All computations were performed using the Parallel Quantum Solution software. All structures were geometry optimized at the Hartree-Fock level using basses sets 6-31g and 6-31g-d. NMR chemical shifts and vibrational frequencies were included in all computations. The resulting data from both bases sets was the same therefore only the structures from bases set 6-31g-d are discussed.
Construction of the Extended Bowl Structure
Corannulene (C20H10) provides an excellent scaffold to build an extended bowl structure, once an extended bowl was created functional groups were added to try to modulate the opening distance and depth of this bowl structure. The corannulene structure was first extended by adding a partial layer of five membered rings to produce C30H10 (Figure 1A). This structure has a similar curvature to corannulene. Six membered rings were then added between the five membered rings to complete the next laver resulting in the C40H10 structure (Figure 1B). The curvature has increased slightly. The final extended bowl structure was created by adding a full layer of six membered rings, C60H10 (Figure 1C). The curvature from the bottom to the top of the bowl structure is now 90 degrees. All structures resulted in expected carbon sp2 bond distances while vibrational signals matched what would be expected. However, 13C chemical shift data resulted in interesting and unexpected trends. For most planer aromatic structures there are equivalent 13C chemical shifts, for example benzene has only a single peak in the 13C NMR spectrum while naphthalene has three unique 13C chemical shifts. [19] The corannulene structure results in three unique 13C chemical shifts while the C30H10, C40H10, and C60H10 structures results in 4, 5, and 7 unique 13C chemical shift respectively. For the corannulene structure the largest 13C chemical shift is in the five membered ring at the center (bottom) of the bowl. The chemical shifts decrease heading toward the top of the bowl structure. The C30H10 13C chemical shifts match the trend as seen in corannulene except of the hydrogen bound carbons at the top of the bowl which have shifted slightly higher. The C40H10 structure shows an opposite trend in 13C chemical shifts with the largest chemical shift at the top of the bowl. This is most likely due to the curvature of the bowl structure. The 13C chemical shifts for the C60H10 structure shift up and down from bottom to top. The largest chemical shift is one position away from the bottom. This may be due to ring current effects across the bowl structure.
Bowl Dimensions
One of the purposes of this study was to determine if the depth and distance across the open of the bowl could be changed with different functional groups. The corannulene structure was calculated to determine the bowl opening of 8.49 Å and depth of 1.42 Å, this structure agreed with the previously determined structure. [12] The diameter was determined by taking an average of distances from the center of the hydrogens (or other atoms) at the top of the bowl across the bowl structure without taking atomic radius into account. The distance from the top of the bowl to the five carbons at the bottom of the bowl was averaged. This distance and the radius of the bowl was used to calculate bowl depth. The bowl opening for C30H10 has increased to 8.92 Å, which would be expected since this structure has the same curvature as corannulene just extended with five membered rings. The bowl depth has increased to 3.89 Å. The C40H10 bowl has a reduction in bowl opening to 8.19 Å due to the increased curvature of the bowl structure, bowl depth has increased to 4.88 Å. The final C60H10 bowl has the smallest opening of 7.62 Å since the curvature at the top is 90 degrees from the bottom and a final depth of 6.61 Å.
The 10 hydrogens at the top of the C60H10 bowl structure were replaced with different functional groups to determine if the bowl opening cold be modulated. Replacement with -CH3, -Cl, -Br, and -NO2 (structures not shown) resulted in a distorted bowl structure. The top of the bowl structure does not to allow for enough space for these functional groups. It is assumed that it would be difficult if not impossible to synthesize these molecules. Reasonable structures were obtained with the functional groups -F, -OH, and -CN (Figure 2). The C60F10 structure shows a slight increase in opening to 7.96 Å and a bowl depth of 6.82 Å. The C60(OH)10 structure increases the bowl opening to 8.23 Å which allows for hydrogen bonding of most of the hydrogens to the neighboring oxygen. The bowl depth is the same of C60F10 at 6.82 Å. A linear functional group such as –CN has increased the opening the most to 9.85 Å without distorting the bowl structure and the largest bowl depth of 7.90 Å. These functional groups have shown that the bowl opening can be modulated.