Understanding Translational-Rotational Coupling in Liquid Water through Changes in Mass Distribution

A molecular dynamics study of liquid water and models of water has been carried out to understand the effect of changes in the mass distribution on molecular translation and rotation. Calculations on the motion of (H2O)-H-m and (H2O)-O-n, where m and n vary over a range of values by varying the mass at the hydrogen and oxygen positions, show that these form two distinct series. The two series exhibit different translational and rotational properties. Although a decrease in diffusivity when compared to H2O is observed in both the series, in the case of (H2O)-H-m series, an enhancement in the ratio of diffusivities {D[H2O]/D[(H2O)-H-m]} is found as compared to the square root of the inverse mass ratios, whereas the effect of mass distribution for (H2O)-O-n is seen to lead to a reduction in the ratio of diffusivities {D[H2O]/D[(H2O)-O-n]} with respect to the square root of the inverse mass ratios. However, the ratios of diffusivities in both the series deviate from the corresponding mass ratios, which can be attributed to the translation-rotation coupling in liquid water.