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排序方式: 共有107条查询结果,搜索用时 31 毫秒
1.
Güizado-Rodríguez M Ariza-Castolo A Merino G Vela A Noth H Bakhmutov VI Contreras R 《Journal of the American Chemical Society》2001,123(37):9144-9152
Bis(NBH(3)), bis(NBF(3)), and NBF(3)/NBH(3) adducts 1-3 were prepared from 1,3-dimethyl-1,3-diazolidine and characterized by the (1)H, (13)C, (11)B, (19)F, 2D (1)H(-13)C HETCOR and NOESY NMR spectra. The structures and conformations of the adducts were established by the variable-temperature (1)H NMR spectra, the X-ray diffraction method (adduct 2A), and density functional calculations at different theoretical levels. The experimental and theoretical data have revealed that bis adducts 1-3 prefer trans orientations of the borane groups (1A, 2A, 3A) in solution, the solid state, and the gas phase. The studies have shown that the energetic preference of trans adducts with respect to cis compounds, decreasing as 2A (2.9 kcal/mol) > 3A (2.7 kcal/mol) > 1A (1.4 kcal/mol), is dictated by spatially repulsive interactions between the CH(3), BH(3), and BF(3) groups. The results of DFT calculations agree well with an experimental trans/cis isomeric ratio of 9:1 determined in solutions of adduct 1. The calculated geometry and energy, as well as the topological analysis of electronic densities, show that trans adducts 1-3 should exist in gas phase as twist conformations T-2 stabilized by the intramolecular C-H(delta+)...(-delta)H-B or C-H(delta+)...(-delta)F-B interactions. These interactions are characterized as closed-shell. The energy of one proton-hydride and proton-fluoride intramolecular contact, estimated as 1.9 (1A-T-2) and 0.7 (2A-T-2) kcal/mol, respectively, classifies the "elongated" intramolecular interactions CH(delta+)...(-delta)HB and CH(delta+)...(-delta)FB as weak ones. It has been established that, on going from gas phase to a condensed phase (solution and solid), the twist-conformations T-2 transform to conformations T-1, probably by intermolecular dipole-dipole interactions. The data presented in this work show that despite a weakness of the "elongated" proton-hydride and proton-fluoride interactions, they can play a significant role in the stabilization of conformational molecular states, especially when cooperativity is in action. 相似文献
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In this note a representation of the discrete Green's function of a compact discretization of a two point boundary value problem of order n 2 is given which among other things allows a direct proff of the convergence (and divergence) properties. 相似文献
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Potentiometric and volumetric determination of 6-mercaptopurine 总被引:1,自引:0,他引:1
Three methods for the determination of 6-mercaptopurine in the pure state and in tablets are described. One method is based on the reaction of this compound with silver ions, whereby an equivalent amount of nitric acid is liberated, the latter being determined potentiometrically with sodium hydroxide. The other two methods involve titration with chloramine-T, with potentiometric and visual end-point-detection. All methods afford accurate and reproducible results and can be used for routine assay of 6-mercaptopurine in tablets. 相似文献
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Miguel A. González Juan José Palacios Camino R. Vela Alejandro Hernández-Arauzo 《Journal of Heuristics》2017,23(2-3):81-110
Single machine scheduling problems have many real-life applications and may be hard to solve due to the particular characteristics of some production environments. In this paper, we tackle the single machine scheduling problem with sequence-dependent setup times with the objective of minimizing the weighted tardiness. To solve this problem, we propose a scatter search algorithm which uses path relinking in its core. This algorithm is enhanced with some procedures to speed-up the neighbors’ evaluation and with some diversification and intensification techniques, the latter taking some elements from iterated local search. We conducted an experimental study across a well-known set of instances to analyze the contribution of each component to the overall performance of the algorithm, as well as to compare our proposal with the state-of-the-art metaheuristics, obtaining competitive results. We also propose a new benchmark with larger and more challenging instances and provide the first results for them. 相似文献
7.
J. Vela R. Viguera M. Santiago A. García A. Jimnez I. Perian M.R. Snchez R. Pereíguez V.I. Martín E. Galbis R. Snchez R. Jimnez R. Prado-Gotor 《Chemical physics》2006,320(2-3):181-187
The substitution reaction of 4,4′-bpy (4,4′-bipyridine) for the aquo ligand in aquopentacyanoferrate (II) was studied in solutions containing -, β- and γ-cyclodextrins. We conclude that the well-known pseudophase model is equivalent to the Brönsted equation in regard to the interpretation of the cyclodextrins effects on reactivity. The equivalence follows from the fact that both pseudophase and Brönsted approaches can be used to rationalize kinetic data and permit to quantify the inclusion of the aromatic ligand in the cyclodextrins. 相似文献
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Castillo D Astudillo P Mares J González FJ Vela A Tiburcio J 《Organic & biomolecular chemistry》2007,5(14):2252-2256
Cations derived from 1,2-bis(benzimidazolium)ethane can penetrate the cavity of dibenzo-24-crown-8 macrocycles to produce a new family of [2]pseudorotaxanes. These supramolecular structures are held together by a series of charge-assisted hydrogen bonds (+)N-H[dot dot dot]O, ion-dipole and pi-stacking interactions. These new adducts were fully characterised by NMR spectroscopy, ESI mass spectrometry and single-crystal X-ray diffraction. The effect of electron-donating and electron-withdrawing groups on the association constants was also analyzed. Chemical control of the threading/unthreading process was acheived by the alternate addition of acid and base. 相似文献