全文获取类型
收费全文 | 24462篇 |
免费 | 839篇 |
国内免费 | 83篇 |
专业分类
化学 | 15133篇 |
晶体学 | 104篇 |
力学 | 405篇 |
数学 | 3422篇 |
物理学 | 6320篇 |
出版年
2023年 | 174篇 |
2021年 | 235篇 |
2020年 | 391篇 |
2019年 | 287篇 |
2018年 | 254篇 |
2017年 | 235篇 |
2016年 | 627篇 |
2015年 | 556篇 |
2014年 | 578篇 |
2013年 | 1113篇 |
2012年 | 1129篇 |
2011年 | 1384篇 |
2010年 | 678篇 |
2009年 | 606篇 |
2008年 | 1236篇 |
2007年 | 1193篇 |
2006年 | 1189篇 |
2005年 | 1037篇 |
2004年 | 827篇 |
2003年 | 728篇 |
2002年 | 743篇 |
2001年 | 525篇 |
2000年 | 457篇 |
1999年 | 382篇 |
1998年 | 353篇 |
1997年 | 347篇 |
1996年 | 356篇 |
1995年 | 325篇 |
1994年 | 323篇 |
1993年 | 327篇 |
1992年 | 308篇 |
1991年 | 269篇 |
1990年 | 223篇 |
1989年 | 204篇 |
1988年 | 180篇 |
1987年 | 198篇 |
1986年 | 197篇 |
1985年 | 262篇 |
1984年 | 286篇 |
1983年 | 230篇 |
1982年 | 277篇 |
1981年 | 275篇 |
1980年 | 240篇 |
1979年 | 245篇 |
1978年 | 254篇 |
1977年 | 236篇 |
1976年 | 224篇 |
1975年 | 192篇 |
1974年 | 176篇 |
1973年 | 167篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Synthesis of Highly Substituted γ‐Butyrolactones by a Gold‐Catalyzed Cascade Reaction of Benzyl Esters 下载免费PDF全文
Maria Camila Blanco Jaimes Alexander Ahrens Daniel Pflästerer Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):427-433
Easily accessible benzylic esters of 3‐butynoic acids in a gold‐catalyzed cyclization/rearrangement cascade reaction provided 3‐propargyl γ‐butyrolactones with the alkene and the carbonyl group not being conjugated. Crossover experiments showed that the formation of the new C?C bond is an intermolecular process. Initially propargylic–benzylic esters were used, but alkyl‐substituted benzylic esters worked equally well. In the case of the propargylic–benzylic products, a simple treatment of the products with aluminum oxide initiated a twofold tautomerization to the allenyl‐substituted γ‐butyrolactones with conjugation of the carbonyl group, the olefin, and the allene. The synthetic sequence can be conducted stepwise or as a one‐pot cascade reaction with similar yields. Even in the presence of the gold catalyst the new allene remains intact. 相似文献
3.
4.
Prof. Dr. Frank Glorius 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8047-8051
A general and efficient methodology for the direct transition metal free trifluoromethylthiolation of a broad range of biologically relevant N‐heteroarenes is reported employing abundant sodium chloride as the catalyst. This method is operationally simple, exhibits high functional group tolerance, and does not require protecting groups. 相似文献
5.
6.
7.
Photobleaching was studied during recording of confocal scanning laser microscopy. Studies on fluorescent gels of FITC-labeled
dextran were used to evaluate differential bleaching along thez-axis. Differential bleaching along the z-axis was observed and it was seen that this was related to the numerical aperture
of the objective in use. This points to the conclusion that photon energy flux density is an important parameter in photobleaching.
To check if photon energy flux density heterogeneity is affected by local variation in the refractive index of the sample,
photobleaching rates were calculated for different fluorescent objects (sections of seeds, animal cells stained with nuclear
stains, immunocytochemistry preparations) and a pronounced similarity was found between photobleaching rates and DIC images. 相似文献
8.
Molecular n-v complexes of quinoline N-oxides with BF3 have been studied by electronic and IR spectroscopy. In most case the donor centre is the oxygen atom of the N-oxide group.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1087–1092, August, 1994. 相似文献
9.
10.
A firm's logistics cost, including shipping and inventory-carrying, is a substantial percentage of its sales. Nevertheless, typical inventory-control methods ignore or insufficiently represent the shipping cost. This paper describes a recursive algorithm that determines the reorder cycle-time that minimizes total logistics cost. It allows for a realistic accounting of shipping cost, which is modelled here as a function of shipping distance and weight. The algorithm uses a relaxation procedure to identify a suitable initial approximation to the optimal order cycle-time and then, through a series of recursive steps, moves to the optimal result. We demonstrate the algorithm with a single item, with a group of items that share a common order cycle, and with multi-items when item demands are random variables. Experience with this algorithm indicates that it converges to the optimal result in a very few steps. 相似文献