Lewis acid-base adducts: a quantitative Raman analysis of formamide and dimethylsulfoxide mixtures.

Raman spectra of pure liquid dimethylsulfoxide (DMSO) and of binary mixtures of formamide (FA) and DMSO in different compositions were obtained. The vibrations involving the SO functional group in the band envelope at ca. 1050 cm(-1) of pure liquid DMSO are assigned to monomers, dimers and higher aggregates of DMSO. The appearance of a new band at 1024 cm(-1), whose intensity shows large dependence on the FA concentration, is assigned to a FA-DMSO adduct. This has been possible due to the two H-bond donor sites of FA and the strong donor character of DMSO that become the environment propitious for the donor-acceptor reaction. Quantitative analysis performed in the SO stretching region in the binary mixtures gives a 1:1 stoichiometry in this adduct in the limit of infinite dilution. This proportion is in full agreement with our previous determination for the FA-ACN adduct. The experimental evidence of the 1:1 FA-DMSO adduct is presented for the first time using Raman spectroscopy. The results described here open new possibilities to study the acid-base reactions nature of FA adducts.