STRUCTURE AND INVERSION BARRIERS OF CORANNULENE, ITS DIANION AND TETRAANION - AN AB-INITIO STUDY

Ab initio calculations were performed for neutral corannulene (1) and its dianion and tetraanion. The minimum energy conformations, as well as the transition states for bowl-to-bowl inversion, were located and characterized by calculation of vibrational frequencies. For some char ged species, the Hartree-Fock method predicts critical point symmetrie s that are lower than those deduced from Huckel theory. However, inclu sion of electron correlation effects shows this to be an artifact of t he HF method. Consideration of electron correlation is also crucial fo r the estimation of the inversion barriers, while the quality of the b asis set has only moderate impact on these results. The ''best theoret ical estimate'' of the barriers decreases from 14.2 kcal mol(-1) for n eutral 1, to 7.9-9.2 kcal mol(-1) for the dianions, and to 3.2 kcal mo l(-1) for the tetraanion. While the introduction of negative charges i nto corannulene causes overall flattening of its curvature, even in th e most extreme case - the tetraanion - preference for a bowl-shaped ge ometry remains. Within C-5v symmetry, the (3)A(2) triplet state is cle arly favored over the (1)A(1) and (1)E(2) Singlet states of the isolat ed dianion. However, no significant preference of the triplet vs. sing let electronic state emerges if the symmetry of the molecular framewor k of the dianion is allowed to distort. Finally, analysis of the charg e distribution in the tetraanion 5 does not support the ''anion within a trianion'' model for the tetraanion of corannulene.