The structures at the Hartree-Fock level, as well as the energetics, are reported for the unsaturated system C36H16, its Si-doped analogue C32Si4H16, and several smaller, unsaturated fragments. Structural effects on the electronic distribution are discussed in terms of a localized orbital energy decomposition. The standard heats of formation are calculated based on homodesmic and isodesmic reactions and the G2(MP2,SVP) method with a valence double-zeta plus polarization basis. The origin of the observed explosion of the all-carbon system (C36H16) to form carbon nanotubes was investigated by exploring a possible initial reactive channel (dimerization), which could lead to the formation of the observed onion-type nanostructures.