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 DNh. 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 πz 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 πxy system in the carbomer rings thus has minimal effect with respect to the πz system. However, the DFT πxy levels are also correlated with the corresponding HF and Hückel levels: the weak degeneracy lifting with respect to the ideally isolated πxy MOs indicates a weak in-plane homoaromaticity.