Neutron Diffraction Study on Partial Pair Correlation Functions of Water at Ambient Temperature

A new determination is presented for the intermolecular partial pair correlation functions of liquid water at ambient temperature. Time-of-flight (TOF) neutron diffraction measurements with excellent statistical accuracies have been carried out for liquid H2O, HDO, and D2O at 25 degrees C employing a high performance total scattering spectrometer installed at a high-power pulsed spallation neutron source. The intermolecular partial pair correlation functions, g(HH)(inter)(r), g(OH)(inter)(r), and g(OO)(inter)(r), have successfully been deduced from direct Fourier transformation of observed intermolecular interference terms. The nearest neighbor intermolecular H center dot center dot center dot H, O center dot center dot center dot H, and O center dot center dot center dot O distances are determined to be 2.44, 1.84, and 2.86 angstrom, respectively. These values are significantly longer than those predicted by molecular dynamics (MD) simulation studies employing classical water-water potentials. Evidence on the deviation of the electron distribution from the spherical one around atoms in the liquid water has been experimentally derived from the combination of the present partial structure functions and high precision X-ray data of water obtained from synchrotron radiation X-ray diffraction.