Phenanthrene dianion is not planar

Abstract

Physical organic chemistry has produced a lot of arguments in favour of the flexibility of the skeleton of the phenanthrene dianion, 1 2-. Circumstantial evidence for the non-planarity of dianions derived from phenanthrene substituted at the 'bay' position has been published.'S2 It has also been demonstrated that 1 2is a highly paratropic species and is hence regarded as a so-called antiaromatic molecule.3i4 It was suggested that in order to minimize paratropicity the molecular skeleton may undergo a deformation so that the efficiency of the 71conjugation is reduced and antiaromaticity is less pronounced. At the same time semiempirical calculations predict that 12is planar, and only a sophisticated treatment including the counter-ion enabled the non-planarity of 12-/2 Li' to be achieved computationally.2*6 We find that ab initio calculations in minimal basis set (STO3G) as well as semiempirical methods (MIND0/3, MNDO, AMl) describe the ground state of 12as a planar structure. However, when the basis set for ab initio calculations was extended to STO 3-2 1 G and further to STO 4-3 1 G, the planar structure of 12proved to be not a minimum, but a transition state (vim = 5 3 cm-' on HF/3-21G level) between two nonplanar minima with a dihedral angle C(4)<(4a)-C(5a)-C(5) =