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91.
Celso P. de Melo Humberto S. Brandi Alfredo A. S. da Gama 《Theoretical chemistry accounts》1984,65(2):91-98
We have used the transfer matrix technique to study the electronic structure of trans-polyacetylene, within a model Hamiltonian which includes second-neighbors interactions. The results show that the valence and conduction band widths and the energy of the localized state are dependent on the strength of the second neighbors couplings. However, for the physically reasonable range of parameters, the electronic structure of the material is only slightly modified, indicating that a simple Hückel treatment is quite accurate for this system.Work partially supported by the Brazilian Agencies FINEP and CNPq 相似文献
92.
—From Stemmadenia Donnell-Smithii and S. Galeottiana, the following new alkaloids were isolated: (+)-quebrachamine (IV), an optical isomer of the known (−)-quebrachamine; isovoacangine (IIIa), a position isomer of voacangine (Ia), voacamine, tabernanthine (IIIb) and ibogamine. 相似文献
93.
A second-quantization formalism combined with a hypervirial theorem is used to derive new recurrence relations for one-dimensional harmonic oscillator matrix elements. The most general case of 〈m|f(â, â+)|n〉 is considered, and the recurrence relations forf(â, â) = Xk, exp(?βX), and exp(?X2) are given as examples. The relations obtained are considerably simpler than those derived by using only the hypervirial theorem; comparatively, the recurrence relations presented here have the advantage of avoiding the use of the quantum mechanical sum-rules when determining initial matrix elements. The proposed procedure can be used to determine the recurrence relations for other potentials as well as to evaluate the two-center integrals. 相似文献
94.
Leonardo Sena Gomes Teixeira Antônio Celso Spínola Costa Júlio César Rosa Assis Sérgio Luis Costa Ferreira Mauro Korn 《Mikrochimica acta》2001,137(1-2):29-33
A new sensitive method exploiting solid-phase spectrophotometry is proposed for the determination of cobalt in pharmaceutical
preparations. The chromogenic reagent 1-(2-thiazolylazo)-2-naphthol (TAN) was immobilized on C18 bonded silica loaded into
a home-made cell with 1.5 mm of optical path for cobalt determination. Cobalt(II) reacts with TAN on C18 material, at pH 6.0–7.5,
to give a coloured complex which has maximum absorption at 572 nm. In this way, the sample was passed through the cell and
Co(II) ions were quantitatively retained on the solid-phase. After the direct measurement of light-absorption in the solid
phase, only the cobalt was eluted with 0.1 mol L−1 hydrochloric acid. The cell was washed with water and then another sample solution could be passed through the cell. The
procedure allowed the determination of cobalt in the range of 10–160 μg L−1 with coefficient of variation of 4.7% (n=10) and apparent molar absorptivity of 2.62 × 106 L mol−1 cm−1 using sample volume of 3-mL.
Received May 15, 2000. Revision August 28, 2000. 相似文献
95.
J.O. Tocho G.M. Bilmes H.F. Ranea Sandoval 《Applied physics. B, Lasers and optics》1999,69(5-6):473-476
Second-harmonic conversion of the 532-nm pulsed output of a doubled Nd:YAG laser in KDP was characterized by analyzing the
changes in the acoustic signals generated in the crystal at different orientation conditions and for several incident fluences.
Using a piezoelectric transducer, phase-matching condition was determined by maximizing the amplitude of the acoustic signals
detected. The angular orientation for optimum harmonic efficiency was obtained with the same precision compared with the conventional
optical procedure. The origin of the photoacoustic signals is the relaxed energy following the absorption of 266-nm photons.
To determine the mechanisms of the 266-nm absorption processes, we also performed experiments under direct illumination with
the 266-nm emission of the quadrupled Nd:YAG laser. A combination of a linear and nonlinear process occurs. Direct absorption
by KDP as well as the participation of transient defects produced in the material were analyzed.
Received: 27 July 1998 / Revised version: 19 March 1999 / Published online: 19 August 1999 相似文献
96.
Jos Luis Mata‐Mata J. Alfredo Gutirrez M. Angeles Paz‐Sandoval Amira R. Madrigal Antonio Martínez‐Richa 《Journal of polymer science. Part A, Polymer chemistry》2006,44(24):6926-6942
End‐functionalized polyesters have been synthesized by ring‐opening polymerization (ROP) of ?‐caprolactone (CL) initiated with five different ruthenium derivatives in the presence of a series of alcohols as transfer agents. Mechanistic studies were performed for ROP of CL with RuCl2(PPh3)3 ( I ), TpRuCl(PPh3)2 ( II ), and TpRuCl(PHPh2)(PPh3) ( III ) as catalysts in the presence or absence of benzyl alcohol (BzOH). Obtained molecular weights are proportional to CL/BzOH ratio, but there is not a direct relationship with CL/ruthenium complex ratios. 1H and 13C NMR spectroscopy revealed the existence of benzyl ester end‐groups. Catalysis involves (a) dissociation of ruthenium complexes, (b) coordination of the lactone CL, (c) coordination of the BzOH with the formation of a metal alkoxide, (d) transfer from the alkoxyl ligand to the coordinated lactone, and (e) ring‐opening of CL by oxygen‐acyl bond cleavage. The proposed mechanism is supported by 1H, 13C, and 31P NMR, gel permeation chromatography (GPC), and MALDI‐TOF analysis of the polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6926–6942, 2006 相似文献
97.
98.
TTT plots: a perl program to create time-to-target plots 总被引:1,自引:0,他引:1
99.
Many examples of chemical and biological processes take place in large-scale environmental flows. Such flows generate filamental patterns which are often fractal due to the presence of chaos in the underlying advection dynamics. In such processes, hydrodynamical stirring strongly couples into the reactivity of the advected species and might thus make the traditional treatment of the problem through partial differential equations difficult. Here we present a simple approach for the activity in inhomogeneously stirred flows. We show that the fractal patterns serving as skeletons and catalysts lead to a rate equation with a universal form that is independent of the flow, of the particle properties, and of the details of the active process. One aspect of the universality of our approach is that it also applies to reactions among particles of finite size (so-called inertial particles). 相似文献
100.
We show that chaotic attractors are rarely found in multistable dissipative systems close to the conservative limit. As we approach this limit, the parameter intervals for the existence of chaotic attractors as well as the volume of their basins of attraction in a bounded region of the state space shrink very rapidly. An important role in the disappearance of these attractors is played by particular points in parameter space, namely, the double crises accompanied by a basin boundary metamorphosis. Scaling relations between successive double crises are presented. Furthermore, along this path of double crises, we obtain scaling laws for the disappearance of chaotic attractors and their basins of attraction. 相似文献