Structures of Alkali Metal Ion–Crown Ether 1:2 Heterocomplexes Investigated Using Cold Gas-Phase Spectroscopy

The structures of alkali metal ion-crown ether (CE) 1:2 heterocomplexes of benzo-18-crown-6 (B18C6), dibenzo-18-crown-6 (DB18C6), and 18-crown-6 (18C6) were studied under cold (∼10 K) conditions in the gas phase. The structures and intermolecular interactions of M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6) (M = Rb and Cs) were determined based on their ultraviolet (UV) and infrared (IR) spectra, with the assistance of quantum chemical calculations, and compared with those of K⁺ complexes reported previously. The electronic transitions of the M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6) complexes occurred in the 35,500-36,000 cm^-1 range, in the vicinity of those of the DB18C6 and B18C6 monomers, and at lower energies than the M⁺(DB18C6) and M⁺(B18C6) complexes. This indicates that the interaction between the M⁺ ion and DB18C6 or B18C6 is substantially weaker in the M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6) heterocomplexes than in M⁺(DB18C6) and M⁺(B18C6). In the structures determined for M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6), the M⁺ ion was mainly encapsulated by the 18C6 moiety, and the DB18C6 and B18C6 molecules were weakly bound to the M⁺(18C6) complexes. Based on the structural determination, high-energy conformers were trapped and detected owing to the incomplete structural relaxation in the ion trap of the M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6) complexes. A previous study reported that UV excitation of K⁺(DB18C6)(18C6) and K⁺(B18C6)(18C6) complexes produces photofragment K⁺(18C6) and K⁺(B18C6) ions, respectively. Upon UV photodissociation (UVPD) of the M⁺(DB18C6)(18C6) and M⁺(B18C6)(18C6) (M = Rb and Cs) complexes, M⁺(18C6) ions were formed as the main fragments, which is reasonable based on the predominant encapsulation by 18C6 in these complexes. Hence, the UVPD patterns of the Rb⁺ and Cs⁺ complexes highlight the peculiarity of the K⁺(B18C6)(18C6) complex, in which K⁺ is effectively transferred from 18C6 to B18C6 upon UV excitation.