A tentative interpretation of the results of recent X-ray and infra-red studies of liquid water and H ₂ O + D ₂ O mixtures

An idealized model is proposed for the arrangement of the molecules in liquid water which involves essentially a sixfold co-ordination of water molecules with four short OH...O hydrogen bonds of $\sim $ 2$\cdot $9 angstrom length and two long O...O contacts of $\sim $ 3$\cdot $6 angstrom length. An ice-like structure may contribute to a small extent also. This octahedral model has been based on evidence obtained from X-ray and infra-red absorption measurements. The model has been found to be in agreement with the density of water and the melting entropy of ice. The reliability of the radial distribution curves W(r) of liquid water obtained from recent X-ray diffraction measurements is discussed. Infra-red absorption measurements have been made of liquid HDO in excess D$_{2}$O and H$_{2}$O, respectively. The respective O$\chembond{1,0} $H and O$\chembond{1,0} $D stretching vibration frequencies of liquid HDO have been determined. The position (at 3400 cm$^{-1}$) and shape of the relatively sharp single O$\chembond{1,0} $H stretching absorption band of liquid HDO is closely comparable to the corresponding band in liquid interbonding alcohols. The results of the infra-red studies indicate an OH...O distance of 2$\cdot $86 angstrom in liquid water at room temperature.