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 hydrocarbons (PAH) containing five- and six-membered rings (PAH5/6) has been developed. The existing set of groups, those for PAH containing only six-membered rings (PAH6) and for flat PAH5/6, has been extended by addition of a new group, derived from known properties of the fullerene C-60. The constituent groups for all the species in a recently-published mechanism for fullerenes C-60 and C-70 formation are given. Thermochemical 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 shown to be limited in its treatment of ring strain and H-H repulsion, although the estimated properties gave sufficiently reliable results when 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 gamma symmetry, such as corannulene. All MM3(92) and MOPAC calculations (MNDO, AM1, and PM3 in both RHF and UHF forms) yield very different values for Delta H-f(0), on a relative basis the predictions are quite consistent. MM3(92) is found to most accurately predict known experimental heat capacities for C-60.