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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 30 (1986), S. 203-218 
    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: We consider the question of predicting the stability of hypothetical structures that are too large for ab initio calculations, too complex for simple MO calculations, and too few for reliable semiempirical SCF MO schemes. The problem is illustrated on a polycyclic conjugated cation having 21 carbon atoms. The analysis is general and applies to conjugated hydrocarbons and their ions for which it is not apparent whether they exhibit aromatic characteristics (and, if they do, to what extent). For the investigated polycyclic cation we find that to a large extent its aromatic components are cancelled by its antiaromatic components resulting in an essentially nonaromatic system. The analysis, which is based on close examination of various contributing conjugated circuits, allows one to examine the partial role of the individual Kekulé valence structures as well as to foretell the nature of both the positively and the negatively charged system. We conclude that the particular cation should be stable but represents a challenge as it would be a cation with a rather small resonance energy. In contrast, the corresponding anion is relatively stable and, at least from a theoretical point of view, is less interesting.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 12
    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: The development of several novel graph theoretical concepts and their applications in different branches of chemistry are reviewed. After a few introductory remarks we follow with an outline of selected important graph theoretical invariants, introducing some new results and indicating some open problems. We continue with discussing the problem of graph characterization and construction of graphs of chemical interest, with a particular emphasis on large systems. Finally we consider various problems and difficulties associated with special subgraphs, including subgraphs representing Kekulé valence structures. The paper ends with a brief review of structure-property and structure-activity correlations, the topic which is one of prime motivations for application of graph theory to chemistry.
    Type of Medium: Electronic Resource
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  • 13
    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: We consider the problem of location an active fragment (substructure) common to a class of biologically active compounds and presumed responsible for their biological activity (therapeutic or toxic). Our approach is graph-theoretical in that molecules are represented by suitable graph-theoretical invariants. Specially weighted paths in the molecular graph are adopted as descriptive elements. By selecting different sets of atoms one searches for a fragment that best represents the relative activities of the compounds. As an illustration we consider a dozen nitrosamine mutagens and analyze the cases of five-, six-, and sevenatom fragments. The approach clearly indicates that a specific seven-atom fragment (for molecules with up to 11 nonhydrogen atoms) can account for the relative mutagenic activities of the nitrosamines considered.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 14
    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: Molecular descriptors currently used in structure-property-activity studies are based on molecular graphs rather than on a structure as a three-dimensional object. Here we suggest a way to amend graph-theoretic invariants with additional geometric information, thereby providing new molecular descriptors for possible use in quantitative structure-activity correlations. In the approach we assume molecular structures embedded on a regular grid. As an illustration we consider first chains of different length embedded on graphite-like lattice. Subsequently, we consider all possible conformations of hexatriene. Although the cases considered here relate to graphite-like lattice, the approach is general and applies to embedings of molecules on three-dimensional lattices, or, in general, to molecules of arbitrary spatial conformations.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 15
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 34 (1988), S. 127-141 
    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: Using a graph theoretical approach we have analyzed the aromatic stability of macrocylic systems having ten and fewer fused benzene rings. The approach requires construction and examination of Kekule valence structures of the macrocyclic systems considered. The Kekule valence structures for a molecule were systematically derived and the conjugated circuits in the individual Kekule formulas were enumerated subsequently. The counting results is the expression for molecular resonance energy, from which relative aromatic stabilities can be established. We report also on molecular local features, including ring benzene characters and Pauling bond orders.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 21 (1982), S. 647-663 
    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: Using a canonical numbering of vertices for the graph corresponding to a particular rearrangement of tetragonal-pyramidal complexes, all 120 permutations defining the symmetry for the rearrangement are derived. An examination of the permutations points to the symmetric group S5, which has previously been found for isomerizations of trigonal-bipyramidal complexes and in the rearrangement of homotetrahedryl cations.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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  • 17
    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: We developed a theoretical method for studying the aromatic stability of large molecules, molecules having a dozen and more fused benzene rings. Such molecules have so far often been outside the domain of theoretical studies. Combining the statistical approach and a particular graph theoretical analysis, it is possible to derive the expressions for molecular resonance energy for molecules of any size. The basis of the method is enumeration of conjugated circuits in random Kekulé valence structures. The method has been applied to evaluation of the resonance energies of conjugated hydrocarbons having about a dozen fused benzene rings. The approach consists of (1) construction of random Kekulé valence structures, (2) enumeration of conjugated circuits within the generated random valence structures, and (3) application of standard statistical analysis to a sufficiently large sample of structures. The construction of random valence forms is nontrivial, and some problems in generating random structures are discussed. The random Kekulé valence structures allow one not only to obtain the expression for molecular resonance energies (RE) and numerical estimates for RE, but also they provide the basis for discussion of local molecular features, such as ring characterization and Pauling bond orders.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 15 (1979), S. 663-682 
    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: The Desargues-Levi graph represents important chemical transformations: (1) isomerization routes for some carbonium ion rearrangements, (2) isomerization of trigonal bipyramidal structures, and (3) some pseudorotations of octahedral complexes. The symmetry properties of this graph have not been fully investigated in the past. Using the concept of the smallest binary code, all permutations which form the symmetry operations in the graph are registered. The resulting symmetry group can be represented as the direct product of S5 (the full symmetric permutation group on five objects) and Ci (the inversion in the center). There are 14 classes belonging to the following partitionings: 120(1), 1826(1), 1428(1), 1236(1), 123262(1), 2 63(2), 2244(2), 210(3), 54(1), and 102(1). The total of 240 symmetry operations are distributed among the above 14 classes as follows: 1, 10, 15, 20, 20, 20, 20, 30, 30, 15, 10, 1, 24, and 24, respectively. Since partitioning cannot uniquely characterize a class, it is suggested that the distance between vertices in a cycle be introduced as an additional parameter to discriminate among classes having identical partitioning. Also, a suggestion to a generalization of the Mulliken notation for irreducible representations of the point molecular groups valid for more versatile symmetry groups of graphs is indicated.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 19
    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: Many regularities in properties among structurally related compounds are known but a systematic approach to investigate all such cases is rare. A plan is outlined for a systematic study of structural regularities that is based on use of selected graph invariants as prime auxiliary quantities. In this paper carbon-13 chemical shifts in alkanes are examined. Since NMR spectra of paraffins are well understood, the example provides a useful illustration of the approach, introduces basic concepts, and illustrates the kind of the results that the graph-theoretical scheme generates. Differences and similarities between graph-theoretical viewing of the problem of structural regularities and customary direct use of additivities are discussed in some detail.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 30 (1986), S. 185-201 
    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: We present a graph theoretical approach to Möbius systems, extending the concept of conjugated circuits to cyclic arrays that involve a single phase inversion. The approach can be applied without difficulty to polycyclic systems, justifying validity of the 4n rule beyond the narrow class of monocyclic systems. One obtains expressions for the resonance energy of Möbius systems that frequently allow an assessment of the relative stabilities among different systems without use of a full numerical analysis. Given reliable computations allowing one to select parameters, the approach could be used quantitatively; however, its important value is at the conceptual level. It offers insights into structural components contributing to the molecular stability for some ground-state organic systems as well as to the factors that govern numerous chemical reactions. In this paper we have analyzed a dozen conjugated alternant hydrocarbons having 4-, 6-, and 8-membered rings and found the most stable Möbius system for each parent hydrocarbon. The analysis consists in enumeration of conjugated circuits of size 4n + 2 and 4n, discriminating between the conjugated circuits involving a phase inversion and those that do not.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
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