Cation-pi-anion interaction in alkali ion-benzene-halogen ion clusters.

The present investigation on some ternary M(+)-benzene-X(-) aggregates follows previous studies concerning the binary, M(+)-benzene (M = Li, Na, K, Rb, Cs) and X(-)-benzene (X = F, Cl, Br, I), systems. A semimepirical model describing the intermolecular potential energy, formulated as a combination of few leading effective components, is here extended and applied to investigate more complex systems. The balancing of size repulsion with induction and dispersion attraction is described as a combination of ion-bond interactions and an electrostatic component, rationalized on the basis of the benzene quadrupole moment, is also included. For M(+)-benzene-X(-) aggregates, the simultaneous presence of a cation and an anion close to the pi system originates a strong Coulombic attraction and nonadditivity effects of the induction energy, which are carefully considered and explicitly included to obtain a proper analytical functional representation of the ternary compound. The proposed semiempirical methodology, providing the whole potential energy surface in a convenient analytical form, is useful to predict the main features of stable and unstable configurations, saddle points, and energy barriers, allowing for investigation of their influence on molecular dynamics simulations.