Vibrational spectroscopic studies of N,N′-dimethylpropyleneurea–water system: Affected solvent spectra and factor analysis

FT-IR spectra of N,N′-dimethylpropyleneurea (DMPU)–water system have been measured in the 4000–500 cm−1 range by Attenuated Total Reflectance (ATR) technique in the entire composition range. The hydration of DMPU in diluted aqueous solutions has been additionally studied with transmission FT-IR spectroscopy of isotopically diluted HDO in H2O, used as a probe of solute hydration. The spectra have been analysed with the quantitative version of the difference spectra method that leads to isolation of the spectrum of solvent affected by the presence of the solute. DMPU-affected HDO spectra provide information about the energetic state of water molecules in the hydration shell, while affected H2O spectra additionally reflect the structural state of the water hydrogen bond network. The CO stretching band of DMPU has been used to obtain also the H2O-affected DMPU spectrum. The affected H2O and DMPU spectra have been determined in infinite dilution approximation, as well as for increasing concentrations of the solute. These results are confronted with factor analysis of ATR spectra, which shows the presence of three well-defined intermolecular complexes in the studied system. Presumable structures of these complexes have been proposed on the basis of Density Functional Theory (DFT) calculations of optimal cluster geometries.