Valence Bond Account of Triangular Polyaromatic Hydrocarbons with Spin: Combining Ab Initio and Phenomenological Approaches

We present computational analyses, methodological advances, and heuristic conclusions applied on a series of polyaromatic systems condensed in the shape of regular triangles, carrying spin, because of topological reasons. A new clue about the classification of title systems in three equivalency classes is presented. A conjugated hydrocarbon having n-hexagonal rings at one edge, carrying n – 1 unpaired electrons, will be called n-triangulene in the generalization of the experimentally known structures with n = 2 (phenalenyl) and n = 3 (triangulene). To be distinguished from most of the previous computational approaches, done by density functional theory, we challenged the problem in the key of valence bond (VB) paradigm in both ab initio and phenomenological manners. The Heisenberg spin Hamiltonian was used to simulate the computed spectrum of VB states for the phenalenyl radical (n = 2), predicting with the fitted parameters the effective VB description of n = 3 triangulene and other related systems. The outcome has practical importance in the prospects of spin chemistry because the VB ab initio calculations are prohibitive beyond the n = 2 case. The results are made transparent to the chemical intuition using the language of resonance structures.