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 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.