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  • Title: Gas-phase complexes of cyclic and linear polyethers with alkali cations.
    Author: Hurtado P, Hortal AR, Gámez F, Hamad S, Martínez-Haya B.
    Journal: Phys Chem Chem Phys; 2010 Nov 07; 12(41):13752-8. PubMed ID: 20844796.
    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.
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