Structure-enthalpy relationships in polycyclic cata-condensed aromatic hydrocarbons
A large fraction (30 of 62) of the cata-condensed polycyclic benzenoid aromatic hydrocarbons with one to six rings have nonplanar pi carbon atom frameworks, with the minimum angle of nonplanarity approximately 30°. In addition to 17 pairs of enantiomers, four of the nonplanar cata-condensed benzenoids with six rings are redicted to be capable of existing as pairs of twisted diastereoisomers. Many of t l ! e nonplanar benzenoids are thermodynamically more stable than isomeric planar com ounds. The MMX molecular mechanics force field provides reasonable estimates o!AHP(g) and geometric structures for cata-condensed benzenoids, in good agreement with the limited experimental data. AM1 calculations are not useful to determine the AHt(g) of these compounds. A group additivity method that incorporates C-C and C-H bond terms, a resonance energy term and steric parameters can be used to accurate1 model the MMX and the experimental AHt(g) data. Both MMX and the grou abbitivity/resonance energy procedure can also be used for predictive purposes an B to estimate resonance energies.