A theoretical study of the conformations of tetrabenzo[a,cd,f,lm]perylene, an overcrowded LPAH

An overcrowded large polycyclic aromatic hydrocarbon (LPAH) tetrabenzo[a,cd,f,lm]perylene (1) was subjected to an ab initio study. The density functional theory (DFT) B3LYP method was employed to calculate energies and geometries of the stationary point conformations of 1. The global minimum was found to be anti-folded a-C_s-1, while twisted-folded tf-C ₂-1 was only 0.2 kJ/mol higher in energy (B3LYP/6-311G**) , giving their equilibrium ratio 52:48 at 25°C. LPAH 1 was compared with its constitutional isomer, tetrabenzo[a,cd,j,lm]perylene (2). The tf-C ₂-1 conformation was 10.3 kJ/mol less stable than the corresponding tf-C₂-2, while a-C_s-1 was 12.8 kJ/mol more stable than the corresponding a-C_i-2. Nucleus-independent chemical shift (NICS) values, calculated at GIAO-B3LYP/6-31G*// B3LYP/6-31G* are indicative of the aromatic character of the para-terphenyl rings A, F, and A′ of 2, while rings B and C have NICS values close to zero, indicating nonaromatic character. The NICS values are consistent with the Clar pictures of 1 and 2.