Aromaticity and homoaromaticity of annulene ring carbomers

Abstract

A simple electron count shows that the ring carbomers of (anti)aromatic [N]annulenes are (anti)aromatic according to the formal criterion of the Hückel rule. Basic VSEPR theory suggests that the ring carbomers of (anti)aromatic [N]annulenes should be (anti)aromatic according to the structural criterion, namely (dis)symmetry with respect to D_Nh. These predictions are here confirmed by DFT geometrical optimization at the B3PW91/6-31G** level. Calculations of the NICS at the center of the rings produce negative (respectively positive) values for singlet 4n  + 2 (respectively 4n) and triplet 4n structures. The structural and magnetic aromaticity criteria are thus correlated. A relevant measure of energetic topological aromaticity is based on the Hückel method. The DFT π_z levels are shown to vary with the corresponding HF levels with a universal slope and to closely correlate with the Hückel MO analysis. Values of the resonance integrals β are derived and Breslow's resonance energy (RE) is calculated. The latter is emphasized to represent the energetic effect of the ring-closing π overlap and thus the switch-on of a π ring current. The RE values of [N]annulenes vary with those of their carbomers in accordance with the Hückel rule. The π_xy system in the carbomer rings thus has minimal effect with respect to the π_z system. However, the DFT π_xy levels are also correlated with the corresponding HF and Hückel levels: the weak degeneracy lifting with respect to the ideally isolated π_xy MOs indicates a weak in-plane homoaromaticity.