3 - Molecular geometry as a source of electronic structure of π-electron systems and their physicochemical properties

Abstract

This chapter shows the advantages of knowing the geometry of the molecule, available from a traditional or in silico experiment. Geometry-based aromaticity index HOMA (harmonic oscillator model of aromaticity) as well as its components GEO and EN, allow to characterize the local π-electron delocalization. This is documented for alternant benzenoid hydrocarbons: (1) linear acenes, (2) kinked, such as phenanthrene, chrysene, and picene, and (3) condensed, such as triphenylene, perylene, pyrene, coronene, and ovalene, as well as for nonalternant π-electron hydrocarbons such as azulene and cyclopentadiene, and heterocyclic analogs of the latter: pyrrole, thiophene, and furan. Application of the aromaticity index named HOMED (harmonic oscillator model of electron delocalization) for heterocyclic five-membered π-electron rings is also discussed. Moreover, the effect of substituents on the aromaticity of benzenoid hydrocarbons and rings is shown.