Corannulene as a Lewis base: Computational modeling of protonation and lithium cation binding

A computational modeling of the protonation of corannulene at B3LYP/6-311G( d,p)//B3LYP/6-311G(d,p) and of the binding of lithium cations to corannulen e at B3LYP/6-311G(d,p)//B3LYP/6-31G(d,p) has been performed. A proton attac hes preferentially to one carbon atom, forming a sigma -complex. The isomer protonated at the innermost (hub) carbon has the best total energy. Proton ation at the outermost trim) carbon and at the intermediate (bridgehead rim ) carbon is less favorable by ca. 2 and 14 kcal mol(-1), respectively. Hydr ogen-bridged isomers are transition states between the sigma -complexes; th e corresponding activation energies vary from 10 to 26 kcal mol(-1). With a n empirical correction obtained from calculations on benzene, naphthalene, and azulene, the best estimate for the proton affinity of corannulene is 20 3 kcal mol(-1). The lithium cation positions itself preferentially over a r ing. There is a small energetic preference for the 6-ring over the 5-ring b inding (up to 2 kcal mol(-1)) and of the convex face over the concave face (3-5 kcal mol(-1)). The Li-bridged complexes are transition states between the pi -face complexes. Movement of the Li+ cation over either face is faci le, and the activation energy does not exceed 6 kcal mol(-1) on the convex face and 2.2 kcal mol(-1) on the concave face. In contrast, the transition of Li+ around the corannulene edge involves a high activation barrier (24 k cal mol(-1) with respect to the lowest energy pi -face complex). An easier concave/convex transformation and vice versa is the bowl-to-bowl inversion with an activation energy of 7-12 kcal mol(-1). The computed binding energy of Li+ to corannulene is 44 kcal mol(-1). Calculations of the Li-7 NMR che mical shifts and nuclear independent chemical shifts (NICS) have been perfo rmed to analyze the aromaticity of the corannulene rings and its changes up on protonation.