全文获取类型
收费全文 | 681699篇 |
免费 | 8044篇 |
国内免费 | 1868篇 |
专业分类
化学 | 353473篇 |
晶体学 | 9574篇 |
力学 | 30654篇 |
综合类 | 22篇 |
数学 | 96481篇 |
物理学 | 201407篇 |
出版年
2021年 | 5385篇 |
2020年 | 6070篇 |
2019年 | 6563篇 |
2018年 | 14203篇 |
2017年 | 14391篇 |
2016年 | 15428篇 |
2015年 | 8778篇 |
2014年 | 12212篇 |
2013年 | 29104篇 |
2012年 | 25105篇 |
2011年 | 34703篇 |
2010年 | 23528篇 |
2009年 | 23186篇 |
2008年 | 31605篇 |
2007年 | 33589篇 |
2006年 | 23024篇 |
2005年 | 24940篇 |
2004年 | 21019篇 |
2003年 | 18999篇 |
2002年 | 17321篇 |
2001年 | 17655篇 |
2000年 | 13921篇 |
1999年 | 10687篇 |
1998年 | 8980篇 |
1997年 | 8736篇 |
1996年 | 8609篇 |
1995年 | 7442篇 |
1994年 | 7505篇 |
1993年 | 7341篇 |
1992年 | 8029篇 |
1991年 | 8092篇 |
1990年 | 7740篇 |
1989年 | 7535篇 |
1988年 | 7533篇 |
1987年 | 7354篇 |
1986年 | 7011篇 |
1985年 | 9116篇 |
1984年 | 9394篇 |
1983年 | 7472篇 |
1982年 | 7768篇 |
1981年 | 7646篇 |
1980年 | 7115篇 |
1979年 | 7638篇 |
1978年 | 7937篇 |
1977年 | 7714篇 |
1976年 | 7711篇 |
1975年 | 7178篇 |
1974年 | 7119篇 |
1973年 | 7286篇 |
1972年 | 5020篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
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
103.
104.
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
105.
106.
Experimental Techniques - The analysis of residual stress in glass is usually carried out by means of photoelastic methods. This article considers the automation of the white light photoelastic... 相似文献
107.
108.
109.
Novikova D. A. Flisyuk O. M. Martsulevich N. A. Garabadzhiu A. V. 《Russian Journal of General Chemistry》2021,91(7):1387-1392
Russian Journal of General Chemistry - The main sources of emissions of industrial nitrogen oxides have been reviewed. A promising method for the absorption of nitrogen monoxide by a reusable... 相似文献
110.
Ana-Maria Udrea Andra Dinache Jean-Marie Pags Ruxandra Angela Pirvulescu 《Molecules (Basel, Switzerland)》2021,26(8)
Multidrug resistance of bacteria is a worrying concern in the therapeutic field and an alternative method to combat it is designing new efflux pump inhibitors (EPIs). This article presents a molecular study of two quinazoline derivatives, labelled BG1189 and BG1190, proposed as EPIs. In silico approach investigates the pharmacodynamic and pharmacokinetic profile of BG1189 and BG1190 quinazolines. Molecular docking and predicted ADMET features suggest that BG1189 and BG1190 may represent attractive candidates as antimicrobial drugs. UV-Vis absorption spectroscopy was employed to study the time stability of quinazoline solutions in water or in dimethyl sulfoxide (DMSO), in constant environmental conditions, and to determine the influence of usual storage temperature, normal room lighting and laser radiation (photostability) on samples stability. The effects of irradiation on BG1189 and BG1190 molecules were also assessed through Fourier-transform infrared (FTIR) spectroscopy. FTIR spectra showed that laser radiation breaks some chemical bonds affecting the substituents and the quinazoline radical of the compounds. 相似文献