Characterization of molecular shape resonances using different decouplings of the dilated electron propagator with application to2Π CO−and2B2g C2H4−shape resonances

1995 ◽  
Vol 103 (2) ◽  
pp. 676-682 ◽  
Author(s):  
Milan N. Medikeri ◽  
Manoj K. Mishra
Keyword(s):  
Molecular Shape ◽  
Electron Propagator ◽  
Shape Resonances

Molecular weight and contraction factors of hyperbranched poly(urea-urethane)s

e-Polymers ◽  
2006 ◽  
Vol 6 (1) ◽  
Author(s):  
Albena Lederer ◽  
Mona Abd Elrehim ◽  
Falko Schallausky ◽  
Dieter Voigt ◽  
Brigitte Voit
Keyword(s):  
Chemical Structure ◽  
Molar Mass ◽  
Molecular Shape ◽  
Branched Polymers ◽  
Compact Structure ◽  
H Nmr ◽  
Hyperbranched Poly ◽  
End Groups ◽  
Solution Behaviour

AbstractElution fractionations of broadly molar mass-distributed, hyperbranched poly(urea-urethane)s with OH- and Ph- end groups were carried out to obtain a number of macromolecular samples possessing different molar masses at the same chemical structure. The characterization of the polymers was performed by SEC, MALLS, 1H-NMR, MALDI-TOF-MS and viscosity measurements. Modifications of the OH-end groups of the polymers were carried out. We observed a strong influence of the end groups on the solution behaviour of the hyperbranched samples. The molecular shape of the hyperbranched polymers in solution was compared to their linear analogue using the Kuhn-Mark-Houwink- Sakurada relationship. The calculated contraction factors between 0,15 and 0,7 depending on the molar masses correspond to a very compact structure of the modified branched polymers at high molar masses.

Quantum chemical shape: new density domain relations for the topology of molecular bodies, functional groups, and chemical bonding

1994 ◽  
Vol 72 (3) ◽  
pp. 928-935 ◽  
Author(s):  
Paul G. Mezey
Keyword(s):  
Functional Groups ◽  
Chemical Bonding ◽  
Quantum Chemical ◽  
Chemical Characterization ◽  
Finite Sequence ◽  
Molecular Shape ◽  
Wide Range ◽  
Density Values ◽  
Skeletal Model

A density domain (DD) is the formal body enclosed by a molecular isodensity contour (MIDCO) surface. Individual nuclear neighborhoods and various formal molecular fragments can be regarded as fuzzy moieties of electron densities, dominated by one or several nuclei. Such a fuzzy fragment involves a whole range of density values, hence it cannot be described by a single MIDCO, but it can be represented by a sequence of density domains. Within the chemically important range of density values, there are only a finite number of topologically different bodies of density domains. In the Density Domain Approach, chemical bonding is described by the interfacing and mutual interpenetration of local fuzzy charge density clouds. The bonding between fragments of a molecule is characterized by a finite sequence of density domains within a wide range of density values and by the correponding sequence of topological patterns of the mutual interpenetration of these fragments. In earlier works, the DD approach was advocated as an alternative to the conventional "skeletal model" of chemical bonding. The classically motivated line diagrams as representatives of bonding are replaced by the pattern of interpenetration of fuzzy fragment bodies at various density thresholds. In this study, novel DD relations are described, suitable for a quantum chemical characterization of functional groups, the local shape properties of such groups, and their contributions to global molecular shape.

scholarly journals Characterization and Some Insights into the Reaction Chemistry of Polymethylsilsesquioxane or Methyl Silicone Resins

2012 ◽  
Vol 2012 ◽  
pp. 1-17 ◽  
Author(s):  
Maki Itoh ◽  
Fukuyo Oka ◽  
Michitaka Suto ◽  
Simon D. Cook ◽  
Norbert Auner
Keyword(s):  
Gas Chromatography ◽  
Molecular Shape ◽  
Gas Chromatography Mass Spectrometry ◽  
Low Molecular Weight ◽  
Silicone Resin ◽  
Methyl Silicone ◽  
Siloxane Bond ◽  
Cage Structure ◽  
Bond Rearrangement

Structural characterization of a polymethylsilsesquioxane (PMSQ) and a DT-type methyl silicone resin (MeDT) has been carried out by various instrumental analyses including GPC, NMR, gas chromatography, and gas chromatography-mass spectrometry. Although the PMSQ had aMwaround 5000, the resin contained a significant amount of low molecular weight species consisting of T2[MeSi(OH)O2/2] and T3[MeSiO3/2] units, ranging from to including many isomers. One isomer of was isolated of which structure was determined as a cage structure. The species are supposed to consist mainly of cyclotetra- and cyclopentasiloxanes, but presence of strained rings such as cyclotrisiloxane rings also was suggested. In MeDT, species in which the T2units in the molecules from PMSQ is replaced with D2[Me2SiO2/2] were found, for example, , suggesting that general silicone resins consist of similar structures as silsesquioxanes. The Mark-Houwink exponent for these methyl resins was~0.3, indicating the molecular shape to be compact. Investigation on the formation chemistry of the cubic octamers indicates that siloxane bond rearrangement is an important mechanism in the molecule build-up process.

A detailed shape characterization of regular polypeptide conformations

1995 ◽  
Vol 73 (2) ◽  
pp. 241-248 ◽  
Author(s):  
Gustavo A. Arteca
Keyword(s):  
Secondary Structure ◽  
Protein Secondary Structure ◽  
Molecular Size ◽  
Molecular Geometry ◽  
Molecular Shape ◽  
Local Analysis ◽  
Structural Motifs ◽  
Protein Backbones ◽  
Shape Characterization

Regular polypeptide conformations include secondary structural motifs such as α-helices and β-strands. The occurrence of some regular conformation is usually deduced from a local analysis of dihedral angles. However, the value of a dihedral angle in itself does not provide any information on the conformation's "shape." This drawback can be circumvented with global, rather than local, macromolecular shape descriptors. Recently, fractal exponents have been proposed as a source of such descriptors. Yet, this approach does not fully capture all essential shape features, since protein backbones are not fractal. In this work, we deal instead with a more "natural" characterization of the polymer's global shape that uses both the chain's geometry and "topology." For the geometry, we study the behaviour of molecular size and anisometry. For the chain's folding features, we study the self-entanglements in a polymer fold. We compute these descriptors for all relevant secondary structural motifs. By using self-entaglements and molecular geometry, we provide a view of secondary structure that is both conceptually appealing and also more discriminating than previous ones in the literature. Keywords: molecular shape analysis, protein secondary structure, self-entanglements.

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