Temperature dependence of the Oxygen-Oxygen separations in water from high energy x-ray diffraction

We have used state of the art, high energy x-ray diffraction to obtain detailed measurements of the Oxygen-Oxygen (O-O) pair distribution function (g(r)) of liquid water between -20 and 92 degrees Celsius. These measurements show ordinary linear behavior of the first O-O distance, over the full temperature range, even through the density maximum. Conversely we do see interesting, non-linear behavior in the O-O distribution at higher separations distances, particularly around the 4.5{\AA} peak. Another interesting feature of these measurements is the presence of a temperature-independent crossover point in the running O-O coordination number at the location of the first minimum in r$^{2}$[g(r)-1], which defines the end of the first shell. At this 3.4(1){\AA} distance the O-O coordination number is 4.5(2) at all the temperatures studied. We believe this work offers important insight into some of the unusual physical properties of water, and provides a valuable validation point for the many Molecular dynamics models of liquid water.