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排序方式: 共有221条查询结果,搜索用时 718 毫秒
51.
Bartoli G Bosco M Giuliani A Marcantoni E Palmieri A Petrini M Sambri L 《The Journal of organic chemistry》2004,69(4):1290-1297
The Lewis acid promoted allylation of aldehydes has become an important carbon-carbon bond forming reaction in organic chemistry. In this context, we have developed an alternative over existing catalytic processes, wherein aldehydes are subject in acetonitrile to reaction of allylation with allyltributylstannane in the presence of 1.0 equiv of cerium(III) chloride heptahydrate (CeCl(3).7H(2)O), an inexpensive and mild Lewis acid. The allylation has been accelerated by using an inorganic iodide as a cocatalyst, and various iodide salts were examined. The procedure must use allylstannane reagent instead of allylsilane reagent, desirable for environmental reasons, but high chemoselectivity was observed, and this is opposite the results obtained with other classical Lewis acids such as TiCl(4) and Et(2)O.BF(3). 相似文献
52.
Letizia Mencaroni Benedetta Carlotti Fausto Elisei Assunta Marrocchi Anna Spalletti 《Chemical science》2022,13(7):2071
In this study, we report strong experimental evidence for singlet fission (SF) in a new class of fluorene-based molecules, exhibiting two-branched donor–acceptor structures. The time-resolved spectroscopic results disclose ultrafast formation of a double triplet state (occurring in few picoseconds) and efficient triplet exciton separation (up to 145% triplet yield). The solvent polarity effect and the role of intramolecular charge transfer (ICT) on the SF mechanism have been thoroughly investigated with several advanced spectroscopies. We found that a stronger push–pull character favors SF, as long as the ICT does not act as a trap by opening a competitive pathway. Within the context of other widely-known SF chromophores, the unconventional property of generating high-energy triplet excitons (ca. 2 eV) via SF makes these materials outstanding candidates as photosensitizers for photovoltaic devices.We found that a stronger push–pull character favours SF, as long as the ICT does not act as a trap. The unique property of generating high-energy triplets (ca. 2 eV) via SF makes these materials outstanding candidates for photovoltaic applications. 相似文献
53.
Emiliano Manzo M. Letizia Ciavatta Raffaella Puliti Yue-Wei Guo Guido Cimino 《Tetrahedron》2005,61(31):7456-7460
Three novel halogenated C15-acetogenins, compounds 1-3, have been isolated, together with known metabolites, from a South China Sea collection of the anaspidean mollusc Aplysia dactylomela. The structures have been suggested by both NMR analysis and comparison with literature data. The structure of 1 was confirmed by a single crystal X-ray study, which also allowed the establishment of its absolute stereochemistry. 相似文献
54.
Emiliano Manzo Maria Letizia Ciavatta Ernesto Mollo Guido Cimino 《Tetrahedron letters》2005,46(3):465-468
A new γ-pyrone propionate, compound 9, and its peroxy derivative 10 have been isolated from the sacoglossan Placobranchus ocellatus. The structure and the relative stereochemistry of the new molecules, which displayed an unprecedented carbon skeleton characterised by a bicyclo [4.2.0] octane, have been determined by both spectroscopic methods and comparison with model compounds. By analogy with photodeoxytridachione (2), a sunscreen protective role could be also suggested for compound 9 in living P. ocellatus. 相似文献
55.
Taccheo S Valle GD Osellame R Cerullo G Chiodo N Laporta P Svelto O Killi A Morgner U Lederer M Kopf D 《Optics letters》2004,29(22):2626-2628
Laser action is demonstrated in a 20-mm-long waveguide fabricated on an Er:Yb-doped phosphate glass by femtosecond laser pulses. An output power of 1.7 mW with approximately 300 mW of pump power coupled into the waveguide is obtained at 1533.5 nm. Waveguides are manufactured with the 520-nm radiation from a frequency-doubled, diode-pumped, cavity-dumped Yb:glass laser operating at a 166-KHz repetition rate, with a 300-fs pulse duration. 相似文献
56.
A. Menditto S. Palleschi A. Minoprio B. Rossi A. Calibotti F. Chiodo M. Patriarca 《Microchemical Journal》2000,67(1-3)
The presence of chemicals in the environment is a matter of concern in that it poses potential health risks. At present, exposure to toxic chemicals and their biological and biochemical effects can be better estimated by biological monitoring, through the systematic collection of specimens from potentially exposed humans. Biological monitoring of human exposure to environmental pollutants is hampered by the difficulty to assess data reliability. As a consequence, the validity of biological monitoring should depend on the strict implementation of a quality assurance (QA) program, which includes a series of procedures aiming to ensure that laboratory results meet defined standards of quality and are reliable. For the validation and monitoring of methods’ performance, to ensure the trueness of measurements and to warrant the traceability to international standards, reference materials (RMs) and certified reference materials should be used. Internal quality control and external quality assessment (EQA) are part of overall QA and are carried out to verify that analytical errors are compatible with the specific requirements or needs of the user. In particular EQA schemes (EQAS) allow to test independently the analytical performance of participating laboratories. In the last decades, increasing concern has been raised by urban air pollution; lead and benzene, two gasoline components released by motor vehicle exhausts, are known to be toxic to humans. For biological monitoring of lead exposure of the general population, screening campaigns, utilizing lead in blood as a biomarker, have been carried out since the 1970s. Strict strategies were adopted to ensure data comparability, including the preparation of RMs, the organization of EQAS and the cross-exchange and analysis of blood samples between laboratories. Biological monitoring of benzene exposure could be carried out by means of various biomarkers such as benzene in blood and benzene, trans,trans-muconic acid (t,t-MA) and S-phenylmercapturic acid (S-PMA) in urine. At present, few RMs and EQAS are available for these biomarkers. A pilot EQAS for t,t-MA in urine, adopted to assess the reliability of data regarding benzene exposure, has been organized and carried out between 1996 and 1997 in Italy. From the accrued experience, it clearly emerges the importance of strategies designed to guarantee the quality of biological monitoring data. The use of RMs and the participation in EQAS are highly recommended in order to improve the global performance of methods and laboratories involved in biological monitoring. 相似文献
57.
Letizia Pellegrini Shengkun Zhu 《Journal of Optimization Theory and Applications》2018,177(3):788-810
The scalar finite-dimensional case has been discussed in the first part of this work series, which aims at exploiting the image space analysis to establish a general regularity condition for constrained extremum problems. Based on this preliminary result, the present paper dedicates itself to further study the regularity conditions for vector constrained extremum problems in a Euclidean space. The case of infinite-dimensional image will be the subject of a subsequent paper. 相似文献
58.
Chiodo L Salazar M Romero AH Laricchia S Della Sala F Rubio A 《The Journal of chemical physics》2011,135(24):244704
Atomic clusters of TiO(2) are modeled by means of state-of-the-art techniques to characterize their structural, electronic and optical properties. We combine ab initio molecular dynamics, static density functional theory, time-dependent density functional theory, and many body techniques, to provide a deep and comprehensive characterization of these systems. TiO(2) clusters can be considered as the starting seeds for the synthesis of larger nanostructures, which are of technological interest in photocatalysis and photovoltaics. In this work, we prove that clusters with anatase symmetry are energetically stable and can be considered as the starting seeds to growth much larger and complex nanostructures. The electronic gap of these inorganic molecules is investigated, and shown to be larger than the optical gap by almost 4 eV. Therefore, strong excitonic effects appear in these systems, much more than in the corresponding bulk phase. Moreover, the use of various levels of theory demonstrates that charge transfer effects play an important role under photon absorption, and therefore the use of adiabatic functionals in time dependent density functional theory has to be carefully evaluated. 相似文献
59.
Marianna Carbone M. Letizia Ciavatta Gianluca De Rinaldis Francesco CastelluccioErnesto Mollo Margherita Gavagnin 《Tetrahedron》2014
The re-investigation of the Mediterranean opisthobranch Thuridilla hopei resulted in the isolation and chemical characterization of three novel nor-diterpene aldehydes, nor-thuridillonals 2–4, structurally related to co-occurring already reported thuridillins. Analogous with the thuridillins, the new molecules were thought to derive from the same dietary precursor epoxylactone 1. 相似文献
60.
J. Derbali F. AbdelMalek S. S. A. Obayya H. Bouchriha R. Letizia 《Optical and Quantum Electronics》2011,42(8):463-472
The development of technology in photonic crystal (PC) structures has seen rapid progress. Using PCs in biosensing area may
open new venues to achieve single molecule detection, and high resolution scanning. A novel PC sensor with improved performances,
in terms of size, compactness and sensitivity is presented in this paper. The sensing element consists of dielectric cylinders
with varying radius introduced along <01> and <10> directions of the crystal. The results show that the peak wavelength shifts
to the high frequency region when only six cylinders are filled with analytes. Also, the peaks show a larger shift compared
to the structure obtained using the entire PC waveguide as sensing region. The proposed sensor shows a better sensitivity
to water than other analytes, where the peak wavelength tends to shift towards the low frequency region. 相似文献