Theoretical investigations of anion-π interactions:: The role of anions and the nature of π systems

The nature of the anion-pi interaction has been investigated by carrying out high level ab initio calculations of the complexes of halide (F-, Cl-, and Br-), linear organic (CN-, NC-), and trigonal planar organic (NO3- and CO32-) anions with different kinds of pi systems, viz. olefinic (tetrafluoroethene), aromatic (hexafluorobenzene), and heteroaromatic (1,3,5-triazine). In an effort to comprehend the underlying basis of this interaction, we have also carried out a rigorous decomposition of the interaction energies using the symmetry adapted perturbational theory (SAPT) method. Contrary to our expectations, the results indicate that the magnitudes of total interation energies of anion-pi and cation-pi interactions are similar. In contrast to cation-pi interactions, anion-pi interactions are, however, marked by substantial contributions from dispersion energies. As in the case of cation-pi interactions, the role of anions also have a marked influence on the nature and magnitude of the anion-pi interaction with interactions involving halide anions being dominated by electrostatic and the induction energies. On the other hand, dispersion energies are markedly higher in interactions involving organic anions. Apart from aiding an understanding of the origin of anion-pi interaction, we believe that the present results would help understand the basis of biomolecular structures and enzyme-substrate interactions and also aid the design of new drugs and novel ionophores/receptors.