Physical origins of interactions in dimers of polycyclic aromatic hydrocarbons.

Dimers of several polycyclic aromatic hydrocarbons (PAHs): naphthalene, anthracene and pyrene have been investigated by symmetry-adapted perturbation theory based on the density functional description of the monomers [SAPT(DFT)]. Calculations have been performed for a number of radial cross-sections of selected stacked and T-shaped configurations. The interaction energies at the minima of stacked configurations of the benzene, naphthalene, anthracene and pyrene dimers increase with the number of rings, but -- in contrast to literature findings -- the rate of increase is somewhat irregular. In particular, the anthracene molecules interact slightly stronger than indicated by the number of rings and the pyrene molecules interact significantly weaker (although the latter do interact stronger than the former). These trends can be partly rationalized by the physical decomposition of the interaction energies given by SAPT. We find the stacked structures to be significantly more stable than the T-shaped ones, with the relative stability of the former structures increasing as the size of the system increases. This observation extends to the benzene dimer where the two structures are nearly isoenergetic and a tilted T-shaped structure actually becomes the global minimum. For the naphthalene dimer, the greater stability of the stacked configuration than of the T-shaped one is in disagreement with recent experiments observing only the latter structure. For the anthracene dimer, theory is in agreement with experiments.