Gas-phase molecular structure of corannulene, C20H10.: An electron-diffraction study augmented by ab initio and normal coordinate calculations

The molecular structure of corannulene has been investigated by gas-phase electron diffraction with help from ab initio calculations at the B3LYP/6-31(d) level and normal coordinate analysis. The structure is closely similar to both that found in the crystal and that predicted from the molecular orbital calculation, but the carbon skeleton differs significantly from a C-20 fragment of C-60 The molecule of C-5v symmetry is skullcap shaped with five six-membered rings fused to the central five-membered ring and to each other. However, unlike C-60 in which the corresponding hexagons are planar, the outermost pairs of carbon atoms with their hydrogens are bent out, tending to flatten the molecule. The bond lengths (rg/Angstrom) with uncertainties of 2 sigma for the four different types of C-C blonds in corannulene are, beginning with that held jointly by the pentagon and hexagon and proceeding around the hexagon, C-1-C-2 = 1.414(6), C-1-C-6 = 1.414(20), C-6-C-7 = 1.447(16), and C-7-C-8 = 1.380(16). The average C-C bond is about 0.022 Angstrom shorter than in C-60. The structure is discussed.