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.

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Journal Chem. Phys.
Bakker, H.J, Woutersen, S, & Nienhuys, H.-K. (2000). Reorientational motion and hydrogen-bond stretching dynamics in liquid water. Chem. Phys., 258, 233–245. doi:10.1016/S0301-0104(00)00134-8