LIQUID STRUCTURE OF ACETONITRILE-WATER MIXTURES BY X-RAY-DIFFRACTION AND INFRARED-SPECTROSCOPY

The structure of acetonitrile-water mixtures has been investigated by X-ray diffraction with an imaging plate detector and IR spectroscopy o ver a wide range of acetonitrile mole fractions (0.0 less than or equa l to X-AN less than or equal to 1.0) Reichardt E-T(N) and Sone-Fukuda D-II,D-I values were also measured for the mixtures. It has been found from the X-ray data that in pure acetonitrile an acetonitrile molecul e interacts with two nearest neighbors by antiparallel dipole-dipole i nteraction together with a small shift of the two molecular centers an d that two acetonitrile molecules in the second-neighbor shell interac t 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 interacti on in an antiparallel orientation. The IR spectra of O-D and C=N stret ching vibrations, observed for mixtures of acetonitrile AN and water c ontaining 20% D2O, suggested that hydrogen bonds are also formed betwe en acetonitrile and water molecules in the mixtures at X-AN less than or equal to 0.8. The average numbers of the first- and second-neighbor acetonitrile molecules gradually increase with increasing water conte nt with an almost constant first-neighbor distance and slightly decrea sed second-neighbor ones, Thus, acetonitrile molecules are assembled t o form three-dimensionally expanded clusters, the acetonitrile cluster s 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 X-AN < 0.6. The concentration dependence of ETN an d DII,I values determined reflects well the above-mentioned behavior o f water molecules in the mixtures. These findings suggest that both wa ter and acetonitrile clusters coexist in the mixtures in the range of 0.2 less than or equal to X-AN < 0.6, i.e., ''microheterogeneity'' occ urs in the acetonitrile-water mixtures.