Symmetry-Binding Correlations of Crown Ether Complexes with Li+ and Na.

The gas-phase structure of 18-crown-6 in the presence of Li+ and Na+ cations is highly flexible and generally distorted. Using density functional theory calculations, natural bond orbital analysis, and symmetry measures, we reveal the driving forces behind the structural and energy trends of 18-crown-6 and its phenyl substituents. We show that the structural deviation from C 3-symmetry increases with the non-bonded interactions between the occupied spx orbitals of the crowns' oxygen atoms and the unoccupied 2s orbital of the cation. These orbital interactions are strongly correlated with the overall host-guest interaction energy. Our approach highlights the role of non-bonded interactions and paves the way for deeper understanding of structure-reactivity relations of flexible host-guest systems.