全文获取类型
收费全文 | 1463篇 |
免费 | 206篇 |
国内免费 | 99篇 |
专业分类
化学 | 898篇 |
晶体学 | 65篇 |
力学 | 142篇 |
综合类 | 42篇 |
数学 | 146篇 |
物理学 | 475篇 |
出版年
2024年 | 2篇 |
2023年 | 11篇 |
2022年 | 26篇 |
2021年 | 59篇 |
2020年 | 44篇 |
2019年 | 27篇 |
2018年 | 41篇 |
2017年 | 86篇 |
2016年 | 102篇 |
2015年 | 111篇 |
2014年 | 114篇 |
2013年 | 132篇 |
2012年 | 130篇 |
2011年 | 75篇 |
2010年 | 59篇 |
2009年 | 75篇 |
2008年 | 88篇 |
2007年 | 91篇 |
2006年 | 62篇 |
2005年 | 84篇 |
2004年 | 58篇 |
2003年 | 52篇 |
2002年 | 36篇 |
2001年 | 30篇 |
2000年 | 27篇 |
1999年 | 22篇 |
1998年 | 14篇 |
1997年 | 21篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 9篇 |
1990年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1768条查询结果,搜索用时 15 毫秒
991.
Driving Helical Packing of a Cyanine Dye on Dendron Nanofiber: Gel‐Shrinkage‐Triggered Chiral H‐Aggregation and Enhanced Enantiodiscrimination 下载免费PDF全文
Long Qin Fan Xie Xue Jin Prof. Minghua Liu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(32):11300-11305
An intelligent molecular hydrogel with a volume phase transition was constructed to regulate the chiral packing of a well‐known cyanine dye on a dynamically self‐assembled chiral nanofiber by using a pH trigger. During the shrinkage of the gel, the chiral nanofiber hierarchically assembled into a superhelix and simultaneously drove the dye molecules to stack, from a predominantly monomer form, in an unexpected helical H‐aggregation manner. Through such a transformation, the supramolecular chirality of the system was significantly enhanced and a new property of visual discrimination for chiral amines emerged. 相似文献
992.
Rebek Imide Platforms as Model Systems for the Investigation of Weak Intermolecular Interactions 下载免费PDF全文
Dr. Michael Harder Marjorie A. Carnero Corrales Dr. Nils Trapp Dr. Bernd Kuhn Prof. Dr. François Diederich 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(23):8455-8463
A Rebek imide receptor with an acetylene‐linked phenyl ring complexes 2,6‐di(isobutyramido)pyridine in (CDCl2)2 via triple H‐bonding and π–π‐stacking interactions, and the influence of para‐substituents on both rings was investigated by 1H NMR binding titrations. When the phenyl ring was extended to biphenyl and the C(4)‐pyridine substituent varied, interaction energies increased in the order CH3CH2???phenyl<CH3S???phenyl<phenyl???phenyl?N‐methylcarboxamide???phenyl, highlighting the energetic gain from π stacking on amide fragments. The predicted preference of amide–π stacking for an antiparallel alignment of the local dipoles could not be confirmed with the studied system. Different substituents were introduced in the para position of the phenyl ring and their interaction with bound 2,6‐di(isobutyramido)pyridine was investigated. Theoretical predictions that the mere introduction of a substituent has a stabilizing effect on π–π stacking, regardless of its electronic nature, were experimentally confirmed. 相似文献
994.
Lina M. Acosta Jorge Jurado Alirio Palma Justo Cobo Christopher Glidewell 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(12):1062-1068
Dibenz[b,f]azepine (DBA) is a privileged 6‐7‐6 tricyclic ring system of importance in both organic and medicinal chemistry. Benzo[b]pyrimido[5,4‐f]azepines (BPAs), which also contain a privileged 6‐7‐6 ring system, are less well investigated, probably because of a lack of straightforward and versatile methods for their synthesis. A simple and versatile synthetic approach to BPAs based on intramolecular Friedel–Crafts alkylation has been developed. A group of closely‐related benzo[b]pyrimido[5,4‐f]azepine derivatives, namely (6RS)‐4‐chloro‐6,11‐dimethyl‐6,11‐dihydro‐5H‐benzo[b]pyrimido[5,4‐f]azepine, C14H14ClN3, (I), (6RS)‐4‐chloro‐8‐hydroxy‐6,11‐dimethyl‐6,11‐dihydro‐5H‐benzo[b]pyrimido[5,4‐f]azepine, C14H14ClN3O, (II), (6RS)‐4‐<!?tlsb=‐0.14pt>chloro‐8‐methoxy‐6,11‐dimethyl‐6,11‐dihydro‐5H‐benzo[b]pyrimido[5,4‐f]azepine, C15H16ClN3O, (III), and (6RS)‐4‐chloro‐8‐methoxy‐6,11‐dimethyl‐2‐phenyl‐6,11‐dihydro‐5H‐benzo[b]pyrimido[5,4‐f]azepine, C21H20ClN3O, (IV), has been prepared and their structures compared with the recently published structure [Acosta‐Quintero et al. (2015). Eur. J. Org. Chem. pp. 5360–5369] of (6RS)‐4‐chloro‐2,6,8,11‐tetramethyl‐6,11‐dihydro‐5H‐benzo[b]pyrimido[5,4‐f]azepine, (V). All five compounds crystallize as racemic mixtures and they have very similar molecular conformations, with the azepine ring adopting a boat‐type conformation in each case, although the orientation of the methoxy substituent in each of (III) and (IV) is different. The supramolecular assemblies in (II) and (IV) depend upon hydrogen bonds of the O—H...N and C—H...π(arene) types, respectively, those in (I) and (V) depend upon π–π stacking interactions involving pairs of pyrimidine rings, and that in (III) depends upon a π–π stacking interaction involving pairs of phenyl rings. Short C—Cl...π(pyrimidine) contacts are present in (I), (II) and (IV) but not in (III) or (V). 相似文献
995.
William Clegg Ross W. Harrington Kazem Barati Mohammad Hossein Habibi Morteza Montazerozohori Arash Lalegani 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(7):578-583
Reaction of copper(I) thiocyanate and triphenylphosphane with the bidentate Schiff base N,N′‐bis(trans‐2‐nitrocinnamaldehyde)ethylenediamine {Nca2en, (1); systematic name (1E,1′E,2E,2′E)‐N,N′‐(ethane‐1,2‐diyl)bis[3‐(2‐nitrophenyl)prop‐2‐en‐1‐imine]}, C20H18N4O4, in a 1:1:1 molar ratio in acetonitrile resulted in the formation of the complex {(1E,1′E,2E,2′E)‐N,N′‐(ethane‐1,2‐diyl)bis[3‐(2‐nitrophenyl)prop‐2‐en‐1‐imine]‐κ2N,N′}(thiocyanato‐κN)(triphenylphosphane‐κP)copper(I)], [Cu(NCS)(C20H18N4O4)(C18H15P)] or [Cu(NCS)(Nca2en)(PPh3)], (2). The Schiff base and copper(I) complex have been characterized by elemental analyses, IR, electronic and 1H NMR spectroscopy, and X‐ray crystallography [from synchrotron data for (1)]. The molecule of (1) lies on a crystallographic inversion centre, with a trans conformation for the ethylenediamine unit, and displays significant twists from coplanarity of its nitro group, aromatic ring, conjugated chain and especially ethylenediamine segments. It acts as a bidentate ligand coordinating via the imine N atoms to the CuI atom in complex (2), in which the ethylenediamine unit necessarily adopts a somewhat flattened gauche conformation, resulting in a rather bowed shape overall for the ligand. The NCS− ligand is coordinated through its N atom. The geometry around the CuI atom is distorted tetrahedral, with a small N—Cu—N bite angle of 81.56 (12)° and an enlarged opposite angle of 117.29 (9)° for SCN—Cu—P. Comparisons are made with the analogous Schiff base having no nitro substituents and with metal complexes of both ligands. 相似文献
996.
Magnet Design by Integration of Layer and Chain Magnetic Systems in a π‐Stacked Pillared Layer Framework 下载免费PDF全文
Hiroki Fukunaga Prof. Dr. Hitoshi Miyasaka 《Angewandte Chemie (International ed. in English)》2015,54(2):569-573
The control of inter‐lattice magnetic interactions is a crucial issue when long‐range ordered magnets that are based on low‐dimensional magnetic frameworks are designed. A “pillared layer framework (PLF)” model could be an efficient system for this purpose. In this report, A magnet based on a π‐stacked PLF with a phase transition temperature of 82 K, which can be increased to 107 K by applying a pressure of 12.5 kbar, is rationally constructed. Two types of low‐dimensional magnetic framework systems, an electron donor/acceptor magnetic layer and a charge transfer [FeCp*2]+TCNQ.? columnar magnet ([FeCp*2]+=decamethylferrocenium; TCNQ=7,7,8,8‐tetracyano‐p‐quinodimethane), are integrated to fabricate the magnet. This synthetic strategy employing a combination of layers and chains is widely useful not only for magnet design, but also for the creation of multifunctional materials with pores and anisotropic frameworks. 相似文献
997.
Multichannel Conductance of Folded Single‐Molecule Wires Aided by Through‐Space Conjugation 下载免费PDF全文
Long Chen Ya‐Hao Wang Bairong He Han Nie Dr. Rongrong Hu Prof. Fei Huang Prof. Anjun Qin Dr. Xiao‐Shun Zhou Prof. Zujin Zhao Prof. Ben Zhong Tang 《Angewandte Chemie (International ed. in English)》2015,54(14):4231-4235
Deciphering charge transport through multichannel pathways in single‐molecule junctions is of high importance to construct nanoscale electronic devices and deepen insight into biological redox processes. Herein, we report two tailor‐made folded single‐molecule wires featuring intramolecular π–π stacking interactions. The scanning tunneling microscope (STM) based break‐junction technique and theoretical calculations show that through‐bond and through‐space conjugations are integrated into one single‐molecule wire, allowing for two simultaneous conducting channels in a single‐molecule junction. These folded molecules with stable π–π stacking interaction offer conceptual advances in single‐molecule multichannel conductance, and are perfect models for conductance studies in biological systems, organic thin films, and π‐stacked columnar aggregates. 相似文献
998.
Salah Daoud Abdelhakim Latreche Abdelhalim Bencheikh 《Crystal Research and Technology》2016,51(1):115-116
In the present work, we have readjusted some empirical parameters obtained by Kumar et al. in their work which contains some numerical errors. 相似文献
999.
《Journal of computational chemistry》2017,38(10):730-739
Noncovalent interactions involving aromatic rings, such as π···π stacking, CH···π are very essential for supramolecular carbon nanostructures. Graphite is a typical homogenous carbon matter based on π···π stacking of graphene sheets. Even in systems not involving aromatic groups, the stability of diamondoid dimer and layer‐layer graphane dimer originates from C − H···H − C noncovalent interaction. In this article, the structures and properties of novel heterogeneous layer‐layer carbon‐nanostructures involving π···H‐C‐C‐H···π···H‐C‐C‐H stacking based on [n ]‐graphane and [n ]‐graphene and their derivatives are theoretically investigated for n = 16–54 using dispersion corrected density functional theory B3LYP‐D3 method. Energy decomposition analysis shows that dispersion interaction is the most important for the stabilization of both double‐ and multi‐layer‐layer [n ]‐graphane@graphene. Binding energy between graphane and graphene sheets shows that there is a distinct additive nature of CH···π interaction. For comparison and simplicity, the concept of H‐H bond energy equivalent number of carbon atoms (noted as NHEQ), is used to describe the strength of these noncovalent interactions. The NHEQ of the graphene dimers, graphane dimers, and double‐layered graphane@graphene are 103, 143, and 110, indicating that the strength of C‐H···π interaction is close to that of π···π and much stronger than that of C‐H···H‐C in large size systems. Additionally, frontier molecular orbital, electron density difference and visualized noncovalent interaction regions are discussed for deeply understanding the nature of the C‐H···π stacking interaction in construction of heterogeneous layer‐layer graphane@graphene structures. We hope that the present study would be helpful for creations of new functional supramolecular materials based on graphane and graphene carbon nano‐structures. © 2017 Wiley Periodicals, Inc. 相似文献
1000.
It is a difficult problem in general to decide whether a Cayley graph Cay(G; S) is connected where G is an arbitrary finite group and S a subset of G. For example, testing primitivity of an element in a finite field is a special case of this problem but notoriously hard.
In this paper, it is shown that if a Cayley graph Cay(G; S) is known to be connected then its fault tolerance can be determined in polynomial time in |S|log(|G|). This is accomplished by establishing a new structural result for Cayley graphs. This result also yields a simple proof
of optimal fault tolerance for an infinite class of Cayley graphs, namely exchange graphs. We also use the proof technique
for our structural result to give a new proof of a known result on quasiminimal graphs.
Received March 10, 2006 相似文献