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

Abstract

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 νCO modes despite the coordination via the O atom.