Isotopic quantum effects in water structure measured with high energy photon diffraction

High energy electromagnetic radiation scattering techniques have been used to measure the structural differences between light and heavy water: we have studied both intra- and intermolecular effects. These methods and our data analysis are described in detail. We have observed a maximum isotopic effect of 1.6% relative to the magnitude of the x-ray structure factor. Our uncertainties are an order of magnitude smaller than those of previous γ-ray measurements (Root J H, Egelstaff P A and Hime A 1986 Chem. Phys. 109 5164) and this has permitted us to test accurately the available quantum simulation results on water. The SPC and TIP4P potentials reproduce the measured results in r-space moderately well for intermolecular effects at distances greater than 2.5 Å. These results show that H₂O is a slightly more disordered liquid than D₂O at the same temperature.