全文获取类型
收费全文 | 3178篇 |
免费 | 94篇 |
国内免费 | 11篇 |
专业分类
化学 | 2105篇 |
晶体学 | 18篇 |
力学 | 117篇 |
数学 | 418篇 |
物理学 | 625篇 |
出版年
2023年 | 25篇 |
2022年 | 72篇 |
2021年 | 114篇 |
2020年 | 99篇 |
2019年 | 69篇 |
2018年 | 56篇 |
2017年 | 48篇 |
2016年 | 135篇 |
2015年 | 107篇 |
2014年 | 91篇 |
2013年 | 169篇 |
2012年 | 209篇 |
2011年 | 247篇 |
2010年 | 151篇 |
2009年 | 105篇 |
2008年 | 195篇 |
2007年 | 210篇 |
2006年 | 189篇 |
2005年 | 158篇 |
2004年 | 166篇 |
2003年 | 118篇 |
2002年 | 96篇 |
2001年 | 45篇 |
2000年 | 21篇 |
1999年 | 16篇 |
1998年 | 16篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 19篇 |
1994年 | 22篇 |
1993年 | 14篇 |
1992年 | 18篇 |
1991年 | 5篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 9篇 |
1985年 | 16篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1975年 | 7篇 |
1974年 | 4篇 |
1971年 | 4篇 |
排序方式: 共有3283条查询结果,搜索用时 15 毫秒
101.
102.
103.
Ipomoea cairica cell cultures produced a tetrahydrofuran lignan, (+)-pinoresinol, identified by UV, IR, MS and NMR methods, not yet found in the intact plant, and new in the Convolvulaceae family. Pinoresinol was found to have antioxidant and Ca2+ antagonist properties. As it could be requested for its biological activity, we examined the possibility to raise the pinoresinol yield of I. cairica cultures, as well as we continued investigations on lignans' response to optimization. 相似文献
104.
Vistoli G Pedretti A Villa L Testa B 《Journal of the American Chemical Society》2002,124(25):7472-7480
The objective of this study was to determine if and how a solvent influences internal motions in a solute molecule. Acetylcholine was chosen as the object of study given its interesting molecular structure and major biological significance. Molecular dynamics simulations were carried out in the vacuum (10 ns), water (5 ns), methanol (5 ns), and octanol (1.5 ns). Seven clusters of conformers were identified, namely, +g+g, -g-g, +gt, -gt, t+g, t-g, and tt, where the gauche and trans labels refer to the dihedral angles tau(2) and tau(3), respectively. As expected, the relative proportion of these conformational clusters was highly solvent-dependent and corresponded to a progressive loss of conformational freedom with increasing molecular weight of the solvent. More importantly, the conformational clusters were used to calculate instantaneous and median angular velocity (omega and omega(M), respectively) and instantaneous and median angular acceleration (alpha and alpha(M), respectively). Angular velocity and angular acceleration were both found to decrease markedly with increasing molecular weight of the solvent, i.e., vacuum (epsilon = 1) > water > methanol > octanol. The decrease from the vacuum to octanol was approximately 40% for tau(2) and approximately 60% for tau(3). Such solvent-dependent constraints on a solute's internal motions may be biologically and pharmacologically relevant. 相似文献
105.
106.
107.
108.
An unprecedented chiral spiropiperidine oxindole system was synthesized starting from enantiopure quaternary 3-aminooxindole and relying on a ring closing metathesis as the key step. This compound acts as an highly constrained Freidinger γ-lactam, adopting a type II β-turn conformation in solution, as assessed by modelling and spectroscopical studies. 相似文献
109.
In the late sixties the Canadian psychologist Laurence J. Peter advanced an apparently paradoxical principle, named since then after him, which can be summarized as follows: ‘Every new member in a hierarchical organization climbs the hierarchy until he/she reaches his/her level of maximum incompetence’. Despite its apparent unreasonableness, such a principle would realistically act in any organization where the mechanism of promotion rewards the best members and where the competence at their new level in the hierarchical structure does not depend on the competence they had at the previous level, usually because the tasks of the levels are very different to each other. Here we show, by means of agent based simulations, that if the latter two features actually hold in a given model of an organization with a hierarchical structure, then not only is the Peter principle unavoidable, but also it yields in turn a significant reduction of the global efficiency of the organization. Within a game theory-like approach, we explore different promotion strategies and we find, counterintuitively, that in order to avoid such an effect the best ways for improving the efficiency of a given organization are either to promote each time an agent at random or to promote randomly the best and the worst members in terms of competence. 相似文献
110.