Is the C8 ring nonplanar

The second order Moller-Plesset (MP2) perturbation treatment with the 6-31G* basis set was applied to the cyclic structure of C8. It has been found that its minimum energy structure is nonplanar, possessing D2d point symmetry. The nonplanar species is located about 56 kJ mol-1 below a planar cyclic stationary point. The latter structure has C4h symmetry and exhibits two imaginary vibrational frequencies. This finding reveals a distinct difference in the potential hypersurface topology resulting from the correlated MP2 and SCF approaches (as in the SCF/6-31G* treatment the C4h species is a local energy minimum). The MP2/6-31G* geometry, energy, and harmonic vibrational characteristics are reported.