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排序方式: 共有151条查询结果,搜索用时 15 毫秒
41.
42.
Pere M. Deyà Antonio Frontera Guillem A. Suñer David Quiñonero Carolina Garau Antoni Costa Pau Ballester 《Theoretical chemistry accounts》2002,108(3):157-167
The structural and energetic changes associated with C–N bond rotation in a squaric acid derivative as well as in formamide,
3-aminoacrolein and vinylamine have been studied theoretically using ab initio molecular orbital methods. Geometry optimizations
at the MP2(full)/6-31+G* level confirmed an increase in the C–N bond length and a smaller decrease in the C=O length on going
from the equilibrium geometry to the twisted transition state. Other geometrical changes are also discussed. Energies calculated
at the QCISD(T)/6-311+G** level, including zero-point-energy correction, show barrier heights decreasing in the order formamide,
squaric acid derivative, 3-aminoacrolein and vinylamine. The origin of the barriers were examined using the atoms-in-molecules
approach of Bader and the natural bond orbital population analysis. The calculations agree with Pauling's resonance model,
and the main contributing factor of the barrier is assigned to the loss of conjugation on rotating the C–N bond. Finally,
molecular interaction potential calculations were used to study the changes in the nucleophilicity of N and O (carbonyl) atoms
upon C–N rotation, and to obtain a picture of the abilities of the molecules to act in nonbonded interactions, in particular
hydrogen bonds. The molecular interaction potential results confirm the suitability of squaramide units for acting as binding
units in host–guest chemistry.
Received: 13 March 2002 / Accepted: 23 June 2002 / Published online: 21 August 2002 相似文献
43.
Baldini L Ballester P Casnati A Gomila RM Hunter CA Sansone F Ungaro R 《Journal of the American Chemical Society》2003,125(46):14181-14189
Calix[4]arenes equipped with two and four zinc porphyrins have been prepared, and they show remarkable flexibility in their self-assembly properties with the bidentate ligand DABCO. The calix-bisporphyrin forms a 2:2 complex with DABCO, generating a large cavity that has the potential to act as a supramolecular host. The calix-tetraporphyrin, on the other hand, forms four different complexes with DABCO depending on the stoichiometry and concentration. During the course of a titration, all four complexes are populated, leading to large conformational changes and the formation of both intramolecular and intermolecular calix-tetraporphyrin-DABCO sandwich complexes. The system was fully characterized using a combination of UV-visible and (1)H NMR spectroscopy to identify the complexes. At a calix-tetraporphyrin:DABCO ratio of 2:4, the major species is dimeric cage assembly that features a large internal cavity for guest complexation. 相似文献
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46.
Sanchez-Pedreño C García MS Ortuño JA Albero MI Ballester E 《Fresenius' Journal of Analytical Chemistry》2001,369(7-8):680-683
A flow-through bulk optode based on the use of 1-(2-pyridylazo)-2-naphthol (PAN) immobilized in a plasticized poly(vinyl chloride) membrane entrapped in a cellulose support, in conjuntion with the flow injection analysis technique, is proposed for the determination of manganese(II). The calibration graph obtained at 570 nm was linear in the range 0.27-27.5 mg L(-1) (5 x 10(-6)-5 x 10(-4) M) Mn(II) with a detection limit of 0.18 mg L(-1). The coefficients of variation of the sensor response for 5.5 mg L(-1) of Mn(II) were +/-0.22% for consecutive measurements (n = 10), +/-0.48% between days (n = 5) and +/-0.38% between different membranes (n = 6). The sensor was readily regenerated with the carrier acetic acid/acetate buffer of pH 4.5. The method was applied to the determination of manganese in steels, waters and lemon tree leaves. 相似文献
47.
Salvador Toms Rafel Prohens Ghislain Deslongchamps Pablo Ballester Antoni Costa 《Angewandte Chemie (International ed. in English)》1999,38(15):2208-2211
A set of weak CH⋅⋅⋅O interactions is responsible for the recognition of tetraalkylammonium ions (ball-and-stick structure) by the tripodal receptors depicted in the diagram, in which the binding pocket is lined with squaramide units. The association constants of the fluorescent receptors are remarkably high at about 105 M −1 and can be used for signaling the presence of selected choline phospholipids. Z=RNH, where R=alkyl, arylamino; X−=BH4−, Cl−, Br−, I−. 相似文献
48.
David Quiñonero Antonio Frontera Salvador Tomàs Guillem A. Suñer Jeroni Morey Antoni Costa Pau Ballester Pere M. Deyà 《Theoretical chemistry accounts》2000,104(1):50-66
A computational study of the mechanism of host–guest complexation between quaternary ammonium compounds and squaramido-based
tripodal receptors has been carried out. Semiempirical molecular orbital calculations, which are in qualitative agreement
with experimental results have been performed using the PM3 Hamiltonian. Molecular interaction potential (MIP) maps were used
to analyze the suitability of both host and guest binding units for a high-affinity recognition process. MIP calculations
were computed from PM3 wavefunctions of the corresponding ammonium cations and dimethyl squaramide as a model compound for
the hydrogen-bond-acceptor unit of the receptors. MIP analyses are helpful for understanding the host–guest process from the
point of view of the double-complementarity principle.
Received: 23 June 1999 / Accepted: 22 September 1999 / Published online: 17 January 2000 相似文献
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50.
Jeroni Morey Maria Orell Miquel Àngel Barceló Pere M. Deyà Antoni Costa Pablo Ballester 《Tetrahedron letters》2004,45(6):1261-1265
We report the synthesis of two new anion receptors of a covalently linked 1,3,5-triarylbenzoamido-crown ether. Our results show that combined with a picrate salt they act by means of an intermolecular charge transfer process (EDA complex), as naked-eye sensors for basic anions, especially for sodium phytate in DMSO/H2O (1:1). 相似文献