全文获取类型
收费全文 | 164篇 |
免费 | 11篇 |
专业分类
化学 | 119篇 |
数学 | 9篇 |
物理学 | 47篇 |
出版年
2023年 | 2篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2016年 | 4篇 |
2015年 | 6篇 |
2014年 | 4篇 |
2013年 | 6篇 |
2012年 | 14篇 |
2011年 | 9篇 |
2010年 | 4篇 |
2009年 | 12篇 |
2008年 | 9篇 |
2007年 | 7篇 |
2006年 | 6篇 |
2005年 | 5篇 |
2004年 | 6篇 |
2003年 | 6篇 |
2002年 | 2篇 |
2001年 | 7篇 |
2000年 | 10篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1996年 | 10篇 |
1995年 | 3篇 |
1994年 | 10篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1985年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有175条查询结果,搜索用时 828 毫秒
91.
Mechanism of Thioether Oxidation over Di‐ and Tetrameric Ti Centres: Kinetic and DFT Studies Based on Model Ti‐Containing Polyoxometalates 下载免费PDF全文
Dr. Igor Y. Skobelev Dr. Olga V. Zalomaeva Prof. Oxana A. Kholdeeva Prof. Dr. Josep M. Poblet Dr. Jorge J. Carbó 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(41):14496-14506
The oxidation of thioethers by the green oxidant aqueous H2O2 catalysed by the tetratitanium‐substituted Polyoxometalate (POM) (Bu4N)8[{γ‐SiTi2W10O36(OH)2}2(μ‐O)2], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti2(μ‐OH)2 group in [{γ‐SiTi2W10O36(OH)2}2(μ‐O)2]8? ( 1 D ) with H2O2. The second mechanism includes hydrolysis of Ti‐O‐Ti bonds linking two γ‐Keggin units in structure 1 D to produce the monomer [(γ‐SiW10Ti2O38H2)(OH)2]4? ( 1 M ), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H2O2. Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H2O2 by compound 1 M is preferred by 6.5 kcal mol?1 with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M . The calculations also indicate that for the ?monomeric“ mechanism two pathways are operative: the mono‐ and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal Ti?OH group, whereas in the multinuclear path the active group is the bridging Ti2(μ‐OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a β‐oxygen atom transfer from the Ti hydroperoxo group because the α‐oxygen atom transfer leads to an unfavourable seven‐fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti‐containing POMs: the Ti‐monosubstituted α‐Keggin ion [α‐PTi(OH)W11O39]4? and the dititanium‐substituted sandwich‐type ion [Ti2(OH)2As2W19O67]8?. 相似文献
92.
"Open rather than closed" malonate methano-fullerene derivatives. The formation of methanofulleroid adducts of Y3N@C80 总被引:1,自引:0,他引:1
Lukoyanova O Cardona CM Rivera J Lugo-Morales LZ Chancellor CJ Olmstead MM Rodríguez-Fortea A Poblet JM Balch AL Echegoyen L 《Journal of the American Chemical Society》2007,129(34):10423-10430
Cycloaddition of bromomalonates to Y3N@C80 unexpectedly gave rise to fulleroid derivatives with unusually high stability. Complete characterization of these derivatives is described including X-ray crystallography, 1H NMR, 13C NMR, HMQC, UV-visible, HPLC, MALDI-MS, and electrochemistry. Density functional theory calculations are also presented, which provide a rationale for the formation of the fulleroid and reveal the underlying thermodynamic basis for their stability. 相似文献
93.
Fernandez JA López X Bo C de Graaf C Baerends EJ Poblet JM 《Journal of the American Chemical Society》2007,129(40):12244-12253
The Preyssler anion, of general formula [Xn+P5W30O110](15-n)-, is the smallest polyoxometalate (POM) with an internal cavity allowing cation exchange with the solution. The Preyssler anion features a rich chemistry evidenced by its ability to accept electrons at low potentials, to selectively capture various metal cations, and to undergo acid-base reactions. A deep understanding of these topics is herein provided by means of DFT calculations on the title series of compounds. We evaluate the energetics of the release/encapsulation process for several Xn+ cations and identify the effect of the encapsulated ion on the properties of the Preyssler anion. We revisit the relationship between the internal cation charge and the electrochemical behavior of the POM. A linear dependence between the first one-electron reduction energies and the encapsulated Xn+ charge is found, with a slope of 48 mV per unit charge. The protonation also shifts the reduction potential to more positive values, but the effect is much larger. In connection to this, the last proton's pKa = 2 for the Na+ derivative was estimated to be in reasonable agreement with experiment. The electronic structure of lanthanide derivatives is more complex than conventional POM structures. The reduction energy for the CeIV-Preyssler + 1e- --> CeIII-Preyssler process was computed to be more exothermic than that of very oxidant species such as S2Mo18O624-. 相似文献
94.
Dr. Sílvia Osuna Dr. Ramón Valencia Dr. Antonio Rodríguez‐Fortea Prof. Marcel Swart Prof. Miquel Solà Prof. Josep M. Poblet 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(29):8944-8956
In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc3N@Ih‐C80 and its D5h counterpart Sc3N@D5h‐C80, and the (bio)chemically relevant lutetium‐ and gadolinium‐based M3N@Ih/D5h‐C80 EMFs (M=Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels–Alder cycloaddition of s‐cis‐1,3‐butadiene on all the different bonds of the Ih‐C80 and D5h‐C80 cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc3N@C80. Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D 5h ‐b , which is the most similar to the unique [5,6] bond type in the icosahedral cage, I h ‐a , is the most reactive bond in M3N@D5h‐C80 regardless of M. Sc3N@C80 and Lu3N@C80 give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M=Gd, as previously found in similar metallofullerenes. Calculations also show that the D5h isomer is more reactive from the kinetic point of view than the Ih one in all cases which is in good agreement with experiments. 相似文献
95.
Vilà N Aparicio PA Sécheresse F Poblet JM López X Mbomekallé IM 《Inorganic chemistry》2012,51(11):6129-6138
The unusual redox behavior displayed by the two isomers of the Wells-Dawson phosphotungstate anion [Fe(H(2)O)P(2)W(17)O(61)](7-) is presented. The electrochemical measurements have been performed in aqueous media at different pH values from 0.5 up to 8.0. The cyclic voltammetry has also been carried out in organic media to get additional experimental data to establish the effect of the protonation on the redox properties of both isomers. At high pH values (pH ≥ 6) or in an organic medium, the reduction of the Fe center is easier in the case of the alpha-1 isomer, whereas for the alpha-2 isomer such reduction takes place at more negative potentials, as expected. In contrast, at lower pH values (pH ≤ 5), an inversion of this trend is observed, and the reduction of the Fe center becomes easier for the alpha-2 isomer compared to the alpha-1. We were able to highlight the influence of the pH and the pK(a) of the electrolyte on POM-based redox potentials given the pK(a) of the latter. A complementary theoretical study has also been performed to explain the experimental data obtained. In this sense, the results obtained from the DFT study are in good agreement with the experimental data mentioned above and have provided additional information for the electrochemical behavior of both isomers according to their different molecular orbital energies. We have also shown the influence of protonation state of the iron derivative on the relative reduction potentials of both isomers. 相似文献
96.
P Jiménez-Lozano ID Ivanchikova OA Kholdeeva JM Poblet JJ Carbó 《Chemical communications (Cambridge, England)》2012,48(74):9266-9268
Kinetic and DFT studies revealed that protonation of Ti-containing polyoxometalates (Ti-POM) lowers significantly the energy barrier for the heterolytic oxygen transfer from the Ti hydroperoxo intermediate to the alkene, increasing the activity and selectivity of alkene oxidation. 相似文献
97.
98.
99.
100.
El Moll H Nohra B Mialane P Marrot J Dupré N Riflade B Malacria M Thorimbert S Hasenknopf B Lacôte E Aparicio PA López X Poblet JM Dolbecq A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(50):14129-14138
The [ε-PMo(V)(8)Mo(VI)(4)O(36)(OH)(4){Ln(III)(H(2)O)}(4)](5+) (Ln=La, Ce, Nd, Sm) polyoxocations, called εLn(4), have been synthesized at room temperature as chloride salts soluble in water, MeOH, EtOH, and DMF. Rare-earth metals can be exchanged, and (31)P NMR spectroscopic studies have allowed a comparison of the affinity of the reduced {ε-PMo(12)} core, thus showing that the La(III) ions have the highest affinity and that rare earths heavier than Eu(III) do not react with the ε-Keggin polyoxometalate. DFT calculations provide a deeper insight into the geometries of the systems studied, thereby giving more accurate information on those compounds that suffer from disorder in crystalline form. It has also been confirmed by the hypothetical La→Gd substitution reaction energy that Ln ions beyond Eu cannot compete with La in coordinating the surface of the ε-Keggin molybdate. Two of these clusters (Ln=La, Ce) have been tested to evidence that such systems are representative of a new efficient Lewis acid catalyst family. This is the first time that the catalytic activity of polyoxocations has been evaluated. 相似文献