  We study by computer simulations the coupled rotational and translational
dynamics of three important linear diatomic molecules, namely, carbon monoxide
(CO), nitric oxide (NO) and cyanide ion (CN-) in water. Translational diffusion
of these molecules is found to be strongly coupled to their own rotational
dynamics which in turn are coupled to similar motions of surrounding water. We
examined the validity of hydrodynamic predictions and found them to be largely
insufficient, particularly for rotational diffusion. A mode coupling theory
approach is developed and applied to understand the complexity of
translation-rotation coupling.
