Liquid Structure of Acetonitrile−Water Mixtures by X-ray Diffraction and Infrared Spectroscopy

Abstract

The structure of acetonitrile−water mixtures has been investigated by X-ray diffraction with an imaging plate detector and IR spectroscopy over a wide range of acetonitrile mole fractions (0.0 ≤ XAN ≤ 1.0). Reichardt ETN and Sone-Fukuda DII,I values were also measured for the mixtures. It has been found from the X-ray data that in pure acetonitrile an acetonitrile molecule interacts with two nearest neighbors by antiparallel dipole−dipole interaction together with a small shift of the two molecular centers and that two acetonitrile molecules in the second-neighbor shell interact with a central molecule through parallel dipole−dipole interaction. Thus, acetonitrile molecules are alternately aligned to form a zigzag cluster. On addition of water into pure acetonitrile, water molecules interact with acetonitrile molecules through a dipole−dipole interaction in an antiparallel orientation. The IR spectra of O−D and C⋮N stretching vibrations, observed for mixtures of acetonitrile AN and water containing 20% D2O, suggested that hydrogen bonds are also formed between acetonitrile and water molecules in the mixtures at XAN ≤ 0.8. The average numbers of the first- and second-neighbor acetonitrile molecules gradually increase with increasing water content with an almost constant first-neighbor distance and slightly decreased second-neighbor ones. Thus, acetonitrile molecules are assembled to form three-dimensionally expanded clusters; the acetonitrile clusters are surrounded by water molecules through both hydrogen bonding and dipole−dipole interaction. The X-ray radial distribution functions and IR spectra suggest that the hydrogen bond network of water is enhanced in the mixtures at XAN < 0.6. The concentration dependence of ETN and DII,I values determined reflects well the above-mentioned behavior of water molecules in the mixtures. These findings suggest that both water and acetonitrile clusters coexist in the mixtures in the range of 0.2 ≤ XAN < 0.6, i.e., “microheterogeneity” occurs in the acetonitrile−water mixtures.