全文获取类型
收费全文 | 17882篇 |
免费 | 1873篇 |
国内免费 | 1576篇 |
专业分类
化学 | 9442篇 |
晶体学 | 88篇 |
力学 | 213篇 |
综合类 | 173篇 |
数学 | 9777篇 |
物理学 | 1638篇 |
出版年
2024年 | 23篇 |
2023年 | 340篇 |
2022年 | 199篇 |
2021年 | 319篇 |
2020年 | 569篇 |
2019年 | 567篇 |
2018年 | 567篇 |
2017年 | 439篇 |
2016年 | 654篇 |
2015年 | 692篇 |
2014年 | 1064篇 |
2013年 | 1586篇 |
2012年 | 1007篇 |
2011年 | 1218篇 |
2010年 | 1055篇 |
2009年 | 1188篇 |
2008年 | 1284篇 |
2007年 | 1177篇 |
2006年 | 994篇 |
2005年 | 772篇 |
2004年 | 666篇 |
2003年 | 654篇 |
2002年 | 604篇 |
2001年 | 422篇 |
2000年 | 408篇 |
1999年 | 400篇 |
1998年 | 360篇 |
1997年 | 250篇 |
1996年 | 224篇 |
1995年 | 257篇 |
1994年 | 206篇 |
1993年 | 154篇 |
1992年 | 152篇 |
1991年 | 86篇 |
1990年 | 79篇 |
1989年 | 61篇 |
1988年 | 59篇 |
1987年 | 38篇 |
1986年 | 44篇 |
1985年 | 67篇 |
1984年 | 57篇 |
1983年 | 21篇 |
1982年 | 61篇 |
1981年 | 48篇 |
1980年 | 40篇 |
1979年 | 40篇 |
1978年 | 41篇 |
1977年 | 28篇 |
1976年 | 35篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
11.
12.
Back Cover: Reactions of Group 4 Metallocenes with Monosubstituted Acetonitriles: Keteniminate Formation versus CC Coupling (Chem. Eur. J. 8/2015) 下载免费PDF全文
13.
Valentin A. Semenov Dmitry O. Samultsev Leonid B. Krivdin 《Magnetic resonance in chemistry : MRC》2020,58(6):532-539
The density functional theory calculation of 1H and 13C NMR chemical shifts in a series of ten 10 classically known Strychnos alkaloids with a strychnine skeleton was performed at the PBE0/pcSseg-2//pcseg-2 level. It was found that calculated 1H and 13C NMR chemical shifts provided a markedly good correlation with experiment characterized by a mean absolute error of 0.08 ppm in the range of 7 ppm for protons and 1.67 ppm in the range of 150 ppm for carbons, so that a mean absolute percentage error was as small as ~1% in both cases. 相似文献
14.
Renny Mathew Karolina A. Uchman Lydia Gkoura Chris J. Pickard Maria Baias 《Magnetic resonance in chemistry : MRC》2020,58(11):1018-1025
A combined experimental and computational approach was used to distinguish between different polymorphs of the pharmaceutical drug aspirin. This method involves the use of ab initio random structure searching (AIRSS), a density functional theory (DFT)-based crystal structure prediction method for the high-accuracy prediction of polymorphic structures, with DFT calculations of nuclear magnetic resonance (NMR) parameters and solid-state NMR experiments at natural abundance. AIRSS was used to predict the crystal structures of form-I and form-II of aspirin. The root-mean-square deviation between experimental and calculated 1H chemical shifts was used to identify form-I as the polymorph present in the experimental sample, the selection being successful despite the large similarities between the molecular environments in the crystals of the two polymorphs. 相似文献
15.
In this paper, we propose a sufficient and necessary condition for the boundedness of all the solutions for the equation with the critical situation that on g and p, where , is periodic and is bounded. 相似文献
16.
This paper deals with the Cauchy–Dirichlet problem for the fractional Cahn–Hilliard equation. The main results consist of global (in time) existence of weak solutions, characterization of parabolic smoothing effects (implying under proper condition eventual boundedness of trajectories), and convergence of each solution to a (single) equilibrium. In particular, to prove the convergence result, a variant of the so-called ?ojasiewicz–Simon inequality is provided for the fractional Dirichlet Laplacian and (possibly) non-analytic (but ) nonlinearities. 相似文献
17.
Gas-phase 21Ne nuclear magnetic resonance spectra were measured at the natural abundance of 21Ne isotope for samples consisting of pressurized neon up to 60 bar at room temperature and applying the magnetic field of the strength B0 = 11.7574 T. It showed that the nuclear magnetic resonance frequency is linearly dependent on the density of gaseous neon. The resonance frequency was extrapolated to the zero-density point, and it permitted the determination of the 21Ne nuclear magnetic moment, μ(21Ne) = 0.6617774(10) μN. The present value of μ(21Ne) is not influenced by the bulk magnetic susceptibility of neon and interactions between neon atoms; therefore, it is more precise and reliable than the previous result obtained for μ(21Ne). 相似文献
18.
Michiel Janssen Prof. Dirk E. De Vos 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(45):10724-10734
The practical application of Shilov-type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ-hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state-of-the-art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2-fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non-selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented. 相似文献
19.
20.
Ping Chen Yanhong Hao Xinxin Wang Prof. Dr. Dan Yuan Prof. Dr. Yingming Yao Prof. Dr. Lutz Ackermann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(30):7292-7297
Highly position selective alkylations of N-alkylindoles at C7-positions have been enabled by cationic zirconium complexes. The strategy provides a straightforward access to install alkyl groups at C7-positions of indoles without a complex directing group. Mechanistic studies provided support for the importance of Brønsted acids in the catalytic manifold. 相似文献