Is the C₈ ring nonplanar?

The second order Møller-Plesset (MP2) perturbation treatment with the 6-31 G+ basis set was applied to the cyclic structure of C₈. It has been found that its minimum energy structure is nonplanar, possessing D_2d point symmetry. The nonplanar species is located about 56 kJ mol^-l below a planar cyclic stationary point. The latter structure has C_4h 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 C_4h species is a local energy minimum). The MP2/6-31G* geometry, energy, and harmonic vibrational characteristics are reported.