Resonance theoretical description of magnetic zero field splitting energies of benzenoid hydrocarbons. Structural dependance of the parameter D

Abstract

The zero field splitting (ZFS) parameters D of the triplet ground states of benzene, naphthalene, anthracene, and phenanthrene are calculated within the framework of a semiempirical valence bond (VB) theory. The VB theoretical results are utilized to develop a much simpler resonance theoretical model which stresses the relationship between molecular structure, aromaticity, and magnitude of the ZFS parameter. The resonance theoretical approximation of the electronic wavefunction is anlysed in terms of triplet benzene subunits which describe a benzene-type resonance of the “triplet electrons” on a particular hexagon of the molecular structure. The ZFS parameter can be approximated by contributions from all triplet benzene subunits of the molecule. The resonance theoretical model can be applied successfully to larger hydrocarbons where a VB theoretical description becomes too complicated. Moreover, the model allows for a simple qualitative understanding of the dependence of the ZFS parameter D on the molecular structure. Resonance theoretical calculations of 16 benzenoid hydrocarbons are reported and show semiquantitative agreement with experiment.