Structures of small carbon clusters: Cyclic ground state of C6

Accurate ab initio calculations have been performed to investigate the nature of the structures and energies of different C6 isomers. The effects of polarization functions and electron correlation have been included in these calculations. A cyclic singlet structure with six π electrons (planar D3h,1A′) is found to be the ground state. The linear 3Σ−g structure lies about 10 kcal/mol higher in energy. The heat of formation, ΔH°0, of C6 is estimated to be ∼281 kcal/mol.