Infrared spectroscopic study of the solvation of tetramethylurea in protic plus aprotic mixed solvents

The infrared spectra of tetramethylurea have been measured in mixtures of dimethylsulfoxide or acetonitrile with D2O or deuterated methanol (CH3OD). Marked solvatochroism is observed for bands associated with the C=O and C-N stretches of tetramethylurea, with the former moving to lower wavenumber, and the latter to higher, as the concentration of the protic component is increased. The C=O band shows four distinct absorbances, which are attributed to non-, mono, di-, and trihydrogen bonded tetramethylure. The relative populations of the different hydrogen bonded species were determined from analyses of the C=O band shapes. It is found that the variations in the populations of the different hydrogen bonded species can be accounted for by a simple mass action treatment.