Using Raman spectroscopy to investigate donor-acceptor reactions in the formamide/dimethylsulfoxide/acetonitrile system

Raman spectra of binary and ternary solvent systems in different concentrations were carried out. The mixtures containing equimolar quantities of dimethylsulfoxide (DMSO) and acetonitrile (ACN), or those in different compositions, do not show any spectral evidence for the formation of a Lewis acid-base adduct. On the contrary, the formamide (FA), DMSO and ACN mixtures exhibit new bands. in the S=O stretching region, the band at 1025 cm(-1) is assigned to the FA-DMSO adduct whereas, in the C equivalent to N stretching region, the 2257 cm(-1) band corresponds to the FA-ACN adduct. The positions of these two new bands in the present system are in agreement with our recent works. In addition, in the NH2 bending region, the 1613 cm-1 band has been described as a mixture of both adducts. Quantitative analyses performed in the S=O and C equivalent to N stretching regions show that the association extent for the FA-DMSO adduct is greater than for the FA-ACN adduct, and this is in excellent agreement with the acid-base concepts commonly used to nonaqueous solvents. Preferential interactions between FA molecules as well as their dependence on the ternary mixture composition are also discussed. The results described here open up new possibilities to study the acid-base reactions nature involving FA adducts and can be useful for a better description of biological systems in which peptide groups have a special role. Copyright (c) 2007 John Wiley & Sons, Ltd.