Far IR continua caused by large polarizabilities of intramolecular NLi+ ⋅⋅⋅N ⇄ N ⋅⋅⋅ Li+N bonds due to cation motion

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Abstract
Aromatic N bases, compounds 1–4 and aliphatic N bases, compounds 5 and 6 were studied by IR spectroscopy pure and as 1:1 mixtures with LiClO4. In the pure LiClO4 solutions the Li+‐ion motion band caused by vibrations of Li+ in a cage of acetonitrile molecules is found at about 400 cm−1. In the case of the solutions of the aromatic N bases 1 ‐ 3 with LiClO4, continua are observed instead of the ion motion band, beginning at about 450 cm−1 and extending toward smaller wave numbers. They indicate the presence of intramolecular NLi+⋅⋅⋅N ⇄N⋅⋅⋅Li+N bonds in which the Li+ ion may fluctuate. These Li+ bonds are homoconjugated, i.e., structurally symmetrical. The continua demonstrate that these bonds show large polarizability, so‐called Li+ polarizability, due to the Li+ motion. Caused by their large polarizabilities they interact strongly with their environments. With the aliphatic N bases complexes with the Li+ ions are formed, but instead of the continua an ion motion band at 400 cm−1 occurs. This result shows that N⋅⋅⋅Li+⋅⋅⋅N bonds with single minimum are present. The different behavior of the aromatic and the aliphatic compounds is explained since the affinity for the Li+ ions is much higher with the aromatic compared with the aliphatic N bases.