Conformation of Gas-Phase Crown Ether Complexes with Alkaline-Earth and Transition Metal Ions Studied by Cryogenic Ion Mobility-Mass Spectrometry.

The gas-phase structures of dibenzo-24-crown-8 (DB24C8) and dinaphtho-24-crown-8 (DN24C8) complexes with divalent metal ions (Mg, Ca, Sr, Ba, Fe, Ni, and Zn) were investigated by cryogenic ion mobility-mass spectrometry (IM-MS) in combination with density functional theory calculations. Several complexes, particularly those of DN24C8, exhibited multiple coexisting conformers. DFT-optimized structures were classified based on the relative orientation of the two aromatic rings in the crown ether. For most metal ions, conformers, with the aromatic rings closely stacked, were stabilized in the DN24C8 complexes by enhanced π-π interactions, whereas conformers, with the aromatic rings laterally displaced, were favored in the DB24C8 complexes due to intramolecular electrostatic repulsion. In contrast, conformers, with the aromatic rings widely separated, were strongly preferred in the Ni complexes, which was attributed to / isomerism in the octahedral coordination. These results highlight the important roles of ion size, charge, and coordination geometry in determining the conformational preferences of crown ether complexes in the gas phase, providing molecular-level insight into ion selectivity.