4 - Trifluoromethylated Corannulene Derivatives: Thermodynamic Stability and Electron-Accepting Properties

Abstract

Thermodynamic stability and electron-accepting properties of trifluoromethylated corannulene derivatives is analyzed based on DFT calculations and comparison to experimental data. It is shown that CF3 groups in adjacent position (peri and ortho) are repelling each other with destabilizing effects of 43 and 50kJ/mol, respectively. These energy increments can be used to predict relative energies of all isomers based on the number of peri and ortho moieties in the molecule. The most stable are isomers with all their CF3 groups at distant positions, and these are the isomers which are obtained in high-temperature gas-phase radical trifluoromethylation of corannulene. Each CF3 group increases the electron affinity of the corannulene derivative by ca 0.30eV. However, more detailed analysis shows that the increase of the electron affinity depends on the way how CF3 groups are distributed at the rim. Addition patterns with uniform distribution of CF3 groups usually have lower electron affinities than the isomers with nonuniform distribution, and especially with ortho moiety. The position dependence is explained by an analysis of the lowest unoccupied molecular orbital (LUMO) and LUMO+1 orbitals of the derivatives, originating from the twofold degenerate LUMO of pristine corannulene. It is found that the mean energy of the two orbitals does not depend on the position of CF3 groups, but the energy gap between the LUMO and LUMO+1 is indeed a function of the CF3 addition pattern. When the gap is large, the LUMO is stabilized and the isomer has higher electron affinity than the isomer with smaller LUMO/LUMO+1 gap. CF3 groups have nonnegligible (several %) contributions to the LUMO and LUMO+1 and the lowest LUMO energies are found for the orbitals with the largest CF3 contributions. Nonuniform distribution of CF3 groups on the rim usually results in substantially different CF3 contribution to the LUMO and LUMO+1, which therefore leads to the large LUMO/LUMO+1 gap and strong stabilization of the LUMO.