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Original scientific paper

https://doi.org/10.5562/cca3765

Catacondensed Chemical Hexagonal Complexes: A Natural Generalisation of Benzenoids

Cate S. Anstöter ; Department of Chemistry, Temple University, Philadelphia, 19122, USA
Nino Bašić orcid id orcid.org/0000-0002-6555-8668 ; FAMNIT, University of Primorska, Koper, Slovenia
Patrick W. Fowler ; Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
Tomaž Pisanski orcid id orcid.org/0000-0002-1257-5376 ; FAMNIT, University of Primorska, Koper, Slovenia


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Abstract

Catacondensed benzenoids (those benzenoids having no carbon atom belonging to three hexagonal rings) form the simplest class of polycyclic aromatic hydrocarbons (PAH). They have a long history of study and are of wide chemical importance. In this paper, mathematical possibilities for natural extension of the notion of a catacondensed benzenoid are discussed, leading under plausible chemically and physically motivated restrictions to the notion of a catacondensed chemical hexagonal complex (CCHC). A general polygonal complex is a topological structure composed of polygons that are glued together along certain edges. A polygonal complex is flat if none of its edges belong to more than two polygons. A connected flat polygonal complex determines an orientable or nonorientable surface, possibly with boundary. A CCHC is then a connected flat polygonal complex all of whose polygons are hexagons and each of whose vertices belongs to at most two hexagonal faces. We prove that all CCHC are Kekulean and give formulas for counting the perfect matchings in a series of examples based on expansion of cubic graphs in which the edges are replaced by linear polyacenes of equal length. As a preliminary assessment of the likely stability of molecules with CCHC structure, all-electron quantum chemical calculations are applied to molecular structures based on several CCHC, using either linear or kinked unbranched catafused polyacenes as the expansion motif. The systems examined all have closed shells according to Hückel theory and all correspond to minima on the potential surface, thus passing the most basic test for plausibility as a chemical species. Preliminary indications are that relative energies of isomers are affected by the choice of the catafusene motif, with a preference shown for kinked over linear polyacenes, and for attachment by angular connection at the branching hexagons derived from the vertices of the underlying cubic structure. Avoidance of steric crowding of H atoms appears to be a significant factor in these preferences.

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Keywords

Benzenoid, polygonal complex, (catacondensed) chemical hexagonal complex, Kekulé structure

Hrčak ID:

261080

URI

https://hrcak.srce.hr/261080

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