Aromaticity and antiaromaticity in oligocyclic annelated five-membered ring systems

The aromaticity and antiaromaticity of oligocyclic annelated five-membered ring systems, pentalene (1), acepentalene (2), dicyclopenta[cd,gh]pentalene (3), and related compounds, have been evaluated computationally using density functional theory (B3LYP/6-31G*). The nucleus-independent chemical shifts (NICS) and magnetic susceptibility exaltations (A) of these unusual cross-conjugated pi systems indicate 1 and 2 with 8 and 10 pi electrons, respectively, to be antiaromatic. In contrast, 3, with 12 pi electrons, is aromatic. Although the rings are not identical, 3 has a more delocalized electronic structure than 1 and 2. The dianions of 1-3 have delocalized structures and are aromatic. The magnetic criteria show the dications of 1 and 2 to be aromatic but the dication of 3 to be antiaromatic. The fused ring systems 2 and 3 are nonplanar. The calculated inversion barriers of acepentalene (2) and its free dianion 2(2-) are 7.1 and 5.4 kcal/mol, respectively.