全文获取类型
收费全文 | 2771篇 |
免费 | 39篇 |
国内免费 | 11篇 |
专业分类
化学 | 1724篇 |
晶体学 | 20篇 |
力学 | 69篇 |
数学 | 405篇 |
物理学 | 603篇 |
出版年
2022年 | 32篇 |
2021年 | 25篇 |
2020年 | 30篇 |
2019年 | 24篇 |
2018年 | 22篇 |
2017年 | 23篇 |
2016年 | 53篇 |
2015年 | 46篇 |
2014年 | 48篇 |
2013年 | 127篇 |
2012年 | 116篇 |
2011年 | 108篇 |
2010年 | 93篇 |
2009年 | 87篇 |
2008年 | 128篇 |
2007年 | 120篇 |
2006年 | 118篇 |
2005年 | 124篇 |
2004年 | 111篇 |
2003年 | 92篇 |
2002年 | 69篇 |
2001年 | 59篇 |
2000年 | 53篇 |
1999年 | 37篇 |
1998年 | 42篇 |
1997年 | 45篇 |
1996年 | 35篇 |
1995年 | 35篇 |
1994年 | 31篇 |
1993年 | 32篇 |
1992年 | 28篇 |
1990年 | 22篇 |
1988年 | 16篇 |
1987年 | 17篇 |
1986年 | 17篇 |
1985年 | 50篇 |
1984年 | 53篇 |
1983年 | 27篇 |
1982年 | 37篇 |
1981年 | 38篇 |
1980年 | 36篇 |
1979年 | 39篇 |
1978年 | 41篇 |
1977年 | 31篇 |
1976年 | 39篇 |
1975年 | 31篇 |
1974年 | 33篇 |
1973年 | 35篇 |
1972年 | 18篇 |
1968年 | 15篇 |
排序方式: 共有2821条查询结果,搜索用时 15 毫秒
141.
(S)-β-Bisabolene, (S)-1, was synthetized by a synthetic route in which (S)-4-methyl-3-cyclohexene carboxylic acid, (S)-10, which was the key intermediate, was prepared via a highly diastereoselective TiCl4-catalyzed Diels-Alder reaction between isoprene and the acrylate of commercial (R)-pantolactone, followed by hydrolysis. Compound (S)-10 was then converted into ketone (S)-13 using two different procedures. The best one of these, as regards the degree of stereospecificity, involved the reaction of (S)-10 with 2 equiv of 4-methyl-3-penten-1-yllithium, 14, in the presence of CeCl3, and gave (S)-13 having ca. 84% ee. The Zr-promoted methylenation of this ketone afforded highly enantiomerically enriched (S)-1. 相似文献
142.
a short, convenient and large scale synthesis of (S)-(+)-2-(2-methoxyphenyl) propanoic acid, involving a resolution of the corresponding racemic acid with quinine, is reported. 相似文献
143.
Antonio Tognazzi Arduino M. Dattilo Luca Bracchini Claudio Rossi Davide Vione 《International journal of environmental analytical chemistry》2013,93(15):1679-1688
It is known that the compound 2,4-dichloro-6-nitrophenol (2,4DC6NP) is formed upon nitration of 2,4-dichlorophenol, which in turn is a transformation intermediate of the herbicide dichlorprop. However, the chemical and spectroscopic characteristics of 2,4DC6NP, as well as its toxicity, are poorly known. This work shows that 2,4DC6NP behaves as a diprotic acid in aqueous solutions, with pKa values of 3.0?±?0.9 and 4.9?±?0.5. At pH?<?3, 2,4DC6NP would undergo protonation. The absorption spectra suggest that anionic 2,4DC6NP, which prevails at pH?>?5 would have an ortho-quinoid structure that is responsible for the absorption peak centred at 428?nm. Considering that 2,4DC6NP has been detected in the brackish lagoons of the Rhône delta (southern France), where its levels are comparable to those of the parent herbicide, it is necessary to examine the possible effects of 2,4DC6NP on the species living in that environment. For this reason, the acute toxicity of the anionic form of 2,4DC6NP was assessed for the brine shrimp Artemia salina, a zooplankton species that lives both in brackish and in saline aquatic environments. The toxicity test yielded a LC20 value of 8?±?2?mg?L?1 and a LC50 value of 18.7?±?0.8?mg?L?1. Such values are safely higher than the maximum detected concentration of 2,4DC6NP in the Rhône delta lagoons. Further studies should be concentrated on the long-term effects of 2,4DC6NP, and in particular on its potential genotoxicity. 相似文献
144.
145.
Enisa Omanovic-Miklicanin Sandro Valzacchi Catherine Simoneau Douglas Gilliland Francois Rossi 《Analytical and bioanalytical chemistry》2014,406(26):6629-6636
A complete characterization of the different physico-chemical properties of nanoparticles (NPs) is necessary for the evaluation of their impact on health and environment. Among these properties, the surface characterization of the nanomaterial is the least developed and in many cases limited to the measurement of surface composition and zetapotential. The biological surface adsorption index approach (BSAI) for characterization of surface adsorption properties of NPs has recently been introduced (Xia et al. Nat Nanotechnol 5:671–675, 2010; Xia et al. ACS Nano 5(11):9074–9081, 2011). The BSAI approach offers in principle the possibility to characterize the different interaction forces exerted between a NP's surface and an organic—and by extension biological—entity. The present work further develops the BSAI approach and optimizes a solid-phase microextraction gas chromatography–mass spectrometry (SPME/GC-MS) method which, as an outcome, gives a better-defined quantification of the adsorption properties on NPs. We investigated the various aspects of the SPME/GC-MS method, including kinetics of adsorption of probe compounds on SPME fiber, kinetic of adsorption of probe compounds on NP's surface, and optimization of NP's concentration. The optimized conditions were then tested on 33 probe compounds and on Au NPs (15 nm) and SiO2 NPs (50 nm). The procedure allowed the identification of three compounds adsorbed by silica NPs and nine compounds by Au NPs, with equilibrium times which varied between 30 min and 12 h. Adsorption coefficients of 4.66?±?0.23 and 4.44?±?0.26 were calculated for 1-methylnaphtalene and biphenyl, compared to literature values of 4.89 and 5.18, respectively. The results demonstrated that the detailed optimization of the SPME/GC-MS method under various conditions is a critical factor and a prerequisite to the application of the BSAI approach as a tool to characterize surface adsorption properties of NPs and therefore to draw any further conclusions on their potential impact on health. Graphical Abstract
146.
147.
Dr. Maria Petrosino Dr. Francesco Stellato Prof. Roberta Chiaraluce Prof. Valerio Consalvi Dr. Giovanni La Penna Dr. Alessandra Pasquo Dr. Olivier Proux Prof. Giancarlo Rossi Prof. Silvia Morante 《ChemistryOpen》2021,10(11):1133-1141
We present in this work a first X-ray Absorption Spectroscopy study of the interactions of Zn with human BST2/tetherin and SARS-CoV-2 orf7a proteins as well as with some of their complexes. The analysis of the XANES region of the measured spectra shows that Zn binds to BST2, as well as to orf7a, thus resulting in the formation of BST2-orf7a complexes. This structural information confirms the the conjecture, recently put forward by some of the present Authors, according to which the accessory orf7a (and possibly also orf8) viral protein are capable of interfering with the BST2 antiviral activity. Our explanation for this behavior is that, when BST2 gets in contact with Zn bound to the orf7a Cys15 ligand, it has the ability of displacing the metal owing to the creation of a new disulfide bridge across the two proteins. The formation of this BST2-orf7a complex destabilizes BST2 dimerization, thus impairing the antiviral activity of the latter. 相似文献
148.
Combined Experimental and Multivariate Model Approaches for Glycoalkaloid Quantification in Tomatoes
Gabriella Tamasi Alessio Pardini Riccardo Croce Marco Consumi Gemma Leone Claudia Bonechi Claudio Rossi Agnese Magnani 《Molecules (Basel, Switzerland)》2021,26(11)
The intake of tomato glycoalkaloids can exert beneficial effects on human health. For this reason, methods for a rapid quantification of these compounds are required. Most of the methods for α-tomatine and dehydrotomatine quantification are based on chromatographic techniques. However, these techniques require complex and time-consuming sample pre-treatments. In this work, HPLC-ESI-QqQ-MS/MS was used as reference method. Subsequently, multiple linear regression (MLR) and partial least squares regression (PLSR) were employed to create two calibration models for the prediction of the tomatine content from thermogravimetric (TGA) and attenuated total reflectance (ATR) infrared spectroscopy (IR) analyses. These two fast techniques were proven to be suitable and effective in alkaloid quantification (R2 = 0.998 and 0.840, respectively), achieving low errors (0.11 and 0.27%, respectively) with the reference technique. 相似文献
149.
150.
Generalizations and Properties of the Principal Eigenvalue of Elliptic Operators in Unbounded Domains
下载免费PDF全文
![点击此处可从《纯数学与应用数学通讯》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Using three different notions of the generalized principal eigenvalue of linear second‐order elliptic operators in unbounded domains, we derive necessary and sufficient conditions for the validity of the maximum principle, as well as for the existence of positive eigenfunctions for the Dirichlet problem. Relations between these principal eigenvalues, their simplicity, and several other properties are further discussed. © 2015 Wiley Periodicals, Inc. 相似文献