Limitations of Pauling Bond Order Concept

Abstract

It is shown that Kekulé structures do not realistically predict the behavior of π-electron properties of those polycyclic hydrocarbons that have many fixed double bonds. This is caused by the fact that such molecules would be destabilized by delocalization. We analyze a group of polycyclic hydrocarbons with a large number of fixed bonds, whose geometry was determined by means of an unrestricted symmetry-broken UB3LYP/6-311G(d,p) DFT method. We put forward a new concept, the unpaired bond order, and show that it is well correlated with bond lengths, but poorly correlated with Pauling bond orders. Hence, in this way we provide a simple test of the validity of the Pauling-bond-order concept for the molecule being considered. © 2012 Copyright Taylor and Francis Group, LLC.