全文获取类型
收费全文 | 15734篇 |
免费 | 441篇 |
国内免费 | 95篇 |
专业分类
化学 | 10900篇 |
晶体学 | 107篇 |
力学 | 219篇 |
数学 | 2424篇 |
物理学 | 2620篇 |
出版年
2022年 | 114篇 |
2021年 | 143篇 |
2020年 | 245篇 |
2019年 | 218篇 |
2018年 | 134篇 |
2017年 | 116篇 |
2016年 | 359篇 |
2015年 | 280篇 |
2014年 | 340篇 |
2013年 | 654篇 |
2012年 | 690篇 |
2011年 | 797篇 |
2010年 | 481篇 |
2009年 | 426篇 |
2008年 | 725篇 |
2007年 | 704篇 |
2006年 | 722篇 |
2005年 | 676篇 |
2004年 | 594篇 |
2003年 | 474篇 |
2002年 | 521篇 |
2001年 | 274篇 |
2000年 | 305篇 |
1999年 | 243篇 |
1998年 | 249篇 |
1997年 | 221篇 |
1996年 | 248篇 |
1995年 | 255篇 |
1994年 | 258篇 |
1993年 | 207篇 |
1992年 | 201篇 |
1991年 | 190篇 |
1990年 | 168篇 |
1989年 | 166篇 |
1988年 | 156篇 |
1987年 | 143篇 |
1986年 | 132篇 |
1985年 | 219篇 |
1984年 | 180篇 |
1983年 | 165篇 |
1982年 | 216篇 |
1981年 | 188篇 |
1980年 | 165篇 |
1979年 | 193篇 |
1978年 | 163篇 |
1977年 | 154篇 |
1976年 | 153篇 |
1975年 | 124篇 |
1974年 | 126篇 |
1973年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
962.
963.
964.
Robert Werner Lang 《Helvetica chimica acta》1986,69(4):881-886
The title compound 1 , which can be regarded as a synthetic equivalent of substituted 1,1,1-trifluoroacetone 2 , is prepared by Zn-induced reductive addition of 1,1,1-trichloro-2,2,2-trifluoroethane ( 3 ) to formaldehyde ( 4 ). A brief survey of the scope of this C? C-bond formation leading to a variety of new CF3-containing synthetic building blocks is also given. 相似文献
965.
Results of voltammetric, potentiometric, chronoamperometric, ion transport, and extraction studies on neutral carrier based, plasticized poly(vinyl chloride) membranes are summarized. They unambiguously confirm that such bulk membranes dispose of immobile anionic sites. These fixed sites lead to a Donnan exclusion of other anions from the membrane and thus to a permselectivity for cations. The results are in perfect agreement with the predictions of earlier membrane models. A rigorous Poisson-Boltzmann analysis of macroscopic liquid membranes clearly indicates that space-charge at the membrane/solution interfaces does not influence the electrochemical properties and the ion-selectivity behavior at steady state. 相似文献
966.
967.
968.
The N(1)- and N(2)-(2′-deoxyribofuranosides) 1 and 2 , respectively, of 8-aza-7-deazaguanine were prepared via phase-transfer glycosylation in the presence or absence of Bu4NHSO4 as catalyst of 6-amino-4-methoxy-lH-pyrazolo[3,4-d]pyrimidine ( 7c ) with 2-deoxy-3,5-di-O-(p-toluoyl)-α-D -erythro-pentofuranosyl chloride ( 10 ). On a similar route, but without catalyst and employing THF as organic phase, the 6-amino-4-chloronucleosides 11b and 12b were synthesized from 7a and converted into the N(1)-and N(2)-substituted 4-thioxo analogues 17a and 18a , respectively. The ratio of N(1)- to N(2)-glycosylation was 2:1 for 7c and 1:1 for 7a , viz. depending on the nucleobase structure. The rate of the H+-catalyzed N-glycosyl hydrolysis was strongly decreased for the N(2)-(β-D -2′-deoxyribofuranosides) as compared to the N(1)-compounds. However, the N(1)-nucleoside 1 , which is an isostere of 2′-deoxyguanosine, is sufficiently stable to be employed later in solid-phase oligonucleotide synthesis. 相似文献
969.
Kurt V. Schenker Wolfgang Von Philipsborn C. Anderson Evans Werner Skuballa Georg-Alexander Hoyer 《Helvetica chimica acta》1986,69(7):1718-1727
The configurations of the 5,6-double bond in the carbacyclins iloprost ( 3 ; (E)) and isoiloprost ( 4 ; (Z)) are based on a complete assignment of the 13C and 1H resonances determined by 1D and 2D 13C-NMR and 1H-NMR methods. 相似文献
970.
Reduction of sugar methoximes with sodium cyanoborohydride followed by permethylation with CH3SOCH2?/ICH3 affords the corresponding deoxy(methylmethoxylamino)alditol O-methyl ethers, which are amenable to capillary gas chromatography. Derivatization proceeds under mild conditions with high yields. Only a single derivative is formed for each reducing sugar. All aldose derivatives can be separated in a single chromatographic run on glass capillaries coated with Carbowax 20 M or Superox 0.1. Good linearity was found. The introduction of the nitrogen atom in the sugar molecule makes these derivatives suitable for selective detection by the NPD. The El-mass spectra contain intense characteristics ions at m/z 74 for aldoses and at m/z 280 for reducing disaccharides, together with a (M - 15)+ion. These fragments are very suitable for highly sensitive GC-MS analysis by selective single ion monitoring. 相似文献