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  • 1
    ISSN: 1432-2234
    Keywords: Valence bond theory, second quantization formulation of ∼
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Valence bond theory is formulated in terms of second quantized operators and is related to the theory of the unitary group of spin-free orbital transformations. The construction of Weyl basis states, the evaluation of matrix elements, and the application to a linked-diagram valence bond perturbation theory are all discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-2234
    Keywords: Valence-bond model ; π-Networks ; Heisenberg model ; Resonance theory ; Néel states ; Renormalization group ; Spin waves
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Approximate ground-state wavefunctions for valence-bond (or Heisenberg) models are obtained both within Néel-state-based and within Kekulé-state-based resonance-theoretic approaches. Comparisons are made between these and other general approaches, with particular emphasis on organic π-network systems. Attention is drawn to the manner in which the quality of the different approximation schemes changes with variations in structural characteristics of the system. It is suggested that resonance-theoretic ideas are most appropriate for (aromatic benzenoid) systems with low coordination number, whereas Néel-state based ideas are most appropriate for (3-dimensional) structures with higher coordination number (and little “frustration”).
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  • 3
    ISSN: 1432-2234
    Keywords: Kekulé structure enumeration ; Dimer covering enumeration ; 1-Factor enumeration ; Benzenoid π-network polymers ; Long-range order ; Bond localization ; Edge reactivity ; Graph theory ; Transfer matrices
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The problem of covering every site of a subsection of the honeycomb lattice with disjoint edges is considered. It is pointed out that a type of long-range order associated to such coverings can occur, so that different phases can arise as a consequence of the subsection's boundaries. These features are quantitatively investigated via a new analytic solution for a class of strips of arbitrary widths, arbitrary lengths, and arbitrary long-range-order values. Relations to work on the dimer covering problem of statistical mechanics and especially to the resonance theory of benzenoid hydrocarbons are noted.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 74 (1988), S. 349-361 
    ISSN: 1432-2234
    Keywords: Resonance theory ; Conjugated circuits ; Valence-bond theory ; Graphite ; Many-body theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Computations for the “conjugated circuits” model which has previously only been treated for finite and quasi-one-dimensional conjugatedπ-networks are here extended to the graphite lattice. Many-body techniques give the resultant resonance energy per site as a function of a physically relevant long-range order parameter.
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 85 (1993), S. 405-405 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1432-2234
    Keywords: Polymers ; Oligomers ; Benzenoids ; Resonance ; Conjugation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary We have initiated a comparative study of several families of structurally closely related large benzenoid hydrocarbons of increasing number of fused benzene rings. Local and global properties of such molecules have been studied with a particular interest in investigating the closeness of finite-size molecules for modelling infinite polymeric structures having the same structural patterns. We focus in this report on two questions: (1) how large is a finite molecule which well approximates an infinite system? and (2) how do terminal groups influence local properties in the central part of the molecule as we approach the infinite system. Emphasis in this report is on extraction of high-accuracy limiting values for selected molecular properties from limited data on a few smaller members of structurally related molecules.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Diabetologia 35 (1992), S. S8 
    ISSN: 1432-0428
    Keywords: Diabetes mellitus ; diabetic nephropathy ; immuno-genetics ; immuno-intervention ; sulphonylurea ; glucose ; glucose transport ; blood glucose control ; transplantation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary New treatment strategies for subjects with Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetes mellitus are being developed. Pilot studies utilising insulin itself have been reported to prevent Type 1 diabetes in subjects likely, by immunogenetic and physiologic criteria, to develop clinically overt disease, while the results of nicoti-namide trials in these subjects remain preliminary. Immunotherapy with cyclosporin A and azathioprine can slow disease progression and may produce long-term remissions when given within two months of onset of clinically overt disease. In subjects with established disease, familial clustering of diabetic nephropathy may be related to concomitant susceptibility to hypertension and elevated rates of Na/H countertransport. Treatment of hypertension associated with the nephropathy appears to slow renal deterioration. Whether reversal of the metabolic consequences of insulin deficiency or resistance also prevents chronic diabetic complications has not been firmly established. In the presence of a reduced Beta-cell mass, moderate hyperglycaemia may itself contribute to decreased muscle glucose uptake but not glycogen synthesis in Type 1 and Type 2 diabetes. Reversal of chronic hyperglycaemia by pancreas and islet cell transplantation, vanadate, and sulphonylureas are discussed as alternate strategies to insulin treatment in establishing normoglycaemia and furthering our understanding of insulin action and secretion at the cellular level. There remains a need to develop more sensitive biochemical and genetic markers to identify subjects at increased risk for developing chronic diabetic complications.
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  • 8
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Notes: Abstract The electronic structure of a π-network polymer with random numbers of two types of monomer units is considered in a “conjugated circuit” resonance-theoretic framework. A transfer-matrix technique for computing relevant ensemble-average energies is described and applied to a few example simple benzenoid systems. A type of long-range ordering is noted to be relevant, and some of its implications are discussed.
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  • 9
    ISSN: 1572-8897
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Mathematics
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1432-2234
    Keywords: π-network polymers ; Valence-bond model ; Resonance theory ; Long-range order ; Bond localization ; Solitonic excitations ; Transfer matrices
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The poly-polyphenanthrene family of extended π-network strips with members ranging from polyacetylene to graphite is considered in terms of the locally correlated valence-bond or Heisenberg Hamiltonian. Resonance theory wavefunctions which provide a variational upper bound to the ground state energy are developed in a graph-theoretic formalism extendable to more general localized wavefunction cluster expansions. The graph-theoretic formalism facilitates the use of general transfer matrix techniques, which are especially powerful in application to quasi-one-dimensional systems such as are illustratively treated here. It is argued that these strips exhibit states of different long-range spin-pairing orderings. Novel properties associated with these different resulting phases are briefly indicated, including the possibilities of solitonic excitations and the reactivity at the ends of the strips. The qualitative arguments are supported by numerical calculations for strips up to width 8.
    Type of Medium: Electronic Resource
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