Double fjord motif in overcrowded large polycyclic aromatic hydrocarbons: The conformational space of hexabenzo[a,cd,f,j,lm,o]perylene

The overcrowded large polycyclic aromatic hydrocarbon (LPAH) hexabenzo[a,cd,f,j,lm,o]perylene (HBP) was subjected to a theoretical study. The ab-initio Hartree-Fock (HF) and the density functional theory (DFT) B3LYP methods were employed to calculate energies and geometries of the stationary point conformations of HBP. The global minimum was found to be the twisted t-D2; the local minimum anti-folded a-C2h was 20.5 kJ/mol higher in energy (B3LYP/6-31G(d)). Thus, only t-D2-HBP exists at room temperature in solution. The enantiomerization of HBP proceeds via a pathway that connects the chiral t-D2 to the achiral a-C2h through a chiral twisted-folded transition state, tf-C1. The energy barrier for enantiomerization was 134.4 kJ/mol. The central benzene ring F of t -D2 is highly distorted from planarity (torsion angles of 15.4 and -30.5), and has a remarkable resemblance to the twist angle of the central ene in the parent BAE bifluorenylidene (32). Yet, nucleus-independent chemical shift (NICS) values, calculated at GIAO-B3LYP/6-31G(d)//B3LYP/6-31G(d) and the almost equal bond lengths (ca. 142 pm) of the central ring are indicative of the aromatic character of this ring. The NICS values are also indicative of the aromatic character of the peripheral rings and the central rings of the two minima and the transition state, consistent with the Clar picture.