Quantum-Chemical Investigations on the Structural Variability of Anion-π Interactions

The structural versatility of anion-pi interactions was investigated computationally. Employing quantum-chemically optimized structures of a series of C6H6-nFn/Br- complexes and the Coulomb law together with the London formula to calculate the electrostatic and the dispersion energy of the interaction between the anion and the pi-system led to the result that up to the number of n = 2 due to a significantly repulsive electrostatic energy of interaction the dispersion energy is not sufficient to stabilize such structures in the gas phase where the anion is located above the plane defined by the aromatic ring. The energy surfaces resulting from the interaction of bromide anions with isolated arenes bearing varying numbers of fluorine atoms in different positions of the aromatic ring also show a pronounced dependency on the subsitution pattern of the aromatic system. Depending on the nature of the electron withdrawing group and its position, the energy surface can have a sharply defined energetically low minimum, in which the anion is 'fixed'. Other substitution patterns result in very flat energy surfaces, and even a surface with more than two local minima within the scanned area was found. Thus, our study reveals the reason for the experimentally observed structural versatility depending on the substitution pattern in the solid state.