Clar Goblet and related non-Kekulé benzenoid LPAHs. A theoretical study

The results of an ab initio and semiempirical study of Clar Goblet (1), a C₃₈H₁₈ non-Kekulé diradical LPAH, and its constitutional isomers 4 and 5 are reported. Planar D_2h-1 was only 87.4 (triplet) and 83.8 (singlet) kJ/mol less stable than its planar Kekulé isomer C_2v-6 (at (U)B3LYP/6-31G*). Planar C_s-4 was 63.6 (triplet) and 76.5 (singlet) kJ/mol less stable than 6. Overcrowded C₁-5 was 80.1 (triplet) and 98.1 (singlet) kJ/mol less stable than 6. In concealed non-Kekulé 1, the singlet was more stable then the triplet by 3.6 kJ/mol, while in nonconcealed non-Kekulé 4 and 5, the triplets were more stable than the corresponding singlets by 12.9 and 18.1 kJ/mol, respectively, in accordance with theory. The spin density in 1, 4, and 5 is delocalized throughout the positions corresponding to active peri-peri coupling positions of the radical anion of naphthanthrone (2). The bond lengths in 1, 4, and 5 are in the range expected for aromatic compounds, except for the central carbon-carbon bonds, which are considerably elongated. A certain stabilization is evident in the homodesmotic reaction singlet-1 + 10 + 10 → 11 + 3 + 3, indicating a "communication" between the two benzo [cd] pyrenyl radical (3) units of diradical 1. The HOMA indices indicate that in both singlet 1 and triplet 1 all of the rings except the central one have a significant aromatic character. The central ring is essentially antiaromatic, having negative HOMA index (-0.140 at UB3LYP/6-31+G*). The stabilities of 1^2- and 1²⁺ are decreased relative to 3- and 3+, respectively.