Densities, heat capacities, viscosities, 1H- and 13C-NMR spectra, and solvatochromic parameters of binary mixtures of 1,3-dimethyl-1,3-diazinan-2-one (DMPU) and water

Highlights: • Thermodynamic, transport and spectral properties of mixtures of DMPU and water were investigated. • (DMPU + water) mixtures exhibit large negative excess molar volumes. • The standard isobaric molar heat capacity of DMPU in water is much higher than that of DMPU itself. • Polarizability and H-bond donor properties decrease when the DMPU concentration increases. • Chemical shift of water protons is more influenced by mixture composition than those DMPU protons. The compound 1,3-dimethyl-1,3-diazinan-2-one (N,N'-dimethylpropyleneurea, DMPU) is a dipolar aprotic solvent with very strong electron-donating abilities but which is much less toxic and less carcinogenic than similar solvents such as the widely used HMPA (hexamethylphosphorictriamide). This paper reports densities and viscosities of the binary mixtures of (DMPU + water) in the temperature range (293.15 to 353.15) K at 5 K intervals, over the whole composition range. Isobaric heat capacities of these mixtures were measured by flow calorimetry at 298.15 K. The ¹H- and ¹³C-NMR spectra as well as the solvatochromic parameters (polarizability, π*, hydrogen-bond acceptor basicity, β, and hydrogen-bond donor acidity, α) of the mixtures were also determined. The densities and viscosities were sensitive to traces of water in the organic-rich region while the heat capacities were not. Excess molar volumes were strongly negative, showing an extremum at mole fraction x_DMPU ≈ 0.40. The variation of viscosity with mixture composition was similar to the density with a sharp maximum at x_DMPU ≈ 0.35. Apparent molar volumes, V_ϕ, of the mixtures show that the water-DMPU interactions have a larger effect on V_ϕ of water than on DMPU. While the standard isobaric molar heat capacity of DMPU in water is much higher than that of DMPU itself, the corresponding value for water in DMPU was not much influenced by the presence of DMPU. The ¹H-NMR spectra show that the chemical shift of the water protons is more influenced by mixture composition than those of the DMPU protons. A de-shielding of the DMPU carbonyl carbon resonance was observed in the ¹³C-NMR spectra at low x_DMPU, indicative of H-bonding between the water protons and the non-bonding electrons of the carbonyl oxygen.