全文获取类型
收费全文 | 15019篇 |
免费 | 2599篇 |
国内免费 | 2160篇 |
专业分类
化学 | 15229篇 |
晶体学 | 180篇 |
力学 | 279篇 |
综合类 | 80篇 |
数学 | 101篇 |
物理学 | 3909篇 |
出版年
2024年 | 54篇 |
2023年 | 209篇 |
2022年 | 906篇 |
2021年 | 766篇 |
2020年 | 820篇 |
2019年 | 615篇 |
2018年 | 497篇 |
2017年 | 517篇 |
2016年 | 757篇 |
2015年 | 680篇 |
2014年 | 749篇 |
2013年 | 1367篇 |
2012年 | 931篇 |
2011年 | 803篇 |
2010年 | 721篇 |
2009年 | 799篇 |
2008年 | 865篇 |
2007年 | 917篇 |
2006年 | 838篇 |
2005年 | 765篇 |
2004年 | 757篇 |
2003年 | 706篇 |
2002年 | 489篇 |
2001年 | 433篇 |
2000年 | 436篇 |
1999年 | 398篇 |
1998年 | 364篇 |
1997年 | 268篇 |
1996年 | 225篇 |
1995年 | 225篇 |
1994年 | 188篇 |
1993年 | 118篇 |
1992年 | 123篇 |
1991年 | 65篇 |
1990年 | 50篇 |
1989年 | 61篇 |
1988年 | 43篇 |
1987年 | 33篇 |
1986年 | 33篇 |
1985年 | 41篇 |
1984年 | 25篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 22篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1975年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
921.
Alain Chaintreau Wolfgang Fieber Horst Sommer Alexis Gilbert Keita Yamada Naohiro Yoshida Alain Pagelot Detlef Moskau Aitor Moreno Jürgen Schleucher Fabiano Reniero Margaret Holland Claude Guillou Virginie Silvestre Serge Akoka Gérald S. Remaud 《Analytica chimica acta》2013
Isotopic 13C NMR spectrometry, which is able to measure intra-molecular 13C composition, is of emerging demand because of the new information provided by the 13C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic 13C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular 13C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic 13C NMR was then assessed on vanillin from three different origins associated with specific δ13Ci profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ13Ci in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results. 相似文献
922.
An environmentally friendly process is reported for the palladium(II)‐catalyzed cyclofunctionalization of allylic hydroxylamine derivatives using a catalytic amount of copper(II) acetate and molecular oxygen as the cooxidant. 相似文献
923.
Shahnaz Rostamizadeh Ali Mohammad Amani Reza Aryan Hamid Reza Ghaieni Nasrin Shadjou 《合成通讯》2013,43(20):3567-3576
A simple, inexpensive, and efficient one-pot synthesis of 2,3-dihydroquinazoline-4(1H)-one derivatives under solvent-free conditions using a catalytic amount of iodine with excellent product yields is reported. This methodology provides easy, quantitative access to various 2,3-dihydroquinazoline-4(1H)-one derivatives, using commercially available iodine as a catalyst. 相似文献
924.
Shamoon Ahmad Siddiqui Tabish Rasheed 《International journal of quantum chemistry》2013,113(7):959-965
In present investigation, the interactions of iridium (Ir) atom with fluorine (F) atoms have been studied using the density functional theory. Up to seven F atoms were able to bind to a single Ir atom which resulted in increase of electron affinities successively, reaching a peak value of 7.85 eV for IrF7. The stability and reactivity of these clusters were analyzed by calculating highest occupied molecular orbital (HOMO)–LUMO gaps, molecular orbitals and binding energies of these clusters. The unusual properties of these clusters are due to the involvement of inner shell 5d‐electrons, which not only allows IrFn clusters to belong to the class of superhalogens but also shows that its valence can exceed the nominal value of 2. © 2012 Wiley Periodicals, Inc. 相似文献
925.
Samantha Jenkins 《International journal of quantum chemistry》2013,113(11):1603-1608
The need to make more quantitative use of the total electronic charge density distribution is demonstrated in this short perspective. This is framed in the perspective of the ground breaking early work of Bader and coworkers, along with mathematicians who captured the essential nature of a molecule in a suitably compact form in real space. We see that this simple form is the Poincaré–Hopf relation for molecules and clusters and the Euler–Hopf relation in solids. Thom's theory of elementary catastrophes combined with the Poincaré–Hopf relation provides the inspiration for the new quantum topology. An alternative use of the Poincaré–Hopf relation, molecular recognition, is discussed. Quantum topology is then used to create a topology phase diagram for both molecules and solids. The author adds their perspectives of the huge potential of the quantum topology approach by demonstrating the ease with which new theoretical ideas can be generated. © 2013 Wiley Periodicals, Inc. 相似文献
926.
The standard (po = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, at the reference temperature of 298.15 K, of 2-amino-4-methylpyrimidine ((98.1 ± 1.6) kJ · mol−1), 2-amino-4,6-dimethylpyrimidine ((55.9 ± 1.8) kJ · mol−1) and 4-amino-2,6-dimethylpyrimidine ((60.1 ± 1.8) kJ · mol−1) were calculated from the enthalpies of formation, in the crystalline phase, and enthalpies of sublimation, derived, respectively, from static bomb combustion calorimetry and Knudsen effusion technique results. In order to quantify the resonance effects arising from the substitution on the pyrimidine ring, hypothetical isodesmic reactions were used to analyze the experimental gaseous-phase enthalpies of formation. The aromaticity of benzene, pyridine, pyrimidine and the substituted pyrimidines was investigated in terms of magnetic (NICS), geometric (HOMA), electronic (Shannon aromaticity, QTAIMs ring critical point properties and HOMO–LUMO gap), reactive (hardness), vibrational (Kekulé mode) and spectroscopic (UV–Vis) properties. 相似文献
927.
An efficient one-pot condensation of 4-hydroxylcoumarin, aromatic aldehydes, and 5,5-dimethylcyclohexane-1,3-dione has been achieved with molecular iodine as a catalyst via conventional heating and microwave irradiation techniques, and thus a variety of new tetrahydrobenzo[c]xanthene-1,11-dione derivatives were prepared in good yields. 相似文献
928.
Yiming Ren 《合成通讯》2013,43(11):1670-1676
Esterification of carboxylic acids and transesterification of β-ketoesters with alcohols have been developed using a catalytic amount of iodine in polyethylene glycol (PEG) ionic liquid (IL 1000) to afford the corresponding esters in good yields. By simple separation of the ionic-liquid phase containing the iodine, the system of I2/IL 1000 can be reused several times. 相似文献
929.
Chandrika Nanjappa Gopalpur Nagendrappa Pasura Subbaiah Sujan Ganapathy Shirur Dakappa Shruthi Sunil S. More 《合成通讯》2013,43(22):2529-2545
A new 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)-radical scavenging and antiproliferative agents of pyrrolo[1,2-a]quinoline derivatives have been synthesized. An efficient method for the synthesis of 14 novel diversified pyrrolo[1,2-a]quinoline derivatives has been described using 4-(1,3-dioxolan-2-yl)quinoline and different phenacyl bromides in acetone and followed by reacting with different acetylenes in dimethylformamide/K2CO3. The structure of the newly synthesized compounds was determined by infrared, 1H NMR, 13C NMR, mass spectrometry, and elemental analysis. The in vitro antioxidant activity revealed that among all the tested compounds 5n exhibited maximum scavenging activity with ABTS. Compound 5b has showed good antiproliferative activity as an inhibitor of epidermal growth factor receptor (EGFR) tyrosine kinase. 相似文献
930.
Yutong Zhao Fu Kit Sheong Jian Sun Pedro Sander Xuhui Huang 《Journal of computational chemistry》2013,34(2):95-104
We implemented a GPU‐powered parallel k‐centers algorithm to perform clustering on the conformations of molecular dynamics (MD) simulations. The algorithm is up to two orders of magnitude faster than the CPU implementation. We tested our algorithm on four protein MD simulation datasets ranging from the small Alanine Dipeptide to a 370‐residue Maltose Binding Protein (MBP). It is capable of grouping 250,000 conformations of the MBP into 4000 clusters within 40 seconds. To achieve this, we effectively parallelized the code on the GPU and utilize the triangle inequality of metric spaces. Furthermore, the algorithm's running time is linear with respect to the number of cluster centers. In addition, we found the triangle inequality to be less effective in higher dimensions and provide a mathematical rationale. Finally, using Alanine Dipeptide as an example, we show a strong correlation between cluster populations resulting from the k‐centers algorithm and the underlying density. © 2012 Wiley Periodicals, Inc. 相似文献