全文获取类型
收费全文 | 7982篇 |
免费 | 323篇 |
国内免费 | 3篇 |
专业分类
化学 | 5690篇 |
晶体学 | 68篇 |
力学 | 169篇 |
数学 | 956篇 |
物理学 | 1425篇 |
出版年
2023年 | 68篇 |
2022年 | 44篇 |
2021年 | 91篇 |
2020年 | 144篇 |
2019年 | 132篇 |
2018年 | 163篇 |
2017年 | 137篇 |
2016年 | 322篇 |
2015年 | 258篇 |
2014年 | 263篇 |
2013年 | 566篇 |
2012年 | 533篇 |
2011年 | 597篇 |
2010年 | 273篇 |
2009年 | 245篇 |
2008年 | 543篇 |
2007年 | 529篇 |
2006年 | 434篇 |
2005年 | 416篇 |
2004年 | 305篇 |
2003年 | 302篇 |
2002年 | 284篇 |
2001年 | 189篇 |
2000年 | 185篇 |
1999年 | 119篇 |
1998年 | 82篇 |
1997年 | 81篇 |
1996年 | 104篇 |
1995年 | 68篇 |
1994年 | 75篇 |
1993年 | 92篇 |
1992年 | 76篇 |
1991年 | 44篇 |
1990年 | 39篇 |
1989年 | 33篇 |
1988年 | 29篇 |
1987年 | 28篇 |
1986年 | 24篇 |
1985年 | 63篇 |
1984年 | 56篇 |
1983年 | 38篇 |
1982年 | 35篇 |
1981年 | 31篇 |
1980年 | 25篇 |
1979年 | 12篇 |
1978年 | 23篇 |
1977年 | 17篇 |
1976年 | 11篇 |
1975年 | 17篇 |
1974年 | 15篇 |
排序方式: 共有8308条查询结果,搜索用时 15 毫秒
1.
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
2.
3.
4.
Marius Lupu Adnana Zaulet Prof. Dr. Francesc Teixidor Prof. Dr. Eliseo Ruiz Prof. Dr. Clara Viñas 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(18):6888-6897
The metallacarborane [3,3′‐Co(1,2‐closo‐C2B9H11)2]? has been synthesized. This species allows the formation of redox couples in which both partners are negatively charged. The E1/2 potential can be tuned by adjusting the nature and number of substituents on B and C. The octaiodinated species [3,3′‐Co(1,2‐closo‐C2B9H7I4)2]? is the most favorable, as it is isolatable and stable in air. A DFT study on stability and redox potentials of complexes has been performed. 相似文献
5.
6.
7.
Ricardo Mañé 《Bulletin of the Brazilian Mathematical Society》1997,28(2):141-153
The objective of this note is to present some results, to be proved in a forthcoming paper, about certain special solutions of the Euler-Lagrange equations on closed manifolds. Our main results extend to time dependent periodic Lagrangians with minor modifications.We have chosen the autonomous case because this formally simpler framework allows to reach more easily the core of our concepts and results. Moreover the autonomous case exhibits certain special features involving the energy as a first integral that deserve special attention. They are closely related to the link found by Carneiro [C] between the energy and Mather's action function [Ma].Reprinted by permission of Addison Wesley Longman Ltd. 相似文献
8.
René Meziat Diego Patiño Pablo Pedregal 《Computational Optimization and Applications》2007,38(1):147-171
We propose an alternative method for computing effectively the solution of non-linear, fixed-terminal-time, optimal control
problems when they are given in Lagrange, Bolza or Mayer forms. This method works well when the nonlinearities in the control
variable can be expressed as polynomials. The essential of this proposal is the transformation of a non-linear, non-convex
optimal control problem into an equivalent optimal control problem with linear and convex structure. The method is based on
global optimization of polynomials by the method of moments. With this method we can determine either the existence or lacking
of minimizers. In addition, we can calculate generalized solutions when the original problem lacks of minimizers. We also
present the numerical schemes to solve several examples arising in science and technology. 相似文献
9.
10.