Shape and size effects in π-π interactions: face-to-face dimers.

The shape and size effects in π–π interactions of face-to-face dimers are discussed, by taking three typical groups of π-systems with different shapes (wave-linear, WL; ladder-shaped, LS; and regular-hexagonal, RH) and sizes (2 to 216 π-electrons) as samples, and carrying out a series of static scanning forcefield calculations. These effects are: (1) for differently shaped π-systems with a same quantity of π-electrons, the denser π-electrons lead to the stronger π–π interactions; (2) the interaction orientation, such as the interplanar distance, is controlled by not only van der Waals (vdW) but also electrostatic potential, even though the former is much stronger and has more contributions to total interaction energy than the latter. Furthermore, these interplanar distances are in a range determined by only monomer shapes, i.e., 3.6–4.1 Å for WL, 3.5–3.7 Å for LS, and 3.4–3.7 Å for RH; (3) a centroid-centroid distance corresponding to the global lowest vdW potential point of a dimer with two identical monomers is only determined by the monomer shape, i.e., 3.6, 3.5, and 3.4 Å, for WL, LS, and RH, respectively; (4) rotation will change the interaction energy when both two monomers with big sizes and low symmetries, and vice versa.