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

Abstract

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.

A Short and Efficient Synthesis of the [3]Triangulene Ring System

“…These data support formation of the symmetrical 4,8,12-trihydro- [3]triangulenium ion (9). These data support formation of the symmetrical 4,8,12-trihydro- [3]triangulenium ion (9).…”

“…Zuschriften containing 1m TfOD in DCE-d 4 at ambient temperature,the dark green solution immediately exhibited very simple 1 Hand 13 CNMR spectra, displaying three unique hydrogens and six carbon signals for the major species in solution. These data support formation of the symmetrical 4,8,12-trihydro- [3]triangulenium ion (9). DFT computations on 9 predict a D 3h symmetrical structure,w ith calculated NMR chemical shifts in good agreement with experiment.…”

“…Triangular‐shaped aromatic hydrocarbons (Figure ) comprise a homologous series of open shell structures, beginning with the well‐studied phenalenyl radical ( 1 ) . Higher homologs 2 , 3 and 4 are expected to exist as di‐, tri‐ and tetraradicals, respectively; all have been predicted to possess high spin ground states . Synthetic attempts to make [3]triangulene ( 2 ) were first described by Clar in 1953 although the first example of a triangulene ring system had been reported much earlier by Weiss …”

“…[1][2][3] Higher homologs 2, 3 and 4 are expected to exist as di-, tri-and tetraradicals,respectively;all have been predicted to possess high spin ground states. [4][5][6][7][8][9] Synthetic attempts to make [3]triangulene (2)w ere first described by Clar in 1953 [10][11][12] although the first example of at riangulene ring system had been reported much earlier by Weiss. [13,14] Among milestones in triangulene chemistry,the aromatic [3]triangulene dianion has been prepared from the Clar route.…”

Electronic Properties of Triangle-Shaped Graphene Nanoflakes from TAO-DFT

“…In other words, there are approximately ( n – 1) unpaired electrons for the ground state of n -triangulene. 22,33 Accordingly, the larger n -triangulenes should indeed exhibit increasing polyradical nature in their ground states.…”