Infrared spectroscopic and computational studies on formamide solutions of Ca²⁺. Vibrational frequencies of formamide and modes of coordination to Ca²⁺

Infrared spectroscopy for formamide (FA) solutions of Ca(ClO₄)₂ shows that both CN stretch (ν_CN) and CO stretch (ν_CO) bands of FA undergo upshifts in the presence of Ca²⁺. Modeling of Ca²⁺ in FA solutions is accomplished by quantum chemical calculations for Ca²⁺(FA)_n (n = 1–8) complexes with Polarizable Continuum Model (PCM). The calculations indicate that bidentate Ca²⁺(FA)₄ complexes are not consistent with the observed upshift of the ν_CN band, although a bidentate coordination of four FA molecules via both O and N atoms was assumed in a previous study of the same system. The experimental results are reasonably reproduced by adopting Ca²⁺(FA)₇ and Ca²⁺(FA)₈ complexes with a monodentate coordination of all FA molecules via the O atom. A strong coupling among the CO oscillators is shown to be responsible for the upshifts of the ν_CO modes despite the O atom coordination.