Comparing the stability of tribenzo[ b , n , pqr ]perylene and tribenzo[ b , k , pqr ]perylene

Two isomeric benzenoid hydrocarbons – tribenzob,n,pqr]perylene and tribenzob,k,pqr]perylene played a crucial role in the formulation of the Clar aromatic sextet theory. The basic assumption of this theory is that tribenzob,n,pqr]perylene is more stable than tribenzob,k,pqr]perylene because the former has five, whereas the latter only four aromatic sextets. We now approach this stability problem from a different direction. By means of a recently developed molecular-orbital-based method it is possible to estimate the energy effects of individual cycles, as well as pairs, triplets, etc. of cycles in polycyclic conjugated molecules. From these energy-effects one can better understand which structural details are responsible for the thermodynamic stability of the underlying molecule. In particular, it is possible to rationalize (in a quantitative manner) the causes of differences in the thermodynamic stability of isomers. Our analysis corroborates the conclusion of Clar theory, but points out a number of hitherto overlooked structure-stability connections. Correspondence: Ivan Gutman, Faculty of Science, University of Kragujevac, P.O. Box 60, 34000 Kragujevac, Serbia.