Liquid Water: Molecular Correlation Functions from X-Ray Diffraction

Abstract

For nearly spherical molecules the x-ray scattering from liquids yields structure and correlation functions for molecular scattering centers. The distribution of electron density in a water molecule is very nearly spherical, and orientational correlation between molecules in the liquid is not ``seen'' by x rays. Structure and correlation functions for molecular scattering centers are derived from x-ray data on water and tabulated. They provide a sensitive test for future work on a molecular theory of liquid water.

Water Structure from Scattering Experiments and Simulation

“…(17) that is formulated to avoid truncation errors 110 . Otherwise, the g OO (r) obtained from such a procedure has spurious peaks introduced by the truncation and does not display the proper limiting behavior at small r. 62,66 One possible approach for avoiding these truncation errors for extracting g(r)'s is to use a Q-space continuation method 113 in which the experimentally truncated Q-space is extended to infinity based on the analysis that the first coordination shell in water is more cleanly separable in Q-space than r-space, and in fact dominates the scattering correlations beyond 7-9Å -1 . 41,66,113 A gaussian function is used to describe this first shell coordination in r-space (20) where α, the area under the Gaussian, r max , the position of the maximum in the first peak, and γ, related to the width of the Gaussian, are parameters that are fit to the tail of the scattering data at the largest available values of Q 66,113 .…”

Large-Angle X-ray Scattering Investigation of the Structure of 2-Propanol–Water Mixtures

“…In addition, the peak beyond 8Å for the third-and fourth-neighbors in the 2-propanol chains is weakened with a decrease in x 2pr and almost disappears at x 2pr = 0.20. In the mole fraction range of x 2pr 0.10, a new peak at 7Å appears, which is due to the third-neighbor interactions in the tetrahedral-like structure of water [13,27,28]. The RDFs for the 2-propanol-water mixtures at x 2pr 0.10 are well comparable with that for pure water (x 2pr = 0), where three peaks at 2.8, 4.5, and 7Å, characteristic for the tetrahedral-like structure of water, are observed [13,27,28].…”

“…In the mole fraction range of x 2pr 0.10, a new peak at 7Å appears, which is due to the third-neighbor interactions in the tetrahedral-like structure of water [13,27,28]. The RDFs for the 2-propanol-water mixtures at x 2pr 0.10 are well comparable with that for pure water (x 2pr = 0), where three peaks at 2.8, 4.5, and 7Å, characteristic for the tetrahedral-like structure of water, are observed [13,27,28]. These features suggest that the inherent chain structure of 2-propanol molecules gradually decreases with decreasing mole fraction x 2pr from 1 to 0.10, and then the hydrogen-bonded network of water is mainly formed in the mixtures at x 2pr 0.10.…”

Microstructure of neat alcohols

“…The corresponding experimental data are not available for the comparison, however the experimental value for the enthalpy of liquefaction compares qualitatively with the calculated ones: for methanol ∆H = −37.3kJ/mol (expt), −35.44kJ/mol (OPLS) , while for TBA ∆H = −46.74kJ/mol(expt) and In view of the features reported for alcohols, one may then ask what this type of analysis would give for water-a stronger hydrogen bonding liquid. The structure factor of water has been reported about three decades ago [19] and constantly improved since then. It shows no sign of distinct pre-peak, except perhaps for a weak shoulder at k ≈ 2.0Å −1 .…”