NORMAL COORDINATE ANALYSIS AND FORCE-FIELD OF 18-CROWN-6

Normal vibrations and force field of 18-crown-6 have been studied in comparison with open-chain poly (oxyethylene). The force field was derived from the Raman and IR wavenumbers for uncomplexed 18-crown-6 and complexes with KSCN, urea and LiSCN·H2O, with different ring conformations. The force field for 18-crown-6 has been found to be substantially the same as that to open-chain poly (oxyethylene). The normal coordinate analysis has indicated that the Raman and IR bands of 18-crown-6 are correlated with the local conformation of the ring. The conformation-wavenumber correlations evaluated previously for open-chain poly(oxyethylene) are also applicable to cyclic compounds. The normal modes of the skeletal deformation vibrations of the 18-crown-6 ring are interpreted in terms of the circular mode, the radial mode and the out-of-plane mode in order to descending wavenumber. © 1990.