Vibrational Spectroscopic and Computational Studies on Formamide Solutions of Alkali Metal Ions

Infrared (IR) spectra were measured for formamide (FA, HCONH₂) solutions of Li(ClO₄) and Na(ClO₄). Both CN stretch and CO stretch bands of FA are observed to undergo upshifts in the presence of the metal ions. Quantum chemical calculations were performed for Li⁺(FA)_n (n = 1–7) and Na⁺(FA)_n (n = 1–8) complexes in order to model the metal ions in FA solutions. In previous Raman studies of the Li⁺ system, the so-called chelate configuration was assumed, in which the Li⁺ ion was put into the center of a ring FA dimer. However, the present calculations reveal that such a configuration is in conflict with the observed band shifts. The experimental IR spectra are reproduced by adopting appropriate isomers of Li⁺(FA)₅ and Li⁺(FA)₆ complexes, in which all FA molecules are coordinated to Li⁺ via the O atom, with a configuration such that the Li⁺ ion and NH₂ group are on the same side of the CO bond. These complexes, especially Li⁺(FA)₆, are also successful in replicating characteristic features observed in the previous Raman spectra. Similarly, an O-bound isomer of Na⁺(FA)₆ is consistent with the experimental IR and Raman spectra of the Na⁺ system. A strong coupling among the CO oscillators of FA molecules is shown to be responsible for the upshifts of the ν_CO modes despite the coordination via the O atom.