Conformational Analysis of 18-Azacrown-6 and Its Bonding with Late First Transition Series Divalent Metals: Insight from DFT Combined with NPA and QTAIM Analyses

Density functional theory calculations, together with quantum theory of atoms in molecules (QTAIM) analyses, have been performed to investigate 18-azacrown-6 complexes of the high-spin late first transition series divalent metal ions in the gas phase and, in some cases, in aqueous solution simulated by a polarizable continuum model. Six intramolecular H–H bonding interactions in the meso-complexes are found to arise from folding of the ligand upon its electrostatic interaction with the metal ions, which are largely absent in the lowest-energy <i>C</i>_2<i>h</i> conformer of the free ligand. The ligand-to-metal charge transfer obtained from QTAIM analysis, among other things, is found to be an important factor that controls the stability of these complexes. The inter-relationship between the ligand preorganization energy, the zero-point corrected formation energy of the metal complexes, and the H–H bonding pair distances, as well as the dependence of the electron density and the total energy density at the H–H bond critical points on the H–H bonding pair distances, provides a physical basis for understanding and explaining the stabilizing nature of these closed-shell interactions, which are often viewed as steric clashes that lead to complex destabilization