Harmonic oscillator model of aromaticity (HOMA) in conjugated radicals and cations

Abstract

We propose the extension of Harmonic Oscillator Model of Aromaticity (HOMA), reparametrizing it for radicals and cations. The application is based on a series of experimental structures based on phenalenyl core, found in both radical and cationic forms. Phenalenyl is the first member of the topological spin molecules named triangulenes, looking like triangular graphene flakes, the series being indexed by the number of hexagonal rings at the edge. We discussed the series of triangulenes with n = 2,3 and 4 sizes. In this view, a HOMA parameterization based on computed structures is devised. We found that phenalenyl systems (the n = 2 triangulene) are relatively high aromatic in both radical and cationic forms, with the experimental structures having HOMA indices comprised between 0.89 and 0.99. Along the n-triangulene series, the global aromaticity of radicals decreases, having HOMA = 0.965, 0.932 and 0.925, for computed structures of n = 2, 3 and 4 congeners, respectively, tending to a plateau estimated at HOMA = 0.914 for hypothetical larger systems. The HOMA indices are approximately correlated with the aromatic stabilization estimated in a spin-coupling phenomenology.