Analysis of MO Ground States of Benzenoid Hydrocarbons in Terms of BORT Resonance Structures

In order to determine the most important resonance structures in the description of benzenoid hydrocarbons, Hückel and Pariser-Parr- Pople ground states of all such hydrocarbons with up to five benzene rings were analyzed in terms of BORT resonance structures. Due to the exponential increase of the number of resonance structures with the size of a benzenoid molecule, we used an approximate optimization procedure based on the greedy algorithm. The analysis of cumulative contributions of various sequences of resonance structures to the MO ground states of the considered ben- zenoids reveals that, besides the Kekulé structures, Claus structures are also very important in the description of these systems. The relative importance of Claus structures can be attributed to the local benzene-like character of these 7t-electron systems, and to the fact that benzene ground state is exactly represented as a linear combination of the two Kekulé and one Claus structures. The relative importance of other types of structures is generally much smaller. The real contribution of these other structures is somewhat obscured due to their large number and nonorthogonality