NBO–NEDA, NPA, and QTAIM studies on the interactions between aza-, diaza-, and triaza-12-crown-4 (An-12-crown-4, n = 1, 2, 3) and Li+, Na+, and K+ ions

DFT-B3LYP level of theory is used to optimize aza-, diaza-, and triaza-12-crown-4 ligands and their complexes in the gas phase. In all calculations, Pople’s triple-zeta basis set (6-311G) is used to represent the wave function of ligands and ions. Three alkaline metal ions containing Li+, Na+, and K+ have been chosen to study and analyze their complex formation with above ligands. Natural bond orbital (NBO), natural energy decomposition analysis (NEDA), natural population analysis (NPA), and quantum theory of atoms in molecules (QTAIM) methods have been used to analyze the host–guest interactions of above complexes. Results show that all interactions are electrostatic dominant; i.e., they are closed-shell interactions. In all complexes, oxygen atoms are stronger electron donors than the nitrogen atoms. O⋯M+ has higher electron density and second-order stabilization energy than N⋯M+. Data also show that the relative energy order in NEDA is as electrostatic > polarization > charge transfer.