Inversion barrier, buckybowls

The curvature and rigidity of a buckybowl can be characterized by the bowl depth, POAV (jt-orbital axis vector) pyramidalization angle [58-60], and bowl-to-bowl inversion barrier (AG i v). The experimental data of the first two are available by single-crystal X-ray crystallography of the desired bowl molecule. Because the barriers of many corannulene derivatives lie in the region of 7-20 kcal/mol, the can be conveniently analyzed by variable temperature NMR study of a suitably derivatized molecule. 

On the basis of crystallographic analysis, the corannulene cores in dibenzo[u,g]-corannulene (88) and A-octyldibenzo[ Z,m]-l,2-corannulimide (91) are shallow with a bowl depth of 0.83 and 0.65 A, respectively [120, 123]. The cyclopenta-annulated buckybowl 90 is much deeper, and its bowl depth was determined as 1.03 A. The maximum POAV pyramidalization angle of 90 was found to be 10.7°, which is very close to that of acecorannulene (69) [119]. However, the bowl inversion barrier for 90 was experimentally estimated as 23.5-23.6 kcal/mol, which is much smaller than that of acecorannulene (69, 27.6 kcal/mol). 

Buckybowl 169 is not a fragment of common fullerenes, such as Ceo and C70 (Scheme 53) [178]. The synthesis started with pentacenequinone 167, and the final step was microwave-assisted Pd-catalyzed cyclization of 168. The maximum POAV pyramidalization angle of compound 169 was predicated to be 8.4° based on computational calculations. The bowl-to-bowl inversion barrier of 169 should be greater than 24 kcal/mol according to the result of a variable-temperature NMR experiment. It should also be noted that buckybowl 169 easily accepts electrons and associates strongly with C70. 

The correlations between structures and bowl-to-bowl inversion barriers provide information on the inversion dynamics and design of a static bowl. Increase in the curve of buckybowls may also increase their molecular dipole moment. A high inversion barrier conveys a configurational stability to a chiral buckybowl, avoiding loss of enantiopurity through the bowl-inversion racemization process. 

Dinadayalane, T. C., Sastry, G. N. (2003). Isolated pentagon rule in buckybowls A computational study on thermodynamic stabilities and bowl-to-bowl inversion barriers. Tetrahedron, 59, 8347-8351. 

Bowl-to Bou/I Inversion in Buckybowls 551 Table 12.1. Barriers for ring inversion in substituted corannulenes. 

---