Vibrational study on the solvates formed during interactions of amide with alkaline earth metal ions

We present in this paper very interesting vibrational data for formamide (FA) complexes with Mg and Ca ions. The νCO and νCN modes of FA are shifted to lower (45 cm−1) and higher (30 cm−1) wavenumbers, respectively, in the presence of Mg(II), whereas both modes are upshifted (∼15 cm−1) as Ca(II) is present. The shifts observed for the system containing Mg(II) indicate that the HC−ON+H2 form is majority due to the coordination through the O atom. On the other hand, the HCONH2 form is dominant because of the coordination to Ca(II) via both O and N atoms. Such differences may be explained based on the ionic radii of Mg and Ca, which lead to the formation of FA complexes with coordination numbers equal to 6 and 8, respectively. The results here presented may help us for a better understanding about amide bond hydrolysis reactions catalyzed by metal ions, since the ionic and molecular FA forms have been recognized as active and inactive intermediates, respectively, in the hydrolysis mechanisms proposed so far.