全文获取类型
收费全文 | 412篇 |
免费 | 25篇 |
国内免费 | 3篇 |
专业分类
化学 | 355篇 |
力学 | 4篇 |
数学 | 45篇 |
物理学 | 36篇 |
出版年
2023年 | 5篇 |
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2019年 | 8篇 |
2018年 | 4篇 |
2017年 | 5篇 |
2016年 | 13篇 |
2015年 | 10篇 |
2014年 | 15篇 |
2013年 | 22篇 |
2012年 | 17篇 |
2011年 | 34篇 |
2010年 | 27篇 |
2009年 | 17篇 |
2008年 | 32篇 |
2007年 | 35篇 |
2006年 | 25篇 |
2005年 | 29篇 |
2004年 | 14篇 |
2003年 | 14篇 |
2002年 | 20篇 |
2001年 | 8篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1974年 | 2篇 |
1959年 | 2篇 |
1941年 | 1篇 |
1937年 | 1篇 |
1931年 | 1篇 |
排序方式: 共有440条查询结果,搜索用时 62 毫秒
11.
Dr. Tim Schleif Dr. Jörg Tatchen Julien F. Rowen Frederike Beyer Prof. Dr. Elsa Sanchez-Garcia Prof. Dr. Wolfram Sander 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(46):10452-10458
The Cope rearrangement of selectively deuterated isotopomers of 1,5-dimethylsemibullvalene 2 a and 3,7-dicyano-1,5-dimethylsemibullvalene 2 b were studied in cryogenic matrices. In both semibullvalenes the Cope rearrangement is governed by heavy-atom tunneling. The driving force for the rearrangements is the small difference in the zero-point vibrational energies of the isotopomers. To evaluate the effect of the driving force on the tunneling probability in 2 a and 2 b , two different pairs of isotopomers were studied for each of the semibullvalenes. The reaction rates for the rearrangement of 2 b in cryogenic matrices were found to be smaller than the ones of 2 a under similar conditions, whereas differences in the driving force do not influence the rates. Small curvature tunneling (SCT) calculations suggest that the reduced tunneling rate of 2 b compared to that of 2 a results from a change in the shape of the potential energy barrier. The tunneling probability of the semibullvalenes strongly depends on the matrix environment; however, for 2 a in a qualitatively different way than for 2 b . 相似文献
12.
Jorna Kalim Thibaut Duhail Dr. Ewa Pietrasiak Dr. Elsa Anselmi Dr. Emmanuel Magnier Prof. Dr. Antonio Togni 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(8):2638-2642
The direct trifluoromethylation of a variety of aliphatic alcohols using a hypervalent iodosulfoximine reagent afforded the corresponding ethers in moderate to good yields (14–72 %). Primary, secondary, and even tertiary alcohols, including examples derived from natural products, underwent this transformation in the presence of catalytic amounts of zinc bis(triflimide). Typical reaction conditions involved a neat mixture of 6.0 equivalents of the alcohol with 1.0 equivalent of the reagent, with the majority of reactions complete within 2 h with 2.5 mol % of the Lewis acid catalyst. Furthermore, experimental evidence was provided that the C−O bond-forming process occurred via the coordination of the alcohol to the iodine atom and subsequent reductive elimination. 相似文献
13.
Prof. Dr. Wolfram Sander Dr. Saonli Roy Kenny Bravo‐Rodriguez Dr. Dirk Grote Dr. Elsa Sanchez‐Garcia 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(40):12917-12923
The benzyl radical ( 1 ) is a key intermediate in the combustion and tropospheric oxidation of toluene. Because of its relevance, the reaction of 1 with molecular oxygen was investigated by matrix‐isolation IR and EPR spectroscopy as well as computational methods. The primary reaction product of 1 and O2 is the benzylperoxyl radical ( 2 ), which exists in several conformers that can easily interconvert even at cryogenic temperatures. Photolysis of radical 2 at 365 nm results in a formal [1,3]‐H migration and subsequent cleavage of the O?O bond to produce a hydrogen‐bonded complex between the hydroxyl radical and benzaldehyde ( 4 ). Prolonged photolysis produces the benzoyl radical ( 5 ) and water, which finally yield the phenyl radical ( 7 ), CO, and H2O. Thus, via a sequence of exothermic reactions 1 is transformed into radicals of even higher reactivity, such as OH and 7 . Our results have implications for the development of models for the highly complicated process of combustion of aromatic compounds. 相似文献
14.
Inherently Chiral Macrocyclic Oligothiophenes: Easily Accessible Electrosensitive Cavities with Outstanding Enantioselection Performances 下载免费PDF全文
Prof. Francesco Sannicolò Prof. Patrizia R. Mussini Prof. Tiziana Benincori Dr. Roberto Cirilli Prof. Sergio Abbate Serena Arnaboldi Simone Casolo Dr. Ettore Castiglioni Prof. Giovanna Longhi Dr. Rocco Martinazzo Dr. Monica Panigati Dr. Marco Pappini Dr. Elsa Quartapelle Procopio Dr. Simona Rizzo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15298-15302
Linear conjugated oligothiophenes of variable length and different substitution pattern are ubiquitous in technologically advanced optoelectronic devices, though limitations in application derive from insolubility, scarce processability and chain‐end effects. This study describes an easy access to chiral cyclic oligothiophenes constituted by 12 and 18 fully conjugated thiophene units. Chemical oxidation of an “inherently chiral” sexithiophene monomer, synthesized in two steps from commercially available materials, induces the formation of an elliptical dimer and a triangular trimer endowed with electrosensitive cavities of different tunable sizes. Combination of chirality with electroactivity makes these molecules unique in the current oligothiophenes literature. These macrocycles, which are stable and soluble in most organic solvents, show outstanding chiroptical properties, high circularly polarized luminescence effects and an exceptional enantiorecognition ability. 相似文献
15.
Photodynamic Therapy Efficacy Enhanced by Dynamics: The Role of Charge Transfer and Photostability in the Selection of Photosensitizers 下载免费PDF全文
Prof. Luis G. Arnaut Prof. Mariette M. Pereira Dr. Janusz M. Dąbrowski Elsa F. F. Silva Dr. Fábio A. Schaberle Dr. Artur R. Abreu Luís B. Rocha Dr. Madalina M. Barsan Prof. Krystyna Urbańska Prof. Grażyna Stochel Prof. Christopher M. A. Brett 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(18):5346-5357
Progress in the photodynamic therapy (PDT) of cancer should benefit from a rationale to predict the most efficient of a series of photosensitizers that strongly absorb light in the phototherapeutic window (650–800 nm) and efficiently generate reactive oxygen species (ROS=singlet oxygen and oxygen‐centered radicals). We show that the ratios between the triplet photosensitizer–O2 interaction rate constant (kD) and the photosensitizer decomposition rate constant (kd), kD/kd, determine the relative photodynamic activities of photosensitizers against various cancer cells. The same efficacy trend is observed in vivo with DBA/2 mice bearing S91 melanoma tumors. The PDT efficacy intimately depends on the dynamics of photosensitizer–oxygen interactions: charge transfer to molecular oxygen with generation of both singlet oxygen and superoxide ion (high kD) must be tempered by photostability (low kd). These properties depend on the oxidation potential of the photosensitizer and are suitably combined in a new fluorinated sulfonamide bacteriochlorin, motivated by the rationale. 相似文献
16.
Dr. Andrea Marraffa Dr. Piero Presenti Prof. Beatrice Macchi Dr. Francesca Marino-Merlo Prof. Mariella Mella Prof. Paolo Quadrelli 《ChemistryOpen》2020,9(5):528-537
Two new families of N,O-nucleoside analogues containing the anthracene moiety introduced through the nitrosocarbonyl ene reaction with allylic alcohols were prepared. The core structure is an isoxazolidine heterocycle that introduces either atom either a phenyl ring or dimethyl moiety at the C3 carbon. Different heterobases were inserted at the position 5 of the heterocyclic ring. One of the synthesized compounds demonstrated a good capacity to induce cell death and an appreciable nuclear fragmentation was evidenced in treated cells. 相似文献
17.
18.
About Earthquake Forecasting by Markov Renewal Processes 总被引:2,自引:0,他引:2
Elsa Garavaglia Raffaella Pavani 《Methodology and Computing in Applied Probability》2011,13(1):155-169
We propose and validate a new method for the evaluation of seismic hazard. In particular, our aim is to model large earthquakes
consistently with the underlying geophysics. Therefore we propose a non-Poisson model, which takes into account occurrence
history, improved with some physical constraints. Among the prevalent non-Poisson models, we chose the Markov renewal process,
which is expected to be sufficient to capture the main characteristics, maintaining simplicity in analysis. However, due to
the introduction of some physical constraint, our process differs significantly from others already presented in literature.
A mixture of exponential + Weibull distributions is proposed for the waiting times and their parameters are estimated following
the likelihood method. We validated our model, using data of earthquakes of high severity occurred in Turkey during the 20th
century. Our results exhibit a good accordance with the real events. 相似文献
19.
Sorin‐Claudiu Roşca Dr. Chiara Dinoi Dr. Elsa Caytan Dr. Vincent Dorcet Prof. Dr. Michel Etienne Prof. Dr. Jean‐François Carpentier Dr. Yann Sarazin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(19):6505-6509
Strontium and calcium (alkaline earth: Ae) olefin complexes stabilised by secondary Ae???F?C and β‐agostic Ae???H?Si interactions are presented. Olefin coordination onto the alkaline earths is plain in the solid state, and it is thermodynamically favoured over the coordination of THF. The existence of the Ae???olefin interactions is corroborated by solution NMR data and DFT computations. The coordination mode of the olefin varies with steric effects and, if enforced, olefin dissociation can be compensated by the other non‐covalent interactions, as supported by DFT computations. 相似文献
20.
TREN (Tris(2-aminoethyl)amine): an effective scaffold for the assembly of triple helical collagen mimetic structures 总被引:1,自引:0,他引:1
Kwak J De Capua A Locardi E Goodman M 《Journal of the American Chemical Society》2002,124(47):14085-14091
A new scaffold, TREN-(suc-OH)(3) where TREN is tris(2-aminoethyl)amine and suc is the succinic acid spacers, was incorporated to assemble triple helices composed of Gly-Nleu-Pro sequences (Nleu denotes N-isobutylglycine). Extensive biophysical studies which include denaturation studies, CD and NMR spectroscopy, and molecular modeling demonstrated that TREN-[suc-(Gly-Nleu-Pro)(n)-NH(2)](3) (n = 5 and 6) form stable triple helical structures in solution. A comparative analysis of TREN-assembled and KTA-assembled collagen mimetics (KTA denotes Kemp triacid, 1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid) indicates that the flexibility of the TREN scaffold is superior to the KTA scaffold in inducing triple helicity. This effect most likely arises from the flexibility of the TREN scaffold which allows the three peptide chains to adjust their register for a tighter triple helical packing. 相似文献