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排序方式: 共有804条查询结果,搜索用时 31 毫秒
1.
We give a Gray code and constant average time generating algorithm for derangements, i.e., permutations with no fixed points. In our Gray code, each derangement is transformed into its successor either via one or two transpositions or a rotation of three elements. We generalize these results to permutations with number of fixed points bounded between two constants. 相似文献
2.
The synthesis of 2-substituted 1,4-diacetoxybutadiene derivatives with a partial control of stereochemistry is described from two potentially precursor enals by a judicious choice of experimental conditions (Ac2O/DMAP in Et3N). These conditions have been successfully applied in the first total synthesis of caulerpenyne. 相似文献
3.
An alternative definition of entropy for non-commutative systems, equivalent to the one by A. Connes, H. Narnhofer, and W. Thirring, is given. It is based on the concepts of conditional entropy, and stationary couplings with classical systems. It allows to prove that any quantum dynamical system with singular spectrum has zero entropy. 相似文献
4.
It has been recently shown that computation with stochastic numbers as regard to addition and multiplication by scalars can be reduced to computation in familiar vector spaces. In this work we show how this can be used for the algebraic solution of linear systems of equations with stochastic right-hand sides. On several examples we compare the algebraic solution with the simulated solution using the CADNA package. 相似文献
5.
A novel approach to H-phosphonates from hypophosphorous acid using a transfer hydrogenation process was developed. This method is atom-economical, environmentally friendly, catalytic, and efficient, leading easily to H-phosphonate monoesters or ammonium salt in moderate to good yields. 相似文献
6.
E. Manova C. Estournès D. Paneva J.-L. Rehspringer T. Tsoncheva B. Kunev I. Mitov 《Hyperfine Interactions》2005,165(1-4):215-220
Iron–nickel spinel oxide NiFe2O4 nanoparticles have been prepared by the combination of chemical precipitation and subsequent mechanical milling. For comparison, their analogue obtained by thermal synthesis is also studied. Phase composition and structural properties of iron–nickel oxides are investigated by X-ray diffraction and Mössbauer spectroscopy. Their catalytic behavior in methanol decomposition to CO and methane is tested. An influence of the preparation method on the reduction and catalytic properties of iron–nickel samples is established. 相似文献
7.
The fragmentation patterns of 24 substituted phenyltrifluoromethanesulfonates has been determined by exact mass and metastable transition measurements. The influence of the ring substituent(s) on the abundance of the [ArO]+ ions has been investigated at low energies and a new standard parameter for plotting against Hammett's ó constants is proposed. The direct loss of CF3SO3 from the molecular ion of 3,5-(diisopropyl)-phenyl-, 3,5-(dicyclopropyl)phenyl- and 3,5-di(α-methylcyclopropyl)-phenyl-methanesulfonate, and further observations on the fragmentation processes of these compounds are consistent with M.O. calculations on the ‘through bond’ stabilization of the [Ar]+ σ state by meta electron donor substituents. 相似文献
8.
Zojer E Beljonne D Pacher P Brédas JL 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(11):2668-2680
Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems. 相似文献
9.
We present a quantum-chemical analysis of the molecular structure and second- and third-order polarizabilities in a series of promising nonlinear optical (NLO) chromophores, the zwitterionic ammonio/borato diphenylpolyenes, R3N+Ph(C=C)nPhB-R3, whose synthesis has been reported recently. The molecular geometries are obtained via MP2/6-31G optimization, while the NLO properties are calculated with the INDO Hamiltonian using the sum-over-states and finite-field real-space methods. The real-space approach allows the direct evaluation of the NLO-active segments of the molecules, while the sum-over-states results illustrate the virtual excitations and charge-transfer pathways that are essential in the NLO response. Both methods highlight the remarkable and unexpected result that it is the strongly polarized phenylene groups that play the key role in generating a high NLO response. 相似文献
10.
Xavier Jouan Jean-Luc Gardette 《Journal of polymer science. Part A, Polymer chemistry》1991,29(5):685-696
The evolution of the photochemical degradation of ABS has been studied in conditions of long wavelength irradiation (λ's> 300 nm). The main photoproducts involved in the oxidative evolution have been identified by using FTIR spectroscopy and chemical titrations. A particular attention has been devoted to α–β unsaturated ketones that appear as secondary photoproducts. Those ketones present a low photochemical stability when exposed in the range 300–400 nm. Conditions for their formation have been experimentally studied. Formation of oxidation photoproducts has been also studied at the macroscopic level and it has been shown that their repartition in the polymer is heterogeneous. The origins of the heterogenities have been studied. 相似文献