Density functional theory study of the free and tetraprotonated spheroidal macrotricyclic ligands and the complexes with halide anions: F^-, Cl^-, Br^-

Theoretical studies of the macrotricyciic tetramine hexaether (SC), its tetraprotonated form SC-4H⁺, and the corresponding complexes X ^-⊂SC-4H⁺ (This expression represents the structural, properties of the halide inclusion complex formed though the free ligand SC-4H⁺ and the halide anion X^-: the spherical, halide anion X^- is held by a tetrahedral array of ⁺N-H-X ^- hydrogen bonds inside the intramolecular cavity of the tetraprotonated form SC-4H⁺) of SC-4H⁺ with the halide anions: F^-, Cl^-, and Br^- have been performed using density functional theory (DFT) with B3LYP/6-31G method implemented in the Gaussian 03 program package. The optimized geometric structures obtained, from DFT calculations are used to perform Natural. Bond Orbital. (NBO) analysis. The three main types of hydrogen bonds ⁺N-H-F^-, ⁺N-H...Cl^-, and ⁺N-H-Br^- are investigated. The results indicate that hydrogen bonding interactions are dominant and the halide anions: F^-, Cl^-, and Br ^- offer lone pair electrons to the contacting σ* (N-H) antibond orbital of SC-4H⁺. For all the structures, the most pronounced changes in geometric parameters upon interaction are observed in the proton-donor molecule. The intermolecular interaction energies are predicted by using B3LYP/6-31G methods with basis set superposition error (BSSE) and zero-point energy (ZPE) correction.