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1.
Busch BB Paz MM Shea KJ Staiger CL Stoddard JM Walker JR Zhou XZ Zhu H 《Journal of the American Chemical Society》2002,124(14):3636-3646
Trialkyl and aryl organoboranes catalyze the polymerization of dimethylsulfoxonium methylide (1). The product of the polymerization is a tris-polymethylene organoborane. Oxidation affords linear telechelic alpha-hydroxy polymethylene. The polymer molecular weight was found to be directly proportional to the stoichiometric ratio of ylide/borane, and polydispersities as low as 1.01-1.03 have been realized. Although oligomeric polymethylene has been the most frequent synthetic target of this method, polymeric star organoboranes with molecular weights of 1.5 million have been produced. The average turnover frequency at 120 degrees C in 1,2,4,5-tetrachlorobenzene/toluene is estimated at >6 x 10(6) g of polymethylene (mol boron)(-1) h(-1). The mechanism of the polyhomologation reaction involves initial formation of a zwitterionic organoborane.ylide complex which breaks down in a rate-limiting 1,2-alkyl group migration with concomitant expulsion of a molecule of DMSO. The reaction was found to be first order in the borane catalyst and zero order in ylide. DMSO does not interfere with the reaction. The temperature dependence of the reaction rate yielded the following activation energy parameters (toluene, DeltaH(++) = 23.2 kcal/mol, DeltaS(++) = 12.6 cal deg/mol, DeltaG(++) = 19.5 kcal/mol; THF, DeltaH(++) = 26.5 kcal/mol, DeltaS(++) = 21.5 cal deg/mol, DeltaG(++) = 20.1 kcal/mol). 相似文献
2.
Chemical strategies for iron acquisition in plants 总被引:3,自引:0,他引:3
Staiger D 《Angewandte Chemie (International ed. in English)》2002,41(13):2259-2264
Iron is an essential element for plant nutrition. Although iron is the fourth most abundant element (3 %) of the earth's crust, it is not readily available because of its low solubility. Therefore, plants need an active mechanism to extract iron from the soil. They have evolved several chemical strategies to acquire iron ions and the physiology of these mechanisms has been known for a long time. Only recently, the use of molecular genetic approaches has led to a biochemical and molecular characterization of the players involved, thus providing an entry to the manipulation of iron uptake in plants. 相似文献
3.
Complexes of the formula Cu2X3 (OR)(N∩N)2 [X = Cl, Br; R = H, CH3; N∩N = bidentate amineligand] are described, their magnetic and spectrophotometric properties are discussed and the possible structures deduced. 相似文献
4.
Cumulant functions are introduced to describe the statistical state of a radiation field. These functions are simply related
to the optical coherence functions but have some interesting features. It is shown that if the cumulant functions of all orders
greater than some numberN
0 vanish then they also vanish for all orders greater than 2. Thermal field is the only field having this property. This property
holds whether the field is described by a classical stochastic process or by a quantum density operator. Further the particular
operator ordering used in defining these cumulant functions for the quantized field affects only the second order cumulant
function. To describe the statistical state of a vector field such as partially polarized or unpolarized radiation, one would
need to introduce cumulant tensors. 相似文献
5.
MA Freitas CL Hendrickson AG Marshall 《Rapid communications in mass spectrometry : RCM》1999,13(15):1639-1642
We present a novel approach for the determination of activation energy for the unimolecular dissociation of a large (>50 atoms) ion, based on measurement of the unimolecular dissociation rate constant as a function of continuous-wave CO(2) laser intensity. Following a short ( approximately 1 s) induction period, CO(2) laser irradiation produces an essentially blackbody internal energy distribution, whose 'temperature' varies inversely with laser intensity. The only currently available method for measuring such activation energies is blackbody infrared radiative dissociation (BIRD). Compared with BIRD, FRAGMENT: (a) eliminates the need to heat the surrounding ion trap and vacuum chamber to each of several temperatures (each requiring hours for temperature equilibration); (b) offers a three-fold wider range of effective blackbody temperature; and (c) extends the range of applications to include initially cold ions (e.g., gas-phase H/D exchange). Our FRAGMENT-determined activation energy for dissociation of protonated bradykinin, 1.2 +/- 0.1 eV, agrees within experimental error to the value, 1.3 +/- 0.1 eV, previously reported by Williams et al. from BIRD experiments. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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Emmanuelle Villedieu-Percheron Saron Catak Didier Zurwerra Roman Staiger Mathilde Lachia Alain De Mesmaeker 《Tetrahedron letters》2014
An intramolecular 6π/10π-electrocyclization from ketene-iminium salts was developed for the preparation of naphthylamines. Various substituents on the nitrogen, on the aromatic ring, and on the olefin were studied. Tricyclic skeletons were obtained in few steps and good overall yields. The electrocyclization of ketene-iminium salts has been computationally explored by means of DFT calculations and their activation barriers were compared to the parent triene as well as the corresponding dienyl allenes and dienyl ketenes. Electrocyclizations for ketene-iminium salts were shown to be highly exergonic and have much smaller barriers to activation. 相似文献
10.
A. Leclère A. Hahn A. Kretschmann C. Du Rietz S. Hähnel Dr. N. Gerbers Co. m. b. H F. Tödt L. S. Foster I. J. Gruntfest M. Déribéré L. J. Desha R. E. Sherrill L. M. Harrison G. Kloz H. Leonhardt N. Bjerrum G. S. Hartley W. B. Fortuno M. G. Mellon I. M. Kolthoff L. S. Guss H. Eichler H. Gorke E. Köppe F. Staiger A. Thiel F. W. Panepinto M. Kilpatrick T. Teorell E. Stenhagen W. L. German 《Analytical and bioanalytical chemistry》1939,116(9-10):336-347