A novel field of ab initio studies: complexation of simple anions within neutral cryptands

We present here an ab initio study on anion guest-macrocycle host relationships. The complexation of different simple inorganic anions (F−, Cl−, BF4− and ClO4−) with the neutral cryptand, the macrotricycle (H3BN)4[(CH2)n]6, (n=3-6), has been studied using ab initio Hartree-Fock calculations and density functional theory using the B3LYP hybrid functional. As the side-lengths of the pure cryptands increase with the oligomeric sidechain (n), the cryptand cavity increases as n3. F− is the most strongly preferred anion for all cryptands, only for the largest host cavity (n=6) complexes are formed exothermally for all anion guests. Morokuma-Kitaura decomposition has been performed to analyse the total interaction energies within the created supermolecules in physically relevant terms including the importance of treatment of the basis set superposition error. According to this scheme the polarisation/charge transfer parts differ in importance amongst the complexes in facilitating a strong binding of the anion guests, but the dominant balance being that of Coulombic attraction and Pauli repulsion.