全文获取类型
收费全文 | 2263篇 |
免费 | 486篇 |
国内免费 | 230篇 |
专业分类
化学 | 1523篇 |
晶体学 | 21篇 |
力学 | 124篇 |
综合类 | 54篇 |
数学 | 283篇 |
物理学 | 974篇 |
出版年
2023年 | 46篇 |
2022年 | 47篇 |
2021年 | 79篇 |
2020年 | 86篇 |
2019年 | 93篇 |
2018年 | 73篇 |
2017年 | 78篇 |
2016年 | 84篇 |
2015年 | 98篇 |
2014年 | 132篇 |
2013年 | 156篇 |
2012年 | 220篇 |
2011年 | 238篇 |
2010年 | 182篇 |
2009年 | 140篇 |
2008年 | 164篇 |
2007年 | 153篇 |
2006年 | 163篇 |
2005年 | 135篇 |
2004年 | 95篇 |
2003年 | 68篇 |
2002年 | 81篇 |
2001年 | 53篇 |
2000年 | 53篇 |
1999年 | 35篇 |
1998年 | 31篇 |
1997年 | 24篇 |
1996年 | 17篇 |
1995年 | 16篇 |
1994年 | 13篇 |
1993年 | 8篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 14篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 12篇 |
1986年 | 6篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1969年 | 2篇 |
1957年 | 2篇 |
1939年 | 1篇 |
1906年 | 1篇 |
排序方式: 共有2979条查询结果,搜索用时 17 毫秒
41.
The double bond migration of butene catalyzed by 1-ethyl-3-methyl-imidazolium fluoride (EmimF) has been studied using quantum chemical method. The geometries of reactant, transition state and product for the isomerization have been optimized by density functional theory (DFT) at the B3PW91/6-31G(d,p), 6-311++G(d,p) and aug-cc-PVDZ levels. The computed results show that the 4-H atom on imidazole ring of EmimF has a good catalytic activity to the double bond migration of butene and the catalytic reaction of 1-butene to 2-butene is a synergetic and elementary process. The apparent activation energy of isomerization is about 197 kJ/mol. 相似文献
42.
43.
Jinzhang Gao Yongjun Liu Wu Yang Lumei Pu Jie Yu Quanfang Lu 《Central European Journal of Chemistry》2005,3(3):377-386
A plasma induced degradation process has been studied to treat 4-nitrotoluene (4-NT) present as an aqueous pollutant. The
plasma was locally generated from a glow discharge around a tip of a platinum anode in an electrolytic solution. The influence
of initial pH and Fe2+ on the degradation was examined. Major intermediates resulting from the degradation process were identified. Amongst the
aromatic intermediates, p-hydroxybenzoic acid was the predominant degradation product. The formation of oxalic acid, malic
acid was also observed. The final products of degradation were NH
4
+
, NO
3
−
and CO2. Based on the analysis of intermediates and the kinetic considerations, the degradation was shown to follow a pseudo-first
order reaction hence, a possible reaction pathway was proposed. 相似文献
44.
trans-RhCl(CO)(TPPTS)2 (TPPTS=tris(m-sulfonatophenyl)phosphine) has been intercalated into Zn-Al layered double hydroxides (LDHs) by the method of ion exchange. The structure, composition and thermal stability of the composite material have been characterized by powder X-ray diffraction, Fourier transform infrared and 31P solid-state magic-angle spinning nuclear magnetic resonance spectroscopy, elemental analysis, thermogravimetry, and differential thermal analysis. The geometry of trans-RhCl(CO)(TPPTS)2 was fully optimized using the PM3 semiempirical molecular orbital method, and a schematic model for the intercalated species has been proposed. The thermal stability of trans-RhCl(CO)(TPPTS)2 is significantly enhanced by intercalation, which suggests that such materials may have prospective application as the basis of a supported catalyst system for the hydroformylation of higher olefins. 相似文献
45.
Novel chiral bisbinaphthyl compounds have been synthesized for the enantioselective fluorescent recognition of mandelic acid. By introducing dendritic branches to the chiral receptor unit, the fluorescence signals of the receptors are significantly amplified because of the light-harvesting effect of the dendritic structure. This has greatly increased the sensitivity of the sensors in the fluorescent recognition. Study of the three generation sensors demonstrates that the generation zero sensor is the best choice for the recognition of mandelic acid because of its greatly increased fluorescence signal over the core and its high enantioselectivity. This sensor is potentially useful for the high throughput screening of chiral catalysts for the asymmetric synthesis of alpha-hydroxycarboxylic acids. 相似文献
46.
水中酚类化合物的液-液-液微萃取/ 高效液相色谱联用分析研究 总被引:3,自引:3,他引:3
建立了液-液-液微萃取/高效液相色谱联用(LLLME/HPLC)测定环境水中痕量酚类化合物2-甲基苯酚、2-硝基苯酚、2,4-二氯苯酚的分析方法,研究了有机相溶剂种类及其体积、料液相pH值与离子强度、接受相的体积、组成及浓度和搅拌速率、萃取时间等因素对分析物萃取效率的影响。实验结果表明,该方法对酚类化合物的富集倍数可达到404~747倍,方法的线性范围为0.2~300μg/L,RSD(n=6)为6.8%~11.4%。测定加标自来水、江水以及生活污水样品的回收率为83%~110%。 相似文献
47.
48.
将壳聚糖用作填充色谱柱的吸附剂,对壳聚糖色谱柱吸附铝(Ⅲ)的最佳试验条件作了试验,并取得如下结果:①铝(Ⅲ)柱吸附时溶液的最佳酸度为pH 5.0;②溶液通过吸附柱达到吸附平衡所需的时间为4 h,测得静态饱和吸附量为5.26×10-4mg.L-1;③溶液通过吸附柱的适宜流速为1 mL.min-1;及④用1 mol.L-1硫酸作洗脱剂可取得最佳效果。上述铝(Ⅲ)的壳聚糖吸附富集条件成功地应用于工业废水中铝(Ⅲ)的络天青S光度测定法,方法的回收率为96.7%,方法简便。 相似文献
49.
Cheng YM Pu SC Yu YC Chou PT Huang CH Chen CT Li TH Hu WP 《The journal of physical chemistry. A》2005,109(51):11696-11706
Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature-dependent emission, and fluorescence upconversion to probe the excited-state intramolecular proton transfer (PT) reaction. Upon excitation, I undergoes ultrafast (<120 fs), adiabatic type of charge transfer (CT), so that the dipolar vector in the Franck-Condon excited state is much different from that in the ground state. In polar solvents such as CH2Cl2 and CH3CN, early relaxation dynamics clearly reveals the competitive rates between solvent relaxation and PT dynamics. After reaching thermal equilibrium, a relatively slow, solvent-polarity-dependent rate (a few tens of picoseconds(-1)) of PT takes places. Firm support of the early relaxation dynamics is rendered by the spectral temporal evolution, which resolves two distinct bands ascribed to CT and PT emission. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, led us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is between that of N* and T*. PT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvent relaxation dynamics induced by CT. Unlike the well-known PT system, 4'-N,N-diethylamino-3-hydroxyflavone, in which equilibrium exists between solvent-equilibrated N(eq)* and T(eq)*, N(eq)* --> T(eq)* PT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature-dependent studies deduce a solvent-polarity-perturbed energy barrier of 3.6 kcal/mol for the N(eq)* --> T(eq)* PT in CH3CN. The proposed dipole-moment-tuning PT mechanism with the associated relaxation dynamics is believed to apply to many PT molecules in polar, aprotic solvents. 相似文献
50.