Interplay of electrostatic and van der Waals forces in coronene dimer

Abstract

AbstractInterplay of electrostatic and van der Waals forces in the coronene dimer is studied. Our results show that the lowest‐energy configuration of the dimer is not necessarily a stack, as it might had been expected a priori. This is a surprising result for the dimer of such a large polycyclic aromatic hydrocarbon (PAH) as coronene (C24H12). The energy of the T‐shaped configuration at all highest feasible levels of density functional theory (DFT) (B3LYP, PBE/6‐31+G(d), D95, cc‐pVDZ, cc‐pVTZ) is lower than the energies of the three plausible stack configurations. To get a better description of the van der Waals interaction, the DFT results were corrected by a phenomenological van der Waals‐type term. This correction gives a slight edge to the parallel‐displaced stack configuration. However, the magnitude of the correction is somewhat arbitrary, depending on the set of parameters used. This makes the definitive conclusion impossible at the currently achievable level of theory. A simple model is proposed that is useful for the qualitative understanding of possible geometries of the coronene dimer and larger coronene clusters. The model represents the coronene dimer as two sets of charged rings interacting via Coulomb and Lennard‐Jones potentials. The model provides an intuitively clear explanation as to why the T‐shaped dimers can be of importance even for some of moderately large PAHs such as coronene, and perhaps for circumcoronene as well. The unexpectedly strong competitiveness of the T‐shaped configuration is connected to the round shape of the coronene molecules. Indeed, rotation or parallel displacement of the non‐round monomers result in significantly smaller Coulomb repulsion as compared with the “face‐to‐face” sandwich configurations. In contrast, for the stacked round molecules rotation and/or displacement are much less effective. Therefore, the round shape of the coronenes leads to an increased role of the electrostatic repulsion in the stack configurations. The proposed model can easily be generalized to other PAH, DNA bases, etc. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007