全文获取类型
收费全文 | 1757篇 |
免费 | 42篇 |
国内免费 | 6篇 |
专业分类
化学 | 1253篇 |
晶体学 | 22篇 |
力学 | 27篇 |
数学 | 77篇 |
物理学 | 426篇 |
出版年
2023年 | 9篇 |
2022年 | 12篇 |
2021年 | 22篇 |
2020年 | 21篇 |
2019年 | 22篇 |
2018年 | 18篇 |
2017年 | 16篇 |
2016年 | 22篇 |
2015年 | 29篇 |
2014年 | 29篇 |
2013年 | 80篇 |
2012年 | 77篇 |
2011年 | 89篇 |
2010年 | 60篇 |
2009年 | 47篇 |
2008年 | 98篇 |
2007年 | 91篇 |
2006年 | 91篇 |
2005年 | 104篇 |
2004年 | 76篇 |
2003年 | 64篇 |
2002年 | 72篇 |
2001年 | 52篇 |
2000年 | 44篇 |
1999年 | 34篇 |
1998年 | 28篇 |
1997年 | 19篇 |
1996年 | 27篇 |
1995年 | 19篇 |
1994年 | 21篇 |
1993年 | 26篇 |
1992年 | 31篇 |
1991年 | 26篇 |
1990年 | 21篇 |
1989年 | 33篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1979年 | 18篇 |
1978年 | 20篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 11篇 |
1974年 | 12篇 |
1973年 | 9篇 |
排序方式: 共有1805条查询结果,搜索用时 7 毫秒
81.
Yoshikawa M Morikawa T Murakami T Toguchida I Harima S Matsuda H 《Chemical & pharmaceutical bulletin》1999,47(3):340-345
The methanolic extract from the flowers of Chrysanthemum indicum L., Chrysanthemi Indici Flos, was found to show inhibitory activity against rat lens aldose reductase. By bioassay-guided separation, the active components, such as flavone and flavone glycosides, were isolated from the extract together with three new eudesmane-type sesquiterpenes, kikkanols A, B, and C. The structures of kikkanols A, B, and C were elucidated on the basis of chemical and physicochemical evidence, which included application of the modified Mosher's method. 相似文献
82.
Atsushi Izumi Masahiro Teraguchi Ryoji Nomura Toshio Masuda 《Journal of polymer science. Part A, Polymer chemistry》2000,38(7):1057-1063
Poly(phenylenevinylene)‐based conjugated polymers with azobenzene groups in the main chains were prepared by the Pd‐catalyzed coupling polymerization of divinylarenes with dihaloarenes. The Pd‐catalyzed coupling polymerization of 4,4′‐divinylazobenzene with dihaloarenes such as 1,3‐dibromobenzene, 1,4‐dibromo‐2,5‐dihexylbenzene, 4,4′‐dibromoazobenzene, and 4,4′‐diiodoazobenzene resulted in polymers with poor solubility. In contrast, soluble polymers containing azobenzene moieties in the main chains were attainable from divinylbenzenes with 4,4′‐dihaloazobenzenes if either or both of the monomers possessed hexyl groups on the aromatic rings. The number‐average molecular weight of the polymer exceeded 10,000 under optimized conditions, and the polymer showed a remarkably redshifted absorption in the visible region (456 nm). 1H NMR and IR spectra supported that the polymers having only trans‐geometry for the double bonds. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1057–1063, 2000 相似文献
83.
A highly selective and sensitive method of fluorometry is described for determination of the fluoride ion at the parts per billion level via the ion-pair complex formation of the fluoride ion with an expanded prophyrin [2,23-diethyl-8,17-bis(2-ethoxycarbonylethyl)-3,7,12,13,18,22-hexamethylsapphyrin (H3sap)]. The ion-pair complex gives out an enhanced fluorescence intensity at 680 nm on excitation at 450 nm. Since the present method is based on a direct reaction of the fluoride ion with the sappyrin, a 200-fold amount of the aluminum (III) ion [10−4M (M = mol dm−3)] and a 2000-fold amount of the iron(III) ion (10−3M) over the fluoride ion did not interfere with determination of the fluoride ion at concentrations as low as 5 × 10−7M in the presence of 1,2-diaminocyclohexane-N,N,N′,N′-teraacetic acid. The proposed method was applied to determination of the fluoride ion in various water samples (tap water, river water, rain water, underground water, and hot spring water) and satisfactory results were obtained. 相似文献
84.
The extraction behaviour of copper(II) and silver(I) with 2-(3,6,10,13-tetrathiacyclotetradec-1-oxy)hexanoic acid (TTCTOHA) was investigated at 25+/-0.1 degrees C and ionic strength of 0.1. The value of the logarithmic distribution coefficient, logK(DR) of TTCTOHA between octan-1-ol and aqueous phases was determined to be 4.13. Copper(II) was extracted with TTCTOHA into octan-1-ol as CuL(2), where L represents the anionic species of TTCTOHA. The logarithmic extraction constant, logK(ex(10)), was determined at -7.42. Silver(I) was extracted with TTCTOHA into octan-1-ol as AgL and Ag(2)L(2). The logarithmic distribution constant, logK(DC), of AgL was estimated to be 0.49. On the other hand, silver(I) was extracted into 1,2-dichloroethane as AgL and the logarithmic extraction constant, logK(ex(10)), was determined to be -2.24. 相似文献
85.
Shunji Aoki Kouichi Higuchi Ai Kato Nobutoshi Murakami Motomasa Kobayashi 《Tetrahedron》1999,55(52):621-14870
Novel mono-O-alkyl-diglycosylglycerols named myrmekiosides A (1) and B (2) were isolated from a marine sponge of Myrmekioderma sp. and their absolute stereostructures were elucidated on the basis of chemical and physicochemical evidence. Myrmekiosides A (1) and B (2) reversed the phenotype of melanoma H-ras transformed NIH3T3 cells at 5 μg/ml. 相似文献
86.
The total synthesis of hedychenone 1 is described. The cycloaddition of the hindered diene 2 and the allenecarboxylate 3 has been shown conclusively to proceed via the [2+2] cycloadduct 5 to give a 2:1 mixture of the desired formal Diels-Alder adducts, the exo and endo isomers 4xn and is thus a stepwise [4+2] cycloaddition. The exo isomer 4x was converted in four steps (reduction, oxidation, olefination, and desilylation) into hedychenone 1. [reaction: see text] 相似文献
87.
A series of nitrogen‐tethered allenynes (‘5‐aza‐1,2‐dien‐7‐ynes’) 1 were transformed to the corresponding 3‐acyl‐4‐alkenylpyrrolidines 3 when treated with a catalytic amount of PtCl2 in MeOH at 70°. Initial Pt‐promoted cyclization forms a nonclassical carbocationic intermediate. In contrast to the cycloisomerization in toluene, which produced the bicyclic cyclobutenes 2 , the intermediate is intercepted by addition of an oxygen nucleophile to achieve the formal hydrative cyclization. 相似文献
88.
Izumi T Naitou Y Shimbo Y Takanishi Y Takezoe H Watanabe J 《The journal of physical chemistry. B》2006,110(47):23911-23919
The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, alpha,omega-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentane (mOAM5AMOm), by optical microscopy, X-ray diffraction, polarization switching, and second-harmonic generation measurements. One compound is 4OAM5AMO4 with a short terminal alkyl chain that forms a single-layer smectic phase (SmCAs) with a random mixing of spacer and tail groups. Another compound is 16OAM5AMO16 with a long terminal alkyl chain that forms a chiral, anticlinic, and antiferroelectric bilayer phase (SmCAb) with the bent molecules tilted to the bilayer. By mixing these two compounds, the SmCAs phase of 4OAM5AMO4 is easily destabilized, leading to the wide content region of the bilayer phases. In the bilayer regime, three other smectic phases are newly induced. Two of them are antiferroelectric and ferroelectric phases in which the molecules lie perpendicularly with respect to the layer. The other shows no polar response to an external electric field and behaves like a smectic A. The new appearance of these bilayer phases is discussed as a mixing effect of long and short tail groups. 相似文献
89.
Pumera M Smíd B Peng X Golberg D Tang J Ichinose I 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(27):7644-7649
A novel protocol for precisely coating individual multiwall carbon nanotubes (MWCNTs) with an ultrathin layer of polypyrrole was developed. The nanocoated MWCNTs were successfully prepared by in situ chemical deposition of polypyrrole in an aqueous suspension of MWCNTs. The coating layer was very uniform and the thickness of the layer was determined by controlling the monomer concentration used, which gave nanometer precision. The products were characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and conductivity and current-voltage measurements. The ultrathin polypyrrole layer could electrically insulate individual MWCNTs. 相似文献
90.
The equilibrium geometry of disilyne is not linear, but is twisted. The potential surfaces of acetylene and disilyne have a critical internuclear distance between the central atoms, where the stable geometry changes from linear to twisted forms the R-dependence of the valence-shell electron energy causes the difference in the structure of the molecules. 相似文献