全文获取类型
收费全文 | 2543篇 |
免费 | 85篇 |
国内免费 | 11篇 |
专业分类
化学 | 1599篇 |
晶体学 | 1篇 |
力学 | 79篇 |
数学 | 505篇 |
物理学 | 455篇 |
出版年
2023年 | 17篇 |
2022年 | 23篇 |
2021年 | 38篇 |
2020年 | 49篇 |
2019年 | 46篇 |
2018年 | 33篇 |
2017年 | 39篇 |
2016年 | 72篇 |
2015年 | 51篇 |
2014年 | 81篇 |
2013年 | 123篇 |
2012年 | 155篇 |
2011年 | 208篇 |
2010年 | 130篇 |
2009年 | 104篇 |
2008年 | 157篇 |
2007年 | 185篇 |
2006年 | 144篇 |
2005年 | 112篇 |
2004年 | 105篇 |
2003年 | 94篇 |
2002年 | 79篇 |
2001年 | 32篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 31篇 |
1997年 | 33篇 |
1996年 | 29篇 |
1995年 | 22篇 |
1994年 | 17篇 |
1993年 | 21篇 |
1992年 | 26篇 |
1991年 | 10篇 |
1990年 | 11篇 |
1989年 | 15篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 18篇 |
1979年 | 9篇 |
1978年 | 18篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1975年 | 9篇 |
1974年 | 10篇 |
排序方式: 共有2639条查询结果,搜索用时 15 毫秒
81.
It is an open question whether there is a strange attractor for the Henon mapping. We show that for certain maps close to
the Henon map there are strange attractors. 相似文献
82.
W. J. van der Hart W. C. M. M. Luijten J. van Thuijl 《Journal of mass spectrometry : JMS》1980,15(9):463-465
It is shown by ion cyclotron resonance measurements that ion/molecule reactions, leading to substitution or reduction product ions from chloro- and nitrobenzene with the title amines, are those between the molecular ions [RNH2]+ or [C6H5X]+˙ and their respective counterparts C6H5X or RNH2. The protonated reagent gas ions [RNH3]+ are not involved in these reactions. In the case of nitrobenzene, adduct ions [C6H5NO2·RNH3]+ do not decompose within the time scale of the measurements. The results obtained are compared with those found under chemical ionization conditions. 相似文献
83.
W. C. M. M. Luijten W. Onkenhout J. van Thuijl 《Journal of mass spectrometry : JMS》1980,15(7):329-330
For compounds C6H5X (X?Cl, Br, I) under chemical ionization conditions, methylamine causes ipso substitution of X by [NH2CH3]+ and by [NH2]+˙. C6H5F is less reactive; it gives some [C6H5NH2]+˙. Nitrobenzene gives an adduct ion [M+CH3NH3]+, a reduction product ion [C6H5NO2]+˙, and an ion at m/z93, probably a substitution product [C6H5NH2]+˙, but no [C6H5NH2CH3]+. It is also shown that the ion m/z94, formed from nitrobenzene with ammonia as reagent gas, is a substitution product rather than a reduction product ion. Carbonyl compounds C6H5. CO. X give adduct ions and some substitution, mainly [C6H5NH2]+˙. 相似文献
84.
Dalia R. Imam Ahmed A. El-Barbary Claus Nielsen Erik B. Pedersen 《Monatshefte für Chemie / Chemical Monthly》2002,133(5):723-734
Summary. 5-Isopropyl-6-naphthyl uracil and 5-isopropyl-6-naphthyl-2-thiouracil were alkylated to give N-1-(ethoxymethyl and methylthiomethyl)
uracil and S2-cyclohexyl-thiouracil, respectively. 5-Ethyl-6-naphthyl uracil and 5-ethyl-6-naphthyl-2-thiouracil afforded N-1-(ethoxymethyl,
methoxy-methyl, methylthiomethyl, acetoxyethoxy methyl and hydroxyethoxy methyl) uracil and S2-((2,2- diethoxyethyl), methoxycarbonylmethyl, ethoxycarbonylpropyl, methylthiomethyl, ethoxymethyl, methyl and cyclohexyl)-thiouracil
upon alkylation.
Received September 25, 2001. Accepted (revised) December 3, 2001 相似文献
85.
Mann DL Ware GM Bonnin E Eitenmiller RR Barna E Christiansen S De Borde JL DeVries J Gilliland P Hemmer J Kalman A Konings E Levin D Salvati L Woollard D 《Journal of AOAC International》2005,88(1):30-37
A liquid chromatographic (LC) method was validated for the determination of total vitamin B6 in infant formula. Total vitamin B6 was quantified by converting the phosphorylated and free vitamers into pyridoxine. Pyridoxine was determined by ion pair reversed-phase LC with fluorescence detection. The method was subjected to an AOAC collaborative study involving a factory-manufactured, milk- and soy-based infant formula. Each was spiked at 3 concentrations in the range of 0-1 microg/g and sent as blind duplicate to participant laboratories. Nine laboratories returned valid data which were statistically analyzed for outliers and precision parameters. The repeatability relative standard deviation (RSD(r)) ranges were 2.0-4.0 and 3.5-5.9% for fortified milk- and soy-based formulas, respectively. The reproducibility relative standard deviation (RSD(R)) ranges were 8.2-8.4 and 6.7-11.2% for fortified milk- and soy-based formulas, respectively. HORRAT values ranged from 0.42 to 0.53, indicating that the precision of the method is acceptable. The mean RSD(r):RSD(R) values were 0.60 and 0.55 for milk- and soy-based formulas, respectively. As expected, RSDs for the unfortified samples were higher, but their HORRAT values (0.81 and 2.06) helped define a realistic limit of quantitation as 0.05 microg/g. Recovery data were quantitative and varied between 81.4 and 98.0% (mean = 89.8%) for each of 6 spiked materials. 相似文献
86.
Vít Kremláček Erik Kertész Prof. Zoltán Benkő Milan Erben Robert Jirásko Prof. Aleš Růžička Prof. Roman Jambor Prof. Libor Dostál 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(52):13096-13097
Invited for the cover of this issue are Zoltán Benkő, Libor Dostál and co-workers at the University of Pardubice and the Budapest University of Technology and Economics. The image depicts signs for the two different pathways representing the two differing reaction types which were clearly observed for 2,1-benzazaphosphole. Read the full text of the article at 10.1002/chem.202101686 . 相似文献
87.
88.
Li Caldeira Balkeståhl Erik Thomé on behalf of the PANDA Collaboration 《Hyperfine Interactions》2014,229(1-3):79-83
Strangeness and charm production provide an excellent probe of QCD in the confinement domain. With the PANDA detector at FAIR, this can be studied in e.g., hyperon production in the p?p → ?Y reactions. In PANDA, all ground state strange hyperons and single charmed Λ’s will be accessible. Simulations show that the differential cross sections and spin observables can be well reconstructed for these reaction channels. 相似文献
89.
Dr. Liangliang Song Prof. Dr. Erik V. Van der Eycken 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(1):121-144
Polycycles are abundantly present in numerous advanced chemicals, functional materials, bioactive molecules and natural products. However, the strategies for the synthesis of polycycles are limited to classical reactions and transition metal-catalyzed cross-coupling reactions, requiring pre-functionalized starting materials and lengthy synthetic operations. The emergence of novel approaches shows great promise for the fields of organic/medicinal/materials chemistry. Among them, transition metal-catalyzed C−H activation followed by intermolecular annulation reactions prevail, due to their straightforward manner with high atom- and step-economy, providing rapid, concise and efficient methods for the construction of diverse polycycles. Several strategies have been developed for the synthesis of polycycles, relying on sequential multiple C−H activation/annulation, or combination of C−H activation/annulation and further interaction with a proximal group, or merger of C−H activation with a cycloaddition reaction, or in situ formation of the directing group. These are attractive, efficient, step- and atom-economic methods starting from commercially available materials. This Minireview will provide an introduction to transition metal-catalyzed C−H activation for the synthesis of polycycles, helping researchers to discover indirect connections and reveal hidden opportunities. It will also promote the discovery of novel synthetic strategies relying on C−H activation. 相似文献