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 ～15 0 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.