• crown ethers;
  • density functional calculations;
  • divalent metals;
  • infrared spectroscopy;
  • molecular recognition


Crown ethers constitute central building blocks for the synthesis of molecular tweezers capable of trapping cationic species. In this study, isolated ternary complexes comprising two [18]crown-6 (18c6) ether molecules and one divalent cation of varying size (Cu2+, Ca2+, Ba2+) are investigated by means of laser vibrational action spectroscopy and computations. In the ternary (18c6)2–Cu2+ complex, one of the crown units folds tightly around the cation, while the second crown ether unit binds peripherally. Such asymmetrical binding manifests itself as a bimodal splitting of the vibrational bands measured for the complex. The size of the cation in the Ca2+ and Ba2+ complexes leads to a progressively more symmetrical coordination of the two crown ether molecules with the metal. In particular, in the spectrum of the (18c6)2–Ba2+ complex, the two components of the vibrational bands are merged into single-maximum envelopes. This is consistent with a C2 arrangement predicted by the computation, in which the cation coordinates with the two crown ether units in a fully symmetrical way.