全文获取类型
收费全文 | 595篇 |
免费 | 28篇 |
国内免费 | 7篇 |
专业分类
化学 | 283篇 |
力学 | 37篇 |
数学 | 213篇 |
物理学 | 97篇 |
出版年
2022年 | 2篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 14篇 |
2018年 | 12篇 |
2017年 | 12篇 |
2016年 | 17篇 |
2015年 | 21篇 |
2014年 | 31篇 |
2013年 | 44篇 |
2012年 | 47篇 |
2011年 | 52篇 |
2010年 | 29篇 |
2009年 | 33篇 |
2008年 | 23篇 |
2007年 | 44篇 |
2006年 | 40篇 |
2005年 | 29篇 |
2004年 | 41篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 4篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 11篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1932年 | 1篇 |
排序方式: 共有630条查询结果,搜索用时 15 毫秒
621.
Tamayo A Lodeiro C Escriche L Casabó J Covelo B González P 《Inorganic chemistry》2005,44(22):8105-8115
Three new fluorescent devices for protons and metal ions have been synthesized and characterized, and their photophysical properties have been explored; these are the macrocycles 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1) and 7-(10-methyl-9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L2) and the bis macrocycle 7,7'-[9,10-anthracenediylbis(methylene)]bis-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L3). All these systems have a pyridil-thioether-containing macrocycles as a binding site and an anthracene moiety as a signaling agent. The coordination properties of these ligands toward Cu(II), Co(II), Ni(II), Zn(II), and Pd(II) have been studied in solution and in the solid state. The addition of these metal ions to dichloromethane solutions of L1, L2, and L3 produce strong changes in the absorption and emission spectra of these ligands. The stoichiometry of the species, formed at 298 K, have been determined from absorption and fluorescence titrations. The Co(II) and Cu(II) complexes of L1 have been studied by EPR spectroscopy. This last complex and its free ligand have also been characterized by X-ray crystallography. 相似文献
622.
Cabezas F Ramírez A Viladomat F Codina C Bastida J 《Chemical & pharmaceutical bulletin》2003,51(3):315-317
Thirteen alkaloids have been isolated from dried bulbs and leaves of flowering Eucharis amazonica (Amaryllidaceae). The alkaloids, 7-methoxyoxoassoanine, 6-O-methylpretazettine and apohaemanthamine, are reported for the first time from a natural source. 相似文献
623.
It has recently been shown that the first passage time problem for a certain class of one-dimensional processes that includes shot noise can be formulated in terms of a set of integral equations. These are found by exact enumeration of all possible trajectories. We show that the equations can be found by more direct means for processes described by the evolution equation
, wheren(t) is time-localized shot noise. 相似文献
624.
Jaume Aceña Sandra Pérez Piero Gardinali José Luis Abad Peter Eichhorn Nubia Heuett Damià Barceló 《Journal of mass spectrometry : JMS》2014,49(12):1279-1289
In this study, four unapproved analogues of Sildenafil (SDF) were photodegraded under synthetic sunlight in artificial freshwater. Homosildenafil (H‐SDF), hydroxyhomo‐sildenafil (HH‐SDF), norneosildenafil (NR‐SDF) and thiosildenafil (T‐SDF) were selected because they are frequently detected as adulterants in natural herbal products. Using UPLC‐Orbitrap (Q Exactive)‐MS, six photoproducts common to H‐SDF, HH‐SDF and T‐SDF and nine unique transformation products of different molecular weights were identified based on their high‐resolution (+)ESI product ion spectra. Mass spectral analysis of deuterated H‐SDF, labeled on the N‐ethyl group, allowed to gain mechanistic insight into the fragmentation pathway of the substituted piperazine ring and to support the postulated photoproduct structures. The mass spectral fragmentation confirmed the stepwise destruction of the piperazine ring eventually producing a sulfonic acid derivative (C17H20N4O5S: 392.1151 Da). In contrast, the photodegradation of NR‐SDF, which lacks a piperazine ring in its structure, formed only two prominent photoproducts originating from N,N‐dealkylation of the sulfonamide followed by hydrolysis. The current work constitutes the first study on the photodegradation of analogs of erectile dysfunction drugs and the first detection of two transformation products (m/z 449 and 489) in environmental samples. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
625.
Paula Mayorga Burrezo Wangdong Zeng Michael Moos Marco Holzapfel Sofia Canola Fabrizia Negri Concepci Rovira Jaume Veciana Hoa Phan Jishan Wu Christoph Lambert Juan Casado 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(41):14609-14613
A complete experimental and theoretical study has been carried out for aromatic and quinoidal perylene‐based bridges substituted with bis(diarylamine) and bis(arylimine) groups respectively. The through‐bridge inter‐redox site electronic couplings (VAB) have been calculated for their respective mixed‐valence radical cation and radical anion species. The unusual similitudes of the resulting VAB values for the given structures reveal the intervention of molecular shapes with balanced semi‐quinoidal/semi‐aromatic structures in the charge delocalization. An identical molecular object equally responding to the injection of either positive or negative charges is rare in the field of organic π‐conjugated molecules. However, once probed herein for perylene‐based systems, it can be extrapolated to other π‐conjugated bridges. As a result, this work opens the door to the rational design of true ambipolar bulk and molecular conductors. 相似文献
626.
For a dynamical system described by a set of autonomous differential equations, an attractor can be either a point, or a periodic orbit, or even a strange attractor. Recently a new chaotic system with only one parameter has been presented where besides a point attractor and a chaotic attractor, it also has a coexisting attractor limit cycle which makes evident the complexity of such a system. We study using analytic tools the dynamics of such system. We describe its global dynamics near the infinity, and prove that it has no Darboux first integrals. 相似文献
627.
Ryszard Kutner Jaume Masoliver 《The European Physical Journal B - Condensed Matter and Complex Systems》2017,90(3):50
In this article we demonstrate the very inspiring role of the continuous-time random walk (CTRW) formalism, the numerous modifications permitted by its flexibility, its various applications, and the promising perspectives in the various fields of knowledge. A short review of significant achievements and possibilities is given. However, this review is still far from completeness. We focused on a pivotal role of CTRWs mainly in anomalous stochastic processes discovered in physics and beyond. This article plays the role of an extended announcement of the Eur. Phys. J. B Special Issue [http://epjb.epj.org/open-calls-for-papers/123-epj-b/1090-ctrw-50-years-on] containing articles which show incredible possibilities of the CTRWs. 相似文献
628.
We study the number of invariant straight lines through the origin of the homogeneous polynomial differential systems of degree
m in
\mathbb Rd{{\mathbb R}^d} or
\mathbb Cd{{\mathbb C}^d} , when this number is finite. This notion extends in the natural way the classical notion of eigenvectors of homogeneous
linear differential systems to homogeneous polynomial differential systems. This number provides un upper bound for the number
of infinite singular points of the polynomial differential systems of degree m in
\mathbb Rd{{\mathbb R}^d} . This upper bound is reached if all the invariant straight lines through the origin are real. 相似文献
629.
Nonlinear Dynamics - For a $$C^{m+1}$$ differential system on $$\mathbb {R}^n$$ , we study the limit cycles that can bifurcate from a zero–Hopf singularity, i.e., from a singularity with... 相似文献
630.
We analyse the existence of symmetric periodic orbits of the two-electron atom. The results obtained show that there exist six families of periodic orbits that can be prolonged from a continuum of periodic symmetric orbits. The main technique applied in this study is the continuation method of Poincaré. 相似文献