全文获取类型
收费全文 | 1446990篇 |
免费 | 25917篇 |
国内免费 | 7842篇 |
专业分类
化学 | 739489篇 |
晶体学 | 20604篇 |
力学 | 76689篇 |
综合类 | 108篇 |
数学 | 249196篇 |
物理学 | 394663篇 |
出版年
2021年 | 13633篇 |
2020年 | 16046篇 |
2019年 | 16182篇 |
2018年 | 14600篇 |
2016年 | 29061篇 |
2015年 | 21196篇 |
2014年 | 30767篇 |
2013年 | 74989篇 |
2012年 | 43941篇 |
2011年 | 44983篇 |
2010年 | 40069篇 |
2009年 | 41439篇 |
2008年 | 42533篇 |
2007年 | 39258篇 |
2006年 | 43176篇 |
2005年 | 36202篇 |
2004年 | 36102篇 |
2003年 | 33081篇 |
2002年 | 33514篇 |
2001年 | 34136篇 |
2000年 | 28263篇 |
1999年 | 24220篇 |
1998年 | 21744篇 |
1997年 | 21574篇 |
1996年 | 21458篇 |
1995年 | 19453篇 |
1994年 | 18931篇 |
1993年 | 18508篇 |
1992年 | 19167篇 |
1991年 | 19316篇 |
1990年 | 18513篇 |
1989年 | 18495篇 |
1988年 | 18135篇 |
1987年 | 17957篇 |
1986年 | 16990篇 |
1985年 | 23324篇 |
1984年 | 24284篇 |
1983年 | 20342篇 |
1982年 | 21942篇 |
1981年 | 21126篇 |
1980年 | 20457篇 |
1979年 | 20914篇 |
1978年 | 22033篇 |
1977年 | 21700篇 |
1976年 | 21468篇 |
1975年 | 20230篇 |
1974年 | 19920篇 |
1973年 | 20321篇 |
1972年 | 14803篇 |
1967年 | 12970篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis 下载免费PDF全文
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
52.
53.
54.
55.
56.
57.
K.V. Stepurska О.О. Soldatkin I.S. Kucherenko V.M. Arkhypova S.V. Dzyadevych A.P. Soldatkin 《Analytica chimica acta》2015
This study was aimed at the development of a conductometric biosensor based on acetylcholinesterase considering the feasibility of its application for the inhibitory analysis of various toxicants. In this paper, the optimum conditions for enzyme immobilization on the transducer surface are selected as well as the optimum concentration of substrate for inhibitory analysis. Sensitivity of the developed biosensor to different classes of toxic compounds (organophosphorus pesticides, heavy metal ions, surfactants, aflatoxin, glycoalkaloids) was tested. It is shown that the developed biosensor can be successfully used for the analysis of pesticides and mycotoxins, as well as for determination of total toxicity of the samples. A new method of biosensor analysis of toxic substances of different classes in complex multicomponent aqueous samples is proposed. 相似文献
58.
59.
Functional zinc(II) phthalocyanines bearing Schiff base complexes as oxidation catalysts for bleaching systems 下载免费PDF全文
Oxidation catalysis is used to increase the performance of hydrogen peroxide in laundry bleach applications. Bleach catalysts provide cost‐effective, energy‐saving and environmentally friendly bleach systems yielding perfect stain removal at lower temperatures. This comparative study is based on the synthesis of bis[bis(salicylhydrazonephenoxy)manganese(III)] phthalocyaninatozinc(II) ( 2 ), bis[bis(salicylhydrazonephenoxy)cobalt(III)] phthalocyaninatozinc(II) ( 3 ) and bis[bis(salicylhydrazonephenoxy)iron(III)] phthalocyaninatozinc(II) ( 4 ) as tri‐nuclear complexes consisting of two Schiff base complexes substituting a zinc phthalocyanine. Complexion on the periphery to obtain complexes 2 , 3 , 4 was performed through the reaction of a Schiff base‐substituted phthalocyanine using MnCl2?4H2O, CoCl2?6H2O or FeCl3?6H2O salts in basic condition in dimethylformamide. Fourier transform infrared, 1H NMR, 13C NMR, UV–visible, inductively coupled plasma optical emission and mass spectra were applied to characterize the prepared compounds. The bleach performances of the three phthalocyanine compounds 2 , 3 , 4 were examined by the degradation of morin as hydrophilic dye. The degradation progress in the presence of catalysts 2 , 3 , 4 /H2O2 combination in aqueous solution was investigated using an online spectrophotometric method. It was found that the catalysts 2 , 3 , 4 exhibited better bleaching performance at 25 °C than tetraactylethylethylenediamine as bleach activator used in powder detergent formulations for stain removal. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
60.