Genetic algorithm density functional theory study of crown ether–dibenzylammonium [2]pseudorotaxanes

Five [2]pseudorotaxanes built from the interaction of a dibenzylammonium cation with five different crown ethers have been investigated by means of high-level ab initio quantum chemical computations. A genetic algorithm has been applied to determine a series of energetically low-lying equilibrium structures for each [2]pseudorotaxane. Different sources for binding interactions such as [N+–H⋯O] and aromatic interactions (π⋯π stacking) are displayed by the optimised structures. In the framework of the genetic algorithm, the equilibrium structures were determined at the level of density functional theory using the meta-GGA (meta generalised gradient approximation) functional TPSS. Subsequent single-point-energy calculations at the level of second-order Møller–Plesset perturbation theory (MP2) in a large basis set (def2-TZVPP) as well as subsequent spin-component-scaled MP2 geometry optimisations were carried out. Binding energies are reported including counterpoise corrections for the basis set superposition error (BSSE) and fragment deformation energies. For two of the five systems studied theoretically, 1:1 complexes (that is, [2]pseudorotaxanes) were identified experimentally by means of 1H NMR spectroscopy, albeit with a very weak association constant for the [2]pseudorotaxane built from dibenzylammonium and the macrocyclic 12,14-diketo-benzo-25-crown-8 molecule.