Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range

Abstract

Four recent x-ray diffraction measurements of ambient liquid water are reviewed here. Each of these measurements represents a significant development of the x-ray diffraction technique applied to the study of liquid water. Sources of uncertainty from statistical noise, Q-range, Compton scattering, and self-scattering are discussed. The oxygen-hydrogen contribution to the measured x-ray scattering pattern was subtracted using literature data to yield an experimental determination, with error bars, of the oxygen-oxygen pair-distribution function, gOO(r), which essentially describes the distribution of molecular centers. The extended Q-range and low statistical noise of these measurements has significantly reduced truncation effects and related errors in the gOO(r) functions obtained. From these measurements and error analysis, the position and height of the nearest neighbor maximum in gOO(r) were found to be 2.80(1) Å and 2.57(5) respectively. Numerical data for the coherent differential x-ray scattering cross-section IX(Q), the oxygen-oxygen structure factor SOO(Q), and the derived gOO(r) are provided as benchmarks for calibrating force-fields for water.