diphenyldisilane bearing two tetramethylene tethers reacts with lithium to form predominantly the disilanyllithium as a result of Si-CPh bond fission, while the pentamethylene homolog undergoes ordinary Si-Si bond cleavage to afford the expected phenylsilyllithium. The compressed Si-Si bond incorporated in the bicyclic ring system may be kinetically stabilized (compression effect), resulting in the unusual Si-C bond fission. When the reaction is carried out in the presence of chlorotrimethylsilane, a Calas-type reaction takes place on the phenyl rings. This result suggests that electron transfer to the phenyl group is the primary process in these bicyclic disilanes, followed by Si-Si or Si-C bond cleavage to afford the corresponding silyllithium species. 相似文献
13.
Kozo Tsuji 《Journal of polymer science. Part A, Polymer chemistry》1973,11(8):2069-2075
When ethylene–acrolein copolymer was irradiated at ?196°C with ultraviolet light, a sharp singlet spectrum with a g value of about 2.001 was predominant. This spectrum is attributed to acyl radicals, which are produced by dissociation of a hydrogen atom from an aldehyde group. At the same time it is supposed that dissociation of formyl groups also took place to give alkyl radicals, CO, and H2. The alkyl radicals reacted with CO molecules to give acyl radicals at ?78°C under vacuum. Peroxy radicals were produced when the sample irradiated at ?196°C in the presence of air was treated at ?78°C. The sample irradiated at ?196°C was warmed to near 0°C and an apparent singlet spectrum with a g value of about 2.004 was observed. This spectrum was tentatively assigned as due to free radicals of the type 相似文献
14.
Allylation of carbonucleophiles with allylic carbamates under neutral conditions has been studied. The C-allylation of carbonucleophile is competitive with the N-allylation of amines, and the structure of amines is crucial for the selectivity. Bulky secondary amines gave the best results. Also a new method of protection-deprotection of amines as carbamates has been developed. Smooth deprotection is possible by the palladium-catalyzed reaction of allyl carbamates with formic acid. This method is particulary useful for primary amines, including optically active amino acids. 相似文献
15.
Direct catalytic enantioselective cross aldol-type reaction of an acetate surrogate was developed using Cu alkoxide-chiral phosphine complexes as catalysts. Chemoselective activation and deprotonation of the donor substrate (acetonitrile) by the soft metal alkoxide in a strongly donating solvent (HMPA) are key to success in this reaction. Useful chemical yields and promising enantioselectivities are produced using either DTBM-SEGPHOS or a tuned BIPHEP as a chiral ligand. [reaction: see text] 相似文献
16.
Summary The ion exchange selectivity of trivalent metal ions has been determined on titanium antimonate cation exchanger prepared by coprecipitation of antimony to titanium at different mole ratios. The selectivity sequence Al3+<Cr3+<Ga3+<In3+<Fe3+ was found for trivalent metal ions at an initial concentration of 10–4 mol dm–3 in nitric acid media. A high separation factor Ga/Al = KdGa/KdAl, 4.8×103, was observed for the Ga3+–Al3+ pair on titanium antimonate with an antimony to titanium ratio of 0.34. The effective separation of Ga3+ and In3+ from Al3+ was achieved using a 3 cm×0.5 cm i.d. column containing titanium antimonate with an antimony to titanium ratio of 0.34. 相似文献
17.
Electron micrographs of parallel arrays of negatively stained ramie cellulose protofibrils were analyzed using the two-dimensional digital autocorrelation function (ACF). The method is based upon the statistical analysis of images in real space. The ACF shows strong parallel streaks of high correlation, and the lateral distance between adjacent streaks allows the mean interfibrillar distance to be estimated as 3.7 nm. The intensity profile along the streaks shows a weak modulation with peaks occurring at integral multiples of 3 or 6 nm. These results provide direct evidence that there is a regular axial texture in the protofibrils, and corroborate the conclusions previously drawn from optical diffraction analysis. Using the difference vectors found in the ACF it has been possible to reduce the picture noise level by linear integration, thereby obtaining an enhanced image. A preliminary result obtained in this way suggests that the projected protofibril morphology associated with the observed axial periodicity is a ribbon-like zigzag structure. Possible applications of the method for future work are discussed. 相似文献
18.
Air containing particulate material was passed continuously for one month through a low-volume air sampler equipped with a cyclone to exclude particulates greater than 10 μm in diameter. The samples and filters were ashed in a low-temperature plasma and, after addition of In2O3 support, palladium internal standard and graphite buffer, the ashes were analyzed by a.c. are emission spectrometry. Standard samples were prepared from a commercial standard containing 49 elements. The procedure allows the determination of 14 trace metals (Ag, Al, Cr, Cu, Fe, Ga, Mn, Mo, Ni, Pb, Sn, Ti, V, Zn) with relative standard deviations of 1.4–15.1%. The procedure has been applied successfully to monthly determinations of the average atmospheric concentrations of these elements in Kobe City for the past three years. 相似文献
19.
Y. Sakuma Y. Ogata N. Tsuji H. Yamanishi T. Iida 《Journal of Radioanalytical and Nuclear Chemistry》2003,255(2):325-327
Liquid scintillation counting is the most popular method for tritium measurement, however, it takes much time and a lot of doing to distill off the impurities before mixing the sample water and liquid scintillation cocktail. We have investigated the possibility of an alternative method to the distillation. We have found out that the filtration can be an alternative to distillation for the environmental water samples before electrolytic enrichment. 相似文献
20.
Watanapokasin Yuwadee Nuchfoang Siriporn Nilwarangkoon Sirinun Sarangbin Somsak Kakizono Toshihide 《Applied biochemistry and biotechnology》2002,98(1-9):301-309
A bacterial strain, SWU-4, capable of using benzothiophene (BT) as a sole carbon and energy source was isolated from a petroleum-contaminated
site in Thailand and identified by 16S rRNA gene sequence analysis to be in the genus of Mycobacterium. The strain was Gram-positive, nonspore former, and grew at 50° C. Colonies of the strain on nutrient agar were rod-shaped,
smooth with a convex surface, slightly mucoid, and yellow pigmented. The thermophilic Mycobacterium sp. strain SWU-4 rapidly degraded 2% (w/v) BT at 50°C. Interestingly, this strain was able to degrade a wide variety of organosulfur
compounds including thiophene, bromo(α)thiophene, and 3-methylthiophene in liquid minimum medium at 50°C, which will be beneficial
for industrial applications. 相似文献