Computational Study of the interaction between Sumanene and Cations as a function of the cation–π separation

Abstract

With the aim of enhancing the comprehension of the cation-π interaction, a computational study of the interaction established between sumanene molecule and various cations was performed. Sumanene is a polycyclic aromatic hydrocarbon with a bowl-shaped structure. The curvature of the molecule causes an asymmetry in the distribution of its molecular electrostatic potential that is more negative in its outer (convex) side. This feature allows testing the role of the electrostatic contribution to cation-π interaction using the same molecule. Five cations with different sizes and shapes were selected for the study: sodium, potassium, ammonium, tetramethylammonium, guanidinium and imidazolium. These are monoatomic cations and models of cationic amino acids side-chains, all of which are known to participate in the formation of cation-π complexes in biological systems. The polyatomic cations were placed in different orientations with respect to the sumanene molecule including the “T-shaped” and “stacked” configurations of the flat cations. The study was accomplished at the RI‑MP2/aug-cc-pVTZ level of calculation, to ensure the correct retrieving of the correlation energy and also that the wavefunction size is appropriate for the modeling of effects more complicated than the electrostatic contribution. The interaction energy (Eint) was computed at different sumanene-cation distances following the C3v symmetry axe of sumanene and exploring it’s both sides: concave and convex. The rigid scans of the potential energy surface indicate that at sumanene-cation distances around the Eint minima, the complexes are more stable with the cation placed by the inner (concave) side of sumanene, with the only exception of the complexes with Na+, the smallest of the cations studied. This result is the opposite of that expected from the pure electrostatic interpretation of the cation-π interaction. As the cation moves away from the sumanene molecule the situation is reversed, and at long distances the outer complexes are more stable than its inner partners. These findings suggest that at long cation-molecule separations the electrostatic contribution dominates because its influence propagates to long distances but at short distances the cation-π interaction is controlled by other stabilizing contributions (induction and dispersion) defining the minimum of the Eint profile. The results obtained contribute to a better understanding the cation-π interaction and emphasize the importance of using the correct level of calculations in its theoretical modeling