Electron Density of Corannulene from Synchrotron Data at 12 K, Comparison with Fullerenes

The electron density of corannulene, C20H10, was derived from a high-resolution synchrotron data set (sin theta/lambda = 1.11 angstrom(-1)) measured at 12 K and from an ab-initio geometry optimization on the B3LYP/6-31G* level. A full topological analysis yielded atomic and bond-topological properties which were compared between experimental and theoretical findings and, as far as steric congruences exist, with corresponding fragments of the fullerene C-70. For the four different types of C-C bonds in corannulene, a rather close bond-order range between 1.3 and 1.8 was found indicating a considerable delocalization in this molecule. As was already found earlier in fullerene cages, the deformation density on the C-C bonds is not arranged symmetrically. There is more density located outside than inside the corannulene bowl so that in total, charge accumulation is shifted to the outer surface of the molecule. The electrostatic potential suggests an H center dot center dot center dot pi stacking in the crystal which directs the relative orientation of the two crystallographically independent corannulene molecules. The positively charged rim region of one molecule is oriented almost perpendicular to the negative potential region at the bottom of a second molecule.