Isotope effects in water as investigated by neutron diffraction and path integral molecular dynamics

The structures of heavy and light water at 300 K were investigated by using a joint approach in which the method of neutron diffraction with oxygen isotope substitution was complemented by path integral molecular dynamics simulations. The diffraction results, which give intra-molecular O–D and O–H bond distances of 0.985(5) and 0.990(5) A , were found to be in best agreement with those obtained by using the flexible anharmonic TTM3-F water model. Both techniques show a difference of ~0.5% between the O–D and O–H intra-molecular bond lengths, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions. Further consideration of the O–O correlations is needed in order to improve agreement with experiment.