Spectroscopic Investigation of the Gas-Phase Conformations of 15-Crown-5 Ether Complexes with K+

Infrared multiple photon dissociation action spectra of the binary and ternary gas-phase complexes formed by 15-crown-5 ether with potassium cations (15c5-K+ and 15c5-K+-15c5) are reported. The spectra span the 800-1500 cm(-1) infrared range. Particularly significant differences are found in the position and structure of the CO-stretching band of the two types of complexes. The computational prediction at the DFT-B3LYP/6-31G* level of theory agrees well with the experimental observations, and provides a correlation between the spectral differences and the structural changes associated with the coordination of the ether oxygens with the alkali cation. The 15c5-K+ complex adopts a pyramidal structure, with the cation lying above the center of mass of the ether ring at a distance similar to its ionic radius. The ternary 15c5-K+-15c5 complex features a less tight coordination of the cation and a relative rotation between the backbones of the two crown ethers, which minimizes the intermolecular repulsions between the oxygens.