EFFECT OF DEFECTS ON MOLECULAR MOBILITY IN LIQUID WATER

LIQUID water is a totally connected random network of hydrogen bonds, the connectivity lying well above the percolation threshold 1-3. But despite this extensive association of hydrogen bonds with strengths greater than the thermal energy, the diffusion and rotation rates of water molecules at ambient temperatures are comparable to those of non-associated simple liquids. Many experiments have indicated that the random tetrahedral network cannot be perfect but must contain defects, which are characterized geometrically by the presence of an extra (fifth) molecule in the first coordination shell, or topologically by the presence of 'bifurcated' hydrogen bonds 4-7. Here we use molecular-dynamics simulations to examine the effect of such defects on molecular mobility in water. We find that they provide pathways of lower energy between different tetrahedral local arrangements, thus acting as 'catalysts'. The anomalous mobility of water under compression 8,9 and the decreased mobility in hydrophobic hydration shells 10,11 can be interpreted on the same basis. We suggest that our results are relevant to studies on 'stretched' water 12,13.