Vibrational Analysis of Azacrown Ether Complex with Li Metal Cation

Abstract

<TEX>$Li^+$</TEX> ion complex of azacrown ether with restricted motion of freedom and pseudo-bilateral symmetry was studied by infrared spectroscopy, which has shown simplified and broadened vibrational features. The C=O and N-H stretching bands, in particular, shows anomalous broadening nearly <TEX>${\sim}50\;cm^{-1}$</TEX>. The density functional calculation at the level of BP86/6-31+<TEX>$G^{**}$</TEX> shows that <TEX>$Li^+$</TEX> ion is trapped and rather free to move around inside the cavity, as much as about <TEX>$0.70\;{\AA}$</TEX>. Through the relocation of <TEX>$Li^+$</TEX> ion inside the cavity, the conformational changes would occur rapidly in its symmetry <TEX>$C_1\;{\rightleftarrows}\;C_2\;{\rightleftarrows}\;C_1$$</TEX>. The potential barrier was obtained to be merely ~2.2 kJ/mol for <TEX>$C_1\;{\rightarrow}\;C_2$</TEX>. During this conformational alteration, the amide backbone twists concurrently its dihedral angle side to side about up to <TEX>${\pm}3$</TEX> degree. Selected vibrational modes were interpreted in terms of the force constant variations of local symmetry coordinates between conformations in the framework of <TEX>$C_1\;{\rightleftarrows}\;C_2\;{\rightleftarrows}\;C_1$</TEX>.