The distribution of acceptor and donor hydrogen-bonds in bulk liquid water

We analyse the distributions of donor and acceptor HBs in liquid water and their relation to deviations from tetrahedral symmetry. Their joint, unconditional and conditional distributions are calculated at several temperatures from classical molecular dynamics trajectories for two water models. Surprisingly, the distribution of acceptor HBs terminating at the water lone-pairs is wider than the donor distribution emanating from its hydrogen atoms. This correlates with a higher probability of forming over-coordinated oxygen (OCO) atoms as compared with bifurcated hydrogen bonds (BHBs). Such bonds are possibly formed by HB exchange reactions competing with HB cleavage and formation. The latter obey the binomial distribution, whereas OCOs and BHBs deviate from it considerably. The conditional HB probabilities indicate that acceptor and donor HBs are correlated, so that forming a HB of one type increases the probability of the other, particularly for polarizable water. Conditional distributions exhibit a common intersection suggestive of two types of water structures: high coordination (tetrahedral) and low coordination (polygons). The temperature effect shows that the enthalpy required to cleave a HB in the AMOEBA polarizable model is 9.4kJmol(-1), close to the experimental value of 10.5kJmol(-1). As a result, the fraction of HBs decreases with T near room temperature with a slope closer to experiment than older simulations.