Vibrational Analysis of Azacrown Ether Complex with Li Metal Cation

摘要

Li⁺ 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 ~150 cm^-1. The density functional calculation at the level of BP86/6-31+G** shows that Li⁺ ion is trapped and rather free to move around inside the cavity, as much as about 0.70 A. Through the relocation of Li⁺ ion inside the cavity, the conformational changes would occur rapidly in its symmetry C₁ ? C₂ ? C₁. The potential barrier was obtained to be merely ~2.2 kJ/mol for C₁ ℃ C₂. During this conformational alteration, the amide backbone twists concurrently its dihedral angle side to side about up to 【3 degree. Selected vibrational modes were interpreted in terms of the force constant variations of local symmetry coordinates between conformations in the framework of C1 ? C2 ? C1.