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  • 1
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 38 (1990), S. 843-850 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Previous energy-based criteria for aromaticity developed from one electron eigenvalues (using methods ranging from Huckel to ab initio calculations) have not correlated well with experimentally derived criteria such as χ⊥nlcl or chemical reactivity data, and are particularly poor for heteroaromatic compounds. Previous work has shown that criteria derived from the lowest π molecular orbitals of heteroaromatic compounds correlate well with χ⊥nlcl and chemical reactivity, prompting us to modify Kollmar's procedure for determining aromatic stabilization energies by localizing the lowest π orbital rather than the entire π bonding manifold. This modified procedure was implemented with both the natural (NLMO) and Boys orbital localization methods, and was applied to a set of ten five-membered ring heteroaromatics and to pyridine and its diazine derivatives. The new criterion ΔπL correlated well with χ⊥nlcl (R2 = 0.86 (NLMO); R2 = 0.92 (Boys)) for five-membered rings, and with theoretical measures of aromaticity for both sets of heteroaromatics. The Boys procedure gave better overall correlations and was markedly superior for pyrazole and pyridazine.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 34 (1988), S. 207-218 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio molecular orbital calculations were performed on a series of phosphazene trimer molecules to elucidate the electronic structure of these systems. The role of the d orbitals was analyzed from comparison of the molecular orbital contour calculations performed both with and without d orbitals in a split valence basis set description. The results indicate that, although the major geometric aspects of these systems can be described without invoking the use of d orbitals, these orbitals are essential to properly describe the electronic structure. The d orbital involvement in out-of-plane π bonding contains elements of both the classical Craig, Paddock, and Mitchell [1-3] and Dewar et al. [4] bonding schemes; the d orbital effects on in-plane bonding modes are also substantial. The charge distribution in these systems is best described as a zwitterionic form, because a significant amount of charge accumulates on the ring nitrogens.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    Journal of Electron Microscopy Technique 13 (1989), S. 277-287 
    ISSN: 0741-0581
    Keywords: Liposome ; Unilamellar vesicle ; Hexagonal HII phase ; Lipidic particle ; Filipin ; Life and Medical Sciences ; Cell & Developmental Biology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Natural Sciences in General
    Notes: Model membrane systems are used extensively in all aspects of membrane research, and freeze-fracture is the preeminent procedure for directly visualizing local structure in these lipid dispersions. Here we describe in detail the formation of liposomes and how freeze-fracture is routinely employed as a complementary technique to biophysical and biochemical procedures in the characterization of multilamellar vesicles (most commonly known as liposomes) and unilamellar vesicles. Many preparative procedures exist for the formation of multi- and unilamellar vesicles. Examples of each system are given and their properties as well as freeze-fracture morphology are discussed. The detection of lipid-phase transitions is considered, in particular, with emphasis on the application of freeze-fracture to the study of lipid polymorphism. We briefly discuss the fracturing of apolar lipids which do not adopt bilayer structures but which can be stabilized into microemulsions by a phospholipid monolayer. Finally, a critical assessment is made of filipin as a morphological marker for cholesterol domains in the plane of the bilayer.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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