全文获取类型
收费全文 | 3643篇 |
免费 | 60篇 |
国内免费 | 11篇 |
专业分类
化学 | 2771篇 |
晶体学 | 80篇 |
力学 | 53篇 |
数学 | 160篇 |
物理学 | 650篇 |
出版年
2021年 | 27篇 |
2020年 | 30篇 |
2019年 | 39篇 |
2018年 | 31篇 |
2017年 | 14篇 |
2016年 | 48篇 |
2015年 | 64篇 |
2014年 | 55篇 |
2013年 | 180篇 |
2012年 | 134篇 |
2011年 | 167篇 |
2010年 | 93篇 |
2009年 | 105篇 |
2008年 | 166篇 |
2007年 | 197篇 |
2006年 | 198篇 |
2005年 | 180篇 |
2004年 | 179篇 |
2003年 | 165篇 |
2002年 | 159篇 |
2001年 | 118篇 |
2000年 | 89篇 |
1999年 | 53篇 |
1998年 | 38篇 |
1997年 | 36篇 |
1996年 | 50篇 |
1995年 | 50篇 |
1994年 | 48篇 |
1993年 | 42篇 |
1992年 | 83篇 |
1991年 | 50篇 |
1990年 | 42篇 |
1989年 | 42篇 |
1988年 | 41篇 |
1987年 | 45篇 |
1986年 | 52篇 |
1985年 | 48篇 |
1984年 | 69篇 |
1983年 | 28篇 |
1982年 | 37篇 |
1981年 | 36篇 |
1980年 | 43篇 |
1979年 | 54篇 |
1978年 | 42篇 |
1977年 | 27篇 |
1976年 | 32篇 |
1975年 | 31篇 |
1974年 | 31篇 |
1973年 | 17篇 |
1972年 | 15篇 |
排序方式: 共有3714条查询结果,搜索用时 31 毫秒
61.
Highly reactive zinc was readily prepared by electrolysis of a DMF solution containing pyrene as a mediator with a platinum cathode and a zinc anode. Preferential reduction of pyrene occurred to generate the corresponding radical anion, which reduced zinc ions generated from anodic dissolution to give zero valent zinc with high reactivity. The reactive zinc was successfully used for an efficient transformation of bromoalkanes into the corresponding organozinc bromides. Organozinc bromides obtained were further used successfully in Pd-catalyzed cross-coupling reaction with various aryl iodides and bromides. 相似文献
62.
A sensitive assay for acyl CoA-cholesterol acyltransferase (EC 2.3.1.26) in rat liver microsomal and mitochondrial preparations is described. The lowered cholesterol concentration in the enzyme reaction with oleoyl CoA is determined spectrofluorimetrically by using the cholesterol oxidase/peroxidase/p-hydroxyphenylpropionic acid system. The assay requires as little as 20 μg of protein in the enzyme preparation. 相似文献
63.
Saito H Sivaguru J Jockusch S Inoue Y Adam W Turro NJ 《Chemical communications (Cambridge, England)》2005,(27):3424-3426
Oxazolidinone-functionalized enecarbamates undergo diastereoselective E/Z photoisomerization upon direct and triplet sensitized irradiations with chiral/achiral sensitizers, showing that the enhanced product diastereoselectivity depends on the solvent and temperature. 相似文献
64.
Since the first heavy alkene analogues of germanium and tin were isolated in 1976, followed by West''s disilene in 1981, the chemistry of stable group 14 dimetallenes and dimetallynes has advanced immensely. Recent developments in this field veered the focus from the isolation of novel bonding motifs to mimicking transition metals in their ability to activate small molecules and perform catalysis. The potential of these homonuclear multiply bonded compounds has been demonstrated numerous times in the activation of H2, NH3, CO2 and other small molecules. Hereby, the strong relationship between structure and reactivity warrants close attention towards rational ligand design. This minireview provides an overview on recent developments in regard to bond activation with group 14 dimetallenes and dimetallynes with the perspective of potential catalytic applications of these compounds.This minireview highlights the recent advances in small molecule activation and catalytic applications of homonuclear dimetallenes, dimetallynes and interconnected bismetallylenes of heavier group 14 elements. 相似文献
65.
K. Komura K. Komura Y. Kuwahara Y. Kuwahara T. Abe T. Abe K. Tanaka K. Tanaka Y. Murata Y. Murata M. Inoue M. Inoue 《Journal of Radioanalytical and Nuclear Chemistry》2006,269(2):511-516
Summary Extremely low activity levels of cosmic ray induced nuclides have been measured in freshly precipitated rainwater by quick
chemical separation coupled with ultra low background gamma-spectrometry. The nuclides detected were 38S (T1/2 = 2.83 h)-38Cl (37.2 m), 39Cl (55.6 m), 24Na (14.96 h), 28Mg (20.9 h), 7Be (53.3 d) and 22Na (2.602 y). The number of atoms in rain water were evaluated to be ranging from 400-1900 l-1 for 39Cl (n = 6, mean: 1200), 30-1500 l-1 for 24Na (n = 16, mean: 520), 80-600 l-1 for 28Mg (n = 13, mean: 260), 1 . 106-4 . 107 l-1 for 7Be (n = 16, mean: 7 . 106) and 2 . 103-1 . 105 l-1 for 22Na (n = 9, mean: 2 . 104). Measurements of activity levels and activity ratios of short-lived cosmic-ray induced short-lived nuclides will open new
method to understand atmospheric processes occurred at the altitude of rain cloud. 相似文献
66.
Brown coals have a considerable number of acidic functional groups of which the main component is carboxyl groups, and the acidity has a wide distribution. In this paper, changes of the acidity distribution were examined by aqueous titration when brown coal was heat-treated to control its acidity distribution. For Loy Yang brown coal from Australia dried at 50 degrees C under vacuum (LY50), the acid dissociation constant, Ka, was distributed over a wide pKa range between 2 and 9. Then, using Gaussian functions, the acidity distribution was divided into four groups, which were characterized by average pKa values: average pKa value of 3.8 (hereafter referred to as Group A), 5.2 (Group B), 6.8 (Group C), and 8.3 (Group D). Among them, Groups A, B, and C were assigned to carboxyl groups. From the changes of the number of carboxyl groups when brown coal was heat-treated up to 400 degrees C, it was found that the way of decrease was different among these acidic groups. The decrease of the amount of carboxyl groups in Group C was significant, and at 325 degrees C most of them disappeared. On the other hand, the carboxyl groups in Group A remained even at a high temperature of 400 degrees C. We estimated approximately the structures around carboxyl groups for LY50 and their structural changes by heat treatment using the known pKa values for simple carboxylic acids and the pKa values calculated by the MOPAC program for complicated carboxylic acids. 相似文献
67.
Takaaki Horaguchi Hiroaki Yagoh Kiyoshi Tanemura Tsuneo Suzuki 《Journal of heterocyclic chemistry》1986,23(3):657-663
4,5-Dihydro-3H-naphtho[1,8-bc]furans 4 and 6 which have various substituents (R1 and R2) have been synthesized from 8-oxo-5,6,7,8-tetrahydro-1-naphthyloxyacetic acids 1 and 3 or their ethyl esters 2 . The reaction of acids 1 and 3 with sodium acetate in acetic anhydride gave a mixture of furans 4 and 6 and lactones 5 and 7 . The ratios of the products were varied according to the types of substituents (R1 and R2) in acids 1 and 3 . As the substituent R1 (R2 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, production of furans 4 became more difficult. However, when a phenyl group was used as the substituent, furan 4 was obtained in good yield. Similarly, as the substituent R2 (R1 = hydrogen) in acids 1 was changed from hydrogen to a methyl, ethyl or isopropyl group, furan formation was more difficult. In contrast, acids 3 which had electron-withdrawing substituents such as chlorine, bromine or a nitro group at the 4-position afforded furans 6 in good yield. 4,5-Dihydro-3H-naphtho[1,8-bc]furans 4 and 4,5-dihydro-3H-naphtho[1,8-bc]furan-2-carbocylic acids 8 were synthesized from the reaction of esters 2 and potassium hydroxide in dioxane. When the substituents R1 or R2 in esters 2 were varied from hydrogen to a methyl, ethyl or isopropyl group the total yields of furans 4 and furancarboxylic acids 8 were reduced. 相似文献
68.
The title compound shows dual fluorescence depending on polarity of the solvent used. The fluorescence in the nonpolar solvent, cyclohexane, has a maximuni at 370 nm, while the fluorescence in the polar solvent, methanol, has a maximum at 400nm. The short and long wavelength fluorescences were assigned to the syn and anti conformers, respectively. From these observations one is allowed to conclude that a novel solvent dipole-induced rotational isomerism takes place in the excited singlet state. 相似文献
69.
K Nakamori I Koyama T Nakamura T Yoshida M Umeda K Inoue 《Chemical & pharmaceutical bulletin》1990,38(11):3116-3119
The effect of taurine in protecting biomembrane attacked by hypochlorous acid (HOCl) was examined using canine erythrocytes which had been pre-treated with HOCl. In the treatment, most of the HOCl was consumed as a result of its reaction with a number of electrophilic substances, such as free amino groups (-NH2) in the membrane, whereas hemoglobin inside the cells was not oxidized. The lysis of HOCl-treated erythrocytes was dependent on the concentration of HOCl and on the incubation time at 37 degrees C. Taurine inhibited the lysis at 37 degrees C in a dose dependent manner. During the incubation of HOCl-treated erythrocytes with taurine, an appreciable amount of monochlorotaurine (TauNHCl) was detected in the supernate. This suggests that taurine might remove the oxidized chlorine from HOCl-treated erythrocytes, resulting in the production of TauNHCl. The effect of taurine on the removal of Cl+ moiety was further examined using Sepharose gel with free amino groups. Taurine removed Cl+ moiety from HOCl-treated Sepharose gel, and the yield of TauNHCl depended on the concentration of taurine and the incubation time. These results indicate that taurine might inhibit the hemolysis by scavenging the oxidized chlorine moiety from the HOCl-treated erythrocytes. Inhibition of the HOCl-induced hemolysis was also observed with other amino acids.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
70.
The deterioration of zinc, zinc—calcium and manganese phosphate coatings and oxalate coatings on steel on heating was investigated by conversion electron Mössbauer spectrometry. and the chemical change of the coatings was analysed on the basis of the thermal characteristics of Zn3(PO4)2·4H2O, Zn2Fe(PO4)4·4H2O, CaZn2(PO4)2·2H2O, Fe3(PO4)2·8H2O. (Mn, Fe)5H2(PO4)4·4H2O and FeC2O4·2H2O. The steel substrate beneath the coatings influenced the thermal decomposition and evaporation of coating materials under the various heating atmospheres. The heat resistance of these coatings and the state of the substrate were also investigated. 相似文献