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. (C) 2000 Elsevier Science B.V. All rights reserved.