全文获取类型
收费全文 | 322390篇 |
免费 | 19593篇 |
国内免费 | 10860篇 |
专业分类
化学 | 181839篇 |
晶体学 | 4888篇 |
力学 | 16567篇 |
综合类 | 542篇 |
数学 | 35437篇 |
物理学 | 113570篇 |
出版年
2022年 | 4906篇 |
2021年 | 5635篇 |
2020年 | 5961篇 |
2019年 | 6012篇 |
2018年 | 5943篇 |
2017年 | 5711篇 |
2016年 | 8986篇 |
2015年 | 7290篇 |
2014年 | 9821篇 |
2013年 | 17390篇 |
2012年 | 16644篇 |
2011年 | 18614篇 |
2010年 | 12892篇 |
2009年 | 12603篇 |
2008年 | 14965篇 |
2007年 | 14213篇 |
2006年 | 13231篇 |
2005年 | 11541篇 |
2004年 | 9859篇 |
2003年 | 8563篇 |
2002年 | 8097篇 |
2001年 | 9480篇 |
2000年 | 7330篇 |
1999年 | 6263篇 |
1998年 | 5181篇 |
1997年 | 4896篇 |
1996年 | 4824篇 |
1995年 | 4263篇 |
1994年 | 4139篇 |
1993年 | 3678篇 |
1992年 | 4183篇 |
1991年 | 4042篇 |
1990年 | 3722篇 |
1989年 | 3339篇 |
1988年 | 3232篇 |
1987年 | 3211篇 |
1986年 | 2996篇 |
1985年 | 3740篇 |
1984年 | 3672篇 |
1983年 | 3052篇 |
1982年 | 3103篇 |
1981年 | 2885篇 |
1980年 | 2811篇 |
1979年 | 3040篇 |
1978年 | 3087篇 |
1977年 | 3029篇 |
1976年 | 2991篇 |
1975年 | 2829篇 |
1974年 | 2853篇 |
1973年 | 2951篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Abstract— The bioluminescence emission spectrum of the light emitted from the elytra of certain poly-noid worms is similar to the fluorescence of the scales and to the fluorescence of flavins. In extracts the fluorescence is confined to the particulate fraction. Stimulation of bioluminescence from such particles was observed upon the addition of Fe2+ ions. 相似文献
982.
The structures and configurations of two new β-carbolinic alkaloids, strychnohirsutine 1 and tetra-dehydrostrychnohirsutine 2, isolated from stem and root bark of Strychnos hirsuta, were reported. 1H NMR data of N,O-diacetylstrychnohirsutine 5 and 1H and 13C NMR data of O-acetyltetradehydrostrychnohirsutine 4 were listed. Alkaloid 1 was converted into 2 by catalytic dehydrogenation. 相似文献
983.
Useful methodology is described for the synthesis of dehydroalanine residues (II) within peptides. The unnatural amino acid (Se)-phenylselenocysteine (I) can be incorporated into growing peptide chains via standard peptide synthesis procedures. Subsequent oxidative elimination affords a dehydroalanine at the desired position. The oxidation conditions are mild and tolerate functionalities commonly found in peptides, including variously protected cysteine residues. To illustrate its utility, cyclic lanthionines have been synthesized by this method. 相似文献
984.
985.
(Use of an interference blank in the determination of nitrate by ion-selective potentiometry.) During selective potentiometry of nitrate in meat product additives, a new interference originating from polyphosphate ions was encountered. The selectivity coefficients measured were 5 × 10?2 for tripolyphosphate, 10?2 for chloride, 8 × 10?4 for pentapolyphosphate and 6 × 10?4 for pyrophosphate. An “interference blank” techniques is proposed; nitrate is destroyed in the medium to be studied, so that calibration is possible in the presence of interfering substances regardless of their identity or concentration. The method is successfully applied to monitoring of meat product additives. 相似文献
986.
The analysis of a coffee concentrate is described: 202 constituents have been identified, 154 of which for the first time. Some of these compounds are new chemicals; their synthesis is described. Most identifications are based upon mass spectrometry combined in many cases with IR. spectrometry and comparison with reference compounds. MS. and IR. data are given in detail. The analysis shows that a large number of minor constituents is present, many of which could not be identified. 相似文献
987.
B. D. Berezin 《Chemistry of Heterocyclic Compounds》1966,1(6):641-643
A qualitative analysis of certain peculiarities of the porphine molecule and its complexes compared with the phthalocyanine molecule is made by evaluating the -electron charges of bonds in these compounds by means of G. V. Bykov's semi -empirical linear equation. Acid-base, oxidation-reduction, and coordination properties of porphine are examined. It is shown that differences in chemical properties of porphines and tetraporphines can be explained by differences in -electron charges of bonds. 相似文献
988.
J. Reid Shelton D. E. Agostini J. B. Lando 《Journal of polymer science. Part A, Polymer chemistry》1971,9(10):2789-2799
Synthesis of the naturally occurring polyester, D -poly-β-hydroxybutyrate (PHB) was accomplished by using an optically active monomer. Polymerization of D -(+)-β-butyrolactone (β-BL) of 73% optical purity with a catalyst system of Et3Al–H2O produced a polymer with a similar optical activity and essentially identical to the natural polymer as isolated from bacterial cells. This paper describes the synthesis and characterization of this optically active polyester along with a suggested mechanism to account for the observed stereospecific polymerization of β-BL with this catalyst system. 相似文献
989.
[structure: see text]. The most powerful DNA microarrays would be prepared by photolithography with free 3'-ends that could be processed enzymatically. A photoremovable group that could be removed in quantitative yield would ensure high purity of the synthesized probes. We have developed new pyrimidine building blocks for 5' --> 3' DNA synthesis with high cycle yields using the NPPOC (3'-nitrophenylpropyloxycarbonyl) protecting group. These phosphoramidites were proved in automated photochemical DNA synthesis on a modified synthesizer. 相似文献
990.
在非水溶剂(二甲基亚砜(DMSO)及N,N_二甲基甲酰胺(DMF))中的伏安曲线.呈现三对电流峰,表明存在三个可逆或准可逆酞菁配体的单电子转移过程,而在水溶液中则不呈现准可逆行为,且波形改变很大.此外,还比较了四磺酸基酞菁锌的水溶液在自然光和红光(600~700nm)照射下的循环伏安曲线,其氧化还原峰的数目和峰电位基本不变,但在红光照射下比自然光照射下的峰电流明显增大. 相似文献