全文获取类型
收费全文 | 1264篇 |
免费 | 54篇 |
国内免费 | 11篇 |
专业分类
化学 | 906篇 |
晶体学 | 4篇 |
力学 | 16篇 |
数学 | 222篇 |
物理学 | 181篇 |
出版年
2024年 | 1篇 |
2023年 | 10篇 |
2022年 | 16篇 |
2021年 | 32篇 |
2020年 | 33篇 |
2019年 | 22篇 |
2018年 | 29篇 |
2017年 | 18篇 |
2016年 | 49篇 |
2015年 | 36篇 |
2014年 | 37篇 |
2013年 | 65篇 |
2012年 | 99篇 |
2011年 | 88篇 |
2010年 | 63篇 |
2009年 | 55篇 |
2008年 | 103篇 |
2007年 | 90篇 |
2006年 | 93篇 |
2005年 | 76篇 |
2004年 | 60篇 |
2003年 | 43篇 |
2002年 | 41篇 |
2001年 | 17篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 6篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 6篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 2篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有1329条查询结果,搜索用时 15 毫秒
61.
Javier Benito Mónica García-Mardones Víctor Pérez-Gregorio Ignacio Gascón Carlos Lafuente 《Journal of solution chemistry》2014,43(4):696-710
In this work, thermophysical properties of n-ethylpyridinium bis(trifluoromethylsulfonyl)imide have been studied at atmospheric pressure in the temperature range 288.15–338.15 K. Density, speed of sound, refractive index, surface tension, isobaric molar heat capacity, electrical conductivity and kinematic viscosity have been measured; from these data the isobaric expansibility, isentropic compressibility, molar refraction, entropy and enthalpy of surface formation per unit of surface area, and dynamic viscosity have been calculated. Moreover, we have characterized the thermal behavior of the compound. Results have been analyzed paying special attention to the structural and energetic factors. The magnitude and directionality of the cation–anion interactions have been studied using ab initio quantum calculations, which allow a better understanding of the physicochemical behavior of the ionic liquid. Finally, density values and radial distribution functions were also estimated ab initio from classical molecular dynamics simulations, providing acceptable density predictions. 相似文献
62.
Stereoselective hydrogenation of methylcyclohex-2-ene-1,4-diols used as important intermediates for the preparation of ampelomins and deoxy-carbasugars was studied. These olefins were obtained in few steps from a chiral cis-diol resulting from microbial oxidation of toluene. Although the stereoselective hydrogenation of this type of substrates is difficult, high yields were obtained for heterogeneous hydrogenation using Adam’s catalyst, where steric hindrance controlled the stereochemical outcome of the process. On the other hand, for homogeneous hydrogenation of similar olefins using Crabtree’s catalyst, coordination with the allylic alcohols allowed for a controlled hydrogen addition from the more hindered face. In this manner two protocols for the hydrogenation of these types of substrates resulting in complementary stereoselectivities are described. 相似文献
63.
Multiresidue analysis of quinolones in water by ultra‐high perfomance liquid chromatography with tandem mass spectrometry using a simple and effective sample treatment
下载免费PDF全文
![点击此处可从《Journal of separation science》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Manuel Lombardo‐Agüí Carmen Cruces‐Blanco Ana M. García‐Campaña Laura Gámiz‐Gracia 《Journal of separation science》2014,37(16):2145-2152
A rapid and simple analytical method has been developed for the determination of 19 quinolones in environmental water samples using ultra high performance liquid chromatography with tandem mass spectrometry. Chromatographic and detection conditions have been optimized and the separation was achieved in less than 4 min. The separation was carried out using a new‐generation column filled with superficially porous particles, resulting in lower backpressure and better resolution than totally porous particle columns. The quinolones were detected by electrospray ionization in positive mode using multiple‐reaction monitoring mode for acquisition. A sample treatment based on liquid–liquid extraction and phase separation via salting‐out was employed to achieve a fast and simple extraction that enables the multiresidue analysis. The method has been validated for an environmental well water sample from a mountain area. Very low limits of detection (between 10 and 90 ng/L) with relative standard deviations lower than 16.5% and recoveries higher than 73% were achieved. Moreover, well waters from different origins (mountain and coast areas and irrigated land) have been evaluated and similar results were obtained. 相似文献
64.
65.
Jose M. Cornejo-Bravo Claudia D. Becerra Kenia Palomino Héctor Magaña Ignacio Rivero Eduardo López-Maldonado 《Soft Materials》2021,19(1):89-99
ABSTRACT We prepared nano/microgels by precipitation copolymerization of N-isopropylacrylamide (NIPAAm), and one of three different carboxyalkyl methacrylamides [methacryloylamido hexanoic acid (M5), 8-methacryloylamido octanoic acid (M7), and 11-methacryloylamido undecanoic acid (M10)], either in the acid forms or as carboxylates (potassium salts). The hydrodynamic diameter (Dh) of the nano/microgels prepared with the carboxylates was smaller (≈100 nm for M10 copolymers), compared to the size of homopolymeric NIPAAm microgels prepared by dispersion polymerization (around 600 nm), indicating that the carboxylates act as surfactants reducing the size of the seeds during the polymerization process. These materials presented a swollen-shrunken transition temperature (T tr) similar to the T tr of the homopolymeric NIPAAm microgels, without pH sensitivity. On the other hand, the copolymeric microgels prepared from the acid form of the comonomers have a similar or bigger size than NIPAAm microgels. For these copolymers, the T tr can be tuned by the type and proportion of acid comonomer used and present pH sensitivity. This is important for biomedical applications such as positive temperature control release. Polyelectrolyte titration demonstrates that the nano/microgels prepared with the carboxylates behave as hard spheres, while the microgels prepared with the weak acid behave as porous materials. 相似文献
66.
Johura Ansary Francesca Giampieri Tamara Y. Forbes-Hernandez Lucia Regolo Denise Quinzi Santos Gracia Villar Eduardo Garcia Villena Kilian Tutusaus Pifarre Jos M. Alvarez-Suarez Maurizio Battino Danila Cianciosi 《Molecules (Basel, Switzerland)》2021,26(8)
In recent times, scientific attention has been paid to different foods and their bioactive components for the ability to inhibit the onset and progress of different types of cancer. Nigella sativa extract, powder and seed oil and its main components, thymoquinone and α-hederin, have showed potent anticancer and chemosensitizing effects against various types of cancer, such as liver, colon, breast, renal, cervical, lung, ovarian, pancreatic, prostate and skin tumors, through the modulation of various molecular signaling pathways. Herein, the purpose of this review was to highlight the anticancer activity of Nigella sativa and it constitutes, focusing on different in vitro, in vivo and clinical studies and projects, in order to underline their antiproliferative, proapoptotic, cytotoxic and antimetastatic effects. Particular attention has been also given to the synergistic effect of Nigella sativa and it constitutes with chemotherapeutic drugs, and to the synthesized analogs of thymoquinone that seem to enhance the chemo-sensitizing potential. This review could be a useful step towards new research on N. sativa and cancer, to include this plant in the dietary treatments in support to conventional therapies, for the best achievement of therapeutic goals. 相似文献
67.
María Consuelo Díaz-Maroto Manuel Lpez Vias Lourdes Marchante María Elena Alan Ignacio Javier Díaz-Maroto María Soledad Prez-Coello 《Molecules (Basel, Switzerland)》2021,26(1)
The effects of different storage conditions, light exposure, temperature and different commercially available cork stoppers on the phenolic, volatile and sensorial profile of Verdejo wines were studied. Two natural corks of different visual quality and a microgranulated cork stopper were investigated over one year at two different storage conditions. One simulating light exposure and temperature in retail outlets and the other simulating optimal cellar conditions (darkness and 12 °C). The wines stored under commercial conditions showed greater losses of total and free SO2 and higher levels of brown-yellowish tones, related to the oxidation of flavan-3-ols. Although these wines underwent a decrease in the total content of stilbenes, a significant increase in trans-piceid was observed. In addition, these wines suffered important changes in their volatile and sensory profile. Volatile compounds with fruity and floral aromas decreased significantly, while volatile compounds related to aged-type characters, as linalool oxides, vitispirane, TDN or furan derivatives increased. Wines stored in darkness at 12 °C underwent minor changes and their sensory profiles were similar to wine before bottling. The high-quality natural corks and microgranulated corks better preserved the quality of the white wines from a sensory point of view. These results showed that temperature and light exposure conditions (diffuse white LEDs and 24 ± 2 °C) in retail outlets considerably decrease the quality of bottled white wines and, consequently, their shelf life, due to the premature development of aged-type characters. 相似文献
68.
A Combined Experimental and Theoretical Study on the Formation of Ag Filaments on β‐Ag2MoO4 Induced by Electron Irradiation
下载免费PDF全文
![点击此处可从《Particle & Particle Systems Characterization》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Juan Andrés Mateus M. Ferrer Lourdes Gracia Armando Beltran Valeria M. Longo Guilherme H. Cruvinel Ricardo L. Tranquilin Elson Longo 《Particle & Particle Systems Characterization》2015,32(6):646-651
A combined experimental and theoretical study is presented to understand the novel observed nucleation and early evolution of Ag filaments on β‐Ag2MoO4 crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The growth process, chemical composition, and the element distribution in these filaments are analyzed in depth at the nanoscale level using field‐emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM) with energy‐dispersive spectroscopy (EDS) characterization. To complement experimental results, chemical stability, structural and electronic aspects have been studied systematically using first‐principles electronic structure theory within a quantum theory of atoms in molecules (QTAIM) framework. The Ag nucleation and formation on β‐Ag2MoO4 are a result of structural and electronic changes of the AgO4 tetrahedral cluster as a constituent building block of β‐Ag2MoO4, consistent with Ag metallic formation. The formation of Ag filament transforms the β‐Ag2MoO4 semiconductor from n‐ to p‐type concomitant with the appearance of Ag defects. 相似文献
69.
Intermolecular and Regioselective Access to Polysubstituted Benzo‐ and Dihydrobenzo[c]azepine Derivatives: Modulating the Reactivity of Group 6 Non‐Heteroatom‐Stabilized Alkynyl Carbene Complexes
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Jairo González Dr. Aránzazu Gómez Ignacio Funes‐Ardoiz Dr. Javier Santamaría Diego Sampedro 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(23):7061-7068
We highlight the versatility of non‐heteroatom‐stabilized tungsten–carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2‐benzazepinium isolable intermediates 5 . By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2 , Fischer‐type alkoxycarbenes 1 , and phenylimines 4 . The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non‐heteroatom‐stabilized carbene complexes 3 ([4+3] vs. [2+2]‐heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2‐benzazepinium intermediates allows access to 3H‐benzo[c]azepines 6 and 3H‐1,2‐dihydrobenzo[c]azepines 7 – 9 with high control of the substitution of the nine positions of the heterocycle. 相似文献
70.