Reorientational motion and hydrogen-bond stretching dynamics in liquid water

The reorientational motion of the molecules in liquid water is investigated by measuring the anisotropy decay of the O-H stretching mode of HDO dissolved in D₂O using femtosecond mid-infrared pump-probe spectroscopy. We observe that the anisotropy shows a non-exponential decay with an initial fast component of which the amplitude increases with increasing lengths of the O-H^.O hydrogen bond. The experimental results can be accurately described with a model in which the dependence of the reorientation rate on the hydrogen-bond length and the stochastic modulation of this length are accounted for. It is found that the O-H group of a water molecule can only reorient after the O-H^.O hydrogen bond has sufficiently lengthened. As a result, the effective rate of reorientation of the molecules in liquid water is determined by the rate at which the length of the hydrogen bonds is modulated.