全文获取类型
收费全文 | 2056篇 |
免费 | 124篇 |
国内免费 | 215篇 |
专业分类
化学 | 1607篇 |
晶体学 | 17篇 |
力学 | 12篇 |
综合类 | 10篇 |
数学 | 116篇 |
物理学 | 190篇 |
综合类 | 443篇 |
出版年
2024年 | 3篇 |
2023年 | 19篇 |
2022年 | 34篇 |
2021年 | 35篇 |
2020年 | 38篇 |
2019年 | 29篇 |
2018年 | 42篇 |
2017年 | 45篇 |
2016年 | 56篇 |
2015年 | 55篇 |
2014年 | 83篇 |
2013年 | 168篇 |
2012年 | 114篇 |
2011年 | 98篇 |
2010年 | 72篇 |
2009年 | 78篇 |
2008年 | 111篇 |
2007年 | 119篇 |
2006年 | 93篇 |
2005年 | 88篇 |
2004年 | 101篇 |
2003年 | 100篇 |
2002年 | 319篇 |
2001年 | 71篇 |
2000年 | 43篇 |
1999年 | 41篇 |
1998年 | 43篇 |
1997年 | 33篇 |
1996年 | 27篇 |
1995年 | 30篇 |
1994年 | 29篇 |
1993年 | 24篇 |
1992年 | 24篇 |
1991年 | 21篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1970年 | 1篇 |
排序方式: 共有2395条查询结果,搜索用时 31 毫秒
991.
992.
Trifluoromethanesulfonic Anhydride as a Low‐Cost and Versatile Trifluoromethylation Reagent
下载免费PDF全文
![点击此处可从《Angewandte Chemie (International ed. in English)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Yao Ouyang Dr. Xiu‐Hua Xu Prof. Dr. Feng‐Ling Qing 《Angewandte Chemie (International ed. in English)》2018,57(23):6926-6929
A large number of reagents have been developed for the synthesis of trifluoromethylated compounds. However, an ongoing challenge in trifluoromethylation reaction is the use of less expensive and practical trifluoromethyl sources. We report herein the unprecedented direct trifluoromethylation of (hetero)arenes using trifluoromethanesulfonic anhydride as a radical trifluoromethylation reagent by merging photoredox catalysis and pyridine activation. Furthermore, introduction of both the CF3 and OTf groups of the trifluoromethanesulfonic anhydride into internal alkynes to access tetrasubstituted trifluoromethylated alkenes was achieved. Since trifluoromethanesulfonic anhydride is a low‐cost and abundant chemical, this method provides a cost‐efficient and practical route to trifluoromethylated compounds. 相似文献
993.
N,N′-Bis(9-phenyl-9-thioxanthenyl)ethylenediamine proved to be an extremely efficient host compound for pyridine and the isomeric methylpyridines. Furthermore, this host displayed selective behaviour during equimolar guest competitions, consistently favouring 3-methylpyridine in binary, ternary and quaternary experiments. Selectivity orders were 3-methylpyridine >> 4-methylpyridine > 2-methylpyridine, and 3-methylpyridine > pyridine > 4-methylpyridine > 2-methylpyridine, for equimolar ternary and quaternary solutions, respectively. When guest concentrations in binary competitions were varied, 3-methylpyridine remained the favoured guest, even at low 3-methylpyridine concentrations. Single crystal X-ray diffraction showed that all four complexes were isostructural (monoclinic, P21/n) while guests occupy discrete cavities in the crystal. Only 3-methylpyridine experiences (guest methyl)C–H???π(host) and (guest methyl)C–H???H–Ar(host) interactions, explaining the observed affinity of the host for this guest. DSC analyses provided further affirmation for the host preference: endotherm peak temperatures for the guest release processes correlated directly with the selectivity order for the three methylpyridine isomers. 相似文献
994.
The p-state mean-field Potts glass with bimodal bond distribution (±J) is studied by Monte Carlo simulations, both for p = 3 and p = 6 states, for system sizes from N = 5 to N = 120 spins, considering particularly the finite-size scaling behavior at the exactly known glass transition temperature T
c. It is shown that for p = 3 the moments q
(k) of the spin-glass order parameter satisfy a simple scaling behavior,
being the appropriate scaling function and T the temperature. Also the specific heat maxima have a similar behavior,
, while moments of the magnetization scale as
. The approach of the positions T
max of these specific heat maxima to T
c as N is nonmonotonic. For p = 6 the results are compatible with a first-order transition, q
(k) (q
jump)k as N but since the order parameter q
jump at T
c is rather small, a behavior q
(k) N
-k/3
as N also is compatible with the data. Thus no firm conclusions on the finite-size behavior of the order parameter can be drawn. The specific heat maxima c
V
max
behave qualitatively in the same way as for p = 3, consistent with the prediction that there is no latent heat. A speculative phenomenological discussion of finite-size scaling for such transitions is given. For small N (N 15 for p = 3, N 12 for p = 6) the Monte Carlo data are compared to exact partition function calculations, and excellent agreement is found. We also discuss ratios
, for various quantities X, to test the possible lack of self-averaging at T
c. 相似文献
995.
改性天然高分子酸缓蚀剂的长效缓蚀性能研究 总被引:1,自引:0,他引:1
文中研究了吡啶季铵盐接技天然高分子技术,制取的药剂(FNP-C)是性能良好的酸缓蚀剂,而且在1mol/L.的盐酸中具有长效缓蚀性能.同时采用色谱一质谱联用仪分析了FNP-C在酸中的降解规律,探索了其长效缓蚀作用机理 相似文献
996.
Wu Lei Qingli Hao Lude Lu Xujie Yang Xin Wang 《Journal of chemical crystallography》2004,34(6):387-391
A new adduct bis(pyridine)-bis(O,O-diisopropyl dithiophosphato-S,S)nickel(II), [Ni(i-Pr2dtp)2(py)2] (dtp = dithiophosphate, py = pyridine) has been obtained by re-crystallization from CH3COCH3. The crystal and molecular structure of the title complex has been determined by X-ray crystallography. It crystallizes in the monoclinic system, space group P21/c, with lattice parameters a = 6.4890(13) Å, b = 16.386(3) Å, c = 14.830(3) Å, = 99.74(3)°, V = 1554.1(5) Å3, and Z = 2. The crystal structure consists of discrete molecules of [Ni(i-Pr2dtp)2(py)2], which displays a slightly distorted octahedral geometry for the NiS4N2 chromophore. Two O,O-diisopropyl dithiophosphate ions act as bidentate ligands with their S atoms coordinated to Ni atom. Each forms a four-membered chelate ring in the equatorial plane. The N atoms from two pyridine ligands are axially coordinated to the Ni atom. The Ni
S bond distances are 2.4492(8) Å and 2.5711(9) Å, and the Ni
bond distances are 2.127(3) Å. Thermal analyses show that the thermal decomposition of this adduct is so unstable that loss of two pyridine ligands begins at about 65°C. Electronic and IR spectra data is in agreement with the structural data. 相似文献
997.
Thomas I.A. Gerber Dibanisile G. Luzipo Peter Mayer 《Journal of chemical crystallography》2005,35(1):39-41
The title compound was prepared from the reaction of (n-Bu4N)[ReOCl4] with a twofold molar excess of 2-(hydroxymethyl)pyridine in benzene. It has a distorted octahedral coordination geometry, and it crystallizes in the monoclinic space group P21/n, with cell dimensions a = 14.4545(4), b = 7.0168(2), c = 14.5269(4) Å, = 116.995(1), V = 1312.85(7) Å3, and Z = 4. 相似文献
998.
999.
Summary The influence of the solvent on the spin-equilibrium of [Fe(bzimpy)2](ClO4)2 (bzimpy=2,6-bis(benzimidazol-2-yl)pyridine) has been investigated in several nonaqueous solvents by means of UV-Vis spectroscopy and magnetic susceptibility measurements. In methanol, a bis-tris ligation equilibrium is found. Both, the FeL
2
2+
and FeL
3
2+
species show spin-crossover behaviour in solution. Photometric and magnetic properties of the complex in various solvents are related. The spin-equilibrium of [Fe(bzimpy)2](ClO4)2 shows the strongest sensitivity towards changes of the solvent that has ever been observed for spin-crossover compounds. Increase in solvent donor number (DN) from DN=14.1 (Acetonitril) to DN=19.1 (Methanol) causes a shift from exp=4.21 (78% High-Spin (HS)) to exp=3.0 (56% HS). In solvents with DN 30 deprotonation of the complex occurs yielding a low-spin species. The solvent effects are discussed in terms of the donor-acceptor concept.Dedicated to Professor Harry J. Emeléus with heartiest congratulations to his 90th birthday on 22th June 1993. 相似文献
1000.