Tetrahedral structure or chains for liquid water.

It has been suggested, based on x-ray absorption spectroscopy (XAS) experiments on liquid water [Wernet, Ph., et al. (2004) Science 304, 995-999], that a condensed-phase water molecule's asymmetric electron density results in only two hydrogen bonds per water molecule on average. The larger implication of the XAS interpretation is that the conventional view of liquid water being a tetrahedrally coordinated random network is now replaced by a structural organization that instead strongly favors hydrogen-bonded water chains or large rings embedded in a weakly hydrogen-bonded disordered network. This work reports that the asymmetry of the hydrogen density exhibited in the XAS experiments agrees with reported x-ray scattering structure factors and intensities for Q > 6.5 A(-1). However, the assumption that the asymmetry in the hydrogen electron density does not fluctuate and is persistent in all local molecular liquid water environments is inconsistent with longer-ranged tetrahedral network signatures present in experimental x-ray scattering intensity and structure factor data for Q < 6.5 A(-1).