VIBRATIONAL-SPECTRA OF SOLUTIONS OF FORMAMIDE IN DMSO

The infrared and Raman spectra of HCONH2, HCOND2, and an equimolar mixture of the two in dimethylsulphoxide (DMSO) at concentrations ranging from the pure formamide to a very dilute solution have been measured. The carbonyl stretching band of formamide is asymmetric. At very low concentrations in DMSO the band becomes symmetric. The behaviour of the asymmetry upon dilution can be explained in terms of two ''sites''. One corresponding to formamide molecules which have no hydrogen bonds to their carbonyl groups and another to a hydrogen-bonded species forming a chain of formamide molecules. The carbonyl stretching band of the equimolar mixture does not become symmetric upon dilution. This is because it consists of the contribution from four different molecules. Subtraction of the contributions from HCONH2 and HCOND2 produces the spectrum of cis- and trans-HCONHD.