全文获取类型
收费全文 | 1786篇 |
免费 | 83篇 |
国内免费 | 10篇 |
专业分类
化学 | 1443篇 |
晶体学 | 23篇 |
力学 | 23篇 |
数学 | 130篇 |
物理学 | 260篇 |
出版年
2023年 | 16篇 |
2022年 | 21篇 |
2021年 | 25篇 |
2020年 | 17篇 |
2019年 | 43篇 |
2018年 | 32篇 |
2017年 | 19篇 |
2016年 | 55篇 |
2015年 | 46篇 |
2014年 | 45篇 |
2013年 | 92篇 |
2012年 | 124篇 |
2011年 | 128篇 |
2010年 | 63篇 |
2009年 | 63篇 |
2008年 | 127篇 |
2007年 | 107篇 |
2006年 | 122篇 |
2005年 | 117篇 |
2004年 | 90篇 |
2003年 | 91篇 |
2002年 | 70篇 |
2001年 | 13篇 |
2000年 | 29篇 |
1999年 | 15篇 |
1998年 | 10篇 |
1997年 | 21篇 |
1996年 | 24篇 |
1995年 | 6篇 |
1994年 | 13篇 |
1993年 | 10篇 |
1992年 | 21篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 12篇 |
1986年 | 4篇 |
1985年 | 25篇 |
1984年 | 14篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 17篇 |
1980年 | 9篇 |
1979年 | 7篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1976年 | 18篇 |
1975年 | 12篇 |
1974年 | 5篇 |
排序方式: 共有1879条查询结果,搜索用时 0 毫秒
121.
122.
123.
Polymer-protected platinum/ruthenium colloidal dispersions were prepared by refluxing mixed solutions of hexachloroplatinic(IV) acid and ruthenium(III) chloride in a mixture of ethanol/water (1/1 v/v) in the presence of poly(N-vinyl-2-pyrrolidone). The electronic spectra and transmission electron micrographs suggested that the colloidal dispersions are almost composed of the mixture of the small monometallic Pt and Ru clusters over all the ratio of Pt/Ru compositions. Extended X-ray absorption fine structure analyses and high resolution electron microprobe analyses indicated that no Pt/Ru alloy clusters exist in the dispersions, and the aggregation occurs between small monometallic Pt clusters (diameter ca. 15 A) and partially oxidized Ru microclusters (diameter less than 10 A). Electron diffraction measurements also suggested that the diffraction pattern of aggregated Pt/Ru cluster particles prepared by the simultaneous reduction of Pt and Ru ions is the same as that of the physical mixture of the small monometallic Pt and Ru clusters separately prepared. Therefore, it can be concluded that the aggregated Pt/Ru cluster particles, with 10 to 60 A in diameter, are built up by small monometallic Pt clusters and partially oxidized Ru microclusters, and that Pt/Ru alloy clusters are not formed. 相似文献
124.
5,12‐Diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxalines: Solid‐State Fluorescence,AIE Properties,and Orbital Switching by Substituent Effect 下载免费PDF全文
The compound 5,12‐diacetyl‐5,12‐dihydroquinoxalino[2,3‐b]quinoxaline 1 a and its derivatives were prepared, and their solid‐ and solution‐state spectroscopic properties were studied; 1 a shows stronger fluorescence in solution than in the solid state due to aggregation caused by self‐quenching. Phenyl‐ or alkoxy‐substituted derivatives 1 b – d show solid‐state fluorescence with moderate quantum yields of about Φ=0.12–0.15, although the corresponding values are 0.01–0.07 in solution. The spectroscopic properties of alkoxy‐substituted derivatives were hardly changed compared to 1 a and 1 b , although 1 a and 1 b have similar absorption and fluorescence maxima in solution and in the solid state. DFT calculations indicate that orbital switching occurs between HOMO and HOMO‐1 and HOMO‐2 due to orbital interactions with introduced substituents. Crystal structure analysis revealed that the molecules have bent structures around tertiary nitrogen atoms and form a characteristic dimeric structure. 相似文献
125.
Shinoda K Hasegawa T Sato H Shinozaki M Kuramoto H Takamiya Y Sato T Nikaidou N Watanabe T Hoshino T 《Chemical communications (Cambridge, England)》2007,(40):4140-4142
A biosynthetic intermediate of violacein produced by the mixed enzymes of VioABDE was elucidated to be 5-(5-hydroxy-1H-indol-3-yl)-3-(1H-indol-3-yl)-1H-pyrrole-2-carboxylic acid, named protoviolaceinic acid, indicating that VioC, responsible for the final biosynthetic step, works to oxygenate at the 2-position of the right side indole ring, and that the oxygenation reaction to form the central pyrrolidone core proceeds in a non-enzymatic fashion. 相似文献
126.
Repeated reaction between chiral quaternary ammonium dimer and disodium disulfonate gave the ionic polymer, which contains chiral quaternary ammonium sulfonate structure as a repeating unit. This chiral ionic polymer showed an excellent catalytic activity in asymmetric alkylation reaction. With the chiral ionic polymer catalysts, high level of enantioselectivities up to 97% ee were attained. 相似文献
127.
Kinugawa T Yabushita A Kawasaki M Hama T Watanabe N 《Physical chemistry chemical physics : PCCP》2011,13(35):15785-15791
Photodissociation of amorphous ice films of carbon dioxide and water co-adsorbed at 90 K was carried out at 157 nm using oxygen-16 and -18 isotopomers with a time-of-flight photofragment mass spectrometer. O((3)P(J)) atoms, OH (v = 0) radicals, and CO (v = 0,1) molecules were detected as photofragments. CO is produced directly from the photodissociation of CO(2). Two different adsorption states of CO(2), i.e., physisorbed CO(2) on the surface of amorphous solid water and trapped CO(2) in the pores of the film, are clearly distinguished by the translational and internal energy distributions of the CO molecules. The O atom and OH radical are produced from the photodissociation of H(2)O. Since the absorption cross section of CO(2) is smaller than that of H(2)O at 157 nm, the CO(2) surface abundance is relatively increased after prolonged photoirradiation of the mixed ice film, resulting in the formation of a heterogeneously layered structure in the mixed ice at low temperatures. Astrophysical implications are discussed. 相似文献
128.
We propose a color vision model that can be used to predict color appearance in mesopic vision as well as photopic and scotopic vision. It is based on a two-stage model which consists of the cone and opponent stages and it assumes rod intrusion at the opponent stage. The model has the following features to describe the color appearance in mesopic vision. First, it includes a gradual and nonlinear shift in spectral luminous efficiency from V (LD) to V (LD) to cope with the spectral sensitivity difference between photopic and scotopic vision and the nonlinearity of rod influence on the luminance channel. Second, the model assumes decrease of the chromatic component with decreasing illuminance to explain the reduction of saturation at low illuminance levels. Third, it assumes that red/green and yellow/blue components change with illuminance levels independently, thus explaining hue shifts with decreasing illuminance. We applied the model for color appearance simulation of natural scenes in a mesopic visual environment. 相似文献
129.
Mack J Bunya M Shimizu Y Uoyama H Komobuchi N Okujima T Uno H Ito S Stillman MJ Ono N Kobayashi N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(16):5001-5020
The optical spectra of a series of core-modified tetrabenzoporphyrins were analyzed to determine the effects of core modification, ligand folding, and partial benzo substitution at the ligand periphery on the electronic structure by using magnetic circular dichroism (MCD) and NMR spectroscopy, X-ray crystallography, cyclic and differential pulse voltammetry, and TD-DFT calculations. Planar 21-carba-, 21-thia-, 21,23-dithia-, and 21-oxa-23-thiatetrabenzo[b,g,l,q]porphyrins reported previously were studied together with the previously unreported 21-oxa- and 21-carba-23-thiatetrabenzo[b,g,l,q]porphyrins. The optical properties of these compounds are compared to those of tetrabenzo[b,g,l,q]-, 5,10,15,20-tetraphenyl-, 5,10,15,20-tetraphenyltetrabenzo[b,g,l,q]-21-thia-, 5,10,15,20-tetraphenyltetrabenzodithia-, 5,10,15,20-tetraphenyldibenzo[g,q]-21,23-dithia-, 5,10,15,20-tetraphenyldibenzo[b,l]-21,23-dithia-, 5,10,15,20-tetraphenyltribenzo[g,q,l]-21-thia-, and 5,10,15,20-tetraphenylbenzo[b]-21-thiaporphyrins. Michl's perimeter model and Gouterman's four-orbital model are used to conceptualize the results and to account for red shifts commonly observed in the spectral bands of nonplanar porphyrinoids. 相似文献
130.
Lusternik–Schnirelmann category of a manifold gives a lower bound of the number of critical points of a differentiable map
on it. The purpose of this paper is to show how to construct cone-decompositions of manifolds by using functions of class
C
1 and their gradient flows, where cone-decompositions are used to give an upper bound for the Lusternik–Schnirelmann category
which is a homotopy invariant of a topological space. In particular, the Morse–Bott functions on the Stiefel manifolds considered
by Frankel (1965) are effectively used to construct the conedecompositions of Stiefel manifolds and symmetric Riemannian spaces to determine
their Lusternik–Schnirelmann categories. 相似文献