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排序方式: 共有196条查询结果,搜索用时 406 毫秒
121.
122.
Ortwin Bobleter Wolfgang Schwald Roland Concin Hanno Binder 《Journal of carbohydrate chemistry》2013,32(3):387-399
The sulfuric acid hydrolysis rate of cellobiose between pH 2 and 3 is directly proportional to the acid concentration. In good agreement with other authors, an activation energy of 133 kJ/Mol was found under these acidic conditions. The relation of the reaction rate constants for the glucose formation and glucose degradation (k1/k2) shows, in contrast to the hydrolysis of cellulose, little dependence on the temperature. Hydroxymethylfurfural, and to a lesser extent furfural, are glucose degradation products, which are also consumed but at a lower reaction rate than glucose. At pH values between 3 and 4.7 (pure water) strong deviations of the hydrolysis rates were observed. The formation of organic acids decreases the pH but has no influence on the reaction rate. This fact indicates that hydrothermolysis follows a reaction mechanism different from that of acidic hydrolysis. 相似文献
123.
Paul A Hull JF Norris MR Chen Z Ess DH Concepcion JJ Meyer TJ 《Inorganic chemistry》2011,50(4):1167-1169
Significant rate enhancements are found for benzyl alcohol oxidation by the Ru(V)═O(3+) form of the water oxidation catalyst [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] compared to Ru(IV)═O(2+) and for the Ru(IV)═O(2+) form with added bases due to a new pathway, concerted hydride proton transfer (HPT). 相似文献
124.
We demonstrate the existence of quantized "bulk" plasmons in ultrathin magnesium films on Si(111) by analyzing plasmon-loss satellites in core-level photoemission spectra, recorded as a function of the film thickness d. Remarkably, the plasmon energy is shown to vary as 1/d2 all the way down to three atomic layers. The loss spectra are dominated by the n=1 and n=2 normal modes, consistent with the excitation of plasmons involving quantized electronic subbands. With decreasing film thickness, spectral weight is gradually transferred from the plasmon modes to the low-energy single-particle excitations. These results represent striking manifestations of the role of quantum confinement on plasmon resonances in precisely controlled nanostructures. 相似文献
125.
Selective CH Functionalization of Methane,Ethane, and Propane by a Perfluoroarene Iodine(III) Complex
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Dr. Michael M. Konnick Dr. Brian G. Hashiguchi Dr. Deepa Devarajan Nicholas C. Boaz Prof. T. Brent Gunnoe Prof. John T. Groves Niles Gunsalus Prof. Daniel H. Ess Prof. Roy A. Periana 《Angewandte Chemie (International ed. in English)》2014,53(39):10490-10494
Direct partial oxidation of methane, ethane, and propane to their respective trifluoroacetate esters is achieved by a homogeneous hypervalent iodine(III) complex in non‐superacidic (trifluoroacetic acid) solvent. The reaction is highly selective for ester formation (>99 %). In the case of ethane, greater than 0.5 M EtTFA can be achieved. Preliminary kinetic analysis and density functional calculations support a nonradical electrophilic CH activation and iodine alkyl functionalization mechanism. 相似文献
126.
Hongyin Gao Qing‐Long Xu Muhammed Yousufuddin Daniel H. Ess Lszl Kürti 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(10):2739-2743
We disclose an efficient and operationally simple protocol for the preparation of fused N‐heterocycles starting from readily available 2‐nitrobiaryls and PhMgBr under mild conditions. More than two dozen N‐heterocycles, including two bioactive natural products, have been synthesized using this method. A stepwise electrophilic aromatic cyclization mechanism was proposed by DFT calculations. 相似文献
127.
Hanno Lefmann 《Discrete and Computational Geometry》2008,40(3):401-413
We consider a variant of Heilbronn’s triangle problem by investigating for a fixed dimension d≥2 and for integers k≥2 with k≤d distributions of n points in the d-dimensional unit cube [0,1]
d
, such that the minimum volume of the simplices, which are determined by (k+1) of these n points is as large as possible. Denoting by Δ
k,d
(n), the supremum of this minimum volume over all distributions of n points in [0,1]
d
, we show that c
k,d
⋅(log n)1/(d−k+1)/n
k/(d−k+1)≤Δ
k,d
(n)≤c
k,d
′/n
k/d
for fixed 2≤k≤d, and, moreover, for odd integers k≥1, we show the upper bound Δ
k,d
(n)≤c
k,d
″/n
k/d+(k−1)/(2d(d−1)), where c
k,d
,c
k,d
′,c
k,d
″>0 are constants.
A preliminary version of this paper appeared in COCOON ’05. 相似文献
128.
Hanno Essén 《International Journal of Theoretical Physics》1990,29(2):183-187
The global symmetry implied by the fact that one can multiply all masses with a common constant is made into a local, gauge symmetry. The matter action then becomes Conformally invariant and it seems natural to choose for the corresponding scalar gauge field the action for a conformally invariant (massless) scalar field. The resulting conformally invariant theory turns out to be equivalent to general relativity. Since this means that the usual Einstein-Hilbert action is not, in fact, a true gauge action for the space-time geometry, the full theory ought to be supplied with such a term. Gauge-theoretic arguments and conformal invariance requirements dictate its form. 相似文献
129.
Hanno Rund 《Foundations of Physics》1983,13(1):93-114
A certain class of geometric objects is considered against the background of a classical gauge field associated with an arbitrary structural Lie group. It is assumed that the components of these objects depend on the gauge potentials and their first derivatives, and also on certain gauge-dependent parameters whose properties are suggested by the interaction of an isotopic spin particle with a classical Yang-Mills field. It is shown that the necessary and sufficient conditions for the invariance of the given objects under a finite gauge transformation are embodied in a set of three relations involving the derivatives of their components. As a special case these so-called invariance identities indicate that there cannot exist a gauge-invariant Lagrangian that depends on the gauge potentials, the interaction parameters, and the4-velocity components of a test particle. However, the requirement that the equations of motion that result from such a Lagrangian be gauge-invariant, uniquely determines the structure of these equations. 相似文献
130.