Van der Waals Complexes of Polar Aromatic Molecules: Unexpected Structures for Dimers of Azulene

Full geometry optimizations at the dispersion corrected DFT-BLYP/TZV2P level of theory have
been performed for dimers of azulene that may serve as a model system for the van der Waals complexes
of polar π systems. The structures and binding energies for 11 dimers are investigated in detail. The DFT-D
interaction energies have been successfully checked against results from the accurate SCS-MP2/aug-cc-pVTZ approach. Out of the nine investigated stacked complexes, eight have binding energies larger than
7.4 kcal/mol (SCS-MP2) that exceed the value of 7.1 kcal/mol for the best naphthalene dimer. T-shaped
arrangements (CH···π) are significantly less stable. Two out of the three best structures have an antiparallel
alignment of the monomer dipole moments in the complex, although the best ones with a parallel orientation
are only about 0.5 kcal/mol less strongly bound which points to a minor importance of dipole−dipole
interactions to binding. Quite surprisingly, the energetically lowest structure (ΔE = −9.2 kcal/mol)
corresponds to a situation where the two seven-membered rings are located almost on top of each other
(7−7) and the long molecular axes are rotated against each other by 130°. The 7−7 structural motif is
found also in other energetically low-lying structures, and the expected 5−7 (two-side) arrangement is
less strongly bound by about 2 kcal/mol. This can be explained by the electrostatic potential of azulene
that only partially reflects the charge separation according to the common 4n + 2 π electron rule. General
rules for predicting stable van der Waals complexes of polar π systems are discussed.