title: 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), (OCH(2)CH(2))(5), and the polyethylene glycol linear chains PEG4 and PEG9, H(OCH(2)CH(2))(n=4,9)OH, 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.
authors.affiliation: Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, 41013 Seville, Spain.
authors.forename: Paola
authors.initials: P
authors.lastname: Hurtado
authors.forename: Ana R
authors.initials: AR
authors.lastname: Hortal
authors.forename: Francisco
authors.initials: F
authors.lastname: Gámez
authors.forename: Said
authors.initials: S
authors.lastname: Hamad
authors.forename: Bruno
authors.initials: B
authors.lastname: Martínez-Haya