全文获取类型
收费全文 | 934篇 |
免费 | 46篇 |
国内免费 | 6篇 |
专业分类
化学 | 660篇 |
晶体学 | 2篇 |
力学 | 6篇 |
数学 | 115篇 |
物理学 | 203篇 |
出版年
2023年 | 9篇 |
2022年 | 20篇 |
2021年 | 23篇 |
2020年 | 27篇 |
2019年 | 16篇 |
2018年 | 28篇 |
2017年 | 10篇 |
2016年 | 25篇 |
2015年 | 29篇 |
2014年 | 25篇 |
2013年 | 53篇 |
2012年 | 63篇 |
2011年 | 66篇 |
2010年 | 27篇 |
2009年 | 26篇 |
2008年 | 52篇 |
2007年 | 41篇 |
2006年 | 56篇 |
2005年 | 30篇 |
2004年 | 32篇 |
2003年 | 39篇 |
2002年 | 31篇 |
2001年 | 18篇 |
2000年 | 10篇 |
1999年 | 13篇 |
1998年 | 9篇 |
1997年 | 14篇 |
1996年 | 7篇 |
1995年 | 10篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 15篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1975年 | 8篇 |
1974年 | 8篇 |
1973年 | 8篇 |
1970年 | 3篇 |
排序方式: 共有986条查询结果,搜索用时 15 毫秒
971.
972.
Molybdenacyclobutane complexes can be prepared by reaction of Cp*Mo(NO)(=CHCMe3) (formed spontaneously by loss of neopentane from Cp*Mo(NO)(CH2CMe3)2 (1) under ambient conditions) with cyclic olefins, including cyclopentene, cycloheptene, and cyclooctene. The cyclopentene metallacyclobutane orients the metallacycle bridgehead protons cis to each other. A trans arrangement is observed for the cyclooctene congener, and both cis and trans orientations occur for the cycloheptene species. The reaction of 1 with cyclohexene initially forms a metallacyclobutane that then undergoes an allylic C-H activation to form an allyl-alkyl-coupled product with the concomitant loss of H2. The analogous allyl-alkyl structure is also observed as the thermodynamic product for the reactions of cycloheptene and cyclooctene with 1. The cyclooctene allyl-alkyl compound (5C) can be converted to an eta2-diene complex by heating with pyridine. Alternatively, heating of 5C in the presence of a variety of olefins displaces the diene ligand that can then be isolated in its unbound state. 相似文献
973.
The Lewis base mediated carbon-carbon bond forming reactions between pentafluorophenylsilanes and iminium cations were studied theoretically and experimentally. The complexation of silanes with anionic Lewis bases was analyzed computationally using DFT methods at the B3LYP/6-31+G(d) level. The pentafluorophenyl group was found to exhibit a significant stabilizing effect on the formation of pentacoordinate silicon species, with (C6F5)3SiF and C6F5SiF3 being the strongest Lewis acids. Comparison of geometrical isomers of trigonal bipyramidal siliconates R2(C6F5)SiXY- (R = Me, F, Cl; X, Y = F, Cl, ClO4) revealed that the heteroatoms and the C6F5 group prefer to occupy apical and equatorial positions, respectively. Calculations of C6F5 group transfer processes from silanes to N,N-dimethyliminium cation lead to the following conclusions: (1) when employing weak Lewis bases X- = Cl-, ClO4- as activators, R2(C6F5)SiF are more reactive than R2(C6F5)SiX (X = Cl, ClO4); (2) the C6F5 group is much more reactive in an apical position than in an equatorial position of the siliconate; (3) a solvent greatly increases the reaction barrier by stabilizing the reactants; and (4) the efficiency of C-C bond formation can be improved by rendering the process intramolecular. It was shown experimentally that reactions of (C6F5)3SiF with PhCH=NMe2+TfO- can be promoted by a series of weak Lewis bases, from which the chloride anion was the most effective. Even more facile was the reaction of N-(silyloxyethyl)iminium ion (C6F5)3SiOCH2CH2N(Me)=CHPh+TfO-, likely proceeding via intramolecular transfer of a C6F5 group. The optimal conditions for pentafluorophenylation of iminium ions involved the use of benzyltriethylammonium chloride in refluxing dichloroethane. 相似文献
974.
975.
Miriam Unger Harumi Sato Yukihiro Ozaki Heinz W. Siesler 《Macromolecular Symposia》2011,305(1):90-100
Variable-temperature FT-IR spectra of poly(3-hydroxybutyrate) (PHB), poly(ε-caprolactone) (PCL) and a PHB/PCL (50:50 wt.%) blend were analyzed by two-dimensional correlation spectroscopy (2DCOS). For this purpose the ν(CO) region was employed to characterize in some detail the crystallization behavior of the investigated polymer systems during cooling from the melt. The asynchronous 2D correlation spectra clearly captured the existence of three components in the crystallinity-sensitive region of the CO stretching mode for PHB and PCL, respectively: a well-ordered, an inter-mediate and a less ordered crystalline state. Furthermore, by 2DCOS application a sequential order of the observed structural changes could be proposed for the whole temperature range during the crystallization of both polymers. In the case of the PHB/PCL (50:50 wt.%) polymer blend, we have split up the spectral data set in the sub-sets between 200–120 °C and 70–30 °C for a more detailed 2DCOS analysis. In this way we could separate the crystallization process of PHB and PCL in the polymer blend. 相似文献
976.
M Sanz BF de Simón E Cadahía E Esteruelas AM Muñoz T Hernández I Estrella E Pinto 《Journal of mass spectrometry : JMS》2012,47(7):905-918
The phenolic composition of heartwood extracts from Fraxinus excelsior L. and F. americana L., both before and after toasting in cooperage, was studied using LC-DAD/ESI-MS/MS. Low-molecular weight (LMW) phenolic compounds, secoiridoids, phenylethanoid glycosides, dilignols and oligolignols compounds were detected, and 48 were identified, or tentatively characterized, on the basis of their retention time, UV/Vis and MS spectra, and MS fragmentation patterns. Some LMW phenolic compounds like protocatechuic acid and aldehyde, hydroxytyrosol and tyrosol, were unlike to those for oak wood, while ellagic and gallic acid were not found. The toasting of wood resulted in a progressive increase in lignin degradation products with regard to toasting intensity. The levels of some of these compounds in medium-toasted ash woods were much higher than those normally detected in toasted oak, highlighting vanillin levels, thus a more pronounced vanilla character can be expected when using toasted ash wood in the aging wines. Moreover, in seasoned wood, we found a great variety of phenolic compounds which had not been found in oak wood, especially oleuropein, ligstroside and olivil, along with verbascoside and isoverbascoside in F. excelsior, and oleoside in F. americana. Toasting mainly provoked their degradation, thus in medium-toasted wood, only four of them were detected. This resulted in a minor differentiation between toasted ash and oak woods. The absence of tannins in ash wood, which are very important in oak wood, is another peculiar characteristic that should be taken into account when considering its use in cooperage. 相似文献
977.
Pedro Silvino Pereira Carlos Vinicius Barros Oliveira Ana Josicleide Maia Maria Celeste Vega-Gomez Miriam Roln Cathia Coronel Antnia Eliene Duarte Henrique Douglas Melo Coutinho Abolghasem Siyadatpanah Roghayeh Norouzi Seyed Jafar Adnani Sadati Polrat Wilairatana Teresinha Gonalves Silva 《Molecules (Basel, Switzerland)》2022,27(9)
Cymbopogon winterianus, known as “citronella grass”, is an important aromatic and medicinal tropical herbaceous plant. The essential oil of C. winterianus (EOCw) is popularly used to play an important role in improving human health due to its potential as a bioactive component. The present study aimed to identify the components of the essential oil of C. winterianus and verify its leishmanicidal and trypanocidal potential, as well as the cytotoxicity in mammalian cells, in vitro. The EOCw had geraniol (42.13%), citronellal (17.31%), and citronellol (16.91%) as major constituents. The essential oil only exhibited significant cytotoxicity in mammalian fibroblasts at concentrations greater than 250 μg/mL, while regarding antipromastigote and antiepimastigote activities, they presented values considered clinically relevant, since both had LC50 < 62.5 μg/mL. It can be concluded that this is a pioneer study on the potential of the essential oil of C. winterianus and its use against the parasites T. cruzi and L. brasiliensis, and its importance is also based on this fact. Additionally, according to the results, C. winterianus was effective in presenting values of clinical relevance and low toxicity and, therefore, an indicator of popular use. 相似文献
978.
Miriam Di Martino Lucia Sessa Martina Di Matteo Barbara Panunzi Stefano Piotto Simona Concilio 《Molecules (Basel, Switzerland)》2022,27(17)
Azo molecules, characterized by the presence of a -N=N- double bond, are widely used in various fields due to their sensitivity to external stimuli, ch as light. The emergence of bacterial resistance has pushed research towards designing new antimicrobial molecules that are more efficient than those currently in use. Many authors have attempted to exploit the antimicrobial activity of azobenzene and to utilize their photoisomerization for selective control of the bioactivities of antimicrobial molecules, which is necessary for antibacterial therapy. This review will provide a systematic and consequential approach to coupling azobenzene moiety with active antimicrobial molecules and drugs, including small and large organic molecules, such as peptides. A selection of significant cutting-edge articles collected in recent years has been discussed, based on the structural pattern and antimicrobial performance, focusing especially on the photoactivity of azobenzene and the design of smart materials as the most targeted and desirable application. 相似文献
979.
Andrei Hutanu Miriam López Ferreiro Joost van Haasteren Oliver Höcker Cristina Montealegre Marina Mäser Alex Keresztfalvi Jose Monti Maria Anna Schwarz 《Electrophoresis》2023,44(19-20):1595-1606
The use of nucleic acids (NAs) has revolutionized medical approaches and ushered in a new era of combating various diseases. Accordingly, there is an increasing demand for accurate identification, localization, quantification, and characterization of NAs encapsulated in nonviral or viral vectors. The vast spectrum of molecular dimensions and intra- and intermolecular interactions presents a formidable obstacle for NA analytical development. Typically, the comprehensive analysis of encapsulated NAs, free NAs, and their spatial distribution poses a challenge that is seldom tackled in its complete complexity. The identification of appropriate physicochemical methodologies for large nonencapsulated or encapsulated NAs is particularly intricate and necessitates an evaluation of the analytical outcomes and their appropriateness in addressing critical quality attributes. In this work, we examine the analytics of non-encapsulated or encapsulated large NAs (>500 nucleotides) utilizing capillary electrophoresis (CE) and liquid chromatography (LC) methodologies such as free zone CE, gel CE, affinity CE, and ion pair high-performance liquid chromatography (HPLC). These methodologies create a complete picture of the NA's critical quality attributes, including quantity, identity, purity, and content ratio. 相似文献
980.
Thorben Lenk Sahar Rabet Miriam Sprick Dr. Gabriele Raabe Prof. Dr. Uwe Schröder 《Chemphyschem》2023,24(5):e202200614
Electrocatalytic hydrogenation of furfural on metal surfaces has become an important research subject due to the potential of the reaction product 2-methylfuran as a renewable energy resource. Identifying effective determinants in this reaction process requires a thorough investigation of the complex electrode-electrolyte interactions, which considers a variety of the influential components. In this work, in operando electrochemical Raman Spectroscopy and Molecular Dynamics simulations were utilized to investigate different characteristics of the interface layer in the electrocatalytic hydrogenation of furfural. Hereby, the influence of applied potentials, electrode material, and electrolyte composition were investigated in detail. The studied parameters give an insight into furfural's binding situation, molecular orientation, and reaction mechanism. 相似文献