A nonplanar cyclic minimum-energy structure of singlet C9

The second order Moller-Plesset (MP2) perturbation treatment with the 6-31G* basis set was applied to cyclic singlet structures of C9. It has been found that the minimum is a Cs nonplanar structure located only about 26 kJ mol-1 above the related linear structure. The finding was further confirmed at the MP2/DZP and MP2/6-311G* levels (although with the MP2 frozen-core option only). Two other low-energy saddle points of C2v symmetry are reported and some results for the regular ring. The Cs minimum should easily undergo an inversion. The analytically computed MP2/6-31G* frequencies of the Cs structure are probably influenced by an artefact of the method. Relation to the available observed data is discussed.