Gas–phase complexes of cyclic and linear polyethers with alkali cations

Abstract

The flexibility of polymer backbones constitutes one key aspect in their molecular recognition properties. This investigation characterizes the structure of the gas-phase complexes formed by the cyclic and linear polyethers with the heavier alkali metal cations. In particular, the cyclic 15-crown-5 ether (15c5), (OCH₂CH₂)₅, and the polyethylene glycol linear chains PEG4 and PEG9, H(OCH₂CH₂)_n=4,9OH, are considered. Infrared multiple photon dissociation spectroscopy is applied to probe the polymer vibrational modes within the spectral range 800–1500 cm⁻¹, in combination with Density Functional Theory calculations. The experimental spectra of the 15c5-M⁺ (M = K, Rb, Cs) complexes correlate with distorted asymmetric backbone structures in which the cation lies above the ether ring, and four oxygens are oriented toward the cation. In contrast, the PEG4-K⁺ complex features an inclusion-like fivefold coordination structure in which the cation and four oxygens are quasi-coplanar and one terminal oxygen lies out of plane. For the PEG9-K⁺ complex, the ether chain builds stable cages involving a coordination of eight oxygens with the cation, sustained by hydrogen bonds between the terminal hydroxyl groups.