Further developments in determining the number of resonance structures in benzenoid free radicals: analytical...

Relationships for counting the number of resonance structures (SC) of benzenoid free radicals are discussed. The structural origin of some of the coefficients to the SC analytical expressions for series of benzenoid free radicals generated by successive attachment of specific aufbau units is determined. It is shown that a proper pruning fragment for the purpose of a priori determination of the coefficient in the highest degree n term of the SC analytical expression belonging to a given radical series has the property of bond fixation when the operation is performed on every member of that series, resulting in all remnant structures having the same SC value.