Structure of Aqueous Mixtures ofN,N-Dimethylacetamide Studied by Infrared Spectroscopy, X-ray Diffraction, and Mass Spectrometry

Infrared (IR) spectroscopy and X-ray diffraction measurements have been performed at 298 K on mixtures of N,N-dimethylacetamide (DMA) and water over the entire range of DMA mole fractions (xDMA). Mass spectra have also been measured on the clusters that have been isolated by the adiabatic expansion of liquid droplets of the mixtures. The IR overtone band of the CO stretching vibration for the DMA−D2O mixtures and the O−D stretching band of HDO for the DMA−H2O mixtures containing 5% D2O both shifted to lower frequencies as the water content increased, accompanied by two inflection points, at xDMA = 0.1 and 0.6. These results suggested that hydrogen bonding between the CO group and the water molecules and structural changes of predominant solvent clusters were taking place at these specific mole fractions. The X-ray radial distribution functions have indicated that the structure of the predominant solvent clusters in the mixtures could be classified into four regimes: (1) 0 < xDMA ≤ 0.1, where the water clusters predominate; (2) 0.1 < xDMA ≤ 0.3, where more water clusters than DMA clusters are formed; (3) 0.3 < xDMA ≤ 0.6, where the DMA structure becomes predominant but the water clusters still remain; and (4) 0.6 ≤ xDMA ≤ 1, where the DMA clusters are dominant in the mixtures. On the basis of the present results on the microscopic structure of the mixtures, the anomalies of thermodynamic parameters such as the enthalpy of mixing are discussed.