全文获取类型
收费全文 | 1434篇 |
免费 | 59篇 |
国内免费 | 9篇 |
专业分类
化学 | 956篇 |
晶体学 | 7篇 |
力学 | 22篇 |
数学 | 212篇 |
物理学 | 305篇 |
出版年
2023年 | 19篇 |
2022年 | 22篇 |
2021年 | 41篇 |
2020年 | 30篇 |
2019年 | 30篇 |
2018年 | 27篇 |
2017年 | 33篇 |
2016年 | 53篇 |
2015年 | 40篇 |
2014年 | 49篇 |
2013年 | 81篇 |
2012年 | 85篇 |
2011年 | 105篇 |
2010年 | 69篇 |
2009年 | 88篇 |
2008年 | 95篇 |
2007年 | 93篇 |
2006年 | 90篇 |
2005年 | 86篇 |
2004年 | 45篇 |
2003年 | 48篇 |
2002年 | 53篇 |
2001年 | 18篇 |
2000年 | 25篇 |
1999年 | 15篇 |
1998年 | 14篇 |
1997年 | 15篇 |
1996年 | 20篇 |
1995年 | 15篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 10篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有1502条查询结果,搜索用时 46 毫秒
41.
42.
The main object of this paper is to study an extension of the half normal distribution defined by adding a positive truncation to it. The new model is more flexible than the half-normal distribution and contains the half normal distribution as a special case. Properties of this distribution, such as moments, hazard function and entropy are studied and parameters estimation is dealt with by using moments and maximum likelihood. A real data application indicates good fit performance of the new model when compared to other competitors in literatures. 相似文献
43.
44.
45.
46.
Effect of methoxyl groups on the NMR spectra: configuration and conformation of natural and synthetic indanic and tetralinic structures
下载免费PDF全文
![点击此处可从《Magnetic resonance in chemistry : MRC》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Beatriz Lantaño José M. Aguirre Eleonora V. Drago Diego J. de la Faba Nicolás Pomilio Jorge D. Mufato 《Magnetic resonance in chemistry : MRC》2017,55(7):619-633
Here, we studied the influence of the methoxyl groups attached at C‐7 and C‐2′ of natural and synthetic 1‐arylindanes on the chemical shift of the signal of bibenzylic hydrogen and carbon atoms and J1,2 coupling constants. This influence was also analysed in natural 1‐aryltetralins and related compounds that possess methoxyl and/or hydroxyl groups bound at C‐8 and C‐2′. The methoxyl groups attached at C‐7 in indanes or at C‐8 in tetralins produce a deshielding signal at H‐1 and shield at C‐1 and a strong decrease in the value of J1,2 due to the pseudoequatorial location adopted by the aryl group bound at C‐1, avoiding an ‘A1,3 strain’. Furthermore, compounds with hydroxyl or methoxyl groups in C‐2′, in the absence of substituents of C‐7 or C‐8, present a strong deshielding signal at H‐1, strong shield of the C‐1 signal and a decrease in the value of J1,2. This is attributed to the stereoelectronic effects of the methoxyl or hydroxyl groups, which we have called ‘Asarone effect’. NOESY experiments were conducted to confirm the configuration and conformation of some of the compounds included in this work. This study shows that both effects, A1,3 strain and Asarone effect, must be taken into account when the structure of natural indanes and tetralins is analysed by using 1H‐NMR and 13C‐NMR spectra. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
47.
Dr. Damián Padín Prof. Jesús A. Varela Prof. Carlos Saá 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(33):7470-7478
Ruthenium vinyl carbenes derived from Cp/Cp*RuCl-based complexes (Cp=cyclopentadiene, Cp*=1,2,3,4,5-pentamethylcyclopentadiene) have been routinely invoked as key intermediates in tandem reactions involving a carbene/alkyne metathesis (CAM). A priori, these intermediates resemble the Grubbs-type family of catalysts, but they exhibit a completely different reactivity pattern that few, if any, other catalytic system can reproduce so far. The reactivity of these species with α-unsubstituted and α-substituted alkynals showcases the peculiarities of these intermediates. Although Z-vinyl dihydrooxazines are preferentially obtained with the former, Z-vinyl epoxypyrrolidines are obtained with the latter. A combination of spectroscopic and computational data now prove that a η3-coordination mode of the ruthenium vinyl carbene and the presence of a Lewis basic chloride ligand give rise to two markedly different stereoelectronic faces, which are responsible for the unconventional reactivity of these species. 相似文献
48.
Adiran deAguirre Ignacio Funes‐Ardoiz Feliu Maseras 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(12):3938-3942
The computational characterization of the full catalytic cycle for the synthesis of indoline from the reaction between iodoacetanilide and a terminal alkene catalyzed by a nickel complex and a photoactive ruthenium species is presented. A variety of oxidation states of nickel, Ni0, NiI, NiII, and NiIII, is shown to participate in the mechanism. Ni0 is necessary for the oxidative addition of the C?I bond, which goes through a NiI intermediate and results in a NiII species. The NiII species inserts into the alkene, but does not undergo the reductive elimination necessary for C?N bond formation. This oxidatively induced reductive elimination can be accomplished only after oxidation to NiIII by the photoactive ruthenium species. All the reaction steps are computationally characterized, and the barriers for the single‐electron transfer steps calculated using a modified version of the Marcus Theory. 相似文献
49.
São João Nepomuceno (SJN) is an iron meteorite belonging to IVA chemical group. It consists of Fe-Ni metal matrix and silicate inclusions made of a coarse-grained mixture of tridymite, orthopyroxene and clinopyroxene. In spite of the extensive work performed on the IVA group there is still no consensus about their origin and its thermal history is subject of ongoing debates. In this work, we report preliminary results on Fe2?+? distribution in the non-equivalent octahedral sites in orthopyroxene crystals extracted from São João Nepomuceno in order to infer on the thermal history of this meteorite and therefore acquire more information related to the origin of the IVA iron meteorite group. 相似文献
50.
Synchronizing Substrate Activation Rates in Multicomponent Reactions with Metal–Organic Framework Catalysts
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)
Dr. Lina María Aguirre‐Díaz Dr. Marta Iglesias Dr. Natalia Snejko Prof. Enrique Gutiérrez‐Puebla Prof. M. Ángeles Monge 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(19):6654-6665
A study on the influence of the cation coordination number, number of Lewis acid centers, concurrent existence of Lewis base sites, and structure topology on the catalytic activity of six new indium MOFs, has been carried out for multicomponent reactions (MCRs). The new indium polymeric frameworks, namely [In8(OH)6(popha)6(H2O)4]?3 H2O ( InPF‐16 ), [In(popha)(2,2′‐bipy)]?3 H2O ( InPF‐17 ), [In3(OH)3(popha)2(4,4′‐bipy)]?4 H2O ( InPF‐18 ), [In2(popha)2(4,4′‐bipy)2]?3 H2O ( InPF‐19 ), [In(OH)(Hpopha)]?0.5 (1,7‐phen) ( InPF‐20 ), and [In(popha)(1,10‐phen)]?4 H2O ( InPF‐21 ) (InPF=indium polymeric framework, H3popha=5‐(4‐carboxy‐2‐nitrophenoxy)isophthalic acid, phen=phenanthroline, bipy=bipyridine), have been hydrothermally obtained by using both conventional heating (CH) and microwave (MW) procedures. These indium frameworks show efficient Lewis acid behavior for the solvent‐free cyanosilylation of carbonyl compounds, the one pot Passerini 3‐component (P‐3CR) and the Ugi 4‐component (U‐4CR) reactions. In addition, InPF‐17 was found to be a highly reactive, recyclable, and environmentally benign catalyst, which allows the efficient synthesis of α‐aminoacyl amides. The relationship between the Lewis base/acid active site and the catalytic performance is explained by the 2D seven‐coordinated indium framework of the catalyst InPF‐17 . This study is an attempt to highlight the main structural and synthetic factors that have to be taken into account when planning a new, effective MOF‐based heterogeneous catalyst for multicomponent reactions. 相似文献