Exploring gas-phase ion-ionophore interactions: infrared spectroscopy of argon-tagged alkali ion-crown ether complexes.

Argon-tagged alkali metal ion-crown ether complexes were generated in the gas phase and investigated using a combination of infrared predissociation (IRPD) spectroscopy, density functional, and symmetry-adapted perturbation theory (SAPT). The IRPD spectra of M(+)(12-crown-4 ether)Ar and M(+)(18-crown-6 ether)Ar, where M = Li, Na, K, Rb, and Cs, were collected in the CH stretching region, using the argon-messenger technique. The gas-phase neutral vibrations for each crown ether shift to higher frequency when complexed to an alkali metal ion. Similarities in the experimental IRPD spectra of Li(+)(12-crown-4 ether)Ar and Li(+)(18-crown-6 ether)Ar indicate that the binding of Li(+) is similar for both crown ethers. In the 12-crown-4 ether systems, there are trackable changes with ion size in the IRPD spectra until the point is reached where the ion is too large to bind to the interior of the macrocycle. Starting with Na(+), the 18-crown-6 spectra vary only slightly as the ion size is increased. The overall profiles of the IRPD spectra indicate that a similar configuration of the complexes is adopted for the ions larger than Na(+). The calculated SAPT interaction energies mirror the trends exhibited by the IRPD spectra and provide interesting insights on the roles of the different interaction energy components in binding the cation to the crown-ether.