THERMOCHEMICAL PROPERTIES OF CURVED PAH AND FULLERENES - A GROUP ADDITIVITY METHOD COMPARED WITH MM3(92) AND MOPAC PREDICTIONS

A group additivity method for strongly curved polycyclic aromatic hydr ocarbons (PAH) containing five- and six-membered rings (PAH5/6) has be en developed. The existing set of groups, those for PAH containing onl y six-membered rings (PAH6) and for flat PAH5/6, has been extended by addition of a new group, derived from known properties of the fulleren e C-60. The constituent groups for all the species in a recently-publi shed mechanism for fullerenes C-60 and C-70 formation are given. Therm ochemical properties, including standard entropies and heat capacities , for all species in the mechanism are estimated. Properties estimated from group additivity are compared with values obtained from MM3(92) for all ring-only stable species. The group additivity approach is sho wn to be limited in its treatment of ring strain and H-H repulsion, al though the estimated properties gave sufficiently reliable results whe n used in preliminary kinetic modeling studies of fullerenes C-60 and C-70 formation. Semiempirical (MOPAC) calculations were performed for C-60 and C-70 and for selected precursors, including those of C-5 gamm a symmetry, such as corannulene. All MM3(92) and MOPAC calculations (M NDO, AM1, and PM3 in both RHF and UHF forms) yield very different valu es for Delta H-f(0), on a relative basis the predictions are quite con sistent. MM3(92) is found to most accurately predict known experimenta l heat capacities for C-60.