全文获取类型
收费全文 | 46959篇 |
免费 | 5110篇 |
国内免费 | 4137篇 |
专业分类
化学 | 33950篇 |
晶体学 | 429篇 |
力学 | 2269篇 |
综合类 | 282篇 |
数学 | 5521篇 |
物理学 | 13755篇 |
出版年
2024年 | 116篇 |
2023年 | 710篇 |
2022年 | 1271篇 |
2021年 | 1377篇 |
2020年 | 1396篇 |
2019年 | 1404篇 |
2018年 | 1172篇 |
2017年 | 1078篇 |
2016年 | 1887篇 |
2015年 | 1771篇 |
2014年 | 2105篇 |
2013年 | 3282篇 |
2012年 | 3973篇 |
2011年 | 4104篇 |
2010年 | 2479篇 |
2009年 | 2683篇 |
2008年 | 2765篇 |
2007年 | 2665篇 |
2006年 | 2348篇 |
2005年 | 2126篇 |
2004年 | 2113篇 |
2003年 | 1619篇 |
2002年 | 1510篇 |
2001年 | 941篇 |
2000年 | 864篇 |
1999年 | 877篇 |
1998年 | 782篇 |
1997年 | 757篇 |
1996年 | 851篇 |
1995年 | 668篇 |
1994年 | 582篇 |
1993年 | 529篇 |
1992年 | 489篇 |
1991年 | 401篇 |
1990年 | 335篇 |
1989年 | 268篇 |
1988年 | 225篇 |
1987年 | 193篇 |
1986年 | 192篇 |
1985年 | 208篇 |
1984年 | 179篇 |
1983年 | 128篇 |
1982年 | 99篇 |
1981年 | 95篇 |
1980年 | 70篇 |
1979年 | 49篇 |
1978年 | 47篇 |
1977年 | 58篇 |
1975年 | 52篇 |
1974年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
Dr. Antonio M. Rodríguez Dr. Pilar Prieto Prof. Antonio de la Hoz Prof. Ángel Díaz-Ortiz D. Raúl Martín Prof. José I. García 《ChemistryOpen》2015,4(3):308-317
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20–30 kcal mol−1 and a polarity (μ) between 7–20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation. 相似文献
12.
讨论一类非齐次非线性椭圆边界值问题.利用极大值原理证明了该问题解的梯度估计.作为它的应用得到了解的效率比估计. 相似文献
13.
14.
15.
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
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
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
16.
17.
18.
19.
在中国科学技术大学夏季学期的研究型实验课程"化学科研基础训练"中开设"壳聚糖的制备与表征"综合实验,以龙虾壳为原料,通过除蛋白、脱盐、脱色、脱乙酰等一系列反应,制备得到目标产物壳聚糖。运用红外光谱、核磁共振仪、黏度法、滴定等对产品的结构及性能进行表征。 相似文献