An X-Ray Diffraction Study on the Stucture of 18-Crown-6 Ether Complexes with Alkali Metal Ions in Aqueous Solution

Abstract

Abstract The structure of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) and its complexes with alkali metal ions in aqueous solution has been investigated by X-ray diffraction and Raman spectroscopic methods at 25 °C. The X-ray scattering data and Raman spectrum for an aqueous 18-crown-6 solution show that free 18-crown-6 has a conformation of C1 or D3d symmetry. The molecule seems to be flexible and may be present as a mixture of the two conformations in aqueous solution. The structure of the lithium complex was not conclusive because of a weak scattering power of lithium atoms and weak complex formation between lithium ion and 18-crown-6. The sodium 18-crown-6 complex is estimated to have a structure similar to that found in crystal. The 18-crown-6 ring within the complex adopts the C1 conformation, where five oxygen atoms within 18-crown-6 coordinate to sodium ion at the equatorial position and an oxygen atom within 18-crown-6 and a water molecule at the axial one. The structure of the potassium complex is also similar to that in crystal in which the D3d conformation is taken. Potassium ion is located at the center of the mean plane of 18-crown-6 and one or two water molecules solvate potassium ion above and/or below the plane of 18-crown-6. It is suggested that the structure of the caesium complex is with either C1 or D3d symmetry, where caesium ion is apart from the mean plane of 18-crown-6. The rubidium complex was not examined because of a strong fluorescent X-ray emission from rubidium atoms when studied. The 18-crown-6 ring in the sodium and potiassium complexes is rather rigid probably because the cavity of the ring well fits to the metal ions, while the ring coordinating to large caesium ion becomes more flexible than that in the sodium and potassium complexes due to weaker interaction with caesium ion.