The Inhomogeneous Structure of Water at Ambient Conditions

The water molecule, H2O, has deceptively simple structure, but contains all the prerequisites for building complexity. The oxygen atom has a greater affinity for electrons and pulls them away from the hydrogens making them slightly positive. On the back side of molecule oxygen has a lone pair – electrons that do not assist in binding the hydrogens in the molecule, but to which the hydrogens of another water molecule can be attracted to form a so-called hydrogen bond (H-bond). Hydrogen bond is much weaker than the bonding inside water molecule, but it is still strong enough with the possibility to make from one up to four H-bonds per water molecule. The network connected by H-bonds between water molecules makes liquid water so special compared to other normal liquids with about 66 anomalies, e.g. density maximum at 4 °C and large heat capacity. The anomalies of water become extreme in the supercooled region (below freezing point), whilst they are also present at ambient conditions where most of waters’ physical, chemical and biological processes of importance occur. Water at ambient conditions has traditionally been considered as a homogeneous distribution of neartetrahedral H-bonded structures with thermal fluctuations increasing with temperature. This picture has been challenged by recent studies based on x-ray Raman (XRS), x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES), suggesting two distinct local structures with tetrahedral as a minority and highly H-bond distorted asymmetrical as the majority. In particular, the proposed predominant asymmetrical structure has caused intense debate in the last years.