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994.
Joseph Delhalle Jean -Marie André Simone Delhalle Christiane Pivont-Malherbe Françoise Clarisse Georges Leroy Daniel Peeters 《Theoretical chemistry accounts》1977,43(3):215-225
The application of the Linear Combination of Localized Orbitals (LCLO) method to the band structure of polyethylene in the T, TG, TGTG' and G conformations is reported. The method uses full ab initio results on molecules to construct Fock matrix elements and deduce the energy bands. The main advantages of the technique are the low cost and the simplicity. Results are compared with those obtained from published ab initio and empirical calculations.Chargé de Recherches of the Fonds National Belge de la Recherche Scientifique.Titulaire d'une bourse de spécialisation I.R.S.I.A. 相似文献
995.
Perfetti L Georges A Florens S Biermann S Mitrovic S Berger H Tomm Y Höchst H Grioni M 《Physical review letters》2003,90(16):166401
High-resolution angle-resolved photoemission data show that a metal-insulator Mott transition occurs at the surface of the quasi-two-dimensional compound 1T-TaSe2. The transition is driven by the narrowing of the Ta 5d band induced by a temperature-dependent modulation of the atomic positions. A dynamical mean-field theory calculation of the spectral function of the half-filled Hubbard model captures the main qualitative feature of the data, namely, the rapid transfer of spectral weight from the observed quasiparticle peak at the Fermi surface to the Hubbard bands, as the correlation gap opens up. 相似文献
996.
We demonstrate a novel method for improvement of spatial beam quality by use of the angular selectivity of a thick phase grating inserted into various laser cavities. Preliminary experiments with an intentionally degraded beam from a Nd:YVO4 laser have enhanced beam propagation factor M2 by a factor of 4.5. Operation with a broad-area laser diode in an external cavity with a thick grating was achieved. We obtained 100 mW of cw power in a beam with an M2 of 1.3. Moreover, a reduction of the output spectrum by as much as a factor of 3.8 was obtained. 相似文献
997.
The evolution of a spherical gaseous interface accelerated by a plane weak shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. Different gaseous bubbles, i.e., helium, nitrogen, and krypton, were introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in the spherical geometry for negative, close to zero, and positive initial density jumps across the interface. We show that the bubble distortion is strongly different for the three cases and we present the experimental velocity and volume of the developed vortical structures. We prove that at late times the bubble velocities reach constant values which are in good agreement with previous calculations. Finally, we point out that, in our flow conditions, the gaseous bubble motion and shape are mainly influenced by vorticity and aerodynamic forces. 相似文献
998.
The axial and transverse diffusion coefficients of a band of iodine in a chromatographic column were measured optically as a function of time. It was found that the axial diffusion coefficient remains constant even when the edges of the sample band get close to the wall. By contrast, the radial diffusion coefficient decreases progressively with increasing time when the edges of the sample band leave the core region and begin to diffuse inside the wall region. The local axial and transverse diffusion coefficients of the band decrease from the column center toward the wall. Hence, the increase in local height equivalent to a theoretical plate observed in the region close to the wall must be explained by increasing mass transfer resistances and degree of heterogeneity of the bed. 相似文献
999.
Raphaël JankowskiDelphine Joseph Christian CavéFrançoise Dumas Michèle OurevitchJacqueline Mahuteau Georges MorgantNada Bosnjakovi? Pavlovi? Jean d'Angelo 《Tetrahedron letters》2003,44(22):4187-4190
When conducted in DMSO, the Hünig's base-promoted condensation of 3-quinuclidinone with quinoline-4-carboxaldehyde gave an equimolar mixture of epimeric aldols 8 with an excellent yield. 相似文献
1000.
We report molecular dynamics potential of mean force (PMF) simulations on the capture of halide anions X(-) (F(-), Cl(-), Br(-)) by a tetrahedral receptor L(4+) built from four quaternary ammonium sites connected by six (CH(2))(n) chains, leading to the formation of inclusion complexes X(-) subset L(4+). Simulations performed with a reaction field correction of the electrostatics and with PME-Ewald summation gave very similar energy profiles. In aqueous solution, an energy barrier of 12-17 kcal mol(-1) was found for the three anions, mainly due to their dehydration when they enter through the largest triangular face of L(4+). In the inclusion complexes, the anion is anchored near the center of the cavity due to the electrostatic field of the four positively charged ammonium sites, shielded from the surrounding water molecules. It was predicted that L(4+) is selective for Cl(-) over Br(-) which both form stable inclusion complexes, while the F(-) complex should dissociate. The comparison of PMFs in aqueous solution and in the gas phase and the energy component analysis demonstrates the importance of solvent on the nature of these complexes and on the complexation energy profiles. The Cl(-)/Br(-) selectivity obtained from the dissociation pathways in water was in good agreement with the results of free energy perturbation simulations based on the "alchemical route" of a thermodynamic cycle, and consistent with experimental observations. 相似文献