全文获取类型
收费全文 | 691篇 |
免费 | 54篇 |
专业分类
化学 | 612篇 |
力学 | 1篇 |
数学 | 57篇 |
物理学 | 75篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 14篇 |
2020年 | 17篇 |
2019年 | 18篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 32篇 |
2015年 | 19篇 |
2014年 | 27篇 |
2013年 | 31篇 |
2012年 | 39篇 |
2011年 | 71篇 |
2010年 | 27篇 |
2009年 | 29篇 |
2008年 | 44篇 |
2007年 | 49篇 |
2006年 | 54篇 |
2005年 | 50篇 |
2004年 | 30篇 |
2003年 | 19篇 |
2002年 | 27篇 |
2001年 | 8篇 |
2000年 | 10篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 8篇 |
1996年 | 15篇 |
1995年 | 7篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 8篇 |
1991年 | 5篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 6篇 |
1986年 | 2篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
1960年 | 1篇 |
1891年 | 1篇 |
排序方式: 共有745条查询结果,搜索用时 62 毫秒
101.
The rotational barriers and conformational properties of the hydroxyl and mercapto groups attached to the alpha and beta positions of cyclohexa-2,5-dione and cyclohexa-2,5-dienthione have been studied at the B3LYP/ 6-311++G(d,p) level of theory. The results show that the conformational preferences of these studied systems are the result of a subtle interplay between different competing effects (conjugation, hyperconjugation, and steric repulsions). The applicability of the density functional theory reactivity indices and the maximum hardness principle for the present systems has been analyzed. 相似文献
102.
103.
Ortiz-Sanchez JM Gelabert R Moreno M Lluch JM 《The journal of physical chemistry. A》2006,110(14):4649-4656
The proton-transfer dynamics in the aromatic Schiff base salicylidene methylamine has been theoretically analyzed in the ground and first singlet (pi,pi) excited electronic states by density functional theory calculations and quantum wave-packet dynamics. The potential energies obtained through electronic calculations that use the time-dependent density functional theory formalism, which predict a barrierless excited-state intramolecular proton transfer, are fitted to a reduced three-dimensional potential energy surface. The time evolution in this surface is solved by means of the multiconfiguration time-dependent Hartree algorithm applied to solve the time-dependent Schr?dinger equation. It is shown that the excited-state proton transfer occurs within 11 fs for hydrogen and 25 fs for deuterium, so that a large kinetic isotope effect is predicted. These results are compared to those of the only previous theoretical work published on this system [Zgierski, M. Z.; Grabowska, A. J. Chem. Phys. 2000, 113, 7845], reporting a configuration interaction singles barrier of 1.6 kcal mol(-1) and time reactions of 30 and 115 fs for the hydrogen and deuterium transfers, respectively, evaluated with the semiclassical instanton approach. 相似文献
104.
105.
Bornas X Llabrés J Noguera M Pez A 《Nonlinear dynamics, psychology, and life sciences》2006,10(3):301-318
Research within the framework of the nonlinear dynamical systems (NDS) in the field of anxiety disorders has shown that greater irregularity/complexity appears in the output from healthy systems. In this study we measured the Heart rate variability (HRV) and the sample sntropy (SampEn) of the ECG mV time series of fearful flyers (N = 15) and a matched control group (N = 15) when confronted with three combinations of feared stimuli (pictures, sounds, and pictures with sounds) as well as relaxing stimuli (pictures and sounds). Fearful flyers had lower SampEn than controls in all conditions, including baseline. Non-phobics showed significant entropy decreases from baseline in two out of three exposure conditions. No differences on HRV were found between groups, and HRV was not sensitive to condition changes. The main finding of the study is that the SampEn calculated on very short ECG mV recordings (10 to 60 seconds, easy to obtain in clinical settings) may be a useful diagnostic measure since it can distinguish fearful from non-fearful flyers. 相似文献
106.
In the Euclidean Space
\mathbb Rn+1{\mathbb {R}^{n+1}} with a density
ee\frac12 n m2 |x|2, (e = ±1){e^{\varepsilon \frac12 n \mu^2 |x|^2},} {(\varepsilon =\pm1}), we consider the flow of a hypersurface driven by its mean curvature associated to this density. We give a detailed account
of the evolution of a convex hypersurface under this flow. In particular, when e = -1{ \varepsilon=-1} (Gaussian density), the hypersurface can expand to infinity or contract to a convex hypersurface (not necessarily a sphere)
depending on the relation between the bound of its principal curvatures and μ. 相似文献
107.
Density functional calculations on XYYX and X2YY isomers of the X2Y2 species (X: H, Li, Na, F, Cl, Br, I; Y: O, S, Se, Te) show that the XYYX isomers are more stable than the X2YY forms except for X = F and Y = S and Te, for which the F2SS and F2TeTe isomers are slightly more stable. 相似文献
108.
Erik Andris Rafael Navrtil Juraj Jaík Martin Srnec Mnica Rodríguez Miquel Costas Jana Roithov 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(28):9721-9726
Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)CoIII(O)]+ ( 1 ) and [(N4Py)CoIV(O)]2+ ( 2 ). Infrared photodissociation spectroscopy revealed that the Co?O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co?O bond characterized by a stretching frequency of ≤659 cm?1. Accordingly, 2 can abstract hydrogen atoms from non‐activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab‐initio calculations suggest that 2 , formally a cobalt(IV)‐oxo complex, is best described as cobalt(III)‐oxyl. Our results provide important data on changes to metal‐oxo bonding behind the oxo wall and show that cobalt‐oxo complexes are promising targets for developing highly active C?H oxidation catalysts. 相似文献
109.
110.
High‐Energy Long‐Lived Mixed Frenkel–Charge‐Transfer Excitons: From Double Stranded (AT)n to Natural DNA
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. Ignacio Vayá Dr. Johanna Brazard Dr. Miquel Huix‐Rotllant Dr. Arun K. Thazhathveetil Prof. Frederick D. Lewis Dr. Thomas Gustavsson Prof. Dr. Irene Burghardt Dr. Roberto Improta Dr. Dimitra Markovitsi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(14):4904-4914
The electronic excited states populated upon absorption of UV photons by DNA are extensively studied in relation to the UV‐induced damage to the genetic code. Here, we report a new unexpected relaxation pathway in adenine–thymine double‐stranded structures (AT)n. Fluorescence measurements on (AT)n hairpins (six and ten base pairs) and duplexes (20 and 2000 base pairs) reveal the existence of an emission band peaking at approximately 320 nm and decaying on the nanosecond time scale. Time‐dependent (TD)‐DFT calculations, performed for two base pairs and exploring various relaxation pathways, allow the assignment of this emission band to excited states resulting from mixing between Frenkel excitons and adenine‐to‐thymine charge‐transfer states. Emission from such high‐energy long‐lived mixed (HELM) states is in agreement with their fluorescence anisotropy (0.03), which is lower than that expected for π–π* states (≥0.1). An increase in the size of the system quenches π–π* fluorescence while enhancing HELM fluorescence. The latter process varies linearly with the hypochromism of the absorption spectra, both depending on the coupling between π–π* and charge‐transfer states. Subsequently, we identify the common features between the HELM states of (AT)n structures with those reported previously for alternating (GC)n: high emission energy, low fluorescence anisotropy, nanosecond lifetimes, and sensitivity to conformational disorder. These features are also detected for calf thymus DNA in which HELM states could evolve toward reactive π–π* states, giving rise to delayed fluorescence. 相似文献